PSI: bias ply vs radial

[JonW]

Almost all of my cars were originally equipped with bias ply tires. Now most of them are running radials. I'm assuming that a radial tire requires more air pressure than a bias ply. Is there a rule of thumb on how to calcuate the correct PSI of a radial tire based on the bias ply PSI found on the door jamb sticker?

[Bloo]

Assuming 40s-50s-60s-70s American cars with ordinary drop center 13-14-15-16 inch rims, Ignore it and put in 35 pounds. You can go a little higher if the tires maximum rating will allow it. Never less than 30 pounds.

 

 

[joe_padavano]

52 minutes ago, Bloo said:

Assuming 40s-50s-60s-70s American cars with ordinary drop center 13-14-15-16 inch rims, Ignore it and put in 35 pounds. You can go a little higher if the tires maximum rating will allow it. Never less than 30 pounds.

 

 

This is what I do as well. Treadwear looks good at 35 psi

[alsancle]

You don’t say what years your cars are. I’m assuming postwar. The other issue is with pre-war rims. They may not withstand a radial tire.  Late 1930s are generally OK. But not in every case.

[Joao46]

My 65 Karmann Ghia recommended less than 20 psi on the front tires and it had bias ply tires from the factory. I don’t think it was meant to have ‘no less the 30’ on its front tires. Even if radials.

[m-mman]

Tire pressure - the never discussed aspect is How often do you really check? 

It is impossible for a tire to gain additional air.  (Yes temperature will increase pressure but that’s not the point)

 

Pick a number “32” psi as the optimal pressure. If you actually fire up the compressor and get out the gauge every time you drive (and every morning on the tour) then you will always have the optimal pressure. But……?

 

A tire gets its strength from the pressure inside. You can drive a long time with too much air but flat tires can move only a few feet. 
 

Taking the optimal as 32 then if you actually inflate to 35, as air inevitably leaks out, your tire will move INTO the optimal range instead of out of range. (Remember an overinflated tire can travel farther with less damage than an under inflated tire)

 

Of course none of this applies if you take things to the extreme.  If 32 is your target pressure and you inflate them to 90psi then all bets are off.  
 

Whoever checks their spare? I typically put 40 or so in the spare so that when I have a flat and I haven’t checked it in well…. who knows when, I will have an inflated replacement to get me to a destination. 

So optimal tire pressure, it’s a little less about how much and a whole lot about how often.  

[Stude Light]

My Diamondback Auburn radials on my LaSalle run at 45 psi. That is what the tire rating for maximum load states so that is where I run them.

[alsancle]

I keep a logbook for my cars. And I try to check the tire pressure once a month and then note the pressures. This is something I started doing more recently when I started driving the cars more often.  Also, you will find that a car will steer at slow speed much better with fully pressurized front tires.

[Lee H]

9 hours ago, Stude Light said:

My Diamondback Auburn radials on my LaSalle run at 45 psi. That is what the tire rating for maximum load states so that is where I run them.

I would respectfully ask you to reconsider doing that. Assuming you are running the 700/R16, the maximum load rating is 2403 lbs. Your LaSalle’s factory weight is 3740 lbs, or only 935 lbs per tire. That is a considerable difference between the rating and actual. What is likely to happen, is that the tires will practically curl up at the edges, and the center of the tire will wear much, much quicker.

[Larry Schramm]

30 minutes ago, Lee H said:

I would respectfully ask you to reconsider doing that. Assuming you are running the 700/R16, the maximum load rating is 2403 lbs. Your LaSalle’s factory weight is 3740 lbs, or only 935 lbs per tire. That is a considerable difference between the rating and actual. What is likely to happen, is that the tires will practically curl up at the edges, and the center of the tire will wear much, much quicker.

 

 

I have been running all of the tires on my vehicles for decades at the maximum tire pressure as noted on the sidewall and never had the tires wear out in the center.  Also on my 1/2 ton truck by going from the tire pressure on the door placard to the tire max I picked up a mile per gallon better fuel economy.  Now if you go over the max tire pressure on the sidewall I agree with you.

 

The tire pressure on the placard on the car takes into consideration NVH, which is engineering speak for noise, vibration, & harshness. The placard tries to balance all into a vehicle with a "good" ride.  With the higher tire pressures the ride can be a little bit more harsh. 

[Bloo]

This^^

 

Also, the two most common causes of tire failure are underinflation and overloading. If you look at load vs pressure chart for a given tire size and load range, it soon becomes obvious that those two causes are almost the same thing. On a typical tubeless radial tire, the most common sort of leaks people get are nails and rim leaks. Both are slow. If you start out at whatever pressure the chart says can carry the load, there is nowhere to go but down. More pressure means in the event of a leak you have more time and or distance before all hell breaks loose, and more time to catch the problem if you are checking your tire pressure regularly. If you are not checking, hopefully someone points at your low tire at a stoplight.

 

 

[Lee H]

46 minutes ago, Larry Schramm said:

I have been running all of the tires on my vehicles for decades at the maximum tire pressure as noted on the sidewall and never had the tires wear out in the center.

I am subject to correction, but I do not believe there are any cars, new or old, where the pressures recommended by the OEM are equal to the maximum rating on the sidewall. It is frequently many psi less. Just as a ready example, my everyday Honda has a max pressure on the sidewall of 44 psi, and a Honda recommended pressure of 29 psi. I am confident that 44 psi would be considered over inflation, and the center of the tire would wear faster (as well as having a smaller contact patch and reduced braking efficiency).
 

You can prove it to yourself as I did with my ‘62. In your driveway, take some chalk and draw a fat, heavy line across the tread. Now drive forward several revolutions, and look at the chalk prints on the pavement and chalk remaining on the tire. If the pressure is correct, you’ll see even prints and even removal of the chalk on the tire. With over inflation, more will transfer from the center, and under inflated, more toward the edges. There are several resources on the web, just search “tire pressure chalk test”

[Larry Schramm]

There might be some differences between radial and bias ply tires, but as long as you do not go above the tire manufacturers recommended max pressure you should be fine.

 

Vehicle manufacturers recommended pressures are there to optimize ride quality and fuel economy.

[Lee H]

To answer the OP’s question, there is a method to determine the correct tire pressure with a different tire size than the original. This is from Discount Tire. You do need to know the original tire load index or rating, but everything else is pretty self explanatory. This method should result in optimal loading and wear across the contact patch (or at least as good as the OEM designed it). Better than shooting in the dark, IMO.

[Stude Light]

17 hours ago, Lee H said:

I would respectfully ask you to reconsider doing that. Assuming you are running the 700/R16, the maximum load rating is 2403 lbs. Your LaSalle’s factory weight is 3740 lbs, or only 935 lbs per tire. That is a considerable difference between the rating and actual. What is likely to happen, is that the tires will practically curl up at the edges, and the center of the tire will wear much, much quicker.

I went and checked on my tires, they are actually rated at 51 psi for maximum load. I’m not sure why I had 45 in my head. I actually run at 42 psi. How did I come up with that number?
 

1) After some drive testing, that seemed to be best for ride quality.

2) The wheels were designed for bias ply tires which spread the load of the tire patch out to a larger part of the rim vs a radial tire which tends to concentrate the load radially around the tire patch. Pressure and tire stiffness are related and a higher inflation pressure on a radial will help spread that concentrated load out to more of the rim. Helps reduce flexing and potential cracking of these older rims running radials.

3) Prewar rims don’t have the safety ridges so the higher inflation rate improves my safety margin, especially during cornering.

4) 40-45 psi was a recommended range by the manufacturer ( personal conversation).

5) The “radial bulge” is almost eliminated at that pressure.

6) The chalk test showed no difference between 35 psi and 45 psi. Both times the center of the chalk wore off quicker - not sure why.

7) I am confident that I’ll calendar out the tires before I wear them out.

While I respect your recommendation and reasoning, I am more than willing to compromise quicker wear out (if that happens) for 1-7 above.

Scott

[Lee H]

6 hours ago, Stude Light said:

Both times the center of the chalk wore off quicker - not sure why.

Your rims do make this a special situation, I think. I don’t know what your tires were originally rated for, but typically I’ve seen 10% to as much as 50% over the loaded GVW. And you have to accommodate the fact that that era car never had a perfect 50/50 weight balance. I do see that the bias ply’s through Coker are rated 1900#, but even if you use the optimistic 1900 lbs as the OE rating (ca. 103 index), by moving up to the radial version and a  2403#, 111 index, the chart suggests an inflation pressure for those tires of 27 -28 psi. You are probably “curling” the edges even at 35 psi.

[Bloo]

If you want to run 27-28PSI in radial tires in my neighborhood during the summer, carry 2 spares.

[Joao46]

I think it really depends on the vehicle’s weight. 
I have driven on the highway for many hours with lower pressures and had no problem. It weighs 1800 lb.

 

Here is a factory tire placard.

 

 

[Lee H]

3 hours ago, Joao46 said:

I think it really depends on the vehicle’s weight

It absolutely does. 7.00/R16’s have a weight rating of 2403 lbs. That’s nearly 10000# they can carry for the set, and I’m sure much higher than OE.

[JACK M]

This is what I do on the race cars, and it should (and i will suggest) translate to DDs.

Go for a long drive on a freeway. Pull into a rest area and quickly jump out and test the temperatures with your handheld pyrometer.

Check inner tread, center tread, and outer tread on all four tires. Do this quickly, best to make a diagram of all four corners and have someone write down your readings.

Hotter in the center means to much air pressure, hotter on the outer edges means not enough air pressure.

If you get readings like hotter inner, not as hot center and colder yet on the outsides you probably need an alignment. (This scenario is not uncommon on the race cars, and we will change alignment to accommodate).

Even temps all the way across will be the most efficient and where I would run my dailies if I were as anal as you guys seem to be.

If you write the pressures down you should be able to come back to these even readings.

 

[Lee H]

No, no. Anal would be using an IR camera.

[JACK M]

50 minutes ago, Lee H said:

No, no. Anal would be using an IR camera.

Well, we are just poor boy dirt racers doin the best with what we have.

I had to save up for the hand held.    LOL