Heirsh

I just wanted to share some knowledge I've acquired on my car. It may be common knowledge to you all, but to me it was interesting.

If my rotors develop .002" runout or more I can feel a shimmy in the steering wheel. The times I have had this problem it has always been measurable on the rotors. I've used new rotors and now trued rotors to cure it. I seem to develop this problem every 8-10k miles.

I have swapped calipers so we'll see if this changes. My old ones had quite some miles on them, though they seem fine on inspection.

It really doesnt take much to cause a shimmy on my car. I have never had a car that developed a shimmy over and over before like this. So I have to wonder if something else is wrong.

Are the floating caliper type brakes (on the cars I've had previously) not as sensitive to rotor runout?

jimq

My Dodge Ram truck has the same problem. I have replaced the front rotors 2 times in only 40K miles. They will get so bad it feels like the wheel is falling off when stopping. i got some race ones the last time and havent had and shimmy since. Some makes of rotors just seem to flex or warp for some reason.

Heirsh

Well 1 set I was working with are Porsche, I don't know about their age. The other set is Zimmerman.

Does anyone have a recommendation on a rock solid stock rotor? Something that would be less prone to this problem, assuming its a problem with the rotor?

Red rooster

I had a similar issue with the LH front rotor . Put a dial gauge on to measure the run out . It was outside maximum spec. Rotated the rotor on the hub by 90 degrees and tried again. Similar run out but in a different spot on the rotor.
Changed the bearing in that hub and its been good ever since .

Maybe just one of those things but the bearing change worked in this situation.

Good luck with your investigation.

Geoff

Indycam

http://www.carrollsmith.com/main.html

http://www.stoptech.com/tech_info/wp...rakedisk.shtml

Heirsh

I've been assuming all that is fine since it all runs perfect when I have a fresh rotor on the car. But, if the bearing is worn and allowing the hub to move a bit, then any out of balance from the tires or alignment could be working its way onto the rotor like recording a record.

Excellent point, as usual, Geoff. Thank you.

Heirsh

Indy,

Thanks. That was good reading on the material change on the disc making non-uniform hardness and varying the wear at diff points. That was something I have not read before.

The old issue with whether these rotors can be warped is a hot topic.

Indycam

Crazy stuff , I wonder if its true .

TR6

I had a case of the wobblies also and changed rotors and pads which has cured it for now. But its been too recent to make any long term predictions.

Red rooster

Heirsh,
This problem got me really thinking about when I had a similar problem.
I dont know if you have a shop manual but the procedure is to measure rotor
thickness say every 90 degrees . About 2 thou variation is OK.
Then clamp the rotor on the hub without the wheel. There is a special tool , as always, but I used some large nuts that slip over the studs and the original wheel nuts. Put a dial gauge on the rotor outer pad contact point and check the run out.
4 thou is maximum . If the rotor is a bit over , try moving the rotor round by one stud and try again . If necessary move it another stud.
Make really sure that the rotor / stud flange areas are perfectly clean.
If you cant get the run out below 4 thou either the rotor is bent or the hub is not running true.
I watched the dial gauge and made a paint mark for the maimum runout point. Moving the rotor around on the studs moved the maximum run out point the same distance. So the rotor was OK but the hub was not !
Took a little while but highlighted where the problem was .
New hub bearing and run out was perfect , just like the other side .Braking wobble just went away and I was happy that I knew why.

That was 5 years ago !!

Geoff

Heirsh

hey Geoff,

I used a similar technique to clamp the rotor to the hub. I used the wheel nuts with spacers. If you use nothing but the 2 flat head screws you wont get a correct reading (well at least in my case).

I did not try turning them on the hubs. That would be an interesting exercise. I've been assuming my hubs are true. I made that assumption based on measuring the rotors as true on a lathe and then measuring the rotors on the hubs as true as well (well I could measure .0005 runout or so, but for practicality I consider that true). At first I did think my hubs were off. I only casually checked the thickness of the rotors to make sure they were in spec in a couple places.

My bearings could still be a place of interest though. I will investigate if the problem comes back. I've got my figners crossed that these newer calipers I put on will somehow fix it. (Its a dream, I know!)

When/if the problem comes back I'll check the thickness and try rotating the rotor on the hub and see what happens. Thanks for the added points to check.

Brakes shouldn't be such a pain!

dfinnegan

Can you swap the rotors left to right for a test?

Indycam

I never thought to do a dial on the inside and outside at the same time . If it is the case that the rotor is funny , thick in one spot .....