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!

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.

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.

Try it now. Quote marks in the html seem to be problematic.

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".

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.
Ian

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.
;)

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

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?

Ian,

Street tires used in racing and solo have short lives. While the issue about how a car handles at higher inflation pressures is supported by the experience of racers and soloists, there are some issues that aren't.

Above TEin mentions that Showroom Stock racers used to inflate tires to high pressures, too. But this was to overcome an inherent shortcoming of the cars - severe understeer - and the car went faster around the racetrack because of the balance was changed. It's quite possible that the reason this worked was because the rear end tires had lost grip and this was helping the vehicle pivot - but it's also possible that this was the result of the increased spring rate.

I think everyone would agree that racetracks and solo courses are not like the street in that the road surface isn't monitored for debris and brushed off every time some is found, not to mention there aren't any pot holes.

But you bring up 2 good points:

1) That the placard pressure has included in it ride qualities. The Ford / Firestone issue comes up constantly as an example that the placard pressure is somehow faulty. The way you've worded it is better - that the lack of maintenance is partially at fault - and it is certainly true that regular tire pressure checks would have helped the situation.


2) That tire manufacturers CAN'T say that tires are safe above what is written on the sidewall - so they are obligated to issue warnings pointing out the words "maximum". This is a valid point, but that doesn't take into account that the tires are designed for use at these lower pressures - or put a different way - NOT designed for use at these higher pressures (and they would be designed differently for higher pressures)

While I'm on this part let me address another response along similar lines:


If you were to calculate the tension on the ply cords, you would quickly find out that the distance around the perimeter of the tire (in cross section) is a major player. Put another way, a little bicycle tire doesn't put much tension on an individual ply cord even at 100 psi. I'm going to take a SWAG, that plies used in bicycle tires are more limited by the ability to manufacture the tire than by the design of the tire - and that the limiting factor in the tire (as far as pressure is concerned) is the bead.

It was both.




I don't agree. It is not at all uncommon to drop a wheel off the paved edge of the track exiting a corner, and the edge of the track can be jagged and irregular. Also, other cars can go off track, and come back on bringing rocks and debris with them which then gets run over on subsequent laps. Driving on track is much harder on a tire than driving on any street.

:thumbup:

And I don't agree. My experience on track is that dropping a wheel off isn't quite the same thing as hitting a pot hole, and debris big enough to worry about gets removed fairly quickly.

lol

Yes, it gets removed right after you run through it! :rolleyes:

I think the article is really interesting. I thought for sure I was losing treadwear when inflating to 44 psi, but seeing as it makes no difference, and actually helps in some cases other than fuel economy, I am thinking about recommending this to others.

I have to admit I didn't read the article before my previous post, but now that I have read it I've got a few more questions.:thumbup:

Basically the article says that the only downside to higher pressures is the increased chance of puncture when running over odd shaped debris on the road.

I've never really ran over anything odd in my 16 years of driving so far, parts of truck tires would be the biggest or that poor porcipine... Lots of potholes though that I thought were going to keep the tire. I've also read about many people denting their rims on pothole edges. This must mess up the tire in some way too I'd imagine.

So wouldn't it be better to raise tire pressure to counter a common problem, hitting potholes and denting rims?

How big a pressure spike can tires handle when hitting things like potholes? I can see how the pressure spike would be less with lower initial pressure but how much higher is the spike at 45 psi vs 32 psi?
Ian

Also, one more thing, has anyone come across data on tire age vs. how much traction it retains? I think 5 years is about it for tires on a car, regardless of tread depth, am I just imagining that they now suck in the rain or do they actually slide more?

Just my personal experience, but I've never had a tread puncture from running over objects on the roads. I have had a couple of cases (Insight with stock RE92 tires at 50 psi) where something managed to flip up and puncture the sidewall.

I'm going to shorten Ian's post to make it easier to see what I am responding to:

There are 2 different types of impacts that I can discern:

1) A large object that compresses the sidewall to the point where the tire bottoms out against the rim flange - and if there is enough energy/object size/vehicle speed, the rim could be dented. In this case, more pressure would help prevent the tire from bottoming out. Striking a curb would be a good example. Needless to say, low profile tires would be more suspectible to this.

2) An object that is more pointed - but not sharp - where the ply cords of the tire are stressed due to bending (enveloping the object) and the result would be the ply cords breaking. This was the object of the test in the paper.

An example of this would be a chunk of concrete. In this case, it wouldn't matter what the aspect ratio of the tires is. The object size, vehicle speed, object shape would all be important factors in whether the tire would fail or not.


It isn't the pressure spike that is the problem - it's the stress concentrated around the object. BTW, the pressure spike is relatively small - a couple of psi as best I can tell.

This particular test has generated a lot of interest - and misunderstanding - so I modified the article to include a photo of the test setup and a different graph from the paper showing the height of the "nub". Sorry about the photo quality, but the copy I have is just as bad.

Read my post, the less weight the higher you can inflate to. (or if you lower speed) Though one has more effect than another.

Its all about how much stress you put on a tire, something 700lbs (or less) pushing on a tire can exert less compression and forces than something 4000lbs.

Also the way your tires are loaded a severe bump displaces less air compared to the area inside the tire resulting in less of a pressure spike. (big diameter small area tires load differently than wide fat smaller diameter tires)

Also ALL new tires can be inflated to about 60psi but it does affect their wear profile along with weight and speed ratings. I guess it depends on what costs more fuel or tires? In my case I go through tires quickly do to the wonderfull roads around here (all sidewall slices) so I tend to overinflate, I have tried following the guidelines and overinflating and it seems my overinflated super cheapo off brands last longer than namebrand tires. I still can't figure it out, must just be luck (or lack therof) that abused tires are lasting longer. (I rarely overinflate past 50psi, except on tires that state 60psi on the sidewall, usually between 40-50 is optimal for me)

Also there are types nylon inner layers that can be employed in tires to virtually eliminate blowouts and increase max pressure but they are rarely employed, ride and handling are generally considered more important than rolling resistance. Similar trick would be to have a unburstable nylon/teflon/etc inner tube that doesn't expand much, would have the same effect.

Cheers