Hello Classic Zer's,

I have the big brake kit on one of my race cars. Actually a car we're going to run this coming week at Sebring. I have fairly violent shaking through the steering wheel under braking.

1. Although the rotors were new, I had them turned to see if this would help the issue. Issue remains the same.

2. We have tried different sets of balanced tires.

I suspect possibly a geometry issue up front. Has anyone ever run across this?

Greg Ira

Greg, were the rotors turned while in place on the car? Have you measured the rotor runout on the car with everything installed?

In all my years of brake work, this has ALWAYS been a rotor issue. I can't quite see how a geometry problem could cause a violent shake.

You had to switch to 280 hubs, right? Did you check runout on the hubs?

Hybrid Z had a thread about rotor runout on the cast iron rotor/aluminum hat setups maybe a year ago and the answer that sounded best to me was to shim the rotor/hat interface to minimize the runout. If you cut the rotors then you create a thin spot in the rotor which won't heat up evenly and may cause further problems. By shimming the thickness of the rotor isn't changed. This is backed up by my own personal experience in cutting rotors. The more you cut rotors the quicker they warp. It got to the point where I won't even cut them anymore, I just replace now.

Are you using solid bushed T/C rods on the car as this will transmit a lot of vibration under braking?.On my own car Greg I have delrin bushes and the harder I apply the stoppers the more the vibration is .If it were the rotors then you would get feedback through the pedal in the form of pulsation.If it is a general rumble and you are running the said set up don't worry to much as this is normal as the original rubber insulates all this vibration.

Are you using solid bushed T/C rods on the car as this will transmit a lot of vibration under braking?.On my own car Greg I have delrin bushes and the harder I apply the stoppers the more the vibration is .If it were the rotors then you would get feedback through the pedal in the form of pulsation.If it is a general rumble and you are running the said set up don't worry to much as this is normal as the original rubber insulates all this vibration.

That sounds like you also have warped rotors.

I ran rod ends which are metal to metal bearings for years, no vibration under braking other than the road surface vibration. There isn't anything in the "geometry" or in solid bushings or rod ends that would cause vibration that feels like a warped rotor. Or if there is, I haven't come across it.

Depending on the AZC rotor size you're using, they sometimes scrape on the outside edge of the front LCA. This occurs somewhat at random and on one customer car only occurred under braking. This caused a lot of vibration and instability until we ground down the end of the LCA.

OK Great. Thanks for all the hints. The whole suspension is rod ends, no delrin, no poly. If all components are true, there should be no vibration, just road feel as described. We've had this set up before, just not these giant brakes. I did have the rotors turned but not on these same Z hubs.

I am a little suspect on shimming the hub. First off there are only four points the hub contacts the rotor. I am worried about having a good contact between the rotor and the hub if I shim it. What if the high or low spot is half way between the bolts on the hub?

We're in the midst of checking the runout. It must be in the hub/rotor area.

Thanks again.

Greg Ira

I wouldn't shim between the hub and the rotor hat. I'd shim between the hat and the rotor. If you have runout on the hub, you can turn the outer face of the hub. If you have runout on the backside of the hub where the rotor bolts up, I'd replace the hub.