I'll be adding some pictures soon to help explain some of these concepts. Until then, sorry for the long explanations.

Bump Steer
Driveline geometry
Tire size and gear ratios
Axle Wrap (coming soon)
Sway bars (coming soon)
Approach, departure and break-over angles (coming soon)
Long-arm suspension (coming soon)
Locking differentials ("lockers") (coming soon)

Bump Steer:

Bump steer is a problem that applies specifically to vehicles with a solid front axle. Vehicles with independent front suspension (IFS) generally do not experience bump steer as the result of a lift.

First, here's a brief explanation of how your steering works on a solid axle rig. Your steering wheel is attached to a steering shaft. At the end of that shaft is a pitman arm. The pitman arm is a lever that swings side to side as you turn the steering wheel. Connected to the pitman arm (the end that swings back and forth) is one end of the track bar. The track bar crosses diagonally down from the pitman arm and is connected to the steering knuckle on the front wheel on the opposite side of the vehicle. Finally, there is a steering linkage that connects both steering knuckles together so both wheels turn at the same time.

Now for some trigonometry. First, imagine the hands of a clock. As the minute hand moves from 40 to 50 minutes, the tip of the hand moves almost completely vertically. However, it also moves a little bit from right to left. Notice that the 8 is mostly below, but also slightly to the right of the 9 on the clock. Keep that in mind...

In stock configuration, the track bar is nearly horizontal, just like the minute hand of the clock between 40 and 50 minutes. When your hit a bump, specifically with the wheel that connects to the track bar, your wheel moves vertically. As it moves vertically, the end of the track bar moves up and down. But, just like the minute hand of the clock, it also moves slightly laterally (horizontally). Since there isn't much shock travel in stock suspension and the track bar is nearly horizontal, there isn't much lateral movement in the track bar.

Now imagine dropping the axle 12" but keeping the same pitman arm. In order for the track bar to connect to the steering knuckle, and for the wheels to remain straight, it must be longer and it will drop at a much steeper angle. Now when you hit a bump and the end of the track bar still moves vertically, but it now moves much more laterally. Imagine the minute hand going from the 35 to 45 minute position. There is much more lateral movement than before.

This exaggerated lateral movement of the track bar has the exact same effect as turning your steering wheel - it pushes out on the steering knuckle. The more severe the angle, the more severe the problem. So, to compensate, you can use a "drop" pitman arm. This arm functions the same way as a stock pitman arm, but it drops the point where the track bar connects, keeping the track bar closer to horizontal, therefore reducing the lateral movement of the track bar you hit a bump.

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Driveline Geometry:

First of all, I won't even presume to know everything there is to know about driveline geometry. I think it's 50% science and 50% art. If you really want to learn about driveline geometry, I recommend reading the tech page at the Tom Woods custom driveshaft webpage. Put on your thinking cap... This guy knows his stuff!

In light of that incredible resource, here's an explanation of driveline induced vibration and some solutions. When the universal joint - or "U-joint" - in a driveshaft is operating at 0^o, both parts of the shaft on either side of the joint spin perfectly along their axis. However, as the joint actuates, or bends, or flexes, or whatever you want to call it, the part of shaft that is deflected doesn't spin perfectly along its axis. Instead, it spins in an elipse around its axis. This non-centric rotation causes vibration. As U-joint angle increases, so does the severity of the vibration. Excessive U-joint angles can quickly lead to premature joint failure, often catastrophic. (Snap! Crunch!) Well, if you're going to lift a vehicle, then you will be introducing radical U-joint angles, and now you've got problems.

Each U-joint uses two yokes, one on each side, to connect to the two portions of the driveshaft it is connecting at different angles. These U-joints and yokes are what allow the driveshaft to function at an angle. So in a typical driveline, you'll have a yoke at the transfer case connected to the yoke on one end of the driveshaft via a U-joint, then another yoke, U-joint and yoke at the pinion (differential) end of the driveshaft.

The alignment of the U-joints at each end of the shaft is particularly important. As I stated before, as a U-joint flexes, the deflected portion of the shaft spins about its axis in an elipse. Imagine if the two ends traveled in elipsis that didn't overlap. It would introduce "wobble" that would make the vibration unbearable. Instead, the U-joints are aligned so that both joints travel in the same elipse. So, the only mass (weight) not spinning perfectly along its axis is the driveshaft. The driveshaft itself, then, is the cause of the vibration. Reduce its mass or the severity of the angles of the U-joints, and you can reduce the vibration.

A constant velocity (CV) driveshart (often called a "double cardan" also) uses three U-joints. Two right next to each other on the transfer case end, and one on the pinion (differential) end. The two U-joints next to each other each handle half of the deflection angle of the driveshaft. The axle is then rotated up at the pinion to allow the pinion to align as straight as possible with the driveshaft (no more than 3^o deflection). By doing this, the very short portion between the two U-joints at the transfer case end of the shaft becomes the only portion of the driveshaft that spins in an elipse. Its mass is much less than that of a full-length driveshaft, effectively reducing the amount of vibration in high-angle driveshafts. The drawback to this is that rotating your axle up at the pinion means that the pinion may not get sufficient lubrication. (You're rotating the pinion above the fill level of the differential. Not good.) It maybe necessary to install a new fill plug in the differential cover to facilitate over-filling of the differential. (Which has its own bad side-effects. But that's not a discussion for here.)

Another solution for reducing driveshaft induced vibration is to drop the transfer case. When you lower the transfer case mounts, the engine / transmission / transfer case assembly pivots on the engine mounts, allowing the transfer case to point down at the differential. This reduces the angle that the U-joint must deflect in order to reach the differential. The axle must then be rotated up slightly in front so that the axis of the differential pinion and transfer case output shaft remain parallel. The drawback to this solution is that it reduces the amount of clearance under your Jeep, which is the whole reason you lifted it in the first place, right?

Fixed vs Slip Yoke

As the suspension does its job, your axle moves up and down. In the case of leaf spring suspension in particular, the axle moves up and down in a (nearly) perfect vertical line. This introduces a problem with your driveshaft. Dig out your math book and look up Pythagorean's Theorem. Remember that a² + b² = c². In the case of your Jeep, "a" is the distance from your transfer case output shaft to a point directly above your rear axle. Then, "b" is the distance from that same point directly above your axle down to the axle itself. So, "c" is the length of the driveshaft. Now, imagine that as your suspension flexes, the distance between your axle and the chassis - "b" gets shorter. As a result, "c" - or the driveshaft - must also get shorter.

There are two ways to compensate for the length change in a driveshaft as the axle moves through its range of motion. The first is to have a 2-piece driveshaft, one fits inside the other. Both shafts have splines, allowing the smaller, inner shaft to slide in and out of the larger, outer shaft.

The other method - and the one used on many Jeep vehicles - is to not change the length of the driveshaft, but instead change the distance "a." By using a yoke that can slide in and out of the transfer case - or a "slip yoke." In this case, as the distance "b" between the axle and the chassis decreases, the distance between the tranfer case U-joint and the point above the axle - "a" - becomes longer, allowing the length of the driveshaft - "c" - to remain constant. As the axle moves through its range of motion, it simply pushes and pulls the yoke in and out of the transfer case along the axis of the yoke and output shaft.

That works great on a stock Cherokee. But on a lifted Cherokee, it causes two big problems. First, and most obvious, the amount of axle travel in a lifted Jeep means lot more movement in the slip yoke. So much so that you can drastically reduce the splint-to-spline contact between the yoke and the output shaft, causing them to strip and fail. (At the worst time, no doubt.)

The second problem is the angle of the driveshaft itself. In stock configuration, the driveshaft is much closer to parallel with the chassis than on a lifted Jeep. That means that the driveshaft is almost perfectly aligned (axially) with the transfer case yoke. So, any movement of the driveshaft is nearly in-line with the axis of the yoke and output shaft. However, in a big-lift Jeep, the driveshaft angle is much steeper. So, as the axle moves through its range of motion, the driveshaft and slip yoke are not nearly as aligned axially. That means that a great deal of force from the driveshaft pushing on the slip yoke is not delivered axially. Instead, the force is delivered transversly (across the yoke, rather than along its axis). That leads to increased wear on the yoke, output shaft and slines.

By replacing your stock slip yoke and fixed driveshaft with a fixed yoke and slip driveshaft, you can eliminate this excessive wear and tear on your transfer case, yoke and U-joints. Another added benefit is that a fixed yoke is several inches shorter than the stock slip yoke. That means an increase in the distance "a," which means a longer driveshaft ("c"), and therefore, reduced driveshaft angles at both ends.

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Tire Size and Gear Ratios:

A Jeep just doesn't look right without big tires crammed underneath it. But there's more to big tires than good looks. Putting bigger tires on (in conjunction with a suspension and/or body lift) has some key benefits. First, it increases your approach, departure and break-over angles. Second, bigger tires offer better traction.

Unfortunately, many Jeep owners just put a lift and bigger tires on without paying any attention to its affect on gearing and final drive ratios. Short and sweet: putting on bigger tires reduces the amount of torque your rig will put out. When it comes to four wheelin', torque is your best friend, and you want to do everything you can to increase it, not throw it away. (On the up-side, however, you'll get great gas mileage!)

Time to dust off the old physics book. Simply put, torque is a force multiplied by a distance. It is always measured in a length*force, such as foot-pounds, inch-pounds, newton-meters, etc.

Your engine produces a certain amount of torque. Whatever that torque may be, it is relayed via your transmission, transfer case and differentials to your axles. As your axles spin, they produce a certain amount of torque. (The gearing in your transmission and transfer case increase the torque of your engine.) When your axles rotate, they (obviously) spin your tires. The torque at the axle is divided by the radius of your tires. The result is an amount of force that your tires deliver to the ground. If you increase your tire radius, you decrease the force your tires exert on the ground. (... and that's bad.)

To demonstrate this, imagine lifting a bucket full of water with your arm fully extended. The muscles in your shoulder produce the torque, and the length of your arm is like the radius of the tire. Now, put that bucket on the end of a long stick and try to lift it again. Even though the bucket isn't any heavier, it will be nearly impossible to lift. Your shoulder is still producing the same amount of torque, but you increased the length of the lever, which reduced the amount of lifting force you create at the end of the lever.

Not only do you reduce torque, but you also throw off your speedometer. Larger tires have a larger circumference. A larger circumference means the tire travels more ground in a single rotation than a smaller tire. So, your speedo thinks it's going slower than it really is. News flash: you're speeding. This can really wreak havoc on automatic transmissions' shift points and on newer computer-controlled engines where many of its functions are dependant on speed.

A simple and effective solution to these problems is to install different ring and pinion gears in your differentials. (Make sure they match in both front and rear axle!!) References to gear ratios can often be confusing. These ratios are measured as XX to 1, or XX:1. That means for every XX rotations of the pinion (which is driven by your driveshaft), your ring (and therefore your axle) makes 1 single rotation. The higher the number XX, the more torque you will produce. However, you're actually geared lower - meaning a lower overall ring speed. So, just remember that "geared low" means you have a higher gear ratio (numerically higher).

By reducing the gearing in your differentials (increasing the ratio) you can compensate for the loss of torque to your wheels. Calculating your new gear ratio is really very simple. Here's how:

current ratio / current tire radius * new tire radius = new ratio

It really is that simple. I'll use my Jeep as an example. Stock differential ratio is 3.55:1. Stock tire radius is about 13.25". New tire radius is about 16.5" (for 33" tires). So, 3.55 / 13.25 * 16.5 = 4.42.

Ring and pinion gears come in sets, and are only available in certain distinct ratios. You can't get a R&P set made to just any ratio. You have to pick from a discrete set. So, in my case, I picked the closest ratio to what I needed, 4.56. If you're new ratio falls in between two available gear ratios and you can't decide, consider this. Do you want more offroad capability, more towing power, but worse gas mileage? Then go for the lower (higher numerically) set. Do you want improved traveling range (driving distance), improved gas mileage, but less torque and pulling power? Then pick the higher (lower numerically) gear set. If you plan on building a serious trail rig, opt for the lower (higher numerically) ratio. Many trail-only rigs will have much, much lower (numerically higher) ratios. That gives them incredible torque and the ability to tackle incredibly tough obstacles at very low, controllable speeds.

Final note: Since it's very difficult to match a gear ratio and tire size exactly to what it was when your rig was stock, your speedometer may still be off. It's a good idea to install a speedometer recalibration unit to dial in the speed signal going to the speedometer and all the other various speed-dependent functions on your vehicle.