Front Rotor problems - help please - Mercedes Benz SLK Forum

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#1 Old 3 Weeks Ago
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Unhappy Front Rotor problems - help please

SLK has been a bit neglected as I've been working on my Beemers but the SLK finally demands some TLC.

I put in new Bilsteins all round last year and they are excellent but I noticed that the inside of the front tyres has started to wear. No problem I thought, I'll wait until they need replacement and get the alignment done at the same time - should really have had it checked when I put the shocks on so it's my fault.

But lately I've started to feel some judder in the steering from the front when slowing down from above about 40 mph. A look at the discs through the wheels and they are in pretty bad shape so I ordered new rotors, pads, and pins.

Today the new rotors etc arrived - all Brembos.

Before taking off the wheels I did a quick check for play in the bearings - I can feel a little movement on both sides but it feels like its in the rack not the bearings and it isn't much.

The pads are hardly worn at all but the rotors look really shot. I check the run out "by eye" and it looks ok so old rotors off and new rotors on.

Now... The run out is so bad on the right that it's rubbing on the caliper. Rotor off, wire brush and brake cleaner and it better but still doesn't look right.

Rotor off and replace with the second new rotor. Same problem and about the same amount of run out. Two rotors with the same amount of wobble - doesn't sound likely. Using a makeshift jig the rotors look flat but I don't have the kit to do a proper test.

Only things I haven't tried are a) putting the old rotor back on and b) putting a new rotor on the left - (screw needs to soak in WD40 overnight). But the old rotor on the left definately doesn have any wobble at all.

So, anyone any experience in diagnosing this type of problem? Have I been sold two faulty rotors? Can wheel bearings be out of alignment but not show any symptoms of wear?

Any advice would be welcome
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#2 Old 3 Weeks Ago
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Check the wheel bearings. I believe they are sealed. If there is a cotter pin, you have the type that has to be greased. In either case, replace them.
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#3 Old 3 Weeks Ago
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I had a similar problem years ago on an Audi s4 that I put rs4 discs and Porsche Cayenne callipers on . I couldn't solve it with any amount of cleaning and re bedding procedures and ended up getting the discs skimmed in situ on the hub of the car , it was surprising how much actually came off the discs as I was stood beside a mate as he did it . Worked great though , it trues both side of the disc to the hub in one go .
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#4 Old 3 Weeks Ago
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Setup a "makeshift" tool to measure the run out ... basically a heavy box with one of the old pins tapes to it.

By looking at the reflection of the end of the pin in the surface of the rotor I could line it up so it just touched the rotor, then turning the rotor to the biggest gap I used a feeler guage to measure the run out.

"Bash test" (striking the rotor/wheel in one place with the heal of the hand) with and without the wheel fitted I came out with different amount sof run out each time - including one reading where the rotor was "perfect".

Clearly I have something moving here!!! But I can only feel the slightest rocking movement with the wheel fitted???

However, I have another problem to be solved - the fixing screw on the other rotor is seized solid.

Two hubs with bearings already fitted is €162.68 delivered. For that price I'll just fit two new hubs whether the bearings can be adjusted or not. And it saves having to drill out the screw

I've included a picture of the worn out rotor just out of grizzly interest ...
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Last edited by [email protected]; 3 Weeks Ago at 01:42 PM. Reason: Spelling
#5 Old 3 Weeks Ago
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When fitting the disk (rotor) onto the hub, by hand, before fitting the securing screw, does the disk spin freely against the hub (Hub stationary, rotate disk), or is the disk catching/sticking to the hub.


I would expect the disk to spin freely/smoothly against the hub (but with a minimal resistance), which would indicate it was not picking up anywhere on the hub and stopping it sitting correctly.. It should fit squarly on the front face of the hub.



If the disk (rotor) is binding on the hub, then some chalk, engineers blue or similar could be used to identify where the disk it making contact with the hub, other than on the front flat face of the hub.
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The hub needs to be really clean before fitting the new disk (rotor). Any rust/ debris on the hub will result in an unacceptable offset in the rotation of the disk.
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I've obviated any potential rust problem by ordering new hubs .

Relatively cheap and quick to fit.

If the hubs were hundreds of euros I'd consider flatting and polishing but they're not - €81 each (c Ł70) complete with pre-fitted bearings - just pack the outer, fit on the stub, check the end float and the jobs done. And I don't have to drill out a rusty screw that I've already spent 2 hours on!!!

I always replace the pads and pins etc to go with a new rotor so I think it's worth considering if there's any doubt about the wheel bearings.

P.S. Why do SLK hubs and rotors rust so much - I did one of my Beemers in the spring and the hubs were "like new". I also don't have to go through the procedure of loosening the wheel nuts and rocking the car just to get a wheel off (I don't like putting grease on the mating surfaces of a rotor or a hub).
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I had the same problem, in the end, I replaced the wheel hub,
You can clean them using a dremel, I did that after I order the new one


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#9 Old 1 Week Ago
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Is this usual for an slk?



Job completed.

Two new hub bearings, two new Brembo disks, and new Brembo pads.

Gentle test drive about 10 miles, all problems solved and lots of stopping power but equally lots of smoke and that track day aroma!!!

IS THIS USUAL FOR AN SLK WITH NEW PADS?

Never experienced this with any other car but this is the first car I've worked on that has fixed calipers so I'll take the pads out and put even more grease in.

N.B.

I tightened the old bearings and set the pre-load just fine and they felt good. BUT on disassmbing them they were short of grease and full of black "gunge", presumably green grease + very fine metal particles. You can never tell what's on the inside from the outside. Now in the bin.

Checked the pre-load with a dial gauge. The "hand-tighten, back-off, spin, check pre-load, repeat three times" worked for me but I feel better having checked with a gauge.

Hub bearings just didn't feel right from new and were, I thought, noisy for new bearings. A short test run and they settled down.

Enough "crud" had collected on the inside of the wheels over the years to stop them seating properly against the new disks.

Four pot calipers are something else - as I pressed one in another would "pop out". Makes sense really but I didn't expect it. Eventually worked out a system where I could close two pots together to get the pads in. Fun, eh?
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Quote:
Originally Posted by [email protected] View Post


Job completed.

Two new hub bearings, two new Brembo disks, and new Brembo pads.

Gentle test drive about 10 miles, all problems solved and lots of stopping power but equally lots of smoke and that track day aroma!!!

IS THIS USUAL FOR AN SLK WITH NEW PADS?

Never experienced this with any other car but this is the first car I've worked on that has fixed calipers so I'll take the pads out and put even more grease in.

N.B.

I tightened the old bearings and set the pre-load just fine and they felt good. BUT on disassmbing them they were short of grease and full of black "gunge", presumably green grease + very fine metal particles. You can never tell what's on the inside from the outside. Now in the bin.

Checked the pre-load with a dial gauge. The "hand-tighten, back-off, spin, check pre-load, repeat three times" worked for me but I feel better having checked with a gauge.

Hub bearings just didn't feel right from new and were, I thought, noisy for new bearings. A short test run and they settled down.

Enough "crud" had collected on the inside of the wheels over the years to stop them seating properly against the new disks.

Four pot calipers are something else - as I pressed one in another would "pop out". Makes sense really but I didn't expect it. Eventually worked out a system where I could close two pots together to get the pads in. Fun, eh?
did u bed the new pads?

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#11 Old 1 Week Ago
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Thanks for the links (I ignored all the stuff about incontinence pads, I don't need 'em yet).

I started the process but I was put off by the smoke.

I'll give the calipers another clean and add more grease then start the process again.

It can't hurt.

P.S. I actually quite like the smell but my wife was not amused.
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Quote:
Originally Posted by jbanks15 View Post
Pads out, lots of grease, bedding in, job done.

No more smoke.

(Still surprised at how little "tolerance" there is between the caliper, rotor, and new pads.
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