Proper Tire Inflation Pressure

[Author nameAuthor names are only available to PAS members.]

I’m trying to figure out what tire pressure I should use on various cars to maximize tread wear. These are bias-ply antique car tires. You guys are a smart group, so here goes:

The problem currently is on a lightweight Franklin, 1924. The tires have been wearing rapidly in the center yet the tire pressures I’ve used are only about 26-27 lbs. Dare I go lower?

On a previous heavy car, 1926 Packard Eight, I had the same problem but when I went down to 26 lbs I got uniform tread wear.

The difficulty is knowing what’s right, because the lower the pressure the worse it is for the tire — heating and blowout risk.

The higher the pressure the faster they wear in the center.

How low can tire pressure be before it causes excessive tread wear?

Tires from the 1920’s are very different from today, so recommendations back then probably don’t apply. Most tires today have an inflation recommendation for maximum load. But maximum load is rarely the case. Especially with a lightweight car.

What I want is a chart which says for a certain tire size carrying so-much weight, this is the proper inflation. The same size tire on a 2700 pound Franklin should be inflated lower than on a 5000 pound Packard or Pierce Arrow, right?

If no charts or graphs exist, how do you determine what this pressure should be? Observing tread wear is a good indication, but as I said, there’s a tradeoff if the pressure gets too low. There ought to be an optimum contact patch size for any given weight load. Knowing the load,, adjust pressure for that optimum contact patch. But has anyone done this?

Your advice appreciated. Or especially references you can recommend.

–Scott

[Author nameAuthor names are only available to PAS members.]

Hi Scott, I haven’t seen a chart showing inflation pressure vs load capability, But I’m sure there is one out there somewhere.

Personally, I run my tires hard. The flexing of the sidewalls generates a lot of heat and heat softens the rubber and then heat promotes the tires coming apart. Even with tire pressures at or just under the max pressure for load that are molded into the tire sidewalls, the tires still drive better and run a lot cooler with a few more PSI of inflation.

The soft tires drive terrible, and the bias ply tires already follow the tar-strips, and worn dips in the road surface, and these characteristics are only made worse by underinflation. There was a service bulletin put out by the Pierce factory about inflation for Series 80 cars. I believe the bulletin stated 38-40psi was recommended.

If you have a very light car, and are seeing the center tread wear as you described, I’d buy and use a ‘no-touch’ infrared ‘laser’ thermometer to monitor the tire temps. I’d start at what pressure you have used in the past, drive the car several miles, then check the tire temps. both tread and sidewall. Then inflate them a few, 2-4psi higher. Run them again and recheck temps.

What I’d be looking for is the inflation that lets the tire run cooler, and definitely the inflation that is the minimum before the tire starts generating a lot of heat.

I’ve found that modern radial tires run MUCH cooler because of the limited tread flexing with a radial design. A radial trailer tire next to a biasply tire will be noticeably cooler with the same or even lower inflation pressure.

Heat is the danger, as well as side sway and steering precision. I prefer the tires to be hard and run cool.

Greg Long

[Author nameAuthor names are only available to PAS members.]

Scott,

It’s a badge of honor to wear out a set of tires before they dry rot on you and break up. It means that you’re driving the cars A LOT and it’s all good.

My two cars (31 model 42 and 34 1240A conv cpe) are run at 45 lbs routinely and I’m using Bedford cord tires. When I first got them I tried running them at 35 lbs and they wiggly and ran hot. I talked to the owner of Universal Tire and he told me that heat is the enemy of these tires and that I needed to put enough air in them to cool them down. After a long drive in the summer the tires should not be more than warm. If you can’t hold your hand on the tires for five seconds without pain, then the tires are too hot and they need more air.

Bill

[Author nameAuthor names are only available to PAS members.]

Bias-belted tires and Radial tires [which have tread-belts] came about in response to the problems with flexing tread surfaces, uneven wear with wider tires, and poor wet and snow traction,

All of these issues are improved with a belt under the tread, to reduce tread flex.

There was an interesting tire advertisement that had a bias and either a radial or bias-belted tire driven over a glass floor, and the tread was filmed from underneath. It was very noticeable that the tread ‘squeezed’ together on the unbelted tread tires as the car’s weight was on them. The groves closed right up.

The belted tread stayed open, the grooves stayed open. It was mentioned that this allowed the tires to be used in winter weather in some snow and slush conditions.

The under and overinflation wear was also lessened with a belted tread.

Fill ’em up, drive ’em hard, and wear ’em out.

Greg Long

[Author nameAuthor names are only available to PAS members.]

Thanks very much for your comments, Greg and Bill. I like Greg’s suggestion of using an IR thermometer to measure tire temperature under different pressure conditions. I really want to put hard numbers on this question, rather than rely on hear-say. I think that if I start out hard — 50 pounds — then drive a certain distance and measure temperature, then repeat the test under the same conditions for, say, 3 pounds less each time, I should see a curve of temperature versus pressure with a distinct knee where the temperature rises rapidly. Just before the knee is where you want to run. And that should depend on the load — the car being driven.

Of course it will be a few months before I can do this, but I’ll report the results back.

I do drive alot and always wear out tires. The fact that they don’t seem to last very long is the problem and hence my motivation. This subject has also come up on motorcycle forums — I used to ride my BMW R75/5 a LOT, and always tried to get maximum tire life. But there too, no one ever seemed to have done an engineering study of tire pressure versus load for maximum tire life.

Interestingly, this subject came up frequently in the ‘teens, say in “Horseless Age” or similar. Back then the philosophy was to run them HARD, like 80 pounds, so they remained round even under load. Of course, tires back then were quite different from today, but the object seems to be the same —- keep the temperature down. With today’s bias tire technology, I’m looking for the minimum pressure to still get good life and minimize tread wear for a given load.

–Scott

[Author nameAuthor names are only available to PAS members.]

Scott

---

In the time of the Horsless Age Mag the tyres (Aussie spelling!) were clinchers & required the high pressures to keep them on the rim. My 1912 Napier is about the same weight as a Pierce 48 & I run clinchers at 80 psi. The tread width is only around 3 inches & the car weighs in at around 4000 lbs. So the weight per square inch on the road is very high. I drive this car a lot & get on average only 5000-6000 miles out of a tyre before it is bald. Usually a lot less as modern clinchers tend to start to peel the beads off at the rim long before then even though the tyres are made from modern materials. VERY annoying & expensive!. I admit to driving the car fairly ‘energetically’. Our Pierce-Arrows are NOT lightweight devices & so short tyre life is to be expected if driven a lot. Think of this as part of the ‘vintage’ driving experience! Your proposed ‘PhD’ study will be interesting. Cheers

---

Jak.

[Author nameAuthor names are only available to PAS members.]

Scott,

Do you belong to the Franklin Club? They would be a great source.

Long ago , tire cos. would list a recommended pressure next to the

tire size you needed. Car engineers selected the tire size that would

be safe for your car. Most likely, many other cars would use the same

size even though they didn’t weigh exactly the same. Probably they

all used the same pressure. Call Coker Tire and get their take on

what you should run.

[Author nameAuthor names are only available to PAS members.]

The pressure per square inch on the road is equal to the PSI in the tire, ignoring any sidewall support.

Thus, for your 4000 pound Napier, at 80 PSI in tires, the contact patch total is 4000/80 = 50 square inches. Thus, 12.5 square inches per tire contact patch on the road….

[Author nameAuthor names are only available to PAS members.]

David,

Please detail the equation in entirety, for those of us who need details.

Also, after you detail the equation, please indicate the utility of same, as in; “the size of the footprint of the tire and the weight of the car should dictate the psi per tire,” or whatever.

Thanks,

Peter

[Author nameAuthor names are only available to PAS members.]

I knew by posting this I’d have some people scratching their heads. There was a question once on the AACA forum about tire pressure on a tiled garage floor, I made the above statement, and was called a fool by one individual…who later did his research and found it was true.

There is no “equation”. A car is supported not by the tires, but by the air pressure in the tires. There is a contact surface, or patch, between the tire and the ground. The area of this patch will vary with PSI in the tire, increasing as PSI drops, decreasing as PSI rises.

Ignoring any tire sidewall support, the pressure inside the tire must equal the pressure on the ground, in PSI. Imagine the tire supported on a column of air at the point of contact.

As far as tire sizes vs. footprint, that’s the whole reason we have different sizes of tires. A 30×3 tire can support a light brass car with appropriate tire patch, while a larger tire is needed for heavier cars.

I know from experience that some people have a hard time accepting this physical rule, but it’s a fact. That’s why you can run 30 psi in your modern large tired car, that’s why a large airplane may have 200 psi or more.

[Author nameAuthor names are only available to PAS members.]

David—-10 out of 10 for remembering your physics lessons!! —jak.

[Author]

Posts