Caster is the tilting of the uppermost point of the steering axis either forward or backward (when viewed from the side of the vehicle). A backward tilt is positive (+) and a forward tilt is negative (-). Caster influences directional control of the steering but does not affect the tire wear and without adjustable control arms, is not adjustable on this vehicle. With too little positive caster, steering may be touchy at high speed and wheel return-to-center may be diminished when coming out of a turn.
As you can see by the drawing, stock caster to pinion angle separation is 6*. This is represented as the pinion at 0* (zero rise) or 90* at the pinion flange face and the caster set at 96*. If we were to rotate the pinion up to stock JK position, the pinion would be tilted up 2* and caster angle now would be 94*. In alignment terms, this is known as 4* of caster. A stock JK has 4* caster and 2* of pinion up tilt for a total of 6* of separation.
Most lift kits (see note 1) change the caster and pinion angle when they are installed. This is to prevent drive shaft vibration and transfer case damage. Many kits feature adjustable control arms. When a lift kit is installed, the owner frequently complains of touchy steering. As mentioned above, this is due to too little caster. This is where adjustable control arms will help. With these you can remove a little of the added pinion up tilt to help get your caster and steering a bit closer to stock.
There are a number of proper, appropriate ways to measure caster angles. Only 2 can be done with a fully assembled front end housing. First, and most obvious, is with an alignment machine. Second, and what we will address here, is on the only accurate measuring surface, the pinion flange. It has been said that you can get a measurement off of the end forgings (or ‘C’) but this is incorrect. End forging are cast and then machined. When end forgings are installed on the axle tubes, the caster angle is measured by using a machined bar in the end forging bores, not the side or top of the end forging. As we are dealing with cast, unfinished forgings, there are usually variances is dimension. The bored surfaces are precisely measured and their relationship to the sides or top of the end forgings may vary.
On the JK, measuring the pinion angle is a simple exercise in deductive reasoning. We know there is 6* of separation. By using a simple angle finder, we can measure the pinion angle at the drive shaft flange (where the angle reads 0 on the above drawing). You’ll want to zero the angle finder first to compensate for ground that may not be level. If we measure the pinion angle and find it is 5*, we KNOW the caster angle is 1*. If the pinion angle reads 4*, we KNOW the caster is at 2*.
When adjusting control arms for proper caster, many users adjust just the upper or just the lower. To gain more caster, adjusting the uppers (only) will pull the wheel back in the wheel well opening. Adjusting the lowers (only) will push the wheel forward in the opening. Ideally, adjusting both upper and lower is the best option as the wheel is placed properly in the wheel well. When adjusting for better caster angle it is crucial to keep an eye on the pinion to driveshaft angle. Remember, the lift kit companies have changed the caster to keep proper (or close) driveshaft to pinion angles. Keeping these angles in their proper location prevents driveshaft vibration and potential catastrophic transfer case failure.
Any adjustment you make should be minimal. Installing a 4” lift and trying to get back to 4* of caster almost guarantees a quick failure of a driveshaft, transfer case bearing or the entire transfer case. Depending on your lift, getting back to 1 or 2* of caster may be a huge success and make a significant difference in the handling of your JK. Anytime you have adjusted control arms it is a good idea to get a 4 wheel alignment. Using control arms of equal length (side to side) is a very important to keeping the Jeep tracking straight. An alignment can make small corrections that will make a huge difference in tracking.
As was mentioned earlier, the stock JK caster angle is 4*. This is a very slight angle by most standards. Many builders use 6-7* as their standard caster. Having more caster than stock is not uncommon when using an aftermarket axle housing such as a ProRock 44 Unlimited. The ProRock 44 Unlimited is designed with 4* of extra caster. This allows users to dial in their pinion correctly and retain factory caster angles (or better). With this housing you will measure as done with the stock housing but use 10* of separation. If your pinion angle is 4* you have 6* of caster. If you have 5* of pinion angle, you have 5* of caster.
For more information about ProRock 44 specifications, click on the link below:
To help understand the differences between the standard ProRock 44 and the Unlimited version, click on this link:
— Scott Frary, Dynatrac