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xcoldricex

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PostSubject: Bump steer   Tue Mar 29, 2011 12:11 pm

just so i don't lose these videos when i need to do this later on:

illustration of bumpsteer:


measuring/adjusting bumpsteer:


won't be cutting at my steering rack - they make bumpsteer kits to adjust the rack endlinks.
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Wallace
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PostSubject: Re: Bump steer   Thu Mar 31, 2011 10:42 pm

That's interesting, how do the endlinks reduce the bump steer?

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xcoldricex

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PostSubject: Re: Bump steer   Thu Mar 31, 2011 11:06 pm

you can adjust the height with spacers.

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Wallace
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PostSubject: Re: Bump steer   Thu Mar 31, 2011 11:29 pm

ahh i see, that's probably the first time I've ever seen anti bump steer parts for anything that wasnt a truck haha, that's sweet!

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dogbreath

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PostSubject: Re: Bump steer   Thu Mar 31, 2011 11:31 pm

Look at a 80s-90s Audi and notice the offset of the wheels. Anti bump-steer!
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xcoldricex

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PostSubject: Re: Bump steer   Sun Apr 10, 2011 4:32 pm

more bumpsteer stuff with some vids


Quote :
Bumpsteer is any change in wheel toe as the suspension moves up and down. Ideally you don't want any of it. As the suspension moves up and down from bumps, body roll, brake dive, whatever, you don't want the car steering itself. All cars have bumpsteer, period. Some have very little, some have a lot. The less bumpsteer a car has, the nicer it is going to be to drive. How much bumpsteer a car has is a function of the geometry of the steering and suspension system.

The exact location of all the pivot points in the steering system of a Mustang to minimize bumpsteer is a complicated problem to solve. Way beyond a 10 minute internet discussion. Here is a ROUGH approximation to the correct solution. When viewed from the front, the tie rods and FCAs should be parallel to one another. To tell when they are parallel to one another, you must be able to see inside the ball joint so you can see where its pivot point is. Unless you have taken your ball joints apart to find this location, there is no way you are going to know where it is. The bottom line is that if you want to adjust the spacers in a bumpsteer kit, you can't do it by eye. You must measure the bumpsteer curve on the car or copy a set of suspension parts, alignment and bumpsteer spacer stack from another Mustang with a known good bumpsteer curve.

The bumpsteer curve refers to a graph which has suspension travel along one axis and wheel toe (in inches at the tire or degrees) on the other axis. Here is an example of one:

http://home.comcast.net/~jhidley/Bumpsteer_graph.JPG

I realize that it is difficult to visualize how moving around different points in the steering and suspension system could affect bumpsteer, so I made a simple suspension model in SolidWorks and animated the results. This makes it easy to see the bumpsteer behavior. The model is intentionally simple to make the bumpsteer behavior simple. The FCA pivot axis is parallel to the centerline of the car, so there is minimal caster change with suspension travel. The FCA is represented as two different links joined at the pivot point of the ball joint. The K-member is just a flat plate to mount the front control arm pivot and the steering rack to.

http://home.comcast.net/~jhidley/bumpsteer_model.jpg

Case #1:

The height of the outer tie rod has been adjusted to give minimum bumpsteer given the locations of the other points in the suspension/steering system. In this case, there is a total of 0.14 degrees of toe change over the entire 6.7" of suspension travel. This is barely visible in either video. Note that the tie rod and the FCA are NOT parallel. If I make them parallel, the bumpsteer gets about twice as bad.

Front view:

http://home.comcast.net/~jhidley/Bumpsteer_good_front.avi

Side view:

http://home.comcast.net/~jhidley/Bumpsteer_good_side.avi

Case #2:

The height of the outer tie rod has been raised 0.7". Notice that the tie rod and FCA are no longer even close to parallel. The length of the tie rod has been adjusted so that toe is still zero at ride height. In this case, the toe changes from 2.8 degrees in at full bump to 2.2 degrees out at full droop. This is total change of 5 degrees from case #1. 35 times as much toe change!

Front view:

http://home.comcast.net/~jhidley/Bumpsteer_bad_front.avi

Side view:

http://home.comcast.net/~jhidley/Bumpsteer_bad_side.avi

As explained on the MM website in the tech section, the 94-95 spindles have almost exactly the same steering arm height as the 87-93 11" brake spindles. If you put these spindles on your Fox Mustang with a stock k-member, your bumpsteer curve is going to be ok. About the same as Ford made it with the original Fox spindles. If you install the 96+ spindles, the straight steering arm of this spindle is going to lower your outer tie rod 1". No problem, just go buy a bumpsteer kit. But wait, you need to RAISE the outer tie rod 1", so unless your bumpsteer kit came with magic bumpsteer spacers that have a negative length and allow the end of the tie rod to occupy the same location as the end of the steering arm, you are out of luck.
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PostSubject: Re: Bump steer   Sun Apr 10, 2011 6:36 pm

Nice info!
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