Installing bigger tires

[wnn]

most people think they can just pop on bigger tires & be good to go but that expensive set of meats can cost you more money in the long run if your not set up for it.

most your gonna have to know is tire dia. the rest can be achieved using the formulas..

Use this formula to determine what RPM your engine will run based on gear ratio, tire height, transmission and MPH. You can try various combinations to come up with an appropriate combination of gear ratio and tire height for you vehicle. The RPM formula should be used as a guideline for comparative purposes only.

RPM =Mph x Axle ratio x trans ratio x 336 / tire diameter



Recommended Engine RPM @ Highway Speed
4 cylinder: 2200 * 3200
V6 cylinder: 2000 * 3200
Small block: 1800 * 2800
Big block: 1800 - 2600
Diesel: 1600-2800

http://www.drivetrain.com/axlegearratio.jpg
http://www.drivetrain.com/transgearratio.jpeg
http://www.drivetrain.com/tirediameter.jpeg

[alpmco]

I have a 4cyl in my Jeep and after going to 31" I lost the use of 5th gear. No power. I had 4.1 gears and upgraded to 4.88. Made a big difference and I get a little better gas mileage. Next tire change will take me to 33" which is the max size I want to go.
One other factor to consider! What axle you have under the vehicle.
Many axles are only good up to a certain tire size regardless of what gears you stick in it. We run into this problem often in the Jeep world with the stock Jeep axles. There are charts out on the Internet that will show max tire size based on axle. Some Jeepers replace their stock axles with heftier Ford or GM axles.

[wnn]

good point i never really took into accountsince i deal mostly in ford ,jeeps do have thin axles..weakest axle i have has to be the 8.8 ford but that was fixed..thanks for pointing that out

[alpmco]

Yeah, this was a good post. Many people just don't know about the gearing or think about it until after the tires are installed.
I'm a lucky one ... my Jeep is an 86 and it has the Widetrack Dana 44 rear with the oversize axle shaft. The front is a Dana 30 which is ok for my needs. A lot of Jeepers use the 8.8 or GM 10 bolt or Dana 60. (I think I have the numbers right?) I hear a bit of complaining about snapping the AMC and Dana 35 axles with oversized tires. Probably more common wiht the 6cyl or larger engine and the hotdogging factor is probably a big contributor to the problem too.
Don't forget about wheel well clearance too. Bigger tires may require more lift, either spring or body. Lots of Jeepers take a sawzall to the wheel wells. :eek:

[wnn]

this is exactly what i ment by costing more money than just a set of tire alotta factors go can go into this. glad to see you posted relavant info Alpmco thanks:D

[bob200587]

I plan on upgrading my 4runner gears to 4.56 as soon as I get the cash together. it's not so bad right now with 31's, but I could use some extra pep.

[Gunfixr]

I plan on upgrading my 4runner gears to 4.56 as soon as I get the cash together. it's not so bad right now with 31's, but I could use some extra pep.

I wouldn't go any higher than that. Any higher will actually weaken the ring gear setup, as there will be so few teeth on the pinion that only one tooth at a time will engage the ring gear, unlike having two teeth at a time with the 4.10 that they come with.
It then becomes real easy to pop one tooth out under load, and cause the whole pinion to be blown.
This is another factor with putting big tires and higher gear ratios in smaller axles.

This is why back when I had a big 4x4 for serious off road work, it was a 3/4 ton. I even changed those axles out. It was a Ford F250 with a 6.9 diesel, running a Borg Warner chain drive transfer case, Dana 60 rear and Dana 50 independent front, using 3.56 gears.
I changed it to a New Process NP205 gear drive transfer case, a Dana 70HD rear, and a Dana 60HD closed knuckle front, with 5.88 gears running 44"x18.5"w Gumbo Monster Mudder tires. The axles were out of a Kaiser military 1.24 ton ammo carrier. I added a Detroit Locker to the rear axle. The transfer case was seperately frame mounted, the truck using 3 drive shafts. I made them all, using 1/4" wall pipe. I did all the work myself. The C6 tranny I rebuilt using extra clutches for more strength.

Shoulda kept that truck, but it was huge, and people now live where I used to play. It ain't real parking lot friendly.

[alpmco]

Right now I'm running a 4.88 in a Dana 30 front and a Dana 44 rear. I have a 2.5L 4cyl, Borg Warner T5 5 speed, Dana 300 Xfer case Lockright lockers front and rear. The tires are 31's but I plan on going to 33's next time around.
Since I live in Florida I really don't need that much ground clearance. No rocks and deep mud is the only concern. I've had water up to my headlights a few weeks ago (Butt deep sitting in the seat) and it keeps chugging along.

[BobS]

One missing piece of information is the BSFC point of the engine. Ideally, you want to gear the vehicle to be just past the torque peak (by about 10-25%, depending on a lot of factors) which is where the most effecient point of the engine is in converting the liquid fuel into torque (HP doesn't matter, other than for fuel consumption calculations). Most gasoline engines today fall in between 0.39 and 0.52 pounds of fuel per horsepower per hour burn rate and diesels fall between 0.32 and 0.40 #/hp/hr.

To calculate the rate of burn, you calculate the gross weight of the vehicle (includes all loading, such as fuel load, passengers, EW of the vehicle, and the payload), determine the rolling resistance of the tires you are running on the type of terrain you are running, the percentage of grade you wish to climb and finally, at what speed you want to maintain. If people are interested further, you can search on SAE J688 or I can post an example with all the calculations.

The point is, there are a lot of factors not yet touched on in modifying your VCI (Vehicle Cone Index-a military term for the on/off road capability of the vehicle).

Best regards,

Bob

[Poopeyhead]

great post. I have a qusetion maybe you could all help me with. I have a 2002 isuzu trooper. I love it and it's awesome off road with the 4 wheel independent suspension. It is auto v6 with the "torque on demand" whatever that is. I'm going to install a 2" suspension lift from calmini ( see it here) http://www.pureisuzu.com/2_trooper_lift1.htm

I was thinking of putting on 31" tires (thats what they recommend). My questions are: should I regear it and if so what size. I live is So Missouri and it's really hilly driving. Your tranny gets a workout.

By the way...did I mention I'm not that technical

Thanks,

Poopeyhead

[BobS]

great post. I have a qusetion maybe you could all help me with. I have a 2002 isuzu trooper. I love it and it's awesome off road with the 4 wheel independent suspension. It is auto v6 with the "torque on demand" whatever that is. I'm going to install a 2" suspension lift from calmini ( see it here) http://www.pureisuzu.com/2_trooper_lift1.htm

I was thinking of putting on 31" tires (thats what they recommend). My questions are: should I regear it and if so what size. I live is So Missouri and it's really hilly driving. Your tranny gets a workout.

By the way...did I mention I'm not that technical

Thanks,

Poopeyhead
Using this info as a reference:

http://www.automotive.com/2002/12/isuzu/trooper/specifications/index.html

...There are a couple of misconceptions you have about your Trooper. The front suspension is independent, but the rear suspension is not-it is what is commonly called a "link-coil" Hotchkiss suspension. This in no way is intended to "put down" the Trooper-they are really nice offroaders (I had 2 for company cars when I worked with Isuzu in Fujisawa - in Japan - years ago).

The weak links in the Trooper design is as follows (I will explain each afterwards):

1. Low plunge front half shafts

2. Low angle Rzeppa joint outer, front joints in the front half shafts

3. Low spring rate front and rear springs (torsion bar in front and coil in the rear)

4. Marginal brake swept area

5. Extreme travel in the rear suspension for suspension roll

_______________________________


To take the easiest first, item 1 and 2 above, The half shafts used in the suspension design are not bad for what they are on a street vehicle. The issues of wear and restricted travel come into play when you lift the vehicle. WHat happens is the angularity between the 3 centerlines (1-where the shaft bolts to the axle at the tripod joint -a form of high travel u-joint-, 2-the centerline of the intermediate shaft that connects the 2 joints -the inner tripod and outer Rzeppa-, and 3 the outer shaft that connects the intermediate shaft to the wheel spindle). To use this design on the street, you have a maximum of travel that the parts can move. What happens when you lift the vehicle, is increase the angularity from it's design position and the distance between the jounce stop (where the control arm hits the frame of the vehicle-usually a rubber bumper) and the ride height, as well as decrease the amount of rebound travel (how far the tire may move downward, which is limited by the length of the shock absorber).

This increased angularity causes the joint to run hotter, melting the joint grease (maing it more viscous) and causing metal to metal failure inside the joint, due the the grease no longer providing a slippery surface for the joints.

Additionally, you run the risk of physically pulling the joints apart if, for some reason, you would break a shock absorber, as no downward stop would exist any longer. This is why off road racecars use limiting straps on the axles-to prevent u-joint damage.

The kit you are looking at SHOULD mitigate most of this as it is not an extreme amount of lift. The only issue I would forsee is a loss of durability of the halfshafts. The only way to completely correct this is to have a custom set of half shafts built of a totally different design-at a cost of over $800.00 for EACH side. Otherwise you just live with it.

________________________________________


For item 3 and 5 above, the soft springs the Isuzu uses is a good thing for offroad use, in that it allows the suspension to do the flexing, not the body. The bad thing is that these soft springs REQUIRE the use of stabilizer bars (sometimes called anti-sway bars) on both ends of the vehicle. The problem with this is that the stabilizer bars, being a torsion design, prevents body roll on the street, which translates to less tire articulation off road, thus restricting mobility.

The lift kit does not, itself alter the spring rate (unless it uses larger diameter wire in the design for the bars and coils), unlike the common thought-by lifting the vehicle, all you are doing is raising the ride height. The result of this lift is, however, and increase in the LEVERAGE of the control arms to the frame attaching points at a static design height. In the CalMini case, you should be ok here.

The stabilizer bar issue is another kettle of fish. The only way to minimize it's effect on the offroad performance is to "break it free" on one side of the vehicle. Dodge and Jeep do this with the AAM "Smart Bar" decoupler design (http://www.aam.com/index.php?s=48)installed by Chrysler. The "poor man's" way of doing this is to replace the end links of the bar with quick disconnect links and hitch pins. This way, when you go offroad, you stop and get out of the vehicle, go to one side of the front and rear stabilizer bar mounting, and pull out the hitch pin, releasing the bar from restricting the articulation of the axle and tire assembly.

_____________________________________

For item #4, the brakes on the Trooper are sized such that a 31 inch tall tire can be OEM fitted and brake acceptably well to meet the FMVSS standards. For offroad use, They would be fine for light 'wheeling. If you decide to "get brave" however, you will be in some deep doo-doo. Get a larger diameter brake kit for the front and rear.

_____________________________________

The stock gear ratio is 4.3:1 in the Trooper, so no changes would be made until you go larger than a 33" tall tire. I would, however, look into getting ARB air locking differentials (http://www.4x4wire.com/isuzu/projects/roundtrooper/ARB/)for the front and rear axle if you are going to do any "hero" type 'wheeling. I hope this helps.

Best regards,

Bob

[soberups]

If I am depending on a rig for SHTF, the last thing I am going to do to it is add a lift kit and big tires. That stuff costs money and it doesnt add much to the utility or durability of the vehicle. A good set of stock-sized mud tires and some heavy duty shocks ought to be sufficient. I would rather spend the money on stockpiling fuel, you cant pour a lift kit into your gas tank.

[yugoshooter]

If I am depending on a rig for SHTF, the last thing I am going to do to it is add a lift kit and big tires. That stuff costs money and it doesnt add much to the utility or durability of the vehicle. A good set of stock-sized mud tires and some heavy duty shocks ought to be sufficient. . And a good winch.

[alpmco]

Depending on the vehicle a lift doesn't cost much ... Well, I guess that's all relative.
I put a 3" lift on my Jeep and total cost with longer break lines and shocks was $300 plus my time to install it. Older Jeeps are easy .. you could even do a shackle only lift but keep in mind each inch of shackle only gives 1/2" of lift and it adds an obstruction to your angle of approach.
If you can weld you can get 4 -5 inches of lift by doing a spring over. You keep the stock springs and move the mounting perch from the bottom of the axle to the top. This also moves the U-bolt nuts to the top side so they don't get hung up on logs and such.

[bob200587]

I wouldn't go any higher than that. Any higher will actually weaken the ring gear setup, as there will be so few teeth on the pinion that only one tooth at a time will engage the ring gear, unlike having two teeth at a time with the 4.10 that they come with.
It then becomes real easy to pop one tooth out under load, and cause the whole pinion to be blown.
This is another factor with putting big tires and higher gear ratios in smaller axles.


I understand that situation, but on the 8" toyota gear you can go as low as 5.29 without having one less tooth. Some of the automatics even came with 4.88 gears from the factory. But I won't need to worry about that unless I decided to do a solid axle swap and bump up to like 35's or bigger....and that's a bit excessive for my purposes. Especially since the 4runner is now my daily driver again.

[Poopeyhead]

BobS,

You are the man. That was more info in one post than I've been able to find in a week of searching the web. I appreciate the help. I won't ever do any serious wheeling. But where we do go camping and hiking there are areas where ground clearance is an issue. Stock ride height just isn't gonna cut it. But mostly it's a daily driver on road. It's so hilly here that even driving the stock setup, the tranny is always shifting.

I would also say that ride height is a benefit in a shtf scenario. There are plenty of people with oversize tires and shocks that could make it down muddy and bumpy roads. But what if there is a felled tree blocking the road, or a roadblock and your forced to go around. Most oversize tires without a lift will rub the fender wells and give you less performance off road. My goal is to be prepared if I need to get into the woods and get around anything or anyone that is blocking my progress.

So after looking at the CalMIni kit, did you think it was a quality setup? There are only a few making lift kits for the trooper. Thanks again for the info.

Poopeyhead

[hitech_hick]

I have built a number of off-road vehicles, and I have gotten to the point to where I will not even think about tires or a lift until I am ready to regear/swap axles.

With all things considered with wind resistance, don't forget about the added rolling resistance of the new tires. Over-sized tires will weigh more than stock tires (unless you get aluminum rims to offset this) requiring more power to turn them. Wider, more aggressive tires will also create more friction between the tires and the road requiring more power.

I am certainly not trying to convince anyone to not build an off-road vehicle here, but I would certainly recommend doing your homework first. I Good set of locker, although pricey, will normally get you further than a lift kit. As for a winch, after putting one on my truck, I don't know how I got along with out one.


hick

[daisycutter]

Another factor most don't think about...I'm in 4x4 daily. I supervise 5-7 logging companies for a living...forester of private timber. I would challenge any takers to compete with my stock tires (Super Duty 17" tires) running iron (tire chains). I have a set of mud service with links the size that goes on trucks and road graders. In snow and mud the extra width gets you over the bank really fast. Narrow gauge tires get down to the traction quicker. Wide tires have a place in the sand and swamps, but not in the Pacific NW. In the spring, I open snow drifts for tree planting and spend all day everyday chained in iron. I'm stuck 3-4 times a day on purpose depending on what it takes to get a road open. For what I do, I would never leave the highway with wide tires. Gearing is a huge issue and the application of throttle control for any given condition is the difference between being augered in and heading to the next mud hole.

[BobS]

BobS,

You are the man. That was more info in one post than I've been able to find in a week of searching the web. I appreciate the help. I won't ever do any serious wheeling. But where we do go camping and hiking there are areas where ground clearance is an issue. Stock ride height just isn't gonna cut it. But mostly it's a daily driver on road. It's so hilly here that even driving the stock setup, the tranny is always shifting.

I would also say that ride height is a benefit in a shtf scenario. There are plenty of people with oversize tires and shocks that could make it down muddy and bumpy roads. But what if there is a felled tree blocking the road, or a roadblock and your forced to go around. Most oversize tires without a lift will rub the fender wells and give you less performance off road. My goal is to be prepared if I need to get into the woods and get around anything or anyone that is blocking my progress.

So after looking at the CalMIni kit, did you think it was a quality setup? There are only a few making lift kits for the trooper. Thanks again for the info.

Poopeyhead
Thanks for the kind words-just remember, this is what I have done for a living and hobby for over 30 years. I have broken more parts and cars than most have ever seen. hehehe

Since you are asking a specific question, it becomes apparent to me you need some backround information before I respond directly. I am not trying to intimidate you, I want to make sure you understand exactly what I am talking about-because the "offroad community" in general, doesn't have a friggin' clue about WHY vehicles move offroad or onroad. Because of this, I am going to break this post into several smaller, more easily digested, sections.

article (c) by RW (Bob) Sheaves, catNET, Incorporated, 2008

Section #1- VCI and what it is

The most important information first-VCI ain't pronounced "vickey", it's "vee cee eye".

:D

Seriously, there is a "thing" called VCI that defines how mobile a vehicle is-on or offroad. VCI is an acronynm for "Vehicle Cone Index". See:

FM 5-420-00-1 Chapter 7 for an explanation of how soils (meaning any terrain vehicles will be required to operate in) affect travel and how it is measured. http://www.globalsecurity.org/military/library/policy/army/fm/5-430-00-1/CH7.htm

You can also purchase the classic referance book:

Theory of Ground Vehicles by J.Y. Wong http://books.google.com/books?id=LH8wd8im13AC&pg=PA123&lpg=PA123&dq=vci+vehicle+cone+index&source=web&ots=4S4y_HKVXt&sig=86SrUV-5_bQ9j47e5at71HH8--4&hl=en&sa=X&oi=book_result&resnum=10&ct=result


Basically, all the toys you can add to a vehicle is meaningless to the vehicle's ability to move, if you ignore the single, most important, basic, concept of VCI-

The single most important part of any vehicle is it's ability to keep it's tire ground contact patch on the terrain. All of the vehicle's dynamic forces are transmitted through this interface to go, stop, turn, climb,and get you out of Dodge.

Many people will argue this. Let them. At the bottom of all the arguments, if you cannot keep the tires on the ground, you have no control over the vehicle and you ain't moving, period. Spinning the tires, sliding the tires, lifting a tire, etc., for whatever reason, will cause the vehicle to stop making controlled progress in forward or reverse.

Reread that last paragraph and think about what I am stating. These are very specific terms I am using and just sliding around this issue is going to cause you to miss some later information.

END Section #1.

Best regards,

Bob

PS-Note to mods-If you wish to reorganize the info I am presenting here in a different format, feel free to do so. If you wish to repackage it for download, please contact me before doing so. Thanks!

[daisycutter]

Furthermore...........10 ply is more survivable and less exposure to puncture. 3 ply side wall a must. Tires with puchy side wall are just asking for destruction. Better have two spares minimum. tough wins over looks every time

[BobS]

Article (c) RW (Bob) Sheaves, catNET Incorporated, 2008

START Section #2

In the last section, I described what makes vehicles move. Simply put, that little flat spot on the bottom of your tires that meets the "road" is the most important way to make your vehicle move.

Now, let's look at what a tire is and how it works....

(WARNING- I am going to use specific engineering terms here only where necessary, but be aware, when I use them, they have a VERY specific meaning, and usually, one that is contrary to "common knowledge". Accept them or not, but these terms are NOT open to debate. They are accepted within that circle that does this stuff for a living. If, for some reason, I have not explaned it clearly, please feel free to ask a question, no matter how stupid you may think it is. The only stupid question is one that remains unasked. As this series of sections gets longer, there won't be time to catch up.)

Reference document links:

http://www.designfax.net/archives/0905/0905applying_FEA.asp

http://www.sensorprod.com/static/tire_tread.php

http://www1.eere.energy.gov/vehiclesandfuels/pdfs/success/rollingresistance4_9.pdf

http://www.offroaders.com/tech/AT-MT-Tires/Yokohama_Geolandar.htm

In general terms, there are 3 parameters of a tire that governs mobility:

1. "Tooth"
2. Ground Pressure
3. Self Cleaning
_____________________

Item 1. "Tooth"

The term "Tooth" refers to the manner and amount of the interface between the tire tread, sidewall, and internal air pressure/volume to the terrain to be traversed.

Think of a set of spur gears:

http://static.howstuffworks.com/gif/gear-spur.jpg

One gear is the tire tread and the other gear is sand. The place where both mesh is the "Tooth" volume. The better the engagement of the teeth of the "gear" (between the tire and terrain), the greater the shear resistance, the greater the traction and resultingly, the greater the available shear torque to move the vehicle. When this shear torque exceeds the available, multiplied engine torque available at the ground contact patch, the vehicle stalls out and cannot climb any further due to limited engine torque input.

Now, in the real world, you will never see the maximum shear point of the tooth. What you will find is that this tooth will break traction becasue you have exceeded the shear of the terrain you are traveling on. This is why tires spin. The terrain cannot resist the engine multiplied torque applied to the terrain by the stronger tire tooth and as a result, the terrain "tears" or properly, exceeds it's shear strength.

VCI is a measurement of the strength of the terrain to resist shear.

One issue to this would be the next logical leap of understanding- the deeper the mesh of the teeth, the better, right? Example: tall, narrow tires, in mud have a lot of tooth depth, right?

Surprisingly, no, this is not true, because of the shear factor-mud, having a large quantity of water, has LESS shear strength per pound than a solid brick of dried mud. This is why you have to understand that soil strength (or properly, VCI) means more than the "stuff" used to overcome problems. If you don't understand the "why" of a problem, you are shooting in the dark as to an answer that will keep you moving. You MAY hit one solution that worked in one try...but only by dumb, blind luck.

The vehicle development engineer's prayer:

Dear God, PLEASE gimme understanding of a problem before I start to break parts AGAIN due to my OWN stupidity!

LOL!

_____________________

Item 2. Ground Pressure

Back in 6th grade science, we all learned that:

"For every force, there is an equal and opposite force"

The force applied to the terrain is a function of the mass of the vehicle, it's attitude (meaning the angle of attack) relative to the terrain, and the tooth component of the VCI measurement.

This force looks like this:


http://www.sensorprod.com/images/tread_sp_site_image01.jpg

CAPTION: Ground contact patch force example.


...and is easily measured by using tools shown here:

http://www.sensorprod.com/static/tire_tread.php


ROT*- The less the ground pressure per square inch, the greater the mobility, up to the limits of VCI.

*ROT=Rules Of Thumb, or the assumptions engineers use to get into trouble when the rules don't apply.
_____________________

Item 3. Self Cleaning

The interface between the tire and the terrain, in the case of low force shear materials, will leave pieces of the softer material enmeshed with the harder material. In the case of a tire and soil, the soil generally loses.

For this reason, the tire tread must be able to expel these pieces of soil quickly, or the excess material wil spin around and act as both a micro-hill to climb (increasing the required power to maintain mobility) and, once climbed, act as "loose ball bearings" (or properly, "friction reducers") between the tire tread and soil, reducing the friction between tread and soil.

In most of today's on/off road tread designs:

http://www.offroaders.com/tech/AT-MT-Tires/images/Geolandar-cross-section.gif

CAPTION: Cross section of Yokohama Off Road Tire

...the designs are there to mimic the actions of a grouser bar, used in tracked vehicles. The problem (and why there are so many conflicting design approaches) is that tires used in the offroad community are used on both highway (hard soil and high shear surfaces) as well as dirt, mud, snow, gravel, etc (soft soil and low shear strength surfaces). How the manufacturer decides to bias the efforts to balance the different operating parameters is an argument best left to another article.

_____________________

END Section #2

[BobS]

Article (c) RW (Bob) Sheaves, catNET Incorporated, 2008

START Section #3A

In the last section, I described the factors of how mobility is determined. In this section I will describe some of the methods to alter those factors. This will NOT be all inclusive, but will cover several of the most common techniques. I leave it as an excercise for the reader to follow through, if interested.

The first modification to the vehicle mobility is...........



THE DRIVER.

No joke. The driver is the most important link in keeping the vehicle moving. Attention to the terrain, knowing what the various sound and vibrations in the vehicle mean and where they are coming from, and understanding YOUR limits are more important than any other part in or on the vehicle. Be sure you follw Dirty Harry's maxim:

A man has to know his limitations.

Read, understand, and PRACTICE the information in the following links:

http://www.steelsoldiers.com/index.php?module=pagesetter&type=file&func=get&tid=1&fid=file&pid=174 Vehicle Damage Assessment and Repair

http://www.enlisted.info/field-manuals/fm-21-305-manual-for-the-wheeled-vehicle-driver.shtml Principles For The Wheeled Vehicle Driver

http://www.pirate4x4.com/tech/billavista/Recovery/US%20Army%20driving%20manual%20vehicle%20recovery% 20ch22.pdf Vehicle Recovery Operations

http://www.ukoffroad.com/expedition/offroad_tips.html Offroad driving

http://www.offroadexperience.com/offroadguide1.htm Offroad driving

There are others, but they tend to get too specific for this discussion. Another series of articles can be written if there is sufficient interest.

http://www.4wdonline.com/A/PiCs37/RollOver01.jpg

CAPTION: OOPS!!!

_____________________

END Section 3A.

[BobS]

Article (c) RW (Bob) Sheaves, catNET Incorporated, 2008

START Section #3B

TIRE SYSTEMS

Now that the driver is fixed....ok, let's keep clean thoughts now, shall we?

Let's take a look at the tire interface.

1. Tire inflation pressures
2. Up or Down- How do I do it?
3. CTIS manufacturers specifics
4. Siping- What is it and Why do it?
5. AT or MT?
_______________________

1. Tire inflation pressures

Tire pressure affects a lot of vehicle operations. Fuel economy, wear, ride, handling, stability, costs, are all items that can be affected (to a greater or lesser value) by a simple item called tire pressure.

The OEM specifies a tire pressure that is developed for what is called BPL, or Body Part Loaded. This design point varies from manufacturer to manufacturer in the specifics, but generally, the standard is 2 each 150 lb. human riders (driver and passenger), 1/2 the load capacity for cargo/luggage/stuff, and full liquids (oil, coolant, fuel). This example pressure we will assume as specified to be 28 pounds per square inch in front and rear tires.

If you look on the sidewall of the tire:

http://www.autofarm.net/101/autofarm/Tire-Information-Graphic[1].gif

CAPTION: Notice the small MAX rating

...you see that, in this example, the tire is rated at 35PSI COLD (this means check the tire in the morning BEFORE you drive it) and to carry 1510 pounds at this pressure, at an "H" speed rating (see:

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35

...for an explanation of the speed rating). This link also contains the clearest explanation of the other factors affecting street use.
______________________

2. Up or Down- How do I do it?

By weighing your vehicle on each tire, or weighing each axle and dividing by 2 (not necessarilly accurtate, but for this example, we will not investigate asymetrical loading cases), you will determine the weight on each tire. You can determine the proper tire pressure for your own tires by:

X= Weight corrected tire pressure
P1= Maximum listed tire pressure
W1= Maximum weight loading per tire
W2= Measured actual weight per tire
F= Correction factor

F= W2/W1 then X= P1*F

or

W1 = 1521
W2 = 1339

then

F= W2/W1 or 1339/1521=0.880341... rounded down to 88%, therefor F=0.88

X=P1*F or 35*.88=30.8119.... rounded up to 31psi COLD is your target pressure for optimium operation, according to the OE specifications.

]ROT*- NEVER, EVER, EVER run less than the door listed OE REQUIRED minimum pressure on a street vehicle with stock tire, regardless of weight.

*ROT- you DO remember what I said this meant, don't you? Violate this tire pressure at your own risk!!!![/B]

Now that you know the street pressure you need to run, lets look at offroad driving pressures.

Basically, as we learned in the VCI information, the ground contact patch must maintain even, full size contact with the terrain being traversed in order to propel the vehicle. We can alter the vehicle performance of the VCI by raising or lowering the tire pressure.

For a practical example of the difference (side by side actually) see:

http://www.4x4abc.com/jeep101/soft-sand-deflate.html

ROT*-Lower tire pressure in soft and dry soil, raise it for dense but loose soil.

*ROT-This ROT assumes no other traction enhancing devices like tire chains or tire cables, paddle tires, ag equipment tires, etc. In other words, we are assuming only commercially available military or civilian tire pattern tires from major manufacturers, not specialty companies. Don't complain if I don't cover your favorite tire brand-if it ain't OEM supplier built somewhere in the world, it don't matter for this discussion.

To lower the pressure, obviously, you may do it by depressing the valve core and letting out enough air to get to your desired pressure. Which is fine, but how the heck do you know what is right for your vehicle in the conditions? Below are general recommendations for various conditions:

1. If you are going to go through hard pack dirt-leave the tire pressure alone.

2. If you are going through up to 4 inches of soft dirt over a clay, packed, undersoil, drop the tire pressure by 5%.

3. If you are going through soft soil with no clay base, drop your tire pressure by 10%.

4. If you are going through the Sahara's wind blown dunes, God (or Allah, or whomever) be with you, and drop your tire pressure down to around 10-15 pounds per square inch per tire and pray....a LOT! Especially if you don't have beadlocks at this pressure-more on beadlocks in another section).

5. If you are going through mud up to 6 inches deep over a "hard pack" (gravel or rock) base, do NOT change your tire pressure.

6. If your are going through 12" of mud over a "hard pack" base, INCREASE your tire pressure up to 10%, but DO NOT EXCEED THE MAXIMUM on the sidewall of the tire.

7. If you are going through more than 12 inches of mud, break out the tire chains. Changing the tire pressure ain't gonna help. You DID remember to pack properly fitted tire chains didn't you?

You DID remember to take your calculated street pressure as the baseline for calculating these new pressures didn't you? Great, I knew ya could. Now write them down, with the conditions to use them, and tape them with clear waterproof tape to the inside of your glovebox so you don't have to remember them!! Remember the 7P's-(Lord knows I can't...my DI in the military in the late 1960's impressed them on my posterior permanently!)

Prior Planning Prevents Piss Poor Practical Performance

Time for another way to screw it up....I mentioned earlier the term CTIS. Any one of you from the recent military can hold on for a minute-this next section is for the FNG's (yeah, if you don't understand this, we'll just let this acronynm pass right on by. I don't want anyone mad at me....hehehe).

CTIS- this translates to Central Tire Inflation System. Fancy, huh? If any of you have even seen a military HMMWV M998 series vehicle (gotta' LOVE those acronynms the military uses) or a civilian Hummer H1, you have, more than likely, seen those 4 letters on a funny triangular shaped plate attached to each hub and funny bolted together 2 piece wheels, covering an air line and the valve stem with an odd shaped valve on it. The dashboard, if the owner hasn't run you off from getting to close to his or her truck, has a black box attached to it with some pushbuttons on it.

http://www.lynchhummer.com/Components/Compressor.gif

CAPTION: AM General Air compressor system for CTIS.

http://www.lynchhummer.com/Components/CTI.8.gif

CAPTION: Air passage through the HMMWV wheel end hub.

CTIS is used to raise or lower the tire pressure on the fly by the driver from the control pod. There is also one more, tiny, little, advantage. Certain systems developed for the military, are capable of keeping a tire with a few bullet holes in it inflated (the HMMWV's AIN'T capable of this!) and if the system cannot keep up with the air loss, the wheel has an integral (to the tire) "runflat" inset and "beadlock" rings to keep the flat tire in place on the run...OOPS, I meant on the move.

One of the most difficult problems to overcome with a CTI system is not the system itself, but rather, it is one of maintenance and simple COST. Do you enjoy waiting 10 minutes in the morning to wait for your tires to fill up? Especially in a -20 degree F winter wind? These are possibilities of this kind of air pressure system that gets worn and has minimal maintenance. Even new ones are susceptible to failure.

ROT*- KISS

*Do I really, REALLY need to explain this one?


Fortunately, there are other alternatives to CTIS and one of them should be the FIRST modification (after the mentioned tools and spare parts are collected, boxed, and secured to the vehicle) you do to your vehicle in preparing it for venturing off into the boonies.

Once you have gotten comfortable getting towed out of trouble you got into by following this advice a little too liberally, it's time to look at how tread patterns affect the offroad traction and built on your lessons to create yet MORE ways to get stuck.....

Below are examples of various offroad tread design patterns available from Michelin:

http://www.d-90.com/tire/tire10.gif

CAPTION: Michelin XL Tread Pattern

http://www.d-90.com/tire/tire31.gif

CAPTION: Michelin XZL Tread Pattern

http://www.d-90.com/tire/tire32.gif

CAPTION: Michelin XML Tread Pattern

In the first picture, the Michelin XL, you see a tread pattern that was developed specifically for the military, for use in the EU for NATO forces on both CTIS and non-CTIS (more about CTIS later) equipped vehicles. For comparison, this would be considered equivalent to a "Super Swamper" tire from Interco, one of the largest specialty tire manufacturers of DOT legal, offroad and highway specific, tires in the world. This is an excellent mud, snow and rock tire for traction. The limiting factor for the XL, as found out by the US Marine Corps on the LAV25 which was originally equipped with the XL's from GM/MVO (General Motors/Military Vehicles Operations), was that the sidewall was extremly fragile when used in rocky conditions. Interco took this into account with the development of the Super Swamper and reinforced the sidewalls with extra plys of cord in their radial tire revamp in the late 1980's.

One of the problems with a directional tred is that, while optimized for forward travel on the tread, reversing that direction was darn near impossible to back out of, when (not if) you got stuck.

ROT*-NEVER use offroad directional tread designs on any vehicle that must be mobile OR run on the highway for any length of time (usually more than a couple of hours before cooling down).

*If you do, you'll be SORRRRYYYYYY!!!! Imagine having a flat with a directional tread, not having the proper side spare, and installing your spare backwards. Not only will you overheat the tire, it will pound you to death (ok, not really, but you will WISH it did after a couple hours of the vibration) and possibly, if enough heat is generated, throw an alligator (seperate and discard the complete tread into the sky. It's really fun to watch-they can get up there!).

The Michelin XZL pattern was developed originally for offroad heavy equipment that work in rock quarry's. As a result, the pattern took the directional tread design of the XL, modified to to be more sharp edged , raised the durometer of the tread (this means the toughness and hardness of the rubber was increased to increase the resistance to puncture, abrasion, and tearing when used on granite rock), and converted the design to a non-directional tread that would "pull" equally in forward or reverse.

The Michelin XML is the latest military version with the "latest tech", including the non-directional tread pattern, siping available, heavy sidewalls, multiple durometer lugs, and the rest of the doodads and geegaws.

Now for the bad news...these Michelins are not cheap-$500.00 a tire or more is simply not affordable for most people. The point of including them here is that they represent the epitome of the tire design efforts around the world. This way, you can judge for yourself what points are important and choose a good quality tire without mortgaging the kids.
______________________

3. CTIS manufacturers specifics

I will not belabor the specifics of CTIS system design. Suffice to say, you CAN retrofit it to any vehicle but the cost is extreme. If you plan on keeping your vehicle for a long time and travel extensively offroad (for work, as an example) you may be able to justify such an expense. Otherwise, there are other options you can use. Not quite as convenient, but FAR less expensive (do you really want to pay over $5000.00 USD to change the tire pressure in your street driven car?). Below is more information on some of the various systems available.

For the record, yes, I do use CTIS and my company owned vehicle is outfitted with a custom designed CTIS system to allow me to do the work I do (it is hard to chase offroad race cars with a stock truck through the open desert). Let the namecalling begin...LOL!

CTIS explanation from HowStuffWorks.com:

http://auto.howstuffworks.com/self-inflating-tire.htm

CTIS system diagram from Eaton Corporation (Dana/Spicer Division):

http://www.roadranger.com/ecm/idcplg?IdcService=GET_FILE&dID=132574

CTIS Troubleshooting reference information from Eaton Corporation (Dana/Spicer Division):

http://www.roadranger.com/ecm/idcplg?IdcService=GET_FILE&dID=145126

CM Automotive Aftermarket CTIS product system information:

http://www.cmautomotive.com/Brief%20Description.htm

AIR CTI Aftermarket retrofit system:

http://www.aircti.com/?gclid=CNWxptTp75QCFSBciAodK2pYqg
______________________

4. Siping- What is it and Why do it?

I will nnot discuss siping here as there is considerable information on the internet about it. In my experience, for offroad tires (NOT for street tires or dual purpose tires) it is useless and destroys the integrity of the tread lugs the manufacturer has designed into the tires.

A good explanation is found here:

http://en.wikipedia.org/wiki/Siping

...along with how-to links if you feel the need.
______________________

5. AT or MT?

AT=All Terrain
MT= Mud Terrain

Both of these tread pattern descriptors are sort of misleading, but for general use (not engineering use) they are accurate enough to get the point across. In general, and AT treaded tire will be a closed tread design (this means the tread has small spaces between the lugs), has increased rain performance grooves (as described in the SIPING section) for the street, and the lugs are generally smaller, leading to the following performance:

1. Lower rolling resistance (compared to a MT tire-but still a LOT more that a true street tread. But then you ain't reading this to learn about street tires, is ya?)
2. Better street performance in braking effort transferred to the road surface.
3. Less tendency to hydroplane (lift off the road surface because the lugs are rolling too fast to squeeze out the water on top of the road surface during a rain).
4. Better fuel economy than a MT tire (again, far less than a street tread though), due to lower rolling resistance of the internal construction flexing against all the other internal parts of the tire-refer back to the cross section of the Yokohama tire to see how much "stuff" is built into a tire).
5. Lower tire life overall, due to construction, and the fact you are going to be beating it up offroad (fact of life-TANSTAAFL*)

*TANSTAAFL= There Ain't No Such Thing As A Free Lunch-from Robert Heinlien "Time Enough For Love"

MT treaded tires will be a very open tread design (this means the tread has great damn big spaces between the lugs), has zip for rain performance grooves (as described in the SIPING section) for the street, and the lugs are generally huge and sharp edged, leading to the following performance:

1. High heat buildup in the tread and a lower speed rating (usually 55mph maximum for the best MT tires like the Michelin XL above).
2. Very poor street performance in braking effort, due to less of the braking effort being transferred to the road surface, because of the large spaces between the blocks. In other words, the gear teeth cannot mesh and take advantage of the height of the tread against the high surface shear strength of the road surface.
3. Most tendency to hydroplane , unless you cut siping, and then, the siping is still only a partial solution. Think of siping as a bandaid applied to a cut off limb.
4. Worst fuel economy of any tire (again, infinately less than a street tread), due to high rolling resistance of the internal construction flexing against all the other internal parts of the tire and the stiffness of the blocks making up the tread pattern).
5. Really BAD tire life overall, due to construction, and the fact you are going to be beating it up even worse offroad (fact of life, again-TANSTAAFL*)


______________________

Conclusions on tires:

1. Select a multiple ply sidewall (3 or4 ply at least), and multiple ply tread carcass (6-8 ply at least).

2. Select a non-directional tread.

3. Choose a tread pattern based on the service life you expect (30,000 mile life expectancy is too long for a heavily used offroader).

4. Make SURE you take into account the amount of street driving you are going to do-don't buy fashionable knobby tires for the mall (you WILL DIE at some point, why make it so soon due to stupidity?).

5. Don't second guess yourself-you CAN always wear the tires out and try something else, so don't overspend (until you have experience and more money, anyway).


END Section #3B

[BobS]

BobS,

You are the man. That was more info in one post than I've been able to find in a week of searching the web. I appreciate the help. I won't ever do any serious wheeling. But where we do go camping and hiking there are areas where ground clearance is an issue. Stock ride height just isn't gonna cut it. But mostly it's a daily driver on road. It's so hilly here that even driving the stock setup, the tranny is always shifting.

I would also say that ride height is a benefit in a shtf scenario. There are plenty of people with oversize tires and shocks that could make it down muddy and bumpy roads. But what if there is a felled tree blocking the road, or a roadblock and your forced to go around. Most oversize tires without a lift will rub the fender wells and give you less performance off road. My goal is to be prepared if I need to get into the woods and get around anything or anyone that is blocking my progress.

So after looking at the CalMIni kit, did you think it was a quality setup? There are only a few making lift kits for the trooper. Thanks again for the info.

Poopeyhead
OK, now back to your original question....

(I get long winded at times, don't I? hehehe)

My point to all this was that tire selection is far more important than a lift kit. Your lowest point of ground clearance* is at the rear axle carrier housing. The only amount of effective ground clearance increase is going to come from changing the tire diameter. Increasing it (i.e. going to a taller tire) will help, but a lift kit is just a quick and dirty way of fitting larger tires.

Increasing the breakover angle is a bit more complex, but it too will be increased by the change in the static loaded radius of the tires (this is NOT one half of the diameter-it is less than that because of the vehicle loading the tire and compression of the sidewall of the tire. The breakover angle is determined by drawing a line, tangent to the static loaded radius of the front tire, through the lowest point of the underbody structure or driveline, that gives the lowest possible angle. The same procedure is done to the rear tire static loaded radius and the included angle between these 2 lines will give you the breakover angle.

Now, that being said, you may decide you want to install 33" tires (for this discussion, I will assume so, to give the worst possible condition). Obviously they will not clear without a lift kit, right?


WRONG

There are many other ways to fit the tires (remember the front end problems I told you about earlier), but the least costly (but more costly than that Calmini kit) is to cut out the wheelhouse aperture, fabricate a flare (probably by fitting a set of Jeep Cherokee "CutOut" Flares from Bushwacker-they have a close contour to the Isuzu) and beat the living daylights out of the inner wheelhouse to clear the tire when it is turned left and right. The wheels should be stock offset and I would recommend beadlock wheels for their safety when blowing a tire (even if you do not run a set of run-flat inserts, the beadlock will prevent the tire from coming off the wheel when it shreads or collapses from a puncture offroad), which WILL happen at some point-be prepared.

The catch to this work is the cost-you WILL spend more money upfront (I would guess $1500.00, not counting repainting the fenders front and rear after modifications, but over the life of the vehicle, you will have less repairs to the front end (half shaft issues primarilly, as I mentioned) AND you will gain more real ground clearance than you would with a lift kit.

On the positive side also, and perhaps more important if you (or anyone else with your permission) have never driven a lifted vehicle to understand how the handling on the street has changed and how much more quickly you can get into BIG trouble!!!

Best as alwyas,

Bob


*Ground Clearance is ALWAYS determined at the lowest point under the vehicle...and it ain't the transfer case or anything else in the middle of the vehicle. This is one of the fallicies of the "offroaders" that show they havent got a clue what it takes to make a vehicle offroad mobile.

[SHNIPE]

from my own personal experience michelins bring teh suck in comparison to quite a few other brands and they come in 16's where as more replacements (at least locally) are found in 15" rim sizes. not sure about mud but for the rocks with occasional water. they dont cut the mustard. they might wear well but the tradeoff for traction isnt one i would take.

If i were really worried about punctures i would stick with a moderate ply sidewall and work with plugs and spares if it meant being able to getout of where im going. Thats just the going rate here in az with the sharp rocks

[BobS]

from my own personal experience michelins bring teh suck in comparison to quite a few other brands and they come in 16's where as more replacements (at least locally) are found in 15" rim sizes. not sure about mud but for the rocks with occasional water. they dont cut the mustard. they might wear well but the tradeoff for traction isnt one i would take.

If i were really worried about punctures i would stick with a moderate ply sidewall and work with plugs and spares if it meant being able to getout of where im going. Thats just the going rate here in az with the sharp rocks
There is a big difference between Michelin military XML's and civilian tires. XZL's are for rock. Either works better for mobility than any other tire, but there is cost for that mobility.

AZ....you mean like Yuma PG? ;)

Best regards,

Bob

[SHNIPE]

i understand the difference in the tires. Ive been in rigs running both and every tire in between. Mich's fall behind in most categories other than wear. in that category there are quite a few that match it though. My personal favorite would be a set of radial maxxis creepies.

Just for reference heres some 52" XLZ's next to my jeep during the teardown.. No they didnt go on my jeep but that woulda looked sweet.. till i flopped.
http://i162.photobucket.com/albums/t272/SHNIPE/Jeep%20Update%2010-16/Teardown%20pics/DSC02465.jpg


Im near phoenix. Fountain hills actually. Not the yuma proving grounds but our proving grounds are far more telling.

http://i297.photobucket.com/albums/mm226/cjchuck/ABigfallonAA.jpg

http://i297.photobucket.com/albums/mm226/cjchuck/DTopoffallAA.jpg

This one also proves the lifting a tire doesnt always mean loss of forward progress
http://img.photobucket.com/albums/v727/shmude/H2H%209-27/CIMG1492.jpg

http://i297.photobucket.com/albums/mm226/cjchuck/2BMFall.jpg

[SHNIPE]

** Edit: Tires are VERY subjective. Tires from one area dont always work well in others. Choose wahts best for your local

BobS has tons of good info. But tires are like wives... you might hate your best friends wife and think he can find better... but he loves her and shes perfect for him.

[BobS]

Tires ARE subjective....up to a point. Testing on real world terrain, as well as computer program terrain, eliminates as much subjective prejudice as possible. Been doing this too long to go into specifics, but 30 years of it teaches a few things... :D . The biggest thing is that tastes dffer, as you say. Biasing tread design against cost and construction, let alone varied terrain is what each person must do on their own. I can only guide the direction of the search.

Sort like a gynocologist, in your example of wives. I can tell you what part will serve what function, but you have to decide which you want to use.......


Quick comment on the issue of a tire off the ground-Your comment is not really applicable and is definately misleading, in the original context of a stock vehicle without LSD or lockers in the axles-which I mentioned elsewhere. Be careful in making generalizations outside of the original context without full disclosure.

Best regards,

Bob

[Kobayashi Maru]

BIG meats should be reserved, for BIG trucks!

First off, a Suzuki Samurai, is a TOY. You wind the rubber band REAL tight, and you're good in 3" of snow or less, on a paved, maintained, salted road. In daylight hours. Maybe... You do NOT however, put BIG meats on it.

36" M/T's are going on my '92 Dodge Ramcharger - Dana 60 front axle and Chrysler 9 1/4 rear. 3.50 gears means a little off the top-end performance (who offroads in 4th gear or O/D anyways - ASIDE from Baja competitors? :rolleyes:). Professional SUSPENSION lift, means it still steers, still stops (longer brake cables!), and all w/o breaking the drivetrain while doing it. And it will NOT be a daily driver, but is LITERALLY, THE BoB! It gets regular shakedown trials, going to Boy Scout campouts, however. :D

:salut:

[arkfroader]

I own a VERY capable Jeep. Stretched wheelbase, coilovers, heavy duty axles with alloy shafts, 38" tires, beadlocks, ect ect ect. I could drive crosscountry (not taking into account rivers) without ever touching pavement (or a trail for that matter).

That said, in a evacuation situation where the roads are still accessable. I would leave it at home. My low mileage dodge 4x4 truck with a slight tire size increase and Mud Terrain tires is 1000X more dependable and wouldn't draw a bunch of attention. I guess it would be concievable If fuel was not a concern to toss the Jeep on a trailer and take them both just in case;)

BTW SHNIPE nice rig :D

[SHNIPE]

Thanks Ark... good to know there are some other Pirate4x4 guys around. Pics of the jeep??

BIG meats should be reserved, for BIG trucks!

First off, a Suzuki Samurai, is a TOY

Ive seen samis capable of going places and running as big of tires on yota axles as your rig could handle. depends on the weight of the rig and what types of traction are available. Out here in AZ we have lots of buggies with lighter axles that do just fine. we also have some very large rigs that snap the heaviest of duty axles. I personally went with lighter axles but will run 40s with no problems

[arkfroader]

http://img.photobucket.com/albums/v326/lrc4x4/Crawl%20Magazine%20Run/aprl08044.jpg

[hitech_hick]

http://img.photobucket.com/albums/v326/lrc4x4/Crawl%20Magazine%20Run/aprl08044.jpg

You are missing 2 lug nuts, I only count 6...;)

Just kidding, nice looking rig...


hick

[SHNIPE]

Damn good lookin YJ..

[arkfroader]

You are missing 2 lug nuts, I only count 6...;)

Just kidding, nice looking rig...


hick
:D :laughing:

I get that a lot, but I get by pretty good. Skinny 38's, alloys, 100:1, stock motor, and weighs in at less than 3800lbs with me in it. I'm not scared to beat on it and I usually have the smallest tires with the crowd I run with.