Quote:
Originally posted by Superman
I do not agree with those who conclude that lower tire pressures increase grip. I believe this is false. I believe that lower tires pressures increase tire FLEX and rolling resistance. Higher pressures decrease rolling resistance (providing more power, essentially) and they also help the tire resist flexing. I tend to run 34-38 cold tire pressures. At autocross I bleed pressure off but keep them in the 36-38 psi area when warmed up. On the street I just let them build to 40 or 42 psi. Our asphalt is cold.
Another thing that low-tire-pressure officianados believe is that higher pressures cause excess wear in the tire's center. I believe the opposite. Actually I verified the opposite when I rode a street bike. Lower pressures cause the tread to flex and heat up. The center of the tire gets the least air/cooling, so it wears faster when pressures are low. Higher pressures avoid this heating of the tread, and higher pressures DO NOT cause your tires to balloon out in the center as some might suspect. The inside of the tire carcass is round, so the more pressure you put in it, the more firmly the tire retains its designed square shape. It is when you put too little pressure in the tire that centrifugal forces can pooch the center out.
So, it's fair to say I prefer higher tire pressures.
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More interesting than the average tyre pressure thread, certainly, but not all completely accurate. Just wait until the resident board engineers take a look at this thread. Folks who are a whole lot more qualified than me have debated this one backwards, forwards and sideways. The undeniable truth (according to the laws of physics) is that: the lower the pressure, the larger the contact patch. This can be very easily demonstrated and is observable to the naked eye if you drop the pressure in one rear tyre to say 10psi and raise the other to say 50psi.
Now I don't for a moment suggest driving around at 10psi. There is a limit to how low you should go with your pressures and as correctly stated above a lower pressure does increase flex in the sidewall and it can increase rolling resistance. Most (but not all) racers and DEers realise that the lower the tyre pressure the greater the grip BUT for one important caveat: tyres are designed to work with a certain amount of air and below that point they do not work properly or efficiently. With the Pirelli PZeroC track tyres I have been using, the most efficient level is about 31-32 psi (hot). Other track tyres like a little more pressure and some road tyres need a little more pressure to stop tread rollover. Fair points.
I am prepared to go out on a limb though and say that a cold rear pressure of 38psi is (for the vast majority of tyres anyway) too high if you are doing some spirited driving or more importantly a DE session. You should let air out for spirited driving not add more air. Just measure your hot pressures if you don't believe me. For example, I did a freezing wet track day in November a couple of years ago with my road tyres and found that even in these conditions the rear pressures rose from 36psi (cold) to 45psi (hot). This noticably decreases rear grip - I promise you.
I do agree that too low a pressure will wear the centres of wide tyres. I think the reason is centrifugal force that causes the centres to bulge if they are not properly stretched with air pressure. Just a theory, but I believe this is why Porsche recommends quite high rear pressures on later cars with wide rear tyres. The centrifugal effect is more apparent at the potentially high speeds on German autobahns.
HTH
Richard
