Front Rotor Runout - PeachParts Mercedes-Benz Forum

benfield4 · #1 10-01-2012, 02:16 PM

Working on the 89 300 SE and I have a wobble when braking going down a hill or while on a freeway exit ramp. I did a complete brake job on this last year. Sounds like a warped brake rotor. The front drivers side runout is .009 inches. The passenger side is .001-.002. I replaced the drivers side rotor since its runout is excessive. I also had made a mark on the hub where the high side is on the old rotor. The new rotor runout is .006 at the same spot. This has me thinking its not the rotor but he hub or bearing. I set the bearings according to the service manual with the dial indicator. The bearings are original. I have ordered a new set of bearings for it but wonder if the hub could be messed up. I did not and should have ran the dial inicator on the hub itself to measure its runout. So I need to do that. Any other tests or ideas on what could cause the runout?

There is an 87 560SEL at the bone yard that I may pull the hub and bearings off of just in case and use to test with. That wheel shake is darn annoying.

mbdoc · #2 10-01-2012, 04:09 PM

Of course you could have the disc machined while it is mounted to the hub.

97 SL320 · #3 10-01-2012, 06:22 PM

When measuring runout, did you install all of the lug bolts? ( Use some washers / large nuts as spacers to prevent the bolts from bottoming out.)

If you are measuring with just a single rotor allen head attaching bolt, the rotor will be skewed. Hubs with 2 bolts would exhibit less runout but it will still be there.

Rotor runout is a minor cause of braking shake and probably more noticeable with light braking than heavy.

With fixed calipers, the pistons on either side will slide in / out and with sliding calipers the caliper will shuffle absorbing runout during braking. ( Rotor is moving left to right , pistons track this motion. ) Now that said, runout will knock the pistons back when the brakes are released causing low peddle because the pistons now have to travel farther to contact the rotor.

Thickness variation is the major cause of braking shake as there is nothing to absorb the variation. ( Pistons move in and out together. )

If the bearings were bad the runout would not be in the same place and would be observed by shaking the rotor. A rusty hub face would cause the issue you have. The hub could be refaced on a brake lathe but the radius where the hub face hits the hub center needs to be maintained. A sharp corner here could cause a stress riser leading to cracking in the long term.

And, just for completeness, rotors that have grooves in them won't cause vibration alone. Other than looking "bad" they have little affect on braking once the pads wear in.

mbdoc · #4 10-02-2012, 08:18 AM

On a W126 car the disc is bolted thru the back with 5 allen bolts & do not require additional bolts to secure. But most newer car do need that.

benfield4 · #5 10-03-2012, 02:09 AM

Went and pulled the hubs off of that 560. I got them for 7 dollars a piece with the bearings. Cleaned and test fitted the hub and its bearings to my spindle with the new disk and get .002 of runout. That is within spec. Not sure what is up with the original hub. When my new bearings get here I will fit them and the new races to the used hub and see what the runout is.

My discs bolt to the hub from behind with the 5 bolts. So when I measure the runout its with a bolted disc to hub with the bearings set to the proper tension. I only wish they came off without doing the bearings. Oh well it makes us service the bearings.

benfield4 · #6 10-07-2012, 11:04 AM

Fitted the new bearings and races to the new/used hub and still have the .002 runout. Used the MB grease this time and its decent to work with. The old bearings from the donor car had a little rust on them so I feel safer using the new bearings. Its nice to have 3 W126 cars at the pull and save as a parts source. Thanks to everyone who looked at this.