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Bias ply tire pressure


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Every spring I check my pressures after winter storage. I was browsing through my 49-54 Pontiac manual and found info on 7.60-15 (4ply) tire pressure. I cross referenced and found this to be for my H78-15 on the car. Front pressure 22 lbs. and rear 20 lbs. After 3 miles or more above 40 mph pressures increase to front 27 and rear 25. I'm so accustomed to them being old school 32 pounds all round. I'm curious what others are running for pressures. I haven't test rode the car yet but don't to cause any unnecessary wear or tear on perfect treaded tires. 

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I've got five old 50s cars that I drive frequently (including a 1953 Pontiac) all with bias ply tires. I have always kept the pressures at about 28- 30 psi all the time. I don't know if that's correct or not but that's what I have always done. 

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Posted (edited)

Bias tires did indeed run at lower pressure than might be advisable with modern tires. Lower gives a smoother ride but feels squishy. Higher feels "tighter" but less smooth. In any tire, higher pressure means less flexing of the casing, and that means less heat. Most tire failures are related to heat. Bias tires generate a lot more heat than modern tires no matter what you do, and if they have inner tubes too, that hobbles their ability to dissipate heat. There is not a chance I would ever run as low as 22/20 where I live. It gets over 100F/38C here every summer, and that is just begging for a blowout.

 

The caveat is that pressure directly affects how the tread contacts the ground, and for best traction you want the tread dynamically flat at the contact patch. Too much air wears out the center of the tread more quickly, and too little wears off the outer edges too quickly. Over multiple sets of tires you can learn what works best. Radials tend to be far less sensitive to this because there is a usually a steel belt under the tread. With bias, you should be paying much closer attention to get the best traction and tread life. 22/20 is way at the low end of normal, even for bias. When I had bias on my Pontiac, I ran the sidewall maximum, probably about 35. That is quite high for bias*, and I did not have excessive center wear. It was not really noticeable. I don't recall the factory settings offhand, but they were down in the 20s and I feel fairly sure that would have caused more wear at the outer edges than is desirable.

 

*- EDIT: Referring to 600-16 on drop center rims.

 

Edited by Bloo (see edit history)
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Posted (edited)

My two Buicks have a nicer ride when tire pressure is 27-28 psi.  Any higher the ride gets harsh.  However, at high psi the tires do not squeal when cornering at high speeds. Lower pressure one would think I was drifting in the corners at high speed. Just squeaking away. 😂. Maybe I should corner more slowly. Hmmmm...nah forget that. 

Edited by avgwarhawk (see edit history)
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I Can tell you that with a heavy car, you need to be running 50 pounds in the front tires or you can’t steer them at low speed. With some of the smaller cars this might not be a problem.

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All food for thought. The pressure seems low but remember that pressure increases as they warm up. At 32 psi my tread looks awesome but then I'm not travelling across the country every summer. I'll see how the ride goes and go from there and also check the actual after an afternoon of cruising.  

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Posted (edited)
1 hour ago, alsancle said:

I Can tell you that with a heavy car, you need to be running 50 pounds in the front tires or you can’t steer them at low speed. With some of the smaller cars this might not be a problem.

50 psi 😲.  To much pressure. The contact with the road at a minimum. These bias ply already lose contact with the road when cornering.  Radials do not.  Keep them at 27-28 psi. 

Edited by avgwarhawk (see edit history)
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2 hours ago, avgwarhawk said:

50 psi 😲.  To much pressure. The contact with the road at a minimum. These bias ply already lose contact with the road when cornering.  Radials do not.  Keep them at 27-28 psi. 

 

I just put 50 pounds in my PII Rolls today..........27-28 pounds is dangerous on snap ring wheels.......you will peel the tire off the rim. As far as wearing and footprint.......last year we did about 8k miles on the old cars around here.........everything from Ford T's to Model J's. The lowest air pressure I run in ANY pre war tire is 40 pounds, and up to 60 depending on application. I would say that half the cars are 45 front, 40 rear. I'm guessing the above poster is talking about post war stuff........

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Posted (edited)

As a good rule of thumb, on any tire today modern bias or radial on OLD cars.......ask guys who drive tens of thousands of miles per year. Fact is, 95 percent of people in the hobby get less than 100 miles on their cars each year....especially pre war stuff. Here is a photo of a J in the Utah dessert........at 100 degrees outside........run that thing at 27-28 pounds and you will overheat the tire and it will explode in minutes. This tour was 900 miles in five days.........and I had just helped out setting up the car for the owner. New drop center rims which means no snap rings.....non authentic....but the car was having multiple wheel failure issues with snap rings. (Three wheel failures in less than 600 miles.) We made our own rim design out of England. Reused the hubs, and had our own new spokes made. Results.....a wheel twice as strong as what was available new. And will take a radial tire if the owner decided to do so. I am NOT a fan of radial tires on ANY pre war car ......especially without making new wheels. It took a bunch of effort to get him to run bias tires........and the car now had 6k miles on them with no flats or failures.......we run 50 in the front/45 in the rear on this car. Works great.......even tire wear, and even temperatures. 

IMG_3709.jpeg

Edited by edinmass (see edit history)
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Posted (edited)

My comments in my earlier post refer to a 1954 Pontiac, and also my 1936 Pontiac, which has drop center rims very much like the ones on a postwar Pontiac. The load carrying capacity of a tire is directly linked to the pressure. The more pressure, the more load it can carry. For modern tires, there are tables you can look up to see how many pounds the tire can carry at a given pressure. How much pressure is appropriate depends on the car. My 1913 Studebaker has bias clinchers, and cannot be less than 60(!) pounds. Generally speaking I think taller narrower tires are likely to need more pressure, and are likely to be rated for it. Radials almost never want less than 32psi no matter what the door sticker says. They might want a lot more.

 

I have never heard of anyone having a blowout because they put too much air in. It probably happened to someone somewhere, but it is extremely rare if it happened at all. Meanwhile, an overwhelming majority of tire failures are caused by underinflation and overloading. Those two things are really the same thing since load capacity is dependent on pressure. The tire gets too hot from all the sidewall flexing and the rubber reaches a temperature it cannot stand. The tire falls apart. How do you reduce the flexing? More pressure. A little too much pressure is always better than not enough.

 

 

Edited by Bloo (see edit history)
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Thanks Ed. You reminded me, I’m running 45 pounds in the front and 40 pounds in the rear.

 

Even with completely rebuilt front end which is properly lubricated, I can’t steer the car at 2 miles an hour without the 45 pounds Of pressure.

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9 hours ago, edinmass said:

 

I just put 50 pounds in my PII Rolls today..........27-28 pounds is dangerous on snap ring wheels.......you will peel the tire off the rim. As far as wearing and footprint.......last year we did about 8k miles on the old cars around here.........everything from Ford T's to Model J's. The lowest air pressure I run in ANY pre war tire is 40 pounds, and up to 60 depending on application. I would say that half the cars are 45 front, 40 rear. I'm guessing the above poster is talking about post war stuff........

It maybe we are talking apples to oranges.  Snap ring wheels and standard rims. Bias ply tires pre-war and post war. I gather 50 psi is recommended for snap ring wheels pre war cars?  26-28 psi for cars manufactured in the 50s on bias ply. The era car summershady is inquiring. 49-54 Pontiac. 

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Car seems to ride a little smoother with the factory recommended tire pressure. Did some mean cornering and felt OK.

The front did increase by 2 lbs but it's still quite cool up here....we still have snow. 

I'm going to wait until later in the summer and take the readings again after an afternoon cruise.

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Posted (edited)

Using tire wear as an indicator, I use 26 rear 28 front. 

 

22 up front saw better handling in winter but came at the cost of heavy wear at the shoulders of the tread. That's a big old boat anchor up front the tires are carrying around. 

 

Phil

Edited by PhilAndrews (see edit history)
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