Brake vibration and front wheel bearings - PeachParts Mercedes-Benz Forum

ATLD · #1 12-01-2006, 10:37 AM

Gentlemen,

A few questions regarding my recently acquired 86' 560sel. I find that I get a noticeable vibration in the vehicle when braking medium or hard from 30+ mph. Assume this is not normal so replaced front and rear rotors, pads all round, rebuilt calipers, replaced pads with organic, flushed the brake fluid, and pressed in new front bearings etc. The vibration still exists.

I guess my question is twofold:

1) Regarding the front bearing preload. Can anyone go over the procedure? I first noticed that there is no washer between the front out-board bearing and the lock-nut; none on either wheel. None came with the bearing kit. Should there be one? Some of my other German cars have one. Anyone know the p/n? When I set the preload, I cranked down on the lock-nut when rotating the rotor until the resistance was significant, then backed off 1/3 turn. I could not procure enough preload to get the rotor to stop completely like I could on my 123 diesel. Is this normal? It seemed to require excessive torque to turn that lock-nut sufficiently to get this bearing drag. My worry is that there should be a washer and I am bottoming out the threads on the spindle, without achieving proper preload. Any thoughts?

2) With the rotors, pads, calipers and bearings all replaced, what else could cause a significant vibration when breaking only. Could it be the tires? They don't cause vibrations when accelerating and/or when cruising at speeds.

Thanks,

Adam

david s poole · #2 12-01-2006, 11:28 AM

if you have other wear in the front susp and the steering shock is gone replace the steering shock it may cure your problem.most likely cause are the brake reaction rod bushings.

ATLD · #3 12-01-2006, 11:55 AM

Steering shock will be replaced. My concern is that the vibration is periodic and proportional to vehicle speed only while braking with moderate to hard pressure. I would think that this would indicate a rotating unbalance of some sort; everything but the tires have been replaced. No vibration in other situations. Suspension is tight.

Anyone have any thoughts about the front wheel bearing preload?

Thanks,

ATLD

Kestas · #4 12-01-2006, 01:09 PM

First, it seems funny that there's no washer. I can't remember if mine have them or if the locknut does double duty as a washer. I'm guessing you need a washer. I wouldn't expect it in the kit, you're expected to reuse the old one along with the locknut.

There's been many threads about setting proper preload, many in which I participated. A search on your part would be easier than my retyping these instructions all over again.

When seating, the point is to tighten it so it is difficult to turn the rotor, not necessarily stop it. If you got a significant amount of drag, then I think you did it properly. Otherwise, you bottomed out against the final thread.

I'm not so sure you set the preload properly. After seating, the locknut must be backed off until loose, then tightened barely finger-tight before locking the nut. Ideally, a dial gauge must be used to set proper preload. Without a dial gauge, a backyard mechanic can do well using the finger-tight method.

Your vibration could be coming from rust oir grit on the hub/rotor mating surfaces. The surface must be clean and free of grit before mating them.

ATLD · #5 12-01-2006, 03:19 PM

Do you know the part number for the washer (if it exists)? I have not looked at the RM precedure for setting the preload for the 126, used the 123 info that I remembered. Maybe that's my mistake. I'll look into your posts.

The hubs were cleaned thuroughly before I pressed in the new bearings so I don't think that there was dirt or dibris on them that is causing the vibration. The rotors installed were clean and new (Zimmermann cross drilled) and could not have added much to the tolerance. But I'll check runout to see if there is any indication that the rotor is not seated right onto the hub. I'd doubt this because the symptom is the same now that it was with the old rotors.

Could the brake reaction rod bushings cause a vibration when brakeing consistently and with even pressure?

ATLD

SootyPast · #6 12-02-2006, 11:25 AM

Most did not have a washer. There is a feel that you can get when you start loading the bearing. It is different than when in the free area. The very best way to do it imho is to tighten it just to zero play and drive it for 50 miles or so. Then jack the car up and check the play. Snug as needed. This used to be part of the first oil change and inspection on the 126's when they were new.
As far as vibration is concerned. Be certain that all of the years of rust film on the back of the wheels @ the hub mating surfaces are completely clean before you go checking other stuff. You can take some emory cloth to it. The next thing I would check would be the tires themselves.

t walgamuth · #7 12-02-2006, 11:59 AM

on the bearings i tighten them till significant resistance is brought to bear to seat everything, then back it off till the rotor turns freely. finger tight sounds a little light to me.

tom w

ATLD · #8 12-03-2006, 10:08 AM

Thanks for all your feedback. I'll see what I can do. It's just that I was almost sure the vibration was rotor or bearing related since the vibration occured only when braking, but oh-well. Makes the diagnosis more fun.

Adam

sokoloff · #9 12-03-2006, 08:29 PM

And of course as a last resort you could always follow the procedures in the manual and use a dial indicator as is recommended.

Len

ATLD · #10 12-04-2006, 09:37 AM

I'll do that too, but I feel that presently I have more bearing preload (less) than required. I would think that vibration would be more likely caused by excess freeplay. The bearings have only been in a week. I will recheck preload this next weekend (as is customary with new bearings) and set to factory. I'll also check runout with the new rotors to see if there is some possibility that they are not flatly mounted.

Thanks for all your suggestions,

ATLD