Ern

Hi all, I have developed flat spots on my tires from sitting rather than braking, I think. I have a number of collectable cars to drive and sometimes my Porsche sits for a month or more. The tires are about 1/16 of an inch out of round. After balancing they still are not smooth running. I may have to purchase a new set of tires and these have only 7,000 miles on them. The tires are Goodyear Eagles P225/50/R16. Any ideas on saving the tires or for preventing this from happening again.

Thanks, Ern

pookee

Look for a shop in your area that sets up racecars. many of them have equipment to shave tires. I think they might be able to help you out.

Ern

Thanks for the ideas. I know I should put the car up on blocks, but then It will sit even more as I have to get it down to drive it, lazy I guess. I have called a number of tire shops and have not found one that shaves tires yet. The Goodyear tire guy said that radials shouldn't develope flat spots from sitting that won't go away after driving. He thinks I flat spotted them from braking. I did smoke them once braking but I don't think that is the problem as I have measured the tread depth and it is the same on the high spots as the low spots. My plan is to pump them up to about fifty pounds, drive them for 50 miles on a hot day, measure them. Then I will call around until I find someone that will shave them and then have them balanced.

Danno

The Goodyear guy is right. A radial tire is like an inside-out suspension bridge. The air-pressure expands puts tension on the sidewall fibers. This stretches tire outwards. The spot that is actually in contact with the ground is pushed inwards cancelling out the air pressure (that's why the tire collapses at that point).

This is similar to spokes on a bicycle wheel. Put a really, really heavy guy on bike and feel the spokes right above the contact patch, they're loose! The tension on the 4-6 spokes at the bottom is cancelled out by the weight and is taken up by ALL the other spokes on the wheel (the wheel doesn't "hang" from the top spokes, but rather ALL of them).

Same thing with a radial tire, the tension on the tire's carcass is actually less at the contact patch than the other areas of the tire. Get your tires shaved and it'll be fine.

pookee

I think what's happening is that the rubber itself compresses in the area of contact and changes dimension in relationship to the rest of the tire. it would'nt take much to feel on a tight suspension car.

Ern

Dang, I went out to take my 928 for my long drive today, the temp was 75 degrees and the clutch pedal went to the floor and stayed there. The fluid is nearly full but there is a stain on the inside of the car on the firewall and the carpet. I think I have air in the clutch master cylinder and I will have to rebuild it. I can't figure out how to bleed it to see if that is the problem as it is below the brake booster and I am not sure what I am looking at. It looks like the easiest way to do a rebuild is in the car. I have never had to rebuild brake cylinders, or clutch cylinders since the car was new. Not bad for 100K miles and over twenty years.

I am going to get those tires shaved when I get my wheels back on the road. Thanks for the help.

Ern

Danno

Don't you just hate when everything gangs up on you all at once?

Joe R

I have changed out my clutch master and Slave cylinder. The bleeding of the clutch is done at the slave cylinder.
Just to keep from getting flat spots on my 78 928 I drive it to work everyday. Unless I drive my 74 914.
PORSCHE THERE IS NO SUBSTITUTE.

Ern

Thanks for the advice. I took my Mityvac and gave the slave cylinder a few pumps and out came the air. I pumped out about a cup of fluid and haven't had a problem since. I think the air got into the clutch master cylinder as it showed a small leak. I will rebuild the master cylinder in place as I can't figure out how to remove it from the car. It looks like the Brake master cylinder and booster would have to be removed first to gain access to it. I was told that rebuild kits are no longer available and I should just buy a new cylinder and gut it. Sure hate to pay $150 for a couple of rubber parts. The slave looks real easy to replace and it is only $100.

I took the car for a long drive to see if the tire flat spots would go away. First, I set the tire pressure to 20 lbs and took if for a 30 mile drive. I measured the tires and they were somewhat better. I then pumped them up to 50 psi and took the car for another 30 mile drive. There was not much change. They have gone from .050" to .030" out of round. The car's handling gets a little slippery at 50 psi. Tomorrow I will call around for a tire shaver.

Ern

Ern

Here is the follow up on my tires out-of-round condition and how I fixed them:

First - I put the car up on jack stands and ran a dial indicator on the tires in twelve places and punched the data into Excel and ran a graph. I marked the places with white paint down into the tread. The TIR was .050"

Second - I reduced the pressure to 20 psi and drove the car for 30 freeway miles. I put the pressure back to 32 psi and retested. The TIR was now .030"

Third - I made many more experiments like leaving the car up on stands for a week, pumping the pressure up to 50 psi & driving 30 miles, and repeating the 20 psi and 30 miles test all with no futhure change. All the graphs from the subsquent tests showed no appreciable change. The tires had stabilized.

Fourth - I couldn't find any one to grind the tires so I put a 10" course wheel on my 2 hp. buffer/grinder and ground the tires myself. It took about 1 hr. per wheel. The tread pattern on my tires is 1 inch wide and the grinding wheel was 1 inch wide so it worked perfectly. I set the grinder on the floor on a no-skid pad. I positioned the wheel to just clear the low spot with a light under showing clearance and alignment. I rotated the tire slowly at first to not overload the grinder or disturb its position since it was not bolted to the floor.

The TIR is now less than .010" and they drive as smooth as new without rebalancing which I plan to do, anyway. If anyone trys this, wear a mask as rubber dust is everywhere.

My conclusion: Radials get permanent flat spots when storing the car. This can be helped by hard use, like driving at 20 psi. However, they will never go back to their original shape. This is unless my out-of-round condition was due to flat spots caused by heavy braking (not too sure about that). For long storage, the car should be on jack stands.

P.S. On the subject of the clutch pedal going to the floor. I have not had a reoccurance since I bleed the slave cylinder.

Ern <img src="graemlins/roflmao.gif" border="0" alt="[hiha]" />

IBRJA

I just read of your problem with your tires, too bad I'm too late. What I would have done would have been to bring the car to a dealer that sold that brand of tire and tell him of the flat spots. He may have authority from the company to adjust the tire on the spot. What I mean is that he could have taken in the damaged tire, and pro rated your wear of that tire, in trade for a new tire and you pay the difference for the pro rated wear. This will usually be done for a tire less than 3 yrs. of age. The dealer can tell when the tire was manufactured or you may have a purchase receipt proving the point. This method is a lot cleaner than the route you took.

Ern

I started with the dealer but I have had the tires 5 years and have less than 10K miles on them. I have a number of collectable car to drive and don't put many miles on each one of them. I just want them to drive perfect when I do use them and the Porsche was not fun to drive. I do not regret the work I put into solving the problem as this was a learning experience for me. Now that I have a system that works well, I can make any of my cars roll smoothly which is good to know. Thanks all for the advice. Ern <img src="graemlins/roflmao.gif" border="0" alt="[hiha]" />