SWB rear suspension problem

[Donald T]

Today, as I was at a stop sign, another driver honked and told me my right rear tire was deflated. Luckily I was near a tire store and they were able to put my '68 912 on a lift and see that both rear tires were down to the cords on both insides. Two new tires were quickly installed without drama. (Thanks NBT, Portsmouth, NH).

I should have known to check as my '68 912 goes through a set of new rear tires about every 5-6K miles! Attempting to fix this about 6 years ago I went to a reputable classic Porsche specialist on Long Island (NY) and learned that the rear tow-in could not be adjusted to spec using the adjusting screw on the rear spring. The reason, I was told, was that the banana arms lose their ability to keep true. So I purchased another set of banana arms from a respected 912er, installed them (not fun), but still had the same issue.

With a rear toe-in problem I assume I am scrubbing MPGs as well as speed but don't know what to do. Although the car handles very well.

Anyone else have this issue or have a solution? I'd buy another set of rear suspension arms but old, used ones might be just as bad as the ones I've installed.

I'll post this to the 911 Board as well considering the identical suspension.

Don

[cmpski]

Don,
Had you not yet replaced your trailing arms I would have guessed one or both was bent. I'm guessing the replacement ones had new bushings installed. What kind of shape are your spring plate bushings in? Is there any damage to the body where the trailing arms attach to the body?
If you where running a lot of negative camber in the rear the tires would wear on the inside.
When you say wear on both sides do you mean both sides of the same tire or wear on both the left and right tire in the same place?
I would ask here, on the 912bbs.org, and on the early911sregistry.org if they have a shop in your area they recommend. Something is worn out, bent, or twisted back there.
Chris

[Donald T]

Chris,
Actually I did replace the trailing arms and installed new spring plate bushings. It doesn't look like the was any damage to the rear end (although the deck lid does not have the center pillar so it was probably replaced - hit?). There is a fair amount of negative camber so the tire wear is both rear tires on the inside of each. If anyone knows of a good suspension shop in Seacoast, NH, area I'd be very interested. Thanks for the reply, Chris!
Don

[Peter Graham]

If the car has been lowered, there is a trade-off between camber and toe. My 68 is lowered and holds out-of-spec negative camber but the toe angles and thrust angle are in spec. Yes, the inside of each rear tire wears more, but there's no way you should get any less than 30K miles on a set, based on my experience and several cross-country trips. I suspect that whomever aligned your car did not get the toe/thrust correct and combined with too much negative camber, tires are quickly toast. And it seems it takes a Porsche dealership with someone trained in these suspensions to do the job correctly. If it was a dealership that did your last alignment, find another dealership or exclusive Porsche shop!

Just replacing parts to correct a problem can often be expensive without resolution: take your car to someone who knows what they are doing and let them investigate the problem as well as options for correction. It may be as easy as an alignment or as involved as a frame shop, but it is critical that they know what they are doing!

Peter

[bobrestore]

The early banana arms bend very easily, especially when you run big tires.. Check the banana arm by looking at it were the brake line holder is. In this area there will be a lump if its bent. bob

[Donald T]

When I get a chance I'll post a photo of the rear of the car so you can eveball the camber. I'll also shoot the side of the car so you can see the relative height of the car in the front and back (which I think 'looks' about right). Thanks for the feedback!

[WFBowen]

Don:
What is the width of your wheels? I'm certainly no expert at the black art of suspension/steering setup, but non-stock widths and offsets might exacerbate the situation. The fact that both sides exhibited the same measurements makes me think that it's not accident damage, although the bushings could have worn evenly over the years.
As Peter mentioned, it's critical that you take the car to someone who not only has the correct equipment for these cars, but knows how to identify and correct the problem.
Good luck,
Bill

[Donald T]

Bill,
I'm running 185x15. I did take the car to a top Porsche-only specialist and he couldn't get the rear aligned (still charged me $330, btw). He claimed that the trailing arms can become out of true. So I bought another set (used) but had the identical issue. Bushings were replaced. Seems to me that if the trailing arms lost their shape over time this would be a common issue for this bbs (which doesn't seem to be the case). There is a very good classic VW guy in our town and I'm going to see if he can help.
Don

[Jaems]

My front was relined in 1989 and have not been touch. The rear was redone in 2003 when I replaced the bushings with new factory ones. The front is still as tight as the way it left the factory. I can let go of the sterling wheel and there is no movement with the sterling wheel, just like you are parked.

Unless you hit something hard or been hit hard there is no reason that the cars should be out of a line.

The Porsche shop probably didn't know how to a line a early SWB car or didn't have the early specs.

[bobsven]

After replacing the bushings on our 67 912 I took the car to an alignment shop in Houston. his machine had all the data for the 912 but I saved the day by bringing with me the really big metric Allen wrench needed to adjust the spring plate.

Car runs in perfect alignment and after a 4500 mile trip out to the rendezvous in Yosemite the new tires still look new!


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[djdsc]

It is hard to tell from your photo in post #1 but you look to be running a widened chrome wheel. If that is the case the 185's might not be wide enough for the rims and causing an issue that is exacerbated with the set-up. Just a thought.

[Gunzel]

There is a factory tool for checking if a rear banana is bent. You have to take the banana off the car and strip it down. You then put it on the jig and it will show you if its bent. I have the tool to check LWB long bananas, but not SWB. It is not a common tool for a person or shop to have.

mike curnow