Taimar bump steer question

[status]

Having looked at the angle of the upper and lower suspension arms vs the angle of the steering tie rod before I took my Taimar apart, I was concerned that something looked odd. I couldn't understand how, as the suspension compressed, the wheels maintained the correct toe-in. A quick read through all the old threads that I could find seemed to confirm that bump steer was a problem so I decided to do a quick measure and sketch to be sure I understood correctly.



The three red lines are the relative positions, viewed looking front to back, of the passenger side, and presuming that the lower arm should be parallel with the ground (I can't recall whether this is exactly correct, but I think it's the relative positions that matter anyhow). I have included in the sketch the different offsets of the inner pivot points in my chassis, and both offset and parts measurements are approximate in inches (nearest 1/4" was what I probably achieved).

The three white arcs are the path the outer ball joints would move through, given the same compression ie linked by the hub (I chose 3" of compression to ensure I could see something like the worst case difference).

The steering tie rod is angled down by the steering arm shape, which seems to me to be wrong, and forces the outer steering ball joint to move outwards whilst both upper and lower outer joints move inwards, which would point the wheel toe-out under compression.

The yellow tie rod / blue arc are what I think should result in neutral behaviour.

Question #1 - am I talking bks (TM) or am I on the right track?

Question #2 - assuming I'm on the right track, is there a better / best steering arm (I saw TR4 mentioned but can't see a supplier) that would reduce or fix the bump steer? (Not that keen on lowering the rack as that might have other knock-on effects that would need solving)

tia for inputs :-)

Nick

[Slow M]

http://www.baer.com/products/suspension/baer-track...

Best,
B.

[status]

Thanks Bernard. They look interesting :-)

[madsvlund]

You are right on track :-) When I did the redesign og my M front suspension, did I move the sterringrack downward to reduce bump steer. Remember to keep a little positive bumpsteer (toe out on compression) if you get into negative bump steer, will the car oversteer under cornering. MEasuring bumpsteer on a double A arm suspension is a bit complicated:




Final setup, note the washers under the steering rack, this was to fine tune bump steer, later replaced with an spacer.

Edited by madsvlund on Wednesday 11th September 22:29

[Slow M]

Nick,

You're welcome.

Also, have a look at how the issue was handled on Mike Zappa's car. If you can't find pictures, I could dig some out of my archives. It'll take a few days, though.

Best,
B.

[status]

Thank you sir!

Found this thread that you'd already commented in, complete with a pic and more explanation that I'd missed. I obviously need to sharpen my search skills a bit :-)

Although both look adjustable, to me, the approach on Mike's car appears potentially stronger than the Baer solution.

cheers
Nick

[status]

Hey Mads

Thanks for the inputs - useful (and interesting) that you highlight that a _little_ bump steer is a good thing. I'll keep that in mind (I guess having an adjustable setup will help me fine tune)

madsvlund said:

When I did the redesign on my M front suspension....

Each time I looked at your pics I saw something more :-) so I took a look at your web site and 'redesign' is an understatement! Although I don't think I will go quite as far, what you have done is food for thought for the future for me :-)

cheers
Nick

[Fiscracer]

Interesting that you all appear to be doing it with the car off the ground and at full droop - a point you'll almost never be at in the real world. If you are going to change things please do it at ride height ie fully built car with engine and driver in

[status]

Fiscracer said:

Interesting that you all appear to be doing it with the car off the ground and at full droop - a point you'll almost never be at in the real world. If you are going to change things please do it at ride height ie fully built car with engine and driver in

Hi Richard

The thing that worried me was the relative differences in the arcs that the 3 outer joints traveled through, and I had presumed that my assumption of the lower arm being about parallel with the ground was "about right".

Sadly my Taimar is spread all around the garage in bits so I can't check, but if you have the approx angle of the lower arm relative to the ground for a fully built/loaded car, I can pretty easily rework my diagram using that.

cheers
Nick