YJs and Bigger Tires

So You Want Bigger Tires So… What To Expect? And Parts Mike's Response Says:

If you have seen our Project YJ you might have noticed it has a Rancho 3” Lift Kit and ProComp 31” tires. This combination has worked well on and off highway. This is a setup built for moderate offroad use and extended highway driving. The gears were upgraded to the New 4:88 from Drivetrain Direct with their New Super 35 Axle. This combination and a Detroit Locker has served our pourposes well under all our driving conditions.

What we have noticed: The Procomp tires due to their rubber compound have shown noticable tread ware

The Rancho Lift has lost some of it origional 3” Plus lift. The rear has settled to a 2” lift and the front is about 2.75” of lift. The rear springs have been loaded down due to the amount of equipment we have to carry across country and on the trails. There is no winch on the front bumper so that has kept the front in pretty good shape lift wise.

PM Says: Larger tires have some very positive attributes: They allow more ground clearance and they normally are wider and this adds to stability. Their liabilities are:

Bigger Tires: The Engine must work harder causing more Stress and less miles per gallon fuel economey and poorer engine power and performance. PM Says: This can be dealt with by changing gear ratio, the larger the tire the lower (higher numerically) the gear ratio has to be changed. This is due to the added leverage and greater weight that is applied by the tire against the axle. The consequence is like an earthquake, it will increase the “magnitude” of the % of change in greater then one to one relativity from small to large. ie: from a 30” tire to a 33” tire is 9%, you can offset this by using a gear ratio change of APPROX 9%, if change from the 30” to a 35” tire the difference is 14% but the gear ratio differnece to keep relitvely close in performance will require at least an 18% rate and if you have a small engine you will need 23% lower gear, if you keep increasing tire size the % becomes a much greater. In this scenario we are only addessing overall street gear ratio, for offroad you then are concerned with lowering the crawl ratio, though lower geared transmission and transfer case modifications.

Bigger Tires: The Engine will have a Lower rpm because of the Bigger Tires. PM Says: This is due to the…the increased diameter which means that the distance traveled in one revolution is further, thus the engine will be turning less rpm to acheive the same mph/distance.

Bigger Tires: The speedometer and odometer will not be correctly calaberated. PM Says: To oliveate this ..you must change the speedometer driven gear in the transfer case or transmission or add an external ratio correction device

Bigger Tires: The tires will rub against the steering arms and springs. PM Says: If you don't have the clearance for the larger tire. You can use a different offset (spacing) from the inside of the wheel to the wheel mounting surface that moves the tire away from the interference.

Bigger Tires: The tires might not fit in the wheelwells PM Says: This should be addressed prior to installation. A lift kit (suspension, body or combination of both) will allow the larger tire to be installed.

Bigger Tires: Cause additional stress on your breaking components. PM Says: Once you decide that you want the larger tires break out your wallet to purchse heavier duty axle and drivetrain components

Bigger Tires: You must regear your differentials. PM Says: You betsya! For Bigger Tires: You should consider a lift kit. 3” and lower will result in less modifications. 4” Plus will result in more modifications such as extended brake lines, SYE, with new drive shaft, dropped pitman arm, etc.. Spring over axle (leaf spring SOA) give a minimum of 5.5”s of lift. Coil spring lifts have come around to the point that the after market manufactures of lift lits are supply complete systems that are known as “long arm”, this is in an effort to lift the vehicle so that you are not going beyond the limits of the length of the control (4 link) arms. On any lifted vehicle driveline angles are critical and must be checked and addresses if they go beyond the working limits of the ujoint and driveshaft yokes.

Bigger Tires: You should consider a lift kit. 3” and lower will result in less modifications. 4” Plus will result in more modifications such as extended brake lines, SYE, with new drive shaft, dropped pitman arm, etc.

PM Says: Lift Kit Alternitives: Body lifts over 1” not recommended.

Because…they tend to cause problems with breaking the body mounts, shift and clutch linkage can only move so far without having to abandon stock parts, brake lines and steering shafts as well as the radiator location has to be dealt with.

Shackle lifts (.50 (1/2”) - 1.25 (1 1/2”) are ok. Over 1.25 not recommended. Because… Fact: NEVER lift your leaf spring vehicle by using shackles that are more then 3/8” longer then stock. Long shackles break frames, twist springs, wearout bushing quickly and cause your vehicles caster angle on the front to change causing steering (wander), the rear shackles will cause ujoint failure and vibration.

Bigger Tires: Wheel backspacing? PM Says: Wheel backspacing on stock rims is about 5.25”, NO, Front wheel drive vehicles have this type of what is known as “positive” backspacing. The later model vehicles run somewhere in the neighborhood of 4” positive backspacing on a 7” wheel, this is in an effort to allow weight reduction by using smaller axles, smaller steering components and lighter wheel bearings. The older vehicles with heavy running gear run 3 1/2” back spacing, this is what attributes to the additional stickout you see on YJ/TJ vehicles using after market wheels 8” and wider

Bigger Tires: Should have aftermarket rims with backspacing around 3.75 - 4”? PM Says: This depends on the particular vehicle. The Rock Crawler guys that are running Dana 60 front axles are using 10” and wider wheels with 2-2 1/2 inch backspacing, this is necessary due to the fact they are trying to run 15” wheels and they will not fit over the 1 ton Dana 60 brakes. The 15” wheel is the standby of offroaders, I believe that it is time to change and move up to 16 or 17 inch wheels. This allows for a lighter tire and wheel assembly and this is a poitive. Less rolling resistance.

The Process of YJ Evolution

The Evolution: Stock: Moderate Mods: Serious Up-Grade: EXtream Simi Pro Level:

Just Empty Every Pocket http://www.jjournal.net/jeep/features/BiggerTires/

 
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