Optimal tire size

[Breeze]

OK, now you're giving me an ice cream headache

[SCCA HP Racer]

Note that the Grand Prix GXP actually has smaller tires on the rear to help quell understeer.

I agree that the Sol is over-tired and I am betting that the tire size chosen was more of a styling decision than engineering. 18 inch wheels are not optimal for performance, they are heavier and have their mass more spread out compared to a 15 inch wheel (slowing acceleration).

In racing we do tune tire pressures. It is amazing how much difference a gain or loss of 1/2 lb in one tire makes, it completely changes the way the car breaks away. The reason for the change is because by changing pressures, you are altering the the spring rate on that wheel. More pressure=higher spring rate.

Bottom line, compound and pressures will make more difference in ultimate handling than small changes in size.

[achieftain]

Have you seen articles yet on the new airless tire/wheel from Michelin? First use will be offroad but they have not ruled out onroad use at some point. These things will never go flat, prolly only separate from the rim. No way to make any "pressure" adjustment after assembly.

[SCCA HP Racer]

I thought this was an alternate run flat design? Had a platstic type insert inside the tire so the sidewalls do not have to be as stiff as a current runflat. Or are you talking about something else?

[achieftain]

They call it Tweel and it ain't pretty. Courtesy SAE Off Highway Engineering March 2005 www.offhighway-online.org

[SCCA HP Racer]

That is really cool and could be a huge impact on the auto market.

This is what I was thinking of:

http://www.familycar.com/CarCare/RunFlatTires.htm

[achieftain]

I've read of that too. With run flat not having a spare isn't an issue anymore. Plus virtual elimination of accidents caused by instantaneous loss of tire pressure from road hazards.

[SolsticeMan]

Originally Posted by SCCA HP Racer ...In racing we do tune tire pressures. It is amazing how much difference a gain or loss of 1/2 lb in one tire makes, it completely changes the way the car breaks away. The reason for the change is because by changing pressures, you are altering the the spring rate on that wheel. More pressure=higher spring rate. ...

Yep, the spring rate is part of the mix, but I think the footprint effects are more dominant with tire pressure changes.

The reason I'm confident of this is I've seen spring rates for tires, and they aren't 1/2 PSI sensitive to pressure. Change pressure from 31 to 30 psi (3+% change) on a low profile tire, the radial rate might change by only 3/4 or 1%, depending on how stiff the sidewall is (lower sidewall height usually means stiffer sidewall and lower radial rate/pressure sensitivity). HOWEVER, that 1 psi change has a large effect on the pressure distribution within the tire patch.

This is VERY evident in the run-flat tire. The radial rate change for a 1PSI change in pressure is not even really that measureable. The high radial rate is only about half or less as sensitive to pressure changes (which makes sense - to run at zero tire pressure, the sidewalls have to be so stiff to support the weight of a car, and therefore make up a huge part of the radial rate). But 1, 2, and 3 PSI changes do have noticeable effects on grip, turn-in, etc. This is because even though there is little change in the spring rate, there are changes to the pressure distribution within the tire patch.

[SkyCaptain]

Originally Posted by achieftain It's 235/60 tires and both cars are Grand Prix GTP, 1997 and 2003. The '97 did run just a little faster with 5 less hp but it had a sport-shift selector that would keep the tranny in gear longer. These are stock out of the box figures with no funny smaller pulleys on the s/c, or removing left front headlamp for colder air induction (usually good for a half second in the 1/4 mile, but frowned on for nighttime trips to the market)

2001 special edition here - slightly modified (and will eventually be turbo'd). I assume you're on clubgp as well? GTP's are the most under-rated cars GM has ever put out, IMHO. But you can't blame people for underrating them - 90% of them have 4 doors, and all of them are automatic front wheel drives that never had a commercial to speak of.... Heck, until the engines started showing up in Fiero swaps I didn't have a clue how great a car they were, and I'm a self proclaimed car nut/afficiando.

[achieftain]

Originally Posted by SkyCaptain 2001 special edition here - slightly modified (and will eventually be turbo'd). I assume you're on clubgp as well? GTP's are the most under-rated cars GM has ever put out, IMHO. But you can't blame people for underrating them - 90% of them have 4 doors, and all of them are automatic front wheel drives that never had a commercial to speak of.... Heck, until the engines started showing up in Fiero swaps I didn't have a clue how great a car they were, and I'm a self proclaimed car nut/afficiando.

No time to be there although I'll have to check it out. I do pick up HIgh Performance Pontiac and Pontiac Enthusiasts magazines every so often. It's cool to see what can be done on the track with a grocery getter. I agree the GTP is the unsung could have been hero against the imports, and still can hold its own. What's peculiar is when every major car mag does a comparison of cars like the GTP, they never consider it. If they had been comparing the GTP to the Altima and the X-type and even the Lincoln LS, all along maybe we would have had more engine earlier. I think even the GXP is under 30k with every conceivable option. Shop that around among the so-called competition in performance family sedans.

[mena661]

What's peculiar is when every major car mag does a comparison of cars like the GTP, they never consider it

I always thought that was odd. But explainable considering that BMW wins year after year no matter how much better the competition is.

[numbbers]

For autocross, it is all about contact patch and rubber compound. You are not going fast enough to worry about spring rate in a tire. Pressure is important, because it effects contact patch. Basically, you want the biggest contact patch, and the softest compound you can get. That may mean going down in diameter to get a wider tire under the fender.