More about bump steer, than I ever really wanted to know Options

[Brett W]

That design is still pretty scary. You still have a pretty large moment arm on the mounting stud for the tie rod end. You would have been better off adding another arm lower on the knuckle so you would have put the tie rod in Double shear. Its much stronger.

[Larry.Hubby]

Brant,

The factory shop manual tells you to determine the ride height by measuring the distance, “b”, in the drawing below:



Where “b” is measured from the ground to the center of the “closing cover on the adjusting lever of the torsion bar”, and to set “b” to 225mm + or – 5mm. As you can see from the drawing, b = a – 90mm. As far as the steering geometry, and the bump steer in particular, are concerned, only the spatial relationship among the strut, suspension arm, and tie rod is significant, and this depends on only the 90mm dimension. The dimension, “a”, is simply the rolling radius of whatever tire that particular car happens to be running, and heaven knows, changing that from whatever it was to something else is practically everybody on this board’s favorite trick. If you change “a”, the entire picture above simply moves up or down, but the suspension geometry doesn’t change. So, to use this method of setting the ride height, as essentially everyone who aligns wheels on our cars does, one really needs to take the tire size into consideration, but I suspect that often this is not done. This is the reason I specified the ride height independent of the tire size difference in the original post. The factory manual actually takes a = 315mm as an example, which is close to what you would calculate for the 78 series 165 x 15 tires that most 914’s were delivered with, and then calculates the 225mm dimension by subtracting 90mm from it. When we lower our cars, the change that really matters is that we increase the 90mm by some amount. In the case of my car, this dimension is 57mm, or 2.25”, larger. Every time you change this dimension, you have to adjust the tie rod length to reset the toe to the correct value, and the bump steer characteristics, as well as the optimum rack spacer thickness and/or tie rod end drop for that situation, all change.

Hope this helps.

[Larry.Hubby]

Andy,

I'd calculate a series of the optimum tie rod end drops vs amount of lowering for you if I still had access to the program, but I don't. After thinking about it for a while, I can tell you that the offhanded number I gave you before, .875", for a car lowered as much as mine, but with dropped tie rod ends only, is much too low (sorry 'bout that). The number for a stock 914 is 1.125" and a lowered car should need MORE, not less correction.

[KaptKaos]

Great write up!

I think the ride height was set at the factory in order to comply with US laws for the minimum height of the headlights. Would that have accounted for part of the bump-steer problems?

I have spacers on my rack, which were installed when I put in turbo tie-rods.

[76-914]

(IMG:style_emoticons/default/popcorn[1].gif) with interest

[brant]

awesome
thank you!

[jmitro]

Great thread; found this while searching for steering rack spacers vs tie rod spacers.

Please add to the classics threads

[SirAndy]

Please add to the classics threads

Done and done ...
(IMG:style_emoticons/default/beerchug.gif)

[jim_hoyland]

Terrific information: from stock, how much may the front ( street car only) be lowered before having to do more than a realignment ?

[jmitro]

Terrific information: from stock, how much may the front be lowered before having to do more than a realignment ?

good question; I may eventually purchase a bump steer gauge to determine the bump steer if I lower the car quite a bit.

Another question is...how much bump steer (if any) is desirable? maybe it depends on the intended use of the car - street vs autox vs track?

in my old BMW E36 racecar, I bought a bumpsteer gauge and dialed out any bumpsteer at all with tie rod spacers.

[914_teener]

Great thread allthough without discussion of the https://en.m.wikipedia.org/wiki/Ackermann_steering_geometry

Ackerman effect it makes more sense in the real world. Goes back to the horse and buggy days.

With the toe changes and hard cornering some bumpster IS desirable which is what I.ll add.

It is interesting later in the thread about what happens when we lower our cars....so they look cool.