The last high speed-rated tires?

[Duke W.]

Re: The last high speed-rated tires?

That's a possiblity, though it's not obvious from inspection.

Then again, I'm not the kind of guy who bangs tires into curbs. I learned that from my dad. My mom, on the other hand, never did learn, which is why the Williams family cars - Pontiacs and Cadillacs - had blackwalls - very "European" for the time.

Duke

[Michael J.]

Re: The last high speed-rated tires?

One thing I will say about the P4000s on my '67, they really are bad at picking up gravel size stones and chipping my paint on the rear. The bias ply Firestone SuperSports don' do this at all. Is this due to the Z speed rating, or just a flaw of radials?

[Duke W.]

Re: The last high speed-rated tires?

It is neither the speed rating, nor a "flaw" in the design. It has to do with the difference in structural characterisitcs of radial versus bias-ply tires and goes like this.

ALL pneumatic tires distort noticeably under load - unlike steel railroad wheels which don't noticeably distort (to the naked eye), but behave effectively as "rigid bodies". This is why pneumatic tire revs/mile specs are about 3 percent less than what you calculate using the inflated OD. Distortion under load causes the actual measured revs/mile to be less than a rigid body like a steel railroad wheel.

The distortion in bias-ply tires is primarily in the contact patch area, which closes up the tread grooves resulting in poor wet and snow grip.

Radials distort in the sidewall. The tread remains relatively undistorted, keeping the gooves open, which is why they are so superior in wet and snow conditions relative to bias-plys.

It could also have to do with the wide grooves that are typical of "all season" tires, which the P4000 is classified due to its tread design. The wide grooves can pick up pea sized gravel and fling it out.

The best solution would be to put some clear film on the rear quarters below the bumper. The problem is probably more noticeable on '67s compared to earlier C2s with steel wheels since the +.06" Rally wheel offset moved the wheels outboard 0.38" on each side, and the 215/70R-15s have wider tread relative to the OE tires, which exposes the bottom of the rear quarters to more road debris kicked up by the tires.

The problem is even worse on my '63 since I have '68 Rally wheels with -.28" offset which pushes the wheels out another .34" relative to the '67 Rallys. This puts the wheel cover spinners outboard of the body, making them like the bad guy's chariot in Ben Hur, so I can easily take out the spokes or pursuing chariots and dogs that chase after the car.

Duke

[Michael J.]

Re: The last high speed-rated tires?

Thanks Duke, for another very good explanation. Since I am done with all flight judging on this car, those clear films like they put on new 'Vettes (I'm getting it put on my new ZR1 when I get it next month) would probably be the way to go. I hate to keep using that Dr. ColorChip stuff on my lacquer.