Ackerman help needed

[BryanP]

We have been testing our car and determined that the under steer problem that we have is because we have parallel steering arms. Drag! On the positive side our steering arm is an aluminum bracket that is bolted to the upright so it is not a really big chore to replace it. We are going to manufacture a new steering arm with geometry (2 mounting holes for the rod ends) for both full Ackerman and positive Ackerman. We know that drawing a line through the king pin to a point at the center of the rear axle will give the plane for the location of the steering arm hole for a car with a rear mounted steering arm. Our steering arm is located in front of the kingpin.
Question: Can we simply mirror the geometry for the rear steering arm to achieve the proper hole location for our steering arm? If not what do we do?
Thanks
Bryan

[BryanP]

We have been testing our car and determined that the under steer problem that we have is because we have parallel steering arms. Drag! On the positive side our steering arm is an aluminum bracket that is bolted to the upright so it is not a really big chore to replace it. We are going to manufacture a new steering arm with geometry (2 mounting holes for the rod ends) for both full Ackerman and positive Ackerman. We know that drawing a line through the king pin to a point at the center of the rear axle will give the plane for the location of the steering arm hole for a car with a rear mounted steering arm. Our steering arm is located in front of the kingpin.
Question: Can we simply mirror the geometry for the rear steering arm to achieve the proper hole location for our steering arm? If not what do we do?
Thanks
Bryan

[Denny Trimble]

Extend the line from the rear axle centerline forward; on a front-steer car the tierod point on the steering arm will be outboard of the ball joints.

[BryanP]

Thanks Denny,
Tomorrow we machine.
Glad I asked first.
regards
Bryan<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Denny Trimble:
Extend the line from the rear axle centerline forward; on a front-steer car the tierod point on the steering arm will be outboard of the ball joints. </div></BLOCKQUOTE>

[Z]

Bryan,

Judging from your photos on the other post, you should have the outer-steering-track-rod-joint as close to the tyre as is practical.

The common advice to have "the steer arm lines intersecting at centre of rear axle" isn't very accurate. Different R&P positions can make a big change to "ackermann". A better guideline is to have an angle of about 60 degrees, or a bit less, between steer-arm and steering-track-rod (for your front steer-arm). So moving the R&P back in the chassis will give you more positive ackermann.


Z

[Richard Pare]

Draw your ackerman line to the diff center. With a rack location that places the steering links perpendicular to the car centerline, measure (or calculate) the included angle between the steering links and the ackerman line. For swept back steering links, rotate the ackerman line about the upper ball joint so that you repeat that same included angle - the intersection point will be forward of the diff center.

[BryanP]

Z, I have just read your article The Steer-Axis. The quality of the information in the article is superb. Your presentation of a very complex subject into a series of concepts that can be visualized and verified is world-class.
Thanks
Bryan


<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Z:
Bryan,

Judging from your photos on the other post, you should have the outer-steering-track-rod-joint as close to the tyre as is practical.

The common advice to have "the steer arm lines intersecting at centre of rear axle" isn't very accurate. Different R&P positions can make a big change to "ackermann". A better guideline is to have an angle of about 60 degrees, or a bit less, between steer-arm and steering-track-rod (for your front steer-arm). So moving the R&P back in the chassis will give you more positive ackermann.


Z </div></BLOCKQUOTE>

[alfordda]

Z is the author of The Steer-Axis article from Racecar Engineering? If so, thanks for writing that article I found it really useful, it kind of made things hit home for me.

[Dick Golembiewski]

...and the one on Ackerman (really "dynamic toe")!

The secret's out!

[BryanP]

Thanks for the help Richard.
We are reviewing our front suspension and are taking this opportunity to address 3 issues. Improve our "Ackerman"� angle by moving the control arm pivot point outward and also by moving our SAI pivot point inward. This will create a SAI angle of 10 degrees, which we hope is more appropriate than our current 2 degrees. We will also change our steering arm length to improve bump steer. It will be interesting to experience the results of this next attempt.
This weekend will be our first autocross of the season weather permitting. Unfortunately the upgrades will not be installed for it. We will just have to remember not to get on the throttle mid turn.
Once we are reasonably happy with our suspension geometry it will be time to build a new set of uprights. We have purchased a new set of calipers for the uprights that will get rid of those horrible external crossovers.
Regards
Bryan
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Richard Pare:
Draw your ackerman line to the diff center. With a rack location that places the steering links perpendicular to the car centerline, measure (or calculate) the included angle between the steering links and the ackerman line. For swept back steering links, rotate the ackerman line about the upper ball joint so that you repeat that same included angle - the intersection point will be forward of the diff center. </div></BLOCKQUOTE>