xupkid2

So I have a 1989 Turbo S that is built for club racing (PCA G Prepared class). I am having trouble figuring out a problem. When on track I have a shake in the front end. When I brake it gets worse. When normal driving there is still a slight shake but braking makes it way worse. I feel the shake in the steering wheel vs the brake pedal.

I have brand new rotors and pads that are not warped. I have checked the hubs to see if they are loose and they are tight. The shake feels like a warped rotor but the rotors are dead straight. I have checked the balance on the wheels and they are good. I cant figure out what it could be as everything seems tight. The car has koni double adjustable shocks with 800# springs up front and I believe #600 in the rear. I have removed the camber eccentric bolt upfront and welded in plates so there is not movement in the strut. I adjust camber with camber plates. I run 993TT front brakes, Pagid black pads and 245 Hoosiers upfront. The motor is stock.

Any thoughts on what it could be would be appreciated as I need some paths to diagnose.

Thanks in advance
Brian

JohnKoaWood

When were the wheel bearings last repacked / replaced? Tie rod ends? Ball Joints?

xupkid2

Bearings were replaced over the winter as well as tie rod ends. I also just replaced the control arms with the fabcar arms. I have probably 7 track days on all the new stuff.

hp18racer

Jack up each side of the car in turn, pull on top and bottom of the tire, any play? (bearing?)
Pull on the front and back, any play? (tie rod?)
Do you get the same symptoms with a different set of wheels and tires on the front? (bent rim or unbalanced/delaminated tire?)

Chris White

How did you check the rotors for trueness?

Our cars are very picky about any dirt between the rotor and the hub - it makes it act like the rotor is warped when it isn't. Its cheap enough to fix, take the rotor off andreally clean the mounting surface.

xupkid2

Thanks for the questions. I've checked the hub for play and there is nothing. Also seems the tie rods are tight. I do have another set of wheels and the shake is the same with those unfortunately.

In terms of the brakes I have cleaned the rotor surface. Also after a track session you can usually see if there are any "hot spots" on the rotors when they are warped. The "glaze" for lack of a batter term is completed universal around the entire rotor. What worries me is that the shake is there even when Im not braking but braking makes it much worse.

Van

Did you properly bed in the Pagid pads? If they're not bedded in properly, they will (and continue to) leave deposits on the rotor that causes a vibration under braking.

You say you have 993 TT brakes? Any chance the rotor isn't sitting on the pub properly - causing it to be out of true? Jack the car up, take a wheel off and use a dial indicator to see if it's spinning true.

ritzblitz

Weird alignment issue?

mikey_audiogeek

Which caster blocks are you running?

Antonio

Flat spotted front tires?

User 41221

Are you still running the ABS? I'd look into whether or not is properly bled (it may require higher pressure than normal while bleeding) or have contamination in the lines running to the ABS pump. The only thing about it is that I would think you would definitely feel it through the brake pedal as well as the steering wheel, but ABS issues could definitely cause your symptoms.

Regards,

jerome951

Van hit it on the head: it's brake pad deposits on the rotors. I used to have this problem when I ran older compounds of PFCs. Just cleaning the rotors, short of sandpaper, will not remove it.

Best solution is to install a cheap set of street pads and drive around a bit. This will scrape off the deposited material and give you a smooth surface again. Of course, the shaking will come back when you're back on the track so you might want to look at a different track pad.

xupkid2

I've checked all the suspension and found one of my caster blocks to be a tad loose. I am running the elephant racing solid caster blocks. I also tightened up everything. I found that my camber plates were also not torqued to spec. I found a little bit of movement in my steering intermediate shaft so i am replacing that as well as my full tie rods. I am hoping that all of this fixes it. If not it at least needed to be done. In terms of the brakes I was previously running Pagid yellows and had the same problem. I then put on brand new pagid blacks and the problem continued. My question in terms of deposit on the rotor is, wouldnt I see the deposit or heat spots on the rotor? Also wouldnt the shake come through the pedal and the steering vs just the steering?

Van

Deposits on the rotor are really not visible (from my experience). And, it's more like a "front end shake" - not like a front wheel that's out of balance - and not really a pulsating pedal.

What's happening is that the pad has different amounts of grip (different coefficient of friction) on different parts of the rotor. So, as the brake pad on one wheel is going grab-slide-grab-slide-grab-slide - it has different braking force (traction) with the road. The wheel with the most traction will tug the steering wheel in that direction.

rabjohns

Did you recently stop drinking heavily?