bump steer kit

[Bill Waters]

A number of forum members have used bump steer kits which they have gotten from Summit Racing. The only one I find there is a Baer for 2002-2005 WRX and STI models. I thought I had seen some of you with 2006 -plus donors using the kit. Does anyone know whether the Summit kit for 2005 models fits a 2006 rack? Not apparent to me why there would be a difference.

Thanks as always.

Bill

[Mitch Wright]

Has anyone actually checked the bump steer on the R?

[Bill Waters]

Fair question, Mitch.

My understanding from posts on this forum has suggested that the S using a stock rack will have horizontal tie rods. One would reasonably presume from this that there is minimal toe change in bump or droop. The frame on the R - and hence the rack - mounts about an inch or so lower than the S, so I would assume that the tie rods will not be horizontal. Also, I thought I remembered (but am not sure) that the FFR R uses a bump steer kit.

Bill

[DruOdil]

check out Plavins build thread. He used them and said they would fit the 06.

[Bill Waters]

Will do; thanks, DruOdil.

Bill

[Mitch Wright]

It is on my list to discuss with FFR when I pick up my Kit. I am looking forward to checking both the front and rear.

[Frank818]

First, many people may not know what "bump steer" is.

Here's what I found:

Bump-steer is a change in toe* angle caused by the suspension moving up or down. Bump-steer is built into the geometry of the suspension and steering system, and has nothing to do with turning the steering wheel. The effect of bump-steer is for the wheel to toe-in or toe-out when the suspension moves up or down. This toe change or "steering" occurs any time the suspension moves, whether it is from body roll, brake-dive, or hitting a bump in the road. Bump steer is undesirable because the suspension is steering the car instead of the driver.
The front wheels do not move directly straight up or down when the car hits a bump. Instead, the wheel follows an arc, or curving path, that pushes the wheel slightly inward (towards the centerline of the car) or outward (away from the car) in response to vertical wheel movement. The outer tie-rod (which connects the steering rack to the wheel) also moves in-and-out in an arc as it moves up and down. If the rate which the outer tie-rod arcs in or out does not match the rate the wheel moves in or out, the wheel will be turned by the tie-rod. This is bumpsteer.
The center point of the arc traveled by the wheel (known as the instant-center) is controlled by the location and angle of the moving suspension links. This point moves as the ride height changes. In contrast, the arc of the outer tie-rod is controlled by the position of the steering rack, which is fixed. In order to eliminate bumpsteer, both the length and the center point of the two arcs must be the same. However, since the instant center moves with ride height, bumpsteer cannot be eliminated throughout the entire range of suspension travel. Therefore suspension designers concentrate on minimizing bumpsteer within the range of movement closest to factory ride height. Changing the ride height or other suspension components may move the suspension outside this narrow "optimized" window.

Now if on the S the tie rods are horizontal (in line) with the steering rack, why would we need a bump steer kit?

[wleehendrick]

Now if on the S the tie rods are horizontal (in line) with the steering rack, why would we need a bump steer kit?

We (S builders) shouldn't. But as I understand it, since the R move the suspension pick-up points 1", a bumpsteer kit is required to make a similar translation of the tie-rod ends.

[C.Plavan]

I have the bumpsteer kit on my R- It will fit the 2006 steering rack. We will most likely need to make longer spacers to have "correct" geometry, but I'm not there yet. I just got the tires mounted today on the wheels that took forever to get from Europe. Now I can start working on it again :)

[Bill Waters]

You are correct, Wleehendrick.

Thanks for the info, Chad.

Bill

[DruOdil]

So what would the bump steer kit do to the S ? Would it add to the performance?

[wleehendrick]

So what would the bump steer kit do to the S ? Would it add to the performance?

Hey Dru... If you build an S at normal ride height, bumpsteer is minimized already, so adding a bump-steer kit would then add bumpsteer... not what you want! Only if you mount the suspension at the R's 3.5" ride-height, would you want to add a bump-steer kit.

[D Clary]

If you put the bump steer gauges on an S, you would probably have more bump than you would want in a race car. On the street it just doesn't matter unless it is really extreme. On the track car we try to keep it under .020 which is probably less than the S. As far as I am concerned the bump steer kit is essential.