|
Driving on the street is considerably different than driving for sport or in competition. While tires used in sport driving or racing are subjected to high stresses, they are not expected to last many thousands of miles. This means that fatigue resistance isn’t as high a priority when designing tires for this purpose. Further, a street tire used in a sport application doesn’t have a long lifespan and fatigue resistance isn’t a factor.
Put another way, the experiences people have concerning tires in racing and sport driving have limited application for street usage, so care must be taken when transferring those experiences. You’ve highlighted one aspect of this - In racing, you want a car that responds quickly to steering input – higher pressure does that.
Street cars that are used in sport driving or racing have a limited amount of things that can be done to them – sometimes because the rules forbid such changes. Changing springs, shocks, and sway bar can dramatically change the way a car handles. But if the rules say you can’t change those, then tire pressure is one of the easiest – and perhaps the only – change you can make. Plus, it is pretty costly to experiment with different suspension pieces, where changing pressure is pretty free of cost.
When I was running street tires on my road racing car, I used as high as 42 psi on the front and 38 on the rear (and each corner was a different pressure) on a car where the stock pressures were 27 front / 31 rear. This was the result of repeated experimentation and the result was at least partially compromised by how much money I was willing to spend, the physical limitations of the car’s alignment adjustment, and the rulebook limitations.
So what do tire engineers do with tire pressures on their street cars? What about vehicle engineers?
While I can’t really speak for others, the impression I am left with is that, by and large, they use the placard pressures. However, I don’t think they, as a group, are much different than the general population and you would find similar trends of neglect – just maybe less so.
But personally, I like a car that is more responsive and I am willing to put up with the decreased ride quality. I am also concerned about unexpected impact failures – not all the roads are pothole free. So I use 3 to 5 psi above the vehicle placard.
Further, my official position is that the car manufacturers spend a lot of time testing cars at the placard pressure and unless one is willing to spent the time, has the expertise, and actually performs a huge amount of testing, then it might be best to default to the car manufacturer’s recommendation.
And taking it a step beyond that, knowing how tires are designed, and how the pressure listed on the sidewall of a tire is derived, I can not in good conscience stand idly by and not say anything when folks report using pressures above the sidewall maximum. Needless to say a lot folks take offense at this position – and they shouldn’t. It is merely a technical concern about safety. Unfortunately, there is not a lot of good reliable data available on the subject – one way or the other – about those aspects high pressure usage that concern safety – handling, tire failures, etc.
I am also concerned about misinformation – and there is a lot of it out there. “Old wife’s tales”, myths, rumors, and just plain misinterpretation of available information are things I want to get corrected. It’s not good when folks suffer under an incorrect notion – particularly one that could lead to tragic results.
I have been taken to task for not having good reliable information – particularly PUBLISHED information that can be readily accessed – and rightly so. To that end I have been searching for such information and pointing it out when I find it – regardless of whether it supports high inflation pressures or not. Data is neutral – it is neither good nor bad (except when the data itself is faulty or the process to get it is faulty.)
I am particularly concerned about “noobies” – folks who are new to the subject. I feel it is important that both the good and the bad be presented – AND how well founded that information is. It is clear that increased tire pressure improves fuel economy. It is also clear that increased pressure improves the wear rate, improves hydroplaning resistance, improves steering response, and hurts the ride quality. What is less clear is the effect increased pressure has on evenness of wear – although it seems that the improved wear rate more than offsets any other wear issue.
What kind of gets lost in the shuffle is that the rate of these improvements (and harmful effects) diminish as the pressure increases. Put another way, the difference between zero to 5 psi above the placard is more than the change from 5 to 10 above the placard – and it keeps diminishing as the pressure is increased.
But there are issues where there is little data - impact resistance, traction (snow, wet, dry), durability – and while opinions abound, it is difficult to sort out fact from conjecture. So you will see heated discussions on these issues – and that’s good. Debate will help sort this out – at least until someone actually does a closely controlled study and publishes their research.
|