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-   -   Bumpsteer? How to get rid of it?? (https://www.rx7club.com/race-car-tech-103/bumpsteer-how-get-rid-532156/)

Slammed_GSL 04-20-06 10:26 PM

Bumpsteer? How to get rid of it??
 
What is it and how do I get rid of it if I got?????
Lowered my car and the steering is really touchy....84' VW Rack and Pinion installed 3 years ago. I had a alignment done yesterday and got some shake at 60mph.
I have Eibach 700/400/250 coil in the front and #200's with a coil cut on the rear.
Any suggestions from the pro's? I'm just a FB drifter trying to dial my car in.

Thanks!

Travis R 04-21-06 08:36 AM

Bump steer is just as it sounds... as the wheel goes up (bump) it turns even though the steering wheel hasn't moved. It happens because of the geometry of the strut, lower control arm , and the steering toe link.
You'll need to work woth a local shop to get it right, but it is usually fixed by either raising or lowering some part of the steering arm. Either raising/lowering the steering rack or the toe link's outboard mounting point on the upright.
Good luck.

Slammed_GSL 04-21-06 11:16 AM

How do turn in spacers work? I seen them and I seems like they raise car hieght up, is this right?
I have not seen a install pic of spacers on 1G, sorry for the stupid questions. I'm still learning this part of the car setup.

jgrewe 04-21-06 01:12 PM

Turn in spacers go between the steering arm and the strut. It makes the spindle higher in relation to the ball joint and steering arm. They change your front roll center height and can help with bumpsteer. The best way to test bumpsteer is to remove your springs and move the suspension through the range you will spend most of your time and check the change in toe in/out. By moving the height of your steering rack or outer tierod attaching point you can minimize toe change.

abeomid 04-21-06 02:56 PM

The CP racing kit (which I assume is what you have) causes bump steer. THere is no way around it beside changing the kit.

tims 04-21-06 07:51 PM

the only true way to eliminate bump steer is using an active suspension like was used in the early nineties in F1. Bump steer is a fact of life. Now there are ways to minimize the bump steer but this will require you to install a couple of gauges on the suspension and then have some way to adjust the rack height or steering arm lengths or both. spacers on the rack or strut mount will do the trick, but without measuring the bump steer and checking to see if the spacers work takes some effort. this is not hard to measure but it can take complex machine work to cure. good luck and don't be affraid to experiment.

Slammed_GSL 04-22-06 01:34 AM


Originally Posted by abeomid
The CP racing kit (which I assume is what you have) causes bump steer. THere is no way around it beside changing the kit.

It's my own copy of CP's rack, same problems. Gonna try a different brand of rack next year after I get my megasquirt FI manifold fab'd up....one money pit at a time.

carbon man 04-22-06 04:39 AM

The easiest way to check how much bump steer you have is to remove the spring then put the rest of the suspension back with a jack under the suspension, also have the steering locked straight ahead. Have a flat plate mounted where the wheel fits or attach a flat plate on the outside of the wheel. With 2 dial gauges at the same height but one near the front of the plate and the other at the rear and jack the suspension up measuring how much you jack up and note if the toe is changing by the amount the dial gauges change.
Once you know how much bump steer you have when you do changes you will know if you are reducing the problem.
A simple set of rules that may help.
The lenght between the lower innner and outer pivot points on the lower control arm should be the same length as the pivot points on the tie rod and the tie rod and the lower control arm should be parralel to each other.

Hyper4mance2k 04-23-06 12:01 AM

The 88-91 CRX rack seems to be a perfect match for out cars of you ask me. the lock to lock is shorter than the VW and it's rear steer. Trust me I started out as a Honda guy.

KeloidJonesJr. 04-23-06 12:50 AM

Wheel hop? Try bushings...they help ALOT.

B6T 04-23-06 02:05 AM

I always thought that bump steer was caused by the difference in lengths between the tie rod and control arm. Since the two links are different lengths measured from pivot to pivot, the arc they follow through their movement is different because they have different radii. It isn't noticeable in most cars at stock ride height because they design the suspension so that the two arc are intersecting, which is where bump steer has the least effect on the change in toe. But once the car is lowered beyond that point, the slightest change in position of the suspension produces a drastic change in toe angle.

Am I correct or completely out to lunch on this? :weedpoke:

Black91n/a 04-23-06 02:39 AM

That's correct, but steering rack and outer tie rod end placement affect bumpsteer too. If the rack is lower or higer than the control arm, and if the control arm and the tie rod aren't parallel, then you'll get bumpsteer.

Ideally in a Macpherson strut car, the rack will be positionned on the same plane as the control arm inner pivot points, with the tie rods parallel to the control arms. This will minimise any bumpsteer issues.

B6T 04-23-06 08:21 PM

Ah that's it. I knew there was something I was missing. Keeping the arms parallel makes sense.

carbon man 04-24-06 12:33 AM


Originally Posted by B6T
Ah that's it. I knew there was something I was missing. Keeping the arms parallel makes sense.

alsoTRY to get them the same lengths between pivot points

Travis R 04-24-06 12:29 PM

They also need to be in the same plane... just having them parrallel won't get rid of all of it.

Black91n/a 04-24-06 03:28 PM

That's why I said that the rack and the inner pivot of the control arms should be in plane, and the tie rods should be parallel to the arms. This implies that both the inner povits of the rack and the arms are in the same place laterally, making the arms and the tie rods be in plane (unless the rack and tie rods are perfectly straight, something I didn't properly explain before).

I don't remember much about Ackreman (spelling?), but to get that you need to have the tie rods a different length than the control arms do you not? That would cause bump steer would it not? Having none, or insufficient Ackreman is a bad thing too. Ackreman helps get the car around the corner by steering the inner wheel more than the outer wheel, as it has a smaller radius circle to travel along. The outer tie rod pivot points and Ackreman aren't really adjustable on a production car without modifying the hubs, so it's kind of irrelevant for the problem at hand.

To the best of my knowledge, for a production car the best you can do is to position the rack to get the tie rod pivot points in the same plane as the control arm inner pivot points, and try to keep the tie rods in plane with the control arms.

carbon man 04-24-06 04:10 PM

ackerman has no effect on steering straight ahead and the more steering angle the more difference ackerman has.
Positive ackerman is where when you turn all the wheels turn in an ark from the same axis point, this could also be described as the inside front wheel has more steering angle than the outside front wheel. A way to measure that is if you draw a plan of your car to scale from above and you draw a line through the centre of the lower ball joint on the lower control arm through the outer tie rod end the line should go through the diff centre and if you draw the lines from the other side of the car the lines will intersect in the middle of the rear axle line.
Zero ackerman is where the wheels turn exactly the same amount of steering angle through a turn. Drawing the lines though the lower ball joint and outer tie rod on both sides the lines would never cross, they would stay paralell.
Anti ackerman is where the inside wheel in a turn has less steering angle than the outer wheel. Drawing the lines through the ball joint and tie rod the on both sides the lines would joint in front of the car.

I believe positive ackerman is good for driving a road car in a parking lot and anti ackerman is best for most race cars depending on the slip angle of the tyre.


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