Steering Modification

[Cobra #3170]

I tried to post this but may have made an error in the process

Last year my car got killed at the Goodguys Autox shootout, after watching the video about a million times I figured we were losing a ton of time in the U turns.
The course at the shoot out had (7) U turns. My car as configured had a maximum inside tire steer angle of only 23 degrees. I decided to fabricate some new steering arms that would bolt in to the existing holes on the
front of the upright. I made two jigs RH and LH and finished the arms today, I now have 30 degrees inside turn angle and 100 % ackermann steering with excellent bump steer and much faster steering. I can't wait to try it in March at the first autox, should be a big improvement on tight courses. The attached picture shows both jigs and the steering arms bolted in place. [url='http://www.clubcobra.com/photopost/data/500/

[Morris]

Bruce
You'll be glad you added the Ackermann.....the car will roll so much better and continue into and out of the turn without scrubbing off much speed.....

Post 362 we go thru bump steer on the rear of the cobra.....might help......

KMP 259 Phase 2

Will take you thru a complete steering change......we went with a Appleton rack & pinion

Good change.....I look forward to the improvement for you.....

[Cobra #3170]

Thanks Morris,

I went to a 1.1 " brake disk from a 1.25 so I could move the tie rod end out a little further. I had about 70% ackermann before that, but should be better now with near 100%. I too use Appleton power rack, had to shorten the steering arm by 1.75" to get enough steer angle. What I had was good for a road courses but not autox, I am done with road courses now (too old, too slow and too scared).

I have been using a laser mounted to either the brake disk or the wheel and measuring it about 15' away on a 4x8 plywood board. It really shows very small movements in bump steer because of the angle factor at that distance.
Both the front and rear are very good now so all I need to do now is figure out what spring rates will work with treaded 200 TW tires.

[strictlypersonl]

100% Ackerman is nice for low-speed driving, but is not ideal for racing. True race cars actually use negative Ackerman to compensate for the higher slip angle of the outside tire. They don't care if there is tire scrub in the pits.
Article on Ackerman

[mickmate]

Hi Bruce, always good talking geometry and Cobras with you. I like your approach to the mods, I like even more your approach to testing the mods. Keep it up and let us know how it worked.

[Cobra #3170]

Originally Posted by strictlypersonl

100% Ackerman is nice for low-speed driving, but is not ideal for racing. True race cars actually use negative Ackerman to compensate for the higher slip angle of the outside tire. They don't care if there is tire scrub in the pits.
Article on Ackerman

That used to be more true than it is now with extremely low profile tires, I am running 18x30 tires and if you read the post this is for AUTOX which rarely exceeds 70 mph. Some formula Atlantic cars actually ran more than 100% ackermann on tight street circuits to get better turn in. Original 427 Cobras had reverse ackermann because Shelby heated the steering arms and bent them inward to clear the 7.5: mag wheels, those cars under steered like dump trucks. I corrected that situation on my Cobra in 1969 by reversing the left and right steering arms and installing a rod end in place of the large tie rod end for clearance to the brake disk and wheel. The car immediately improved with no other changes in spring or bar rates, granted a Cobra is not a true race car but we are talking Cobras here, not race cars. I once owned a parallel steer March 78B Formula Atlantic and turn in was so bad we had to add toe out to get decent turn in. The toe out made in very unstable in a straight line. Adding ackermann to that car fixed both issues. I don't doubt that ackermann isn't always the answer, it depends on so many other variables like roll stiffness, torsional stiffness, tire aspect ratio etc, etc but in the case of my Cobra it works quite well.

[Cobra #3170]

Originally Posted by mickmate

Hi Bruce, always good talking geometry and Cobras with you. I like your approach to the mods, I like even more your approach to testing the mods. Keep it up and let us know how it worked.

Thanks Mic,

The beauty of this change is that I left the original (although modified) steering arm in place. If my fix doesn't work, I just unbolt the new piece, lengthen the tie rod adjustment and reinstall the tie rod in the original arm. The main reason for the mod was to gain steer angle but I got a little more ackermann as a bonus! I should have an answer by early March and will let you know how it worked.