SAE Paper 800087 – The Effect of Inflation Pressure on Bias, Bias-Belted, and Radial

[CapriRacer]

Originally Posted by IndyIan

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

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:

Originally Posted by jamesqf

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?

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.

[TEiN]

Originally Posted by CapriRacer

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.

It was both.

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.

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.

[CapriRacer]

Originally Posted by TEiN

,,,,,,,,,

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.

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.

[TEiN]

Originally Posted by CapriRacer

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!

[brucey]

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.

[IndyIan]

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.

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?

[jamesqf]

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.

[CapriRacer]

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

Originally Posted by IndyIan

........

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

..........

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.

..........

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?

..............

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.

[rmay635703]

Originally Posted by jamesqf

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?

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