I need some theory on wheel wobble vs. ackerman
I need some theory on wheel wobble vs. ackerman
(OP)
Hi all. I've been here for a few years poking around, just decided to join up and see if I can get something explained to me.....
Background
- I build front suspension for muscle cars(Mopar). My competitors' products cause terrible problems with front wheel wobble at speed because of ackerman that's actually opposite what it should be. I get mountains of calls from frustrated people asking what to do. I can solve the problem every time, but I can't explain to them why it works...
So here's my issue... On the factory 62-76 Mopar suspension, the steering arms bolt on to the spindles- it's a rear steer system. These guys are swapping the steering arms side for side so they point forward and installing a rack-n-pinion. The problem is, the arms are angled inward for proper ackerman in a rear steer system. Once they're swapped, the ackerman angles are actually reversed. The drag racers always try this, assuming their straight line cars don't need turning geometry. However, almost every one of them has trouble with uncontrollable wheel wobble at speed - either on accel. or during decel. - I discovered some time ago that the culprit has been this "negative" ackerman issue - once proper steering arms are fab'd to correct ackerman, the wobble disappears, and I've done this many times for people. I'll also point out that ackerman doesn't have to be perfect - even at zero, the problem's gone - it seems I only have to get it out of that "extremely backwards" condition to fix the issue.
Picturing the steering arm angles in my head, it seems to make sense(kinda)that it would be unstable, but I can't really explain why. So... I'm trying to find the technical explanation for this. For my own knowledge, and for that of the many people who ask me to fix their rides.
Thanks for any help in the matter.
Background
- I build front suspension for muscle cars(Mopar). My competitors' products cause terrible problems with front wheel wobble at speed because of ackerman that's actually opposite what it should be. I get mountains of calls from frustrated people asking what to do. I can solve the problem every time, but I can't explain to them why it works...
So here's my issue... On the factory 62-76 Mopar suspension, the steering arms bolt on to the spindles- it's a rear steer system. These guys are swapping the steering arms side for side so they point forward and installing a rack-n-pinion. The problem is, the arms are angled inward for proper ackerman in a rear steer system. Once they're swapped, the ackerman angles are actually reversed. The drag racers always try this, assuming their straight line cars don't need turning geometry. However, almost every one of them has trouble with uncontrollable wheel wobble at speed - either on accel. or during decel. - I discovered some time ago that the culprit has been this "negative" ackerman issue - once proper steering arms are fab'd to correct ackerman, the wobble disappears, and I've done this many times for people. I'll also point out that ackerman doesn't have to be perfect - even at zero, the problem's gone - it seems I only have to get it out of that "extremely backwards" condition to fix the issue.
Picturing the steering arm angles in my head, it seems to make sense(kinda)that it would be unstable, but I can't really explain why. So... I'm trying to find the technical explanation for this. For my own knowledge, and for that of the many people who ask me to fix their rides.
Thanks for any help in the matter.





RE: I need some theory on wheel wobble vs. ackerman
http://www.auto-ware.com/
RE: I need some theory on wheel wobble vs. ackerman
The problem with all the experts' websites is that they explain normal issues considering normal angles, most of which is common knowledge. Once I get the ackerman into the "normal" range, the problem is fixed. So, my quest is to find why things get screwy when ackerman is totally backwards - this is also a situation that very few people would even run into, because it only concerns the 60s-70s Mopars, and only if they hack in a rack and flip the steering arms - a small group indeed.
RE: I need some theory on wheel wobble vs. ackerman
What speed does it occur at?
Is it sensitive to downforce?
Do different brands of tyre suffer from it equally?
Cheers
Greg Locock
RE: I need some theory on wheel wobble vs. ackerman
RE: I need some theory on wheel wobble vs. ackerman
Anyway at a rough guess you are getting some sort of yaw instability, where one of the two front tyres is passing through zero slip angle, and then the other one is taking over and steering it back. The first tyre then starts steering, and the second one passes through zero slip and becomes a passenger.
But that is only a guess, and at straight ahead the Ackermann effect should be very small anyway.
Cheers
Greg Locock
RE: I need some theory on wheel wobble vs. ackerman
I doubt from previous posts that any effect on geometry has been given to the placement of the rack and the tie rod lengths, positions and arcs vs control arm lengths, positions and arcs.
For instance what is the distance between tie rods on the rack, vs drag link length on the original steering.
What is the height of the tie rod end of the steering arm vs the original.
What is the height of the rack vs the drag link, etc etc.
Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: I need some theory on wheel wobble vs. ackerman
Perhaps Greg has a point on it... I need to find more material on this...
RE: I need some theory on wheel wobble vs. ackerman
If there is significant weight on the front on launch, you are giving away traction.
I have never ever seen a perfectly flat level track with a perfectly consistent surface in my life.
Also we do strike a gust of side wind occasionally. That can also generate some suspension travel and change pre loads on steering linkages
Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: I need some theory on wheel wobble vs. ackerman
RE: I need some theory on wheel wobble vs. ackerman
So the front end has mass and there are a various springs (actual springs, chassis flex, tire/wheel flex) so there is undoubtly a front end side to side resonant frequency. And the damping is likely to be low.
The "Anti-Ackerman" has to generate differential side forces between the two front wheels probably somewhat proportional to the steering angle. And these contact pach forces can generate a torque on the steering axis due to the contact patch trail behind the king pin inclination. So this is a path that can feed energy into the spring /mass system.
Then at some speed there is some disturbance to start the oscillation and the side to side force builds up the oscillations.
RE: I need some theory on wheel wobble vs. ackerman
RE: I need some theory on wheel wobble vs. ackerman
front wheels with a change in diameter and offset/backspacing from factory design move the intersection of the steering axis at the contact patch. This changes the scrub radius and creates a torque about the contact patch. Lateral loading of the chassis due to engine torque would result in a significant change in normal force between the left and right tires. This steering force would provide the energy necessary for oscillation. Natural frequency of oscillation would be unusually high due the hard, narrow and lightly loaded front tires and would therefore occur at a higher than expected MPH, matching up with the period of gyroscopic precession.
The reverse ackerman would result in increase toe changes (toe-in) for a given steering input, compounding the above.
RE: I need some theory on wheel wobble vs. ackerman
http://www.nationaltbucketalliance.com/tech_info/c...
The wobble would be due to the front tires fighting for which one will control the direction of the car. If you ask why it happens even when the car is only going straight down the track it is for the same reason you still need to have a driver steer to keep the car straight. Even the tiniest imperfections lead to the need for steering corrections eventually.
RE: I need some theory on wheel wobble vs. ackerman
Cheers
Greg Locock