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Old 06-27-2008, 11:38 AM   #1 (permalink)
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SAE Paper 800087 The Effect of Inflation Pressure on Bias, Bias-Belted, and Radial

In another thread, I mentioned that there was an SAE paper that might be useful as a resource on using high tire pressures.

SAE Paper 800087 The Effect of Inflation Pressure on Bias, Bias-Belted, and Radial Tire Performance - by B. L. Collier and J. T. Warchol, B. F. Goodrich, February, 1980

I've posted a summary here:

Barry's Tire Tech

Enjoy!

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Old 06-27-2008, 11:52 AM   #2 (permalink)
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Thanks for the link, that is great info. The second image (wear test) is broken when you click it to enlarge. The rest are fine.
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Old 06-27-2008, 01:07 PM   #3 (permalink)
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Thanks for this great summary. The results presented are pretty much in line with what I was able to gather from various other papers I've read.

I liked your editorial comments too and agree with your points. I was a little surprised by the wet traction results too as I have read an other paper that showed slightly decreased handling ability under wet conditions.
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Old 06-27-2008, 02:31 PM   #4 (permalink)
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Quote:
Originally Posted by ttoyoda View Post
Thanks for the link, that is great info. The second image (wear test) is broken when you click it to enlarge. The rest are fine.
Try it now. Quote marks in the html seem to be problematic.
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Old 07-03-2008, 11:45 PM   #5 (permalink)
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Wink

How much difference do you think it makes that the tires in this test are all rated at only 32 psi, yet, all current tires in this size (215/75x15) are rated at higher pressures? Although a few are rated at 35 psi, the majority are at 44 psi, some at 50 psi, and some at 65 psi. I'm no tire engineer, but these pressure rating differences say to me that some advances may have been made in tires since this test was conducted, and the results may not necessarily apply to the tires that are available today.

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Old 07-04-2008, 07:41 AM   #6 (permalink)
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Quote:
Originally Posted by TEiN View Post
How much difference do you think it makes that the tires in this test are all rated at only 32 psi, yet, all current tires in this size (215/75x15) are rated at higher pressures? Although a few are rated at 35 psi, the majority are at 44 psi, some at 50 psi, and some at 65 psi. I'm no tire engineer, but these pressure rating differences say to me that some advances may have been made in tires since this test was conducted, and the results may not necessarily apply to the tires that are available today.


Just to be clear.

The tires tested in this study were rated at 32 psi. Passenger car tires today are rated at 35 psi. However, the maximum pressure on the sidewall (which isn't directly connected to the rating pressure) can show a wide variety of pressures depending on 1) how the tire manufacturer reads the regualtion that covers what is to be imprinted on the sidewall, and 2) how the tire manufacturer reads the tire standards that cover that particular type of tire (meaning passenger car, light truck, etc, and not all season, winter, etc.)

In other words. you must be very careful how you conceptualize these pressures. The technical meaning of each is different and way each should be viewed (in the context of how a tire reacts to pressure) is going to be different.

So the assertion that the maximum pressures imprinted on the sidewall of modern tires must mean there have been technical advances isn't true. (There have no doubt been technical advances, but the pressure isn't any indication of advancement.) In fact, when this study was conducted, the tire standards that are applicable today, were applicable then - and further, tires was labeled in much the same way then.

When I brought up the link to Tire Rack that you provided, there are a mix of tire types (meaning P metric, metric and LT metric), and this is no doubt creating confusing. Tire Rack doesn't publish the letters that go in front of (or behind) the numbers in the tire size and those letters are sometimes important. (Just for reference, if the Load Index has a dual number, then the tire is an LT metric and not a tire designed for use on a passenger car.)


OK, so to answer your question, inflation pressure has a profound effect on the way a tire reacts, and pretty much all of the results of the study are applicable today. What will be different will be the levels of - oh, say - RR or grip, or impact resistance, not the way pressure affects that property.

One of the things I think is really important from this study is how far back I had to go to find ANY information about the effect pressure has on a tire when it is operated HIGHER than the rating pressure. There is very little data for tires operated in these region. My concern is for tires operated above the placard pressure and the further away from that value, the greater my concern - AND, I'm really concerned about tire operated over the max pressure.

And - again - just to be clear, the rating pressure for passenger car tires is 35 psi, and NOT what is written on the sidewall. In some respects you could say that since the rating pressure is 35 psi, then 44 psi is "pushing the envelope" - but one that has been recognized by the tire industry as an acceptable "pushing of the envelope".
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Old 07-04-2008, 11:53 AM   #7 (permalink)
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Just thinking out loud here, isn't solo 1 racing a good sample set for how "over" inflated passenger car tires perform? Braking and acceleration to and from 100mph, powersliding sideways at 80mph? It may not be sustained for hours but the tires can be heated enough that they lose grip rapidly. Then they are pounded over curbs too?

I think this whole debate comes back to the fact that manufactures don't test their tires at higher pressures and therefore can't say they are safe at those pressures, due to legal issues etc.

We have lots of anecdotal evidence that tires can be over inflated from many people here, and people racing solo1 and 2, that tires don't have a problem being used at 40-50 psi even under extreme racing conditions, or coasting down the road at 50 mph.

We also have no official data on how 40-50psi tires perform for braking or turning, but again solo 1 and 2 folks don't inflate the tires to these pressures for fuel efficiency. These pressures result in faster times. Does this translate to the street? I think so but I'm not an independent tester.

I also think that some auto manufacturers pressure recommendations are probably pushing the envelope of safety due to underinflation. They want some vehicles to float along as smoothly as possible so people will buy them. The ford explorer fiasco showed this, if those people had started with tires at 38 psi instead of 28 psi their lack of maintenance would've been far less likely to result in a blowout.
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Old 07-04-2008, 12:29 PM   #8 (permalink)
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I think you're on the right track.

Back in the old days of Showroom Stock racing in FWD cars, modifications to the cars to make them handle were severely restricted by the rules, and some teams were known to run the rear tire pressures as high as 70 psi. We thought that was awful high, but never had any problems with those tires. Later we found out that some other teams ran their rear pressures at 90 psi.
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Old 07-04-2008, 12:29 PM   #9 (permalink)
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Actually many tires can be run WELL above their ratings, just remember the speed you can go decreases along with the weight capacity of the tire when it is run above its rating. (I say this in regard to sustainable operation not one time use)

If you have a city only driver (aka under 50mph) with a light car (well under tire weight rating) you can usually inflate most 2 ply sidewall tires to 60-65PSI (note I don't recommend this, it is possible that it can result in premature blowouts if you hit one too many big bumps, also if you do this on tires that aren't new you might possibly explode one in your face, never happened to me but I'm told it can happen) Also remember as you increase pressure the sidewalls will weather check
exponentially faster.

Many tires like spacesavers as an example aren't always constructed better they just have different weight and speed ratings to make up for the pressure increase. (though I've found 3ply sidewalls on space savers make them very durable if you happen to have a vehicle that accepts the size)

I will definately agree with whats been said, no MAJOR advancements have been made in tires, just creatively changing the ratings and wear profiles, Ooh repackaging is always fun.

Good Luck
Ryan

Quote:
Originally Posted by TEiN View Post
How much difference do you think it makes that the tires in this test are all rated at only 32 psi, yet, all current tires in this size (215/75x15) are rated at higher pressures? Although a few are rated at 35 psi, the majority are at 44 psi, some at 50 psi, and some at 65 psi. I'm no tire engineer, but these pressure rating differences say to me that some advances may have been made in tires since this test was conducted, and the results may not necessarily apply to the tires that are available today.


Last edited by rmay635703; 07-04-2008 at 12:34 PM.. Reason: clarification, spelling
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Old 07-05-2008, 01:10 AM   #10 (permalink)
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Just a thought: Why is it that the skinny tires on my road bike, with sidewalls not much thicker than a stiff sheet of paper, can take - no, require - 100 PSI or more, but the much thicker sidewalls of car tires can't (in some people's opinion, at least) take more than 32 PSI?

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