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Discussion Starter #1
Hello All
I have a 2006 New Beetle TDI with 102,000 miles on it that I bought from a friend last year (with 98,000 mi.). The tires were in good shape but I just noticed when rotating the tires last weekend that both of the back tires are badly scalloped on the inside tread.

The air pressure in the tires matches the data tag on the door that has the front tires at 35 psi and the rear at 39 psi which seems like the maximum load rating pressure for the rear. I use this car to drive to work and never haul anything other that my 170# self. My question is would 39 psi be too high for normal driving causing the unloaded rear end to bounce and scallop the tread. I am also considering replacing the rear shocks.

Thanks Steve
 

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39psi is very high.

The recommended pressure for my 98 is 29psi front and back for highway speeds and the 205/55/R16 tires it came with.
 

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I also run 29 pounds. When I bought the car last July, one tire was in horrible shape; nothing round about it. After replacing, no similar issues yet with rotation every 5k. I'm curious to hear if replacing the shocks helps your case.


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Discussion Starter #5
After reading many postings on this and other websites I plan to run these tires at a lower pressure. As bad as these are I can't do them any worse. This way I can tell if lower pressure will stop the cupping without wearing out the outside tread before I buy new tires.

As for the shocks I'm finding that worn shocks or struts will cause cupping. KeithJ on TDI forums writes:

Cupping is mostly from worn struts/shocks. They will not "wear in", not even on the dragstrip. They have to be sanded smooth, if there is enough tread left. Get new tires.
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And:
To further the reason for cupping, the wheel/brake/axle system has mass. This mass is attached to the body with a spring. So the mass-spring combination forms a mechanical oscillator with frequency(radians per second) being the square root of the spring constant (force/displacement) divided by the mass. This is a much higher frequency and the most energetic one. All it takes is minor road surface imperfection to drive the wheel system into oscillation.

Now there is a second, lower frequency based on the mass of the car and the spring rate. The damping of the shock absorbers in this range (higher amplitude, much lower frequency) rarely is lost. But the high frequency damping component of the shocks/struts can be lost after as little as 30k miles.

If you were to observe a car with a loss of high frequency damping at highway speed, the wheels look like they are vibrating at about 20-50 Hz. Voila, instant cuppinig. And once wear sets in, the shocks/struts are abused to where all damping in this frequency is lost. Oil foams, orifices wear, seals are shot and pressure lost.
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Whew! In a nutshell bad shocks will not dampen out any oscillations of the tire so it seems reasonable that this would cause cupping of the treads. The most likely reason ( and the cheapest) that I can see is over inflation on a light rear end. This could set up a bouncing that shocks can't dampen.

It would be good to hear if anyone else has had this problem and how they solved it.
 

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My suspension was blown entirely, the rear springs were actually broken, but I never noticed any issue with the tires themselves like you describe.
 

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I just replaced the tires on my 2006 tdi for the same reason! The old tires were uniroyal tiger paws from sep '08 and march '09. My beetle has 96,000 miles, i bought it with 87,000 miles. The tires were very noisy as well, it is amazing how quiet the car is now. No one (dealer + 3 tire shops) could tell me what caused the wear. I bought the tires (michelin energy mxv4 205/55r16) at discout tire, they said to bring it back @ 2,000 miles and they will check the wear.

The 4 opinions were: separating tires/incorrect inflation, rear struts (dealer said u dont replace rear struts on an '06), improper rotation and incorrect inflation.

Check your front tires as well: mine had worn unevenly there as well. It only had both front tires toed out, the rest of the alignment was fine.

Let me know if you find out what caused the problem.
 

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I have a 2002 beetle turbo and tires are doing the same thing on the rear inside. I have been told by two places it is a bent axle. Another place said it was just the alignment and possibly shims. The alignment did not work. The left rear was out of spec .25 and the right rear is out 1.75. The shop said proper rotation and shims would fix it. I called an import only shop and they said that far out of spec sounds like a bent frame. I am just lost. I hate our dealership because I think they are not honest and really I came to that from dealing with one person there. Oh and the tires are slanting in on the top so it looks weird in the rear. The left looks better but the right is horrible. It has 225-45 zr17 I believe
 

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I had this happen to an Eagle Talon I had, after 2 sets of tires, I was told I needed the rear suspension replaced to fix it. I took that as it was time to trade the car in, so I bought my first NB a 1998.
 

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Discussion Starter #10
Wow! Great info from 05yellow and mpendelton. This is starting to sound like a chronic problem with the rear tires on New Beetles. It doesn't sound like any particular brand or grade of tire but more like a combo of inflation, shocks and alignment.

Yesterday I measured the pressure on the rear tires before I left work then again when I got home, 39 psi cold and 43 psi hot. This morning I lowered the pressure to 32 psi before I left for work and measured when I arrived, 36 psi hot. The four pound rise in pressure may be enough to cause some bouncing on an unloaded rear end. This would in turn seem to cause the shocks to wear in the same place inside the cylinder. Now I'm just guessing on all of this but it seems to me that 43 psi is way too much pressure.

Reading 05yellowbeetle was like reading my own problem, almost identical down to the mileage. The front driver tire is worn on the outside tread indicating alignment and that's why I removed the tires to rotate them and found the rear in terrible shape. If any of this rings a bell with anyone out there please post your story. This could be interesting.
 

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Took mine to a frame guy today and he did not notice anything bent on the frame. The right rear wheel is really out of whack so he mentioned the Spindle and the bearing assembly.
 

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I had cupping on my tires, the car is an 05', but only has 38k on it (original tires). The wear was even, front and back but, I believe they may have been alittle flat spotted from sitting so much. I put new rear shocks and new struts on it, then new tires and an alignment, now it's smooth as can be and much quitter. I guess we need to drive it more..lol
 

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Took mine to a frame guy today and he did not notice anything bent on the frame. The right rear wheel is really out of whack so he mentioned the Spindle and the bearing assembly.
Take your car to a good alignment shop. They will know if they can do it or not. A good alignment technician can correct rear axle problems. It takes a special wrench and a special shim kit, but they will know exactly what you need if they know how to do it at all. For some reason it doesn't take much to bend these rear axles out of alignment. Both sides on mine were bent,after some spirited driving through a low water crossing slash jump. All is good now.:)

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Discussion Starter #14
Thanks for the good info. I have ordered shocks and will replace them front and back. I removed one of the rear shocks yesterday to make sure the correct one was ordered and found that it didn't take much to push the shock all the way in, much less than I expected. I should get the new rear shocks today so I'll see the difference in the two.

Good to know that it doesn't take much to get the rear axle out so I'll have the alignment guy pay particular attention to that.

More to come.
 

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Well the rear shocks were replaced and sure enough the old ones were much easier to stroke than the new ones. What may be the telling clue is that when the old ones were pushed in hard you could hear air or oil squishing by the piston seal. When the cylinder fully extended there was an air pocket for about the top inch and a half of the stroke. Re-balancing and alignment will be next.
 
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