Violent front end shake under braking!

[Jeremy Soule]

Hello all, I've been running my formula 1st in autoX, C-Mod. class this season. The front and shakes really bad under braking but goes away if I let up a bit on the brakes. I never noticed this condition when road-racing but I was much more easy applying the brakes in that racing environment.

Before the last race I had the front wheels balanced, no change.

I've checked the front ball joint's for free play it's negligible if any ditto tie rod ends. Also no freeplay in steering box.

Checked torque on the four bolts holding the front end to the frame.

The upper beam/torsion bar has a little over a 1/16" of freeplay side to side. Is that acceptable?

I've checked the front beam tubes for straightness with the straight edge (across the end plate/shock towers) and a machinist rule Both the upper and lower beam appear to be swept back 5/32" on the right side and both upper and lower beams on the left side appear to be swept back 3/32". Not sure this is a totally accurate method to check.

The third possible culprit is the extreme angle/laydown of my shock absorbers, see photo.



Your thoughts, comments are appreciated. This shake will tear things up and it's not fast either!

[Purple Frog]

when i used to autocross a tube front end... 100s of moons ago....

I used brass bushings to control side to side motion of the trailing arms in the end of the tubes. Especially the one using the through ARB that doesn't connect in the middle. Eliminate the slop.

Used to buy them from Bill Scott (R.I.P.)

I also used a heavy steering damper off a Kombi.

[Jeremy Soule]

Thanks PF, was thinking of making aluminum washers (shims) to eliminate end play.
And a steering dampener like came standard from the factory, Duh.

[mikey]

Have you checked the rotors and pads? Warped? Pistons cocked in bore? I'd start with the brake system itself...

[Keith Carter]

Have you tried a different wheel/tire set yet?

[BeerBudgetRacing]

Have you checked the rotors and pads? Warped? Pistons cocked in bore? I'd start with the brake system itself...

pad compound? they aren't getting to the same temps.....
something that'll work cold?

[Lynn]

Make sure the frame is not touching the ground under hard braking. Voice of experience.

[xmazdatracy]

I have this so bad that its hard to keep the car in a straight line. It is liner tho. The harder I press the brake the more it will shake. I can see on the rotors that they are warped. I will push less on the middle peddle thingy until I can afford new rotors.

[Mike Wright]

Jeremy,

At Indy this year Garry Sharp had this same issue crop up on his FST. Turned out that the pinch bolts (pointed set screws) that fasten the cast arms (dog legs) to the swar bar were loose, as I understand it. He tightened them up and.......wah-lah....problem solved. If you want more info about this or Garry's contact info just send me a PM. Something similar happened to me also and I got a fix for it from Larry Campbell. Let me know if you need more info.

Mike

[DJ Stewart]

Had a similar experience that ended up being wheel bearings. Worth a look?

[Steve Davis]

If you haven't already installed a steering damper.. do that first. Dampening the pitman arm will do wonders...