The elimination of bump steer seems to generate a lot of angst and discussion among locost builders, and I assume among other types of builders as well.
I can't comment on what is an acceptable maximum amount of bump steer, or whether toe-in or toe-out is best. Here are my observations and my reasons for making them:
This discussion is about bump steer design for the front of a RWD car.
Bump steer for an IRS design can be sorted using the same principals. Some of the considerations are different. I believe that RWD vehicles should always toe-in on bump, to avoid toe-out under acceleration (the rear end will squat), which is unstable (reference, Smith or Hammill).
What does bump steer do? Some say minimal bump steer is vital for a good handling car. I'm not sure what they mean by that - in a turn the steering wheel is turned, so the pivots are no longer aligned and bump steer calcuations and measurements no longer work - it doesn't take much misalignment at all to send them astray. It certainly is true in the rear, where there is no steering input, but the front?
On rough roads, bump steer can cause the car to jump around as suspension movement results in single wheel directional changes.
Under braking the front end will dive into bump, and the wheels will toe-in or toe-out depending on the geometry. It's best they do neither, but I suspect if that's not possible it's better for the road if the wheels toe-in on bump for stability. For the track perhaps toe-out is preferable to help with turn-in. Notice that the MGB, a production car, seems to toe-in on bump (as best as I can measure fully clad suspension joints while under a car). Note the bump steer curve is calculated from the measurements, not actually measured.
I've also read (Hammill?) that the front should be set-up to toe-out on droop so that the inside tire will be at a higher slip angle in a corner.This doesn't seem quite right. First, the wheel is turned, so the original bump steer settings have little effect. Second, toe-in or out is the difference of both wheels not of a single wheel - the driver will find the correct location between the two. Finally, every design I've looked at has virtually no "roll steer" (not really the correct term), the amount of toe does not change at all when the chassis rolls - what's lost on bump on the outside tire is gained on droop on the inside tire. Here's the MGB again in for 0 to 3 degrees of roll:
The following analysis is based on the Locost book locations as best as I can determine them, with some fictional locations for the steering joints.
Prevailing theory is that the rack end pivots should be on a line between the upper and lower control arm pivots, and the tie rod end should be on a line between the upper and lower ball joint centers on the upright, with the tie rod aligned with the virtual roll center of the suspension. This seems contrary to ackerman, which would have the outer steering arm bend slightly, but that's a different discussion. This picture shows how the theory would be put into practice using a rack of 18.5":
This gives a pretty good approximation to minimize bump steer, but surprisingly is not the best. The reason is that instantaneous center moves as the suspension moves, and the tie rod moves off of the ideal location. This figure shows the same suspension under 3" of bump and the toe difference that results:
As you can imagine, the amount of bump steer is not necessarily linear. This graph shows the resulting bump steer using the so called "ideal" location. Notice this is all toe-out for bump and droop.
Some theorists tell you that a horizontal tie rod is vital to minimizing bump steer, with the linkages on the appropriate lines again. This is completely false in every analysis I've done. The geometric reason is the difference between the virtual roll center and the tie rod virtual center. It might be correct for the best handling, but it sure doesn't minimize bump steer. So is it unacceptable or best? I don't know, check it out and decide for yourself:
And bumped, the change in tie rod end length (so the amount the wheel will turn) is highest of all of the choices.
And the full analysis:
So what is better than the theory? A computer search can find locations that will minimize bump steer throughout the range. Here's one that I've done:
Notice that at maximum bump of 3", the amount of toe change is less than the "theoretical ideal".
And it's still better throughout the entire range:
One question is whether the difference between the search and the "ideal" theory is enough to worry about. I suspect not - it's probably too small to measure, and setting the locations of everything this precisely is too much for my skill. What is interesting is whether it's necessary to cut your steering rack to reach the ideal measurement, by theory or computer. Check out this analysis using a 23.5" rack instead of the 18.5" used above. It's only slightly worse than the "theoretical ideal", and much better than the "horizontal ideal" that many swear by.
Here they all are again to compare side by side. Note the "horizontal ideal" is using a different scale because it's so far off from the others.
| "Theoretical ideal" | "Horizontal ideal" |
|---|---|
| Same Rack, Better Spot | Using A Long Rack |
You can get the complete drawings from BumpSteer.tcw for turbocad 7.0 or greater, or BumpSteer.dxf for the common exchange format. Note there are some differences between the results of the computer analysis and the drawings. This is due down to round off errors between the computer calculations, the precision I've used to measure and save results, and the drawings themselves - the tolerences for measuring steering angles is very small.
If you don't have a computer program to find the best location, and a steering rack you're told won't work?
First, you'll probably find there are constraints on mounting the steering rack in the chassis. The tie rod end / steering arm height can be changed only if you have custom steering arms, or you heat and bend the stock arms, or you convert to spherical rod ends and use "bump steer" spacers. You may find you need to shorten your steering rack to get around the chassis members - I did.
Install your suspension and steering components, including your shocks, with the spring removed and the chassis in the air, hubs blocked up to ride height. Guess where you want your rack and temporarily fix it in place - usually with the tie rods horizontal and the rack centered between the steering arms. Roughly set your wheel alignment for toe and camber. Now grab the tie rod end / steering arm connection, and move the wheel through it's full travel (usually limited by the shocks). If you can feel the arm move in and out, you've got bump steer. Try changing the height of your rack or tie rod end on the steering arm until the movement is minimized. You're on your own to guess, if you've any bump steer you can't eliminate,whether it's too much and whether you should set it for toe-in or toe-out - sorry :). For mine I was okay with not being able to feel or see any bump steer, I didn't feel the need to break out the dial gauge.
If you're really having trouble, try what Rorty does:
