Rear brake very hot

[jacovdw]

Naj,

I have been sifting through the literature I have and found nothing indicating when the grease in the rear hub should be replaced.

It's a different story for the front hubs though. According to the manual one should replace the grease in the metal dust caps every 20 000 km's (12 000 miles).
That's about 20 grammes of high temperature wheel bearing grease per cap (if filled to the line).

For the rear hubs you basically have to disassemble it (special tools required) to change the grease and that is quite a tedious job to do every 20 000 km's.

Thus it would be interesting to hear what other members have to say.

[Larry & Norma]

Is it fair to assume the original grease should last the life of the bearing, perhaps 100,000+ miles?
Front bearings suffer much more stress due to steering forces and braking forces.

[mait]

Re: Lubrication of rear wheel bearings.
 These bearings are lubed by the rear axle lubricant. Re-packing as in the case of front bearings is not necessary.
 BBB recommends change rear axle housing oil every 12,000 miles.

Mait

[Larry & Norma]

There is an oil seal between the rear axle oil and the bearing, this prevents the oil washing the bearing grease away!

[jacovdw]

Larry is correct, there is an inner and outer oil seal fitted on the enclosed axle shaft and the bearing is indeed packed with high temperature bearing grease.

With reference to the rear bearings, the service manual states that the bearings should be rejected on principle (even if they appear serviceable) after 100 000 km as the duration of further serviceability is unknown.

I guess in practice it doesn't always work that way as the cost of replacement (in terms of time, effort and money) make many owners neglect that aspect.

[wwheeler]

Thanks for the replies.

I was planning on rebuilding the axle in about 4-5 months and cleaning the entire rear under section including gas tank. That's a big project! I could wait until then, but I would like to see if I can find the cause to this problem first.

I think I will remove the wheel, pads again and see if the rotor is perfectly in-line. Maybe there is something causing the rotor to misalign when tightened down. I don't think the brake regulator in the rear could cause a problem side to side. I am pretty sure this has to do with the rotor alignment. Since the calipers have the piston springs to keep the pads snug on the rotor, I think this is the source of heat. Has anybody else ever seen the piston springs?

Maybe the rotor is warped?

[jacovdw]

...I don't think the brake regulator in the rear could cause a problem side to side...

Highly unlikely to be the cause of your problem.

I still would check the brake rotor and the hub with a dial gauge to check for any deviation.
If I remember correctly, one also need to check if the brake rotor itself is centered in the caliper.

Can't for the life of me remember where I read that in the manual at the moment, but will look for it and report back.

[Shvegel]

Hi,
Interesting problem. I second checking the runout of the rotor on the axle then I would check the runout of the axle flange itself. if the axle flange is bent from a previous accident or curbstrike it will cause even a new rotor to "Wobble" more than is acceptable.

[Larry & Norma]

Are you aware that there is a locating stud for the rear disc/drum on the halfshaft flange.
The hub only fits in one position. Usually if the hub is tightened in
the wrong position the stud just pushes back through the flange but maybe
in your case it did not.

[wwheeler]

I looked for a locating stud but never found one. The rotor without the wheel bolts or pads will spin freely. The other side was the same way and I am not having problems there. However, maybe the stud is sticking through just enough to cause problems.

I don't much like the thought of the axle being bent!!!! BUt a good idea nonetheless.

Hopefully I can pull the wheel off tonight and do some investigation.

Great ideas. Its always easier to find something when you know what you are looking for.

Thanks!

[wwheeler]

There definitely isn't a locating stud on my axle flange nor is there a place for one. After removing the wheel, I found the rotor alignment to be slightly more inboard in the caliper. I couldn't find any obvious warpage with the rotor using a straight edge.

The end play on the right axle flange is about 1.5mm (.060"). The run out is 2mm (.008"). I looked in the BBB and I found a spec for the shaft run out which is .12mm (.0047"). I could not find a spec for the shaft end play. It looks like the run out is only slightly out of spec but the end play seems excessive.

The new rotor arrived and I will put that on and see what the run out is. What is the spec for the rear rotor run out?

Any thoughts?

[jacovdw]

Hello Wallace,

The runout for the brake disk is measured on the outer edge (dial gauge mounted perpendicular) and would be 0.12 mm for the front and 0.15 mm for the rear (maximum).

That end play does sound excessive indeed. Bearings towards the end of their life perhaps?

The specifications I have for the rear axle flange is as follows:
a) maximum permissible vertical runout on centre alignment with shaft installed is 0.12 mm
b) maximum permissible lateral runout of flange is 0.12 mm

Hope this helps.

[Benz Dr.]

The spring type pistons in the caliper may lead to higher pad pressure but I doubt it's causing your disc to run hot. Any lateral deflection on the rotor or the axle shaft would normally move the pads back and should prevent any heating problems.
In most cases, one pad having more wear than the other is usually a stuck piston. I can't imagine it being a wheel bearing. Spinning the wheel while lifted should tell you if the bearing is bad just by the amount of noise you hear. You could also put the car on a lift and use a mechanics stethoscope to listen for any unusual noises.

[wwheeler]

The problem is temporarily solved!!!! The right axle shaft flange had a lateral (wobble) run out of .008-.010" as measured at the edge on the face with a Starret dial. The end play is exactly .046". The rotor had a run out of .012". To me, both of those dimensions are excessive. I installed the new rotor on the flange and secured it. When the axle was pushed in, the rotor was about a 1/32" away from touching the inboard caliper casting. With the axle pulled out to the end play range, the rotor was just about centered.

I had already moved the pistons back in their bores. I held the rotor in the centered postion and applied the brakes to reset the pistons in that spot. When I turned the left tire, the rotor and pads made a shh...shh...shh sound from the rotor run out. I decided to again reset the pistons directly on the spot where the pad didn't make contact with the rotor. After doing that, the rotor/padnoise was almost consistent all the way around!!!!

After doing a quick blast down the highway, both calipers were the same temperature. Normally the right one would very hot at this point. So, this is a temporary fix and will only be permanent when I rebuild the rear axle.

If this would have happened on the front, I don't think that the heat generated would have been as severe. The spring loaded pistons in the rear not only keep the pads snug on the rotor, but also a stronger force is required to push them back.

Thanks to everybody for their suggestions as they were extremely helpful. Oh well, I have more fish to fry. 

[Benz Dr.]

I had a similar problem a while back. This was with a standard caliper. I had a hot running problem as well but this was caused by a defective caliper. The casting was a bit too thick or something like that and we had to put a washer behind the caliper to [position correctly;y.
I changed axles last week to the LSD and I had the exact same problem with a completely different unit. The caliper works fine but is clearly no to spec.

 LDS really makes the car pull hard. I can definitely feel the difference and the front of the car lifts more under hard throttle. Should be fun when the roads clear up.

[jacovdw]

The problem is temporarily solved!!!!...

Glad you got it sorted Wallace.