The science/physics/math/safety behind overinflated tires...

<div class='quotetop'>QUOTE(akim @ Jun 28 2006, 05:32 PM) [snapback]278235[/snapback]</div>

We ran or Graspic DS2s this past winter at the same 44/42 we run during the summer on the Integrities. Traction was phenominal, no problems whatsoever. Snow, ice, slush, rain, OK on all. I call it our 2wd 4x4. We have a circular driveway, one part is a gentle climb, the other is for 4wd in the winter, and only if one must. Heck, the Prius made it up that AND under lousy conditions! This in rural Maine- think boonies, bear, deer, moose, our road gets plowed by Darrel if the town is having a good day, that kind of rural.

 

I believe most would agree that a highly inflated tire will wear less on the shoulders and perhaps wear more in the center than a less inflated tire would.

Often a car's tires wear faster on the shoulders than the center of the tread, due to the car's allignment specifications, out of spec alignment, agressive driving, or lots of cornering). In these cases, which seem common with Prius owners, couldn't higher inflation pressure extend the life of the tire by protecting the shoulders at the expense of the center?

Of course if the tire is so highly inflated that the contact patch shrinks to the point of creating frequent scrubbing from braking and even acceleration as well as turning, such higher pressures could cause more treadwear across the tread. (Perhaps another reason nobody glues tread to flat wheels?

 

While these tirerack articles focus more on comparing underinflation with proper inflation, I found them excellent.


This one includes great photos of the water between a Hydroedge tire and the glass surface it was driven over at different inflation pressures. It also describes the handling characteristics at different pressures from a comparative road test.
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=3


This one focuses on dry traction and handling at different pressures.
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=2


This gives an overview of tire pressure effects and mentions that in autocross or competition, drivers use higher pressures for better cornering.
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=1


This talks more about customizing pressure selection for handling and performance. It recommends increasing inflation 6-10 psi for wet traction.
http://www.tirerack.com/tires/tiretech/tec...e.jsp?techid=58

 

Oh wow. I did not realize it was a cold psi rating. I guess I will turn it up a notch.

Here is a question... Should I rotate my tires since it costs me about $20 each time. Some say it would be cheaper to just buy new tires. Note I do not change them myself. Not a matter of ability unless you count the ability to overcome my laziness.

 

i rotate my own tires. if done in 20 minutes or less, it can be a nice workout. (5 minutes of that is adjusting tire pressures) jacking up car on back jack point will lift both wheels off the ground

 

<div class='quotetop'>QUOTE(Tadashi @ Jun 28 2006, 05:45 PM) [snapback]278338[/snapback]</div>

When I got my car, I took it immediately to Discount Tire (America's Tire) and paid $10 per tire for "lifetime" (of the tire) rotation and balance. Every 5,000 miles I do my own oil and get it to Discount for the free rotation/balance. If you get 40,000 out of the tires, that's 7 services for your $40. Probably any tire chain will do the same, even for tires that you don't purchase there. If you don't have Discount/America's try a Goodyear store--the OEMs are their tires, and they should be willing to service them for something like the above fee.

 

$10 per tire for lifetime of the tire is very very cheap

 

<div class='quotetop'>QUOTE(DaveinOlyWA @ Jun 28 2006, 07:42 PM) [snapback]278410[/snapback]</div>

I think I've heard that Wal-Mart is even cheaper, but some folks prefer not to darken their doorstep....

 

Tire gauges are almost never accurate. Always use the same gauge for your tire inflation pressure. Each gauge will be accurate to itself. I highly suggest a digital gauge with backlit readout that measure in half pound increments. Gauges do not have to be expensive.

I say this because a half pound or pound difference at the drag strip can significantly alter the outcome of a race. The amount of flex in the sidewall can make the difference between a tire hooking up and spinning. It also alters the reaction time of the vehicle. In a front tire the slightest change in diameter alters the "rollout" at the starting line also changing the reaction time of the driver and vehicle.

After eight years of racing I have tried all types of tire gauges and so have many of my fellow competitors. Sooner or later we all misplace a gauge and borrow one to get through the day. Each time the actual tire pressures were checked by different gauges the results would differ by as much as two pounds. However, the difference in pressures was measured by each gauge as the same in relation to the original reading.

In other words, if one gauge read 14.5 pounds cold and another read 15.5 pounds cold, when hot the 14.5 pound cold reading went to 15.5 pounds and the 15.5 pound cold reading went to 16.5 pounds.

Always use the same gauge. just because two gauges of the same make and model are used does not mean the results are the same. If the measurement basis is not accurate each time, then putting 40 pounds of air in one month and going to 42 pounds of air the next month may actually be incorrectly based on a gauge that reads two pounds low...

 

<div class='quotetop'>QUOTE(Brian K @ Jun 28 2006, 04:59 PM) [snapback]278308[/snapback]</div>

Brian K is that a typo? :blink:

 

<div class='quotetop'>QUOTE(Wildkow @ Jun 29 2006, 10:10 AM) [snapback]278554[/snapback]</div>

Sorry, yes a typo. It should be 44/42. Thanks for catching it.

It reads better now.

 

A little simple math should give you an idea of how traction changes with tire pressure. First, simple physics indicates that the pressure in the tires controls the size (and to some degree the shape) of the tire contact patch on the ground. The simple fact is that:

Weigh on wheel = tire pressure X size of contact patch.

So, take a Prius, curb weight = 2890lbs, plus a little cargo and driver (310lbs for ease of computation) so the total weight of our example car is 3200lbs. Now I know that the loading on the four tires is not equal in the real world, but for this example, lets assume it is and that gives us 800lbs per wheel.

800lbs / 35psi = 22.8 sq. in. of contact patch.
800lbs / 40psi = 20 sq. in. of contact patch

or about 13% to 14% less traction surface. This reduction actually improves behavior in wet weather by reducing hydroplaning, but I don't know if the increased friction due to the increase in psi on an asphault surface makes up for the decrease in contact area.

Maybe someone who knows more about the behaviour of the coef. of friction between rubber and asphault could take it from here and calculate the actual difference in traction.

Dave H.

 

The pressure against the road across the contact patch is unlikely to
be uniform, and given the wear patterns that people report, softer
tires evidently bear more heavily on the region nearer the sidewalls
than the middle. Running the design pressure evens that out better.
.
_H*

 

Huh? Where did you come up with that? That formula is so oversimplified it is wrong on its face. To illustrate my point I will place 800 lbs of weight on a bicycle tire with 40 psi in it. Are you saying that my bike tire now has 20 sq. inches of contact patch?

The contact patch will vary with tire circumference, width, stiffness of construction.

Sorry to be rude dude, but your formula is not correct.

 

<div class='quotetop'>QUOTE(pocketpenguin @ Jun 29 2006, 11:35 PM) [snapback]278944[/snapback]</div>

The SHAPE of the contact patch will vary with circumference and width, but the AREA will not.

Unless you have compressed the tires such that the bike is resting on the rim, yes, you will have 20 square inches of contact patch. Say 10 on each wheel. an area of 2 X 5 inches. Not very large, and I expect with a more normal load on a bike, say 180 lbs, with tires at 60 or 80psi, that's only 2 or 3 square inches of contact patch, total, entirely plausible on a bicycle. Most tires (not the run-flat variety) rely on the pressure in the tire to support the car. That is why flat tires rest on the rims. If you have to support N weight and you can only support X weight per square inch (i.e. PSI) you HAVE to have that much surface in contact with the road. Otherwise, please patent it as anti-gravity. Now I DO admit that with different weight on each wheel, the size of the contact patches will be different on each wheel, but the TOTAL area should be about as computed. Remember, the air in the tire doesn't know whether its pressing on the bottom or top of the tire, so the pressure across the contact patch should be fairly uniform on the ground, varying only by those amounts which the sidewall rigidity of the tire contributes. I doubt the rigidity of most tires will support 800lbs.

 

have to agree with pp on this one. formula needs more validation

 

<div class='quotetop'>QUOTE(David H. Hawkins @ Jun 29 2006, 10:29 PM) [snapback]278923[/snapback]</div>

Now where was it that I heard about the tire that went flat and covered the globe? :lol:

The formula would apply well to fluids or gases at rest, but a tire isn't quite either. A related but more complicated formula (with vectors and integrals) may apply to the force of the air on the inside of the tire. This formula is a very rough approximation of the force a tire applies to the road.

 

I have to agree with David on this issue. There could be second-order effects due to tire construction and distribution of pressure across the contact patch, but to a first-order the simple equation looks and feel right. As he said, just because the contact patch is smaller it doesn't necessarily mean the traction is compromised by the same amount. That is dictated by the coefficient of friction and (again) the weight of the car. But certainly the smaller contact patch will result in less loss through friction -- meaning higher milage. Anyone want to dispute that higher pressures mean higher milage?

 

<div class='quotetop'>QUOTE(jbarnhart @ Jun 30 2006, 05:00 PM) [snapback]279319[/snapback]</div>

:huh:
Higher mileage before or after the blow-outs?


Seriously though, I'm not sure if you mean treadwear or fuel efficiency. There will be more tread squirm at extremely high pressures and very small contact patches. This is likely to accelerate treadwear and might possibly increase rolling resistance. On bumpy roads the energy from hitting bumps that doesn't go into deforming the tire will go into the suspension and bouncing the car where the energy isn't recovered as well as by a pneumatic tire, _possibly_ increasing fuel consumption. I'm only referring to very high pressures, where these marginal energy losses outweigh the losses from flexing the tire carcass as it rolls.

I'm not saying that we should lower our tire pressures to improve tread wear or fuel consumption. I'm simply pointing out that there is a optimal tire pressure or golden mean for each objective, above which each suffers. The more interesting and complicated question is empirical. What is the optimal pressure for fuel efficiency and the optimal pressure for low treadwear under specific demands? Still more complicated is our ultimate question involving individual needs, preferences, trade-offs, and tastes. What is the overall optimal pressure for me?

 

<div class='quotetop'>QUOTE(tochatihu @ Jun 27 2006, 11:06 PM) [snapback]277841[/snapback]</div>

Not necessarily true. I think the main reason you don't hear about center tread wear on the Prius is due to the narrower tire that the Prius has. I can tell you that when I was running 255x45x17's on a Mustang I used to have, the center would definitely have more wear when I tried it with a set to over-inflate (above vehicles recommended tire pressure, but under the max PSI of the tire).

I did this to try and eek out any additional mpg I could get out of the car since I was commuting with it as a daily driver at the time. While racing I had a different set of tires on the car, so it wasn't a racing/burn-out issue with the over inflated tires.

dave