Update Strange vibration-heat from rear wheel
 

Hello, long time listener, first time caller. I posed this on the BenzWorld 123 forum but wanted to get a wider variety of input. For my 1984 300D I am investigating the cause of unusual noise, vibration and heat generation from my right rear wheel at hi-way speeds. It's intermittent, only occurring twice, once in June and once last week. Yesterday I jacked up & blocked up both rears and removed the tire, caliper, disc/drum, etc. and inspected everything, including the axle shafts and shock and essentially found nothing out of the ordinary so I put it all back together.

One thing of concern is that (in park, with both rears off the ground) both wheels turned fairly easily (and in the opposite direction) until I snugged the lug nuts for the right tire. At that point, I had to grab the O.D. of each tire with both hands to move it. I loosened the right side lug nuts a little and both sides spun freely again.

With both tires fully tightened, I placed it in neutral and the left spun very easily but the right was still very difficult to spin. Is this normal or is there a clearance issue in my right hub? If so, this could very well be the source of the noise, vibration and heat.

To eliminate the right rim, I installed the spare tire Bundt rim on the right side and it reacted the same. I even tried loosening and tightening the LEFT SIDE tire and it did not affect rolling resistance as the right one did.

When both rims are tightened down is there supposed to be that much more resistance in spinning the wheels than with the right tire loose?

Thanks for anything you can reveal about how this rear end works and what is "normal" for rolling these tires by hand.

Respectfully,
Earl Alllen

Answer
 

Inspect the rear brake pads = remove them from the caliper to verify the edges of the backing plate are not corroded within the caliper box.

This sounds like a common issue, especially in northern states.

Loosening the tire allows the rotor to shift = relieves tension of a stuck brake pad.

If this is the issue, dismount the calipers, use a file to clean the brake caliper box where the edges of the brake pad backing plate make contact.

A very light coat of anti seize on the contact surface may slow corrosion in this area.

.

The discs, calipers, pads & hoses were replaced by the PO not long ago and still appear new. Here is what I have done so far-
I jacked and blocked up both rears and removed both tires.
-could not detect any appreciable axial movement of the hub
-the disc is straight & not warped
-the pads are 5/16" thick and evenly worn
-the parking brakes are assembled properly with plenty of lining left on the shoes
-brake hose is new and satisfactory.
-after removing the caliper, I exercised the pistons by reconnecting the brake line and having hubby slowly pump brakes to push pistons out. One piston moved much easier than the other. I had to clamp the looser piston while pumping the brakes harder to get the other piston to move. After a couple of cycles back and forth they seem to move pretty easily.
-axles on both sides had the same feel & the same amount of movement, when you grab & push and pull the shaft portion there is a inch or so of in-out movement, which I assume is exercising the inner and outer joints.

I did the standard tests to check for sticking caliper and all tests were sat with the tire removed. It is only after completely torqueing down the lug nuts that the wheel becomes extremely difficult to spin (only the right rear).

hot-wheel test drive update
 

I test drove it at 75 MPH for 5 minutes and the right rear rim was 140 degrees and the others were about 100 degrees. Then test ran for 15 minutes at 75 MPH and the right rear rim was at 180 degrees and the rest were at 110.

Additional readings showed the "can" shaped portion of the right rear axle shaft was at 120 degrees; the dust shield plate behind the brakes was 210 degrees; the caliper was 250 degrees and the surface of the disc was 280 degrees. I'm leaning toward replacing that new(er) caliper and/or disc. But I still have the question of excess resistance in rolling the right rear wheel (only after lug nuts are tight).

Answer
 

Ouch.

This indicates there is an issue with piston drag.

You have three options:
#1. Flush the brake system, and hope the issue was contamination or lack of exercise (parked to long).
#2. Rebuild this caliper.
#3. Replace this unit with a rebuilt caliper.

.

The long standing rule in working on brakes is that one always does the same thing to both sides of an axle... so rebuild both sides... or replace both sides with rebuilt...

Thank you. I had to go back and change my initial response to your answer. I realize folks do not know me or what I know. I am a 28 year veteran in the USCG; started out as a Machinery Technician and progressed thru CPO-CWO and now LCDR. I have been owning/maintaining MBZ since 1997 & this is my 6th 123, with a 108 and a 107 thrown in there. So, I am pretty familiar with the long standing rules of working on brakes.

Thru further tests drives and IR thermometer reading, I have narrowed down the source of heat to the interaction between the caliper and the disc. I know, I know, several have said that already :rolleyes:. This brings into question the quality of the parts replaced by the previous owner. Looks like I'll be renewing two calipers, two discs and two hoses.

Although I am still not satisfied with the question of the high resistance to free-rolling that wheel. That resistance disappears when the lug nuts are loosened or the tire is removed. Perhaps when the lugs are tightened and the disc and hub are squeezed together that is when the undue pressure is applied causing the heat. felt. We'll see once I get those parts in.

Wrong length lug bolts? Easy test. If wheel is dragging with the bolts tight. Remove them and put temporary cheap throwaway washers under each bolt. Then test for wheel drag again. Over intrusion into the emergency brake area with the wrong longer bolts is a maybe.

Or pull a bolt from the other side and do a length comparison with one from the difficult side. If the caliper can sllde back and forth it should be okay I would think. Then again take it for granted that I know little as I am just a car hobbiest. That's when I even get time to be one. I am getting pretty tired of not finding the time to work on cars that I enjoy doing. For some reason to me it is a form of relaxation.


I am not trying to diminish in any way what others suggest. Just a perhaps slight possibility I would look into for the time it takes. There are two different lug bolt lengths for the wheels on the old 123s I believe. The alloy wheel lug bolts are too long when used on steel wheels.

The only problem I have with this is I would not expect to build heat from this issue. On that basis a troublesome non floating caliper is a better guess.

You can check if the caliper's mount to the hub assembly is the same on the left as it is on the right, since you mentioned when you tighten the wheels that's when you get resistance (heat). But since nothing happens on the left side I would compare it against that.

Hopefully it's just a case of something as simple as ... a shim used on the inboard rear caliper pad versus none on the outboard, causing undue pressure from the pad to rotor. Or something like that.

My post was actually just trying to correct (for the archive record) the accidental omission by WHunter which might have left the impression in those new to brake work that one side could be addressed by itself properly...
Threads are not just for helping the OP.... but also for people reading them later.... and we have a HUGE variety of mechanical experience visiting and using this site...

Roger on all Mr. leathermang.

Where are you in C. TX? My wife is from Dripping Springs. I joined the Coast Guard out of there and it is still my home of record. We may be retiring somewhere around there in a couple years.

VR/Earl

Earl, check your pm's...