Bump-steer

You are on the way to your favorite trail. You hit a pothole. The steering wheel is jerked violently left then right as the truck is heading to the side of the road. You have bump-steer!

Caster

Installing an aftermarket suspension on your truck has some trade offs. On the plus side you gain room for larger tires and more axle articulation. On the negative side you can upset factory caster and pinion angles.

Caster angle is established in the axle design by tilting the top of the kingpin toward the rear and the bottom of the kingpin forward so that an imaginary line through the center of the kingpins would strike the ground at a point ahead of the point of contact. This imaginary center line is the kingpin axis. Most vehicles have positive caster.

The purpose of caster is to provide steering stability, which will keep the front wheels in the straight-ahead position and also assist in straightening up the wheels after a turn.
Pinion angle and caster are changed when you install longer shackles, lifted springs or an improperly completed spring over axle lift. Move the pinion up and you have negative caster, move the pinion down and you have positive caster. This end result is due to the design of a solid axle and the steering knuckle being attached to the ends of the axle tube. As you push the pinion up or down to change pinion angle, the kingpin axis moves the same amount, changing your caster.

Some off-roaders have tried to adjust poor pinion angles by rotating the pinion up with shims or by moving the spring pads on the axle. If done to excess this can cause unusual or unsafe handing characteristics to your truck by inducing negative caster.

If camber and toe-in are correct, you can perform your own caster test as easily as driving the truck on the road and observing its reaction to a simple test. Before testing, make sure all tires are properly inflated, being very careful to insure the front tires are exactly the same pressure. As you drive down the road at a moderate speed turn the wheels slowly from side to side. If the truck turns easily from side to side but is hard to straighten out, insufficient (negative) caster is the problem. To restore the correct caster angle you can install steel shims or rotate the steering knuckles at the end of the axle tube. It is best to have this angle checked by a competent shop, but a lot can be accomplished by the backyard mechanic.

Toe Adjustment

Toe is the turning out of the front wheels. The purpose of a toe specification is to insure parallel rolling of the front wheels. Proper toe adjustment helps keep the truck going down the road in a straight line. A slight amount of positive toe (as shown in the drawing at right) makes the vehicle feel more stable. Improper toe can cause wander and shimmy as your tires start to show unusual wear pattern.

Toe can be very easily set in your driveway. The measurement should be taken as close to the center of the hub as possible. With a simple steel tape measure take a reading at the front and back of the tire in the center. Use the drawing as an example. When setting toe adjustments in the drive way it is very important to make an adjustment, roll the truck back and forth a few feet then recheck your measurement. Toe should be set toward the high side on the positive end of the scale for larger diameter tires.

Back Spacing

To run those gnarly Thornbirds you will need three inches (or less) back-spacing. Most aftermarket wheels I have seen have 3.5 inch  back-spacing. Nearly all Samurai owners with Thornbirds are actually running 2.75 inch back-spacing. This allows sufficient room to clear the lugs on the springs during full lock turns, but it can have an adverse affect on your axle bearings. Backspacing affects your bearings because the center of the contact patch is offset from the kingpin axis. This induces torque to the knuckles.

Articulation

I recently asked the off-road mailing list, "What is your definition of articulation and wheel travel?" I received this response from Mike Kunz:

The ability of one component of a machine to twist off axis from another component of or from the main frame of said machine. This definition is based upon my handy Compton's Encyclopedia Dictionary, which states in part: "a joint or juncture? in the skeleton?more or less freely movable?" "a moveable joint between rigid parts?" and "a joint or connection between two parts capable of spontaneous separation".

How it is Measured

One of two ways it could be measured, based upon the definition given:

1. Angle of the axles to each other, as viewed down the center of the two axles. This would give a "flex angle".
2. Ramp Travel Index (RTI). Drive up a 20 degree ramp until one wheel comes off the ground and measure how far up you went, divide travel over wheel-base at rest and you have a ratio. If you go up without lifting a tire, you hit 1,000.

1. Now that you have 1,000 RTI, drive up the ramp with the trailing wheel/tire off the ramp, again until you lift a tire, measure and divide again. This will give a ratio over 1,000.
2. Alternatively, set your ramp at 23 degrees and repeat the test in the main paragraph above. This will give you a ratio below 1,000 RTI, but for 23 degrees.

An example: I ran up the 23 degree ramp to a score of 869, and up beside the 20 degree ramp to a score of 1373. Of course, when you move a ramp from place to place and set it on different surfaces, all the old figures are now off, since you can't get it back the same, unless you do it in the same spot of the shop parking lot. Remember that weight loading can affect RTI dramatically, so you have to test with the same weight (gas, tools, water, people, etc.) each time.

Both have downsides/disadvantages, and neither reflects real world utility. As an example, if you had a super flexy set of $2k springs from El Specialty Spring Company, but had a stock front bumper and a double tube rear bumper, you could drive up the ramp fine and achieve a marvelous score, but when you tried to climb a rock face, the bumper ends would hit and stop forward motion. Great ramp, but no utility.

Wheel travel

The distance the wheel will move throughout its arc of travel from full extension to full compression, without damaging the vehicle, in dynamic circumstances. You can only estimate this, since you can't repeat in the shop what the wheel will do when you drive off a four foot waterfall, or crest a knife-edge dune at 45 MPH.

How it is Measured

The estimate is measured by hanging your suspension while the frame is on jack stands, and using a hydraulic jack to push the axle away from the frame, measuring droop, or distance from the frame to the axle in a vertical direction, then compressing the suspension as far as possible, measuring again and the difference is travel. This is easy in torsion bar mini-trucks, just take out the torsion bars and compress the suspension. That will show you how far up it will go. In a leaf spring rig, you must extrapolate the settings by looking at where your suspension was when it was on the ramp, and subtracting that figure from the static measurement to get your difference.