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7 minute read
Bumpsteer FAQ
Compliments of JMortensen @ http://forums.hybridz.org
8-29-2005
Bumpsteer FAQ
Bumpsteer is a condition where the toe changes as the suspension moves through its travel. It is generally not noticeable on street cars unless they are being driven "arrest-me-officer" fast, but is an important factor with regards to how a race car handles at the limit. Bumpsteer curves can be plotted out, and generally they follow the following rules: when the suspension droops the wheels toe in, and when it compresses the wheels toe out.
A car suffering from too much bumpsteer generally has the following symptoms: Bumps in the road hit by one wheel cause the car to jerk in one direction or the other even though the steering wheel does not move. When driving through a bumpy turn the wheel must be constantly moved back and forth in order to keep the car traveling in a continuous arc.
Why does my Z have bumpsteer in the first place?
Bumpsteer is engineered into most cars direct from the manufacturer. They use bumpsteer as a safety mechanism. The more the suspension compresses, the more the front tires toe out. When you turn very sharply and load the outside front suspension that tire toes out and makes the car want to take a straighter path. This will contribute to understeer and understeer is considered to be a safer and easier condition to control than oversteer.
What causes bumpsteer?
Bumpsteer is caused when the tie rod and control arm are different lengths, or sit at different angles, or a combination of the two. In a Z the tie rod and control arms are the same length from the factory, so the issue in the Z is that the two are at different angles.
How do I measure bumpsteer?
Bumpsteer can be measured with a bumpsteer gauge. Wasnt that easy? 
A bumpsteer gauge is basically a glorified pair of dial indicators. Usually a bumpsteer gauge comes with a steel or aluminum plate which gets strapped to the wheel. A stand sits next to the front tires that holds the dial indicators in contact with the plate. The suspension is moved up and down, and the front and back dial indicators are compared to measure the toe change. Unfortunately, you want to do this measurement right in the cars normal ride height, and through a couple inches of suspension movement on either end of that ride height. To accurately measure the front springs and sway bar must be removed, then the car must be placed at ride height with relation to the suspension and moved up or down while monitoring the toe change on the dial indicators.
How do I fix bumpsteer?
There are many ways to fix bumpsteer. Probably the most common modification available is the bumpsteer spacer which can be purchased from just about any Z parts supplier. This is a spacer that fits between the bottom of the strut and the control arm, and restores factory geometryÂ. It doesn't really restore factory geometry, and it really does not fix bumpsteer either. What it will do is return the ROLL CENTER to a position closer to stock on a lowered Z. What bumpsteer spacers really do is raise the roll center of the car which makes it have less body roll in turns with a given spring rate. The bumpsteer spacer does not eliminate bumpsteer.
The next most common modification is the JTR crossmember modification or some variation of it. This involves redrilling the control arm hole up ¾ or 15/16" and out ¼ in order to reduce bumpsteer and add negative camber at the same time. Moving the pivot out ¼ will gain more negative camber, but it also exacerbates bumpsteer, because now the control arm length is now shorter than the tie rod length. Again, this approach will be helpful to the roll center, but will not cure bumpsteer unless the pivot is moved to the correct height. In order to know what the correct height is you really need to measure on YOUR car, because a 30 year old car is going to vary chassis to chassis, plus some people want to run the "bumpsteer spacer" to raise the roll center AND fix the bumpsteer in addition to that.
Now were into the not-so-common methods for dealing with bumpsteer. One method I have used is to slot the front crossmember. I did this by drilling a hole directly above the original control arm pivot hole then connecting the two holes with a cutoff wheel. I then measured the bumpsteer with a gauge and adjusted the pivot up until it was at 0. I've run my Z this way for more than 5 years, autoxing and doing track days on slicks and have not had the pivots move at all.
So far all of these methods have dealt with moving the inner or outer end of the control arm in relation to the tie rod. It is equally possible to move the tie rod.
One way to move the tie rod is to modify the steer knuckle that the outer tie rod attaches to so that it will accept a common bumpsteer spacer kit. These kits are very common in road racing and circle track racing, and generally replace the outer tie rod with a rod end, then use spacers to move the rod end up or down in relation to the steer knuckle. Cary (tube80z) has pointed out that a standard bridge ream can be used to ream out the hole in the steer knuckle. The bridge ream has the same taper as a Ford, so a Pinto bumpsteer spacer kit available from any circle track racing parts supplier will work once the steer knuckle has been modified. One issue with modifying bumpsteer on the tie rod end is clearance between the wheel and the tie rod. Sometimes there is not enough room to move the tie rod down far enough to get rid of the bumpsteer.
Another way to move the tie rod is to raise or lower the steering rack, which then raises or lowers the inner tie rod. This is not an easy option in a stock Z, but can be done with some fabrication if desired.
Where do I want the bumpsteer to be?
Opinions vary. Some like me prefer 0 bumpsteer, some like John Coffey prefer to tune the car by changing the bumpsteer and roll center characteristics by varying the thickness of bumpsteer spacer used. What is a fairly universal idea is that little toe out on compression makes the car more forgiving and toe in on compression is not a good thing.
Is bumpsteer worth messing with?
If you have to ask this question it probably isnt worth it. 99% of street drivers will never notice bumpsteer or will consider it cured by the JTR mod or a bumpsteer spacer. For those that do drive hard enough to notice it can be very worthwhile.
Modifying your suspension is potentially dangerous and you do so at your own risk.
