Rapid rear tire wear

[George Davis]

After a couple years of blissful ignorance, I finally noticed that my rear tires are wearing at a huge rate.  With maybe 8-10,000 miles on a new set, the fronts look great but the rears are close to the wear bars.

In the garage the car sits with a bit of negative camber.  The tread wear is symmetrical on both sides, with the wear being in a "wedge" pattern corresponding to the camber.  See attached sketch.

So the question is, is this rapid wear on the rears normal, or should I be looking for alignment/rear axle issues (other than correcting the camber)?

Thanks for any info you can give!








Download Attachment: Treadwear.jpg
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George Davis
'69 280 SL Euro manual

[mdsalemi]

George,

After my car was restored I've never been completely satisfied that the alignment was correct.  I've found a new local shop here in Michigan that has a sophisticated alignment system.  All they work on is Porsche, BMW and Mercedes.  I am due a new set of tires shortly, and when I go to this place to get them installed, I'll have them put the car on the machine to MEASURE what we have.

Often times if you "send a car in for alignment" you don't get a report of what you HAD prior to them making any adjustments.  In my case I have a noticable camber issue like you indicate (though mine is the opposite direction) and I want accurate measurements of all alignment data to compare to specs before we actually do anything.

The alignment specs for our car indicates that we should have +1 degree 30' +/- 30' for rear wheel camber; that's with about 600kg of rear axle load as well.  Imagine my surprise when I discovered that this shop keeps a set of weights (ex dumbells) to properly load the vehicles prior to alignment.

Arm yourself with the good data book, and find a good alignment shop and more importantly someone willing to work with you to correct the issues and get them right.  Then get a new set of tires!

Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored

[George Davis]

Michael,

good advice and thank you!  But trying to read between the lines, can I assume you also had rapid wear of your rear tires?

I guess what I'm trying to figure out is whether the rapid wear is caused by excessive camber, or is it independent of camber and I should look elsewhere?

George Davis
'69 280 SL Euro manual

[mdsalemi]

Hi George,

No, I don't have rapid tire wear, mostly because there is only a couple of thousand miles on the restored car, and tires were already rotated B to F last year...not to say I would not, but I don't.

I do indeed have the camber issue, but it isn't drastic, just not correct.

I swapped tires with one of our friends, and when these arrive, I will have them installed and then we'll but the car on the "jig" and give a read out.  What I'll do for our benefit is photograph things so you can see what we're talking about.


Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored

[Ben]

Am I not correct in thinking that the only way to change the rear camber is with the spring pads ??

Regards,
Ben in Ireland.
'64 230SL 4sp.
'03 CLK Kompressor

[A Dalton]

Camber change is upper  spring pad thickness and spring base plate position.[ position #3 being highest]
 This is for each side  adjustment and can be different , side to side...
 Total overall adjustment can be changed with compensation spring pads thickness. [ this effects both side]
 I always use the thickest [ 6mm] here, as they cars are old and
usually have weakened spring and neg camber.......

[George Davis]

A little update on my rear axle.  First, I got under it yesterday and pushed and pried and there does not seem to be any play at the axle pivot.  That's good news.

At that point I concluded that the rapid tire wear is probably due to a combination of (a) the track changes that occur as the swing axle moves up and down, which makes the tires scuff in and out all the time; and (b) excessive negative camber.

I then pulled out a spring to see which spring pads are in there, and lucky me, they are already the thickest pads (30mm) available, which means correcting the negative camber won't be so simple.  I think I  need more lift than is possible by rotating the lower spring perches.  The most straightforward solution would be new springs, but I'm too broke/cheap to consider that right now.  I won't work on it for a few weeks, but whatever I come up with I'll post.



George Davis
'69 280 SL Euro manual

[A Dalton]

<< but I'm too broke/cheap to consider that right now>>

 A low cost temp fix [ til you get some $$$ for springs] is to buy a couple of those cheap rubber spring helpers . They go between the coils and actually work quite well.
 Any Auto store has them and you just jack the car to extend the spring coil distance and slip them in..
Best placement is about the 1/2 way distance of the spring..
 You may even notce less ass sink when taking off........
Temp fix, but I have seen guys do this and they are still there years later when I work on their car...