Bump Steer - What is it?
Thread Starter
Pro
Joined: Aug 2002
Posts: 686
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From: SLC, UT
Bump Steer - What is it?
Ok,I'm showing my ignorance here...
So what the heck is bump steer? From the other posts it appears to have something to do with steering and a lowered P-car.
Regards,
Jim
'86 911
So what the heck is bump steer? From the other posts it appears to have something to do with steering and a lowered P-car.
Regards,
Jim
'86 911
Intermediate
Joined: Oct 2002
Posts: 35
Likes: 0
From: Central NJ, USA
Bump steer is basically, driving over a bump/pothole/uneven surface and the vehicle pulls/steers with or away from the bump. The wheel physically will move in your hands as the car does this, i.e. hit a pothole with the front left tire and the car steers left. Why? I'm going to be trying to track down this answer as I fix the 911 I bought today! <img src="graemlins/oops.gif" border="0" alt="[oops]" />
-Vex
-Vex
Former Vendor
Joined: Apr 2002
Posts: 302
Likes: 1
From: Santa Clara, CA
To be a bit more specific, bump steer is a change in toe due to suspension travel. The suspension travel may be caused by a "bump" (though any suspension travel will have the same effect). The toe change has the effect of "steering" the car. Hence the name bump steer.
Although the steering wheel may jerk when going over a bump, the wheel jerk is not what is meant by bump steer.
All 911s bump steer. This is because the tie rods are radius arms with one end fixed to the chassis (at the steering rack) and the other end moving with the suspension. Hence any suspension movement causes the tie rod to trace out an arc, effectively changing the horizontal distance between rack and steering knuckle. This has the undesired affect of steering the wheel.
Properly set up, the suspension geometry is designed to minimize bump steer. This is changed when ride height is altered. Proper geometry for lowered cars can be restored through a variety of methods.
Although the steering wheel may jerk when going over a bump, the wheel jerk is not what is meant by bump steer.
All 911s bump steer. This is because the tie rods are radius arms with one end fixed to the chassis (at the steering rack) and the other end moving with the suspension. Hence any suspension movement causes the tie rod to trace out an arc, effectively changing the horizontal distance between rack and steering knuckle. This has the undesired affect of steering the wheel.
Properly set up, the suspension geometry is designed to minimize bump steer. This is changed when ride height is altered. Proper geometry for lowered cars can be restored through a variety of methods.
RL Technical Advisor
Joined: May 2001
Posts: 11,870
Likes: 75
From: Portland Oregon
[quote]Originally posted by Chuck Moreland:
All 911s bump steer. This is because the tie rods are radius arms with one end fixed to the chassis (at the steering rack) and the other end moving with the suspension. Hence any suspension movement causes the tie rod to trace out an arc, effectively changing the horizontal distance between rack and steering knuckle. This has the undesired affect of steering the wheel.
[/QB]<hr></blockquote>
Well Chuck,...thats not quite the accurate story.
ALL steering tie-rods prescribe that geometric arc as they move up and down; this is indigenous to all suspensions.
The reason that 911's bump steer (and others) is to the geometric difference of that arc made by the steering and front A-arms. This is caused by their dissimilar pivot points. If the inner end of the tie-rods (steering rack) could be placed at the same location as the A-arm pivot, you would have very little, if any, bump steer.
In short, the ball joint and the outer tie-rod prescribe different arcs as they move up and down though their range of motion. This is why simply installing a spacer kit doesn't always correct that problem in all cases, and sometimes one must correct (minimize) bump steer at the tie-rods ends with the ERP bump steer kit.
Hope this clarifies this for you.
All 911s bump steer. This is because the tie rods are radius arms with one end fixed to the chassis (at the steering rack) and the other end moving with the suspension. Hence any suspension movement causes the tie rod to trace out an arc, effectively changing the horizontal distance between rack and steering knuckle. This has the undesired affect of steering the wheel.
[/QB]<hr></blockquote>
Well Chuck,...thats not quite the accurate story.
ALL steering tie-rods prescribe that geometric arc as they move up and down; this is indigenous to all suspensions.
The reason that 911's bump steer (and others) is to the geometric difference of that arc made by the steering and front A-arms. This is caused by their dissimilar pivot points. If the inner end of the tie-rods (steering rack) could be placed at the same location as the A-arm pivot, you would have very little, if any, bump steer.
In short, the ball joint and the outer tie-rod prescribe different arcs as they move up and down though their range of motion. This is why simply installing a spacer kit doesn't always correct that problem in all cases, and sometimes one must correct (minimize) bump steer at the tie-rods ends with the ERP bump steer kit.
Hope this clarifies this for you.
Former Vendor
Joined: Apr 2002
Posts: 302
Likes: 1
From: Santa Clara, CA
Steve I think we are full agreement.
My statement about "all 911s bump steer" was followed by "Properly set up, the suspension geometry is designed to minimize bump steer. "
All 911s DO bump steer. However when set up properly the affects are minimal for a small range of suspension travel. And I think that is what you are trying to say here:
[quote]Originally posted by Steve Weiner-Rennsport Systems:
<strong>
If the inner end of the tie-rods (steering rack) could be placed at the same location as the A-arm pivot, you would have very little, if any, bump steer.
</strong><hr></blockquote>
My statement about "all 911s bump steer" was followed by "Properly set up, the suspension geometry is designed to minimize bump steer. "
All 911s DO bump steer. However when set up properly the affects are minimal for a small range of suspension travel. And I think that is what you are trying to say here:
[quote]Originally posted by Steve Weiner-Rennsport Systems:
<strong>
If the inner end of the tie-rods (steering rack) could be placed at the same location as the A-arm pivot, you would have very little, if any, bump steer.
</strong><hr></blockquote>
Thread Starter
Pro
Joined: Aug 2002
Posts: 686
Likes: 1
From: SLC, UT
Chuck & Steve,
Thanks for the technical definition and description. I've noticed it a "small bit" after hitting a pot hole or ?, however I also notice it in my BMW (very stiff suspension). I always thought it was simply part of design and the way suspension parts move in harmony (or disharmony).
Thanks again,
Regards,
Jim
'86 911
Thanks for the technical definition and description. I've noticed it a "small bit" after hitting a pot hole or ?, however I also notice it in my BMW (very stiff suspension). I always thought it was simply part of design and the way suspension parts move in harmony (or disharmony).
Thanks again,
Regards,
Jim
'86 911
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Track Day
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HEY CHUCK ,
WHO GAVE YOU PERMISSION TO BANDY A PICTURE OF MY
MUG AROUND LIKE THAT?
DON'T YOU KNOW IT'S COPYRITE!
DOGGIE ---- <img src="graemlins/cussing.gif" border="0" alt="[grrrrrrr]" /> <img src="graemlins/cussing.gif" border="0" alt="[grrrrrrr]" />
WHO GAVE YOU PERMISSION TO BANDY A PICTURE OF MY
MUG AROUND LIKE THAT?
DON'T YOU KNOW IT'S COPYRITE!
DOGGIE ---- <img src="graemlins/cussing.gif" border="0" alt="[grrrrrrr]" /> <img src="graemlins/cussing.gif" border="0" alt="[grrrrrrr]" />
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