Tire pressures Radial vs Bias ply does it make a difference

Brakes, Suspension, Rims and Tires

  1. HWYCRZR

    HWYCRZR Senior Member FCBO Gold Member

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    As I am starting to experience the ride on my '68 Polara with "stock size P215 75R 14" radial tires, I question the proper tire inflation. It seems that many modern vehicles recommend around 30 psi whereas our tire pressure recommendations are between 24 & 28 PSI
    I believe I pumped mine up to around 30 PSI and now seem to have more road noise than I did at the lower range. The max load pressure on the tire is 44 PSI.
    Just wondering is the 24 PSI where I want to be?
    Any other thoughts or PSI range you guys are using on your "stock" size tires

    upload_2019-5-17_9-48-28.png
     
  2. BigblueC

    BigblueC Well-Known Member

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    I run 32 front, 28 rear on my car ('65 4 door). The more road noise is simply because the tire is firmer with more pressure. To me, it's more important to pay attention to what the tire is telling you based on wear patterns. Also, your tire should gain approximately 10% pressure when fully heated, like after a decent trip down the highway. An under inflated tire will gain more due to heat build up and potentially cause a blow out. Over inflated will cause wear issues and a rougher ride. At least that's what I've been taught by a tire guy.

    So drive it some and pay attention to the details like pressure and wear/contact patterns and you can dial it in.
     
  3. 67Monaco

    67Monaco Old Man with a Hat

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    I run about 35psi. I like a firmer ride. At lower pressures I feel there's too much side wall bulge and always freaks me out like I have a flat.
     
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  4. commando1

    commando1 Mr. Normal FCBO Gold Member

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    I overfill them to 36 cuz I'm too lazy to check them all the time.
     
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  5. 413

    413 Member

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    Go by the side of the tire you are running. How does a 50 year old door jamb sticker know that the new tires need more pressure? They don’t. Also the Manufacturer never changes the sticker for a stripped car or one with many options that increase weight by a big amount. One size fits all for the sticker. No good if you think about it.

    Tires have gone from cold max pressures of 32 to 35, 41, 44, now 51 on our 2018 car passenger car tire.

    Running higher tire pressure is generally better. I run max or close to it in all cars. It works great TRY IT! Better cornering, less outer edge wear on fronts, more load carrying capacity, less rolling resistance. More reserve air if you have a slow leak.

    If you fill a 32 psi max tire to 24 psi that’s 75% of max. So now you are filling a 51 psi max tire to 24 psi which is 47% of max. So thats only 47% of the load carrying capacity listed on the side of the tire. Is this a good idea? Not on my car.

    Think back to your bicycle tires and running them low on air and how they act react, and what happens when you hit bumps, curbs, etc.
     
    Last edited: May 17, 2019 at 11:57 AM
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  6. BigblueC

    BigblueC Well-Known Member

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    The side of the tire will only tell the MAXIMUM ratings of the individual tire, not what the tire should be running at on vehicle X.
     
  7. 67Monaco

    67Monaco Old Man with a Hat

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    37 for the win.
     
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  8. Snotty

    Snotty Senior Member FCBO Gold Member

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    To this day, I run 35 psi in all my vehicles unless the sidewall calls for something different.
     
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  9. commando1

    commando1 Mr. Normal FCBO Gold Member

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    I love how these guys that overthink it.
     
    Last edited: May 18, 2019 at 12:24 AM
  10. jct

    jct Senior Member

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    32-35 psi
     
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  11. HWYCRZR

    HWYCRZR Senior Member FCBO Gold Member

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    I had them about 34 PSI cold. I think I will start there. I think it was a loose front shock that was aggravating the road noise. Bottom shock mount was not quite as tight as it should have been. I don’t like rattles.
    Thanks for the replies.
     
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  12. CBODY67

    CBODY67 Senior Member

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    Back in the later '60s, when I was in high school and had my faithful K&E Slide Rule, CAR LIFE magazine had some great tech articles. Several on tires. Somewhere along the line, there was one which had tire capacity at incremental inflation pressures, 24psi to 32psi, which was the normal pressure range back then.

    According to most road tests, also in CAR LIFE, the normal f/r weight distribution on a C-body sedan was 55/45.

    On the '66 Newport, the base tire pressure was 24psi, cold. At that inflation pressure, the car's weight should not overpower the tires' capacity. For normal 'round town driving, 24psi worked fine for many.

    Further reading the owner's manual, if highway speeds would be encountered, then "+4 psi" to the base 24psi, for a total of 28psi. That resulted in a "loaded" tire pressure recommendation of 28psi frt/30psi rr.

    In the article on suspension tuning, tire "slip angle" was mentioned. With lower pressure in the front, the car would understeer or slip sooner than the rear (which had more pressure).

    So, with some figurin', if the car was to have more neutral handling characteristics, then the front and rear tires should have similar slip angles, which would relate to their pressure bias, frt to rear.

    Being that Chrysler products were the better handling cars of their time, I did some experimenting on our then-newer '66 Newport 6-Window sedan. What I sought to do was to have each set of tires support their proportional amount of weight, using air pressure differences to do that.

    With the tire pressure chart and the weight distribution information, I came to the conclusion that the front tires needed a bit more air to "support the weight" than the rear tires did. My "even" situation resulted in my 30frt/28rr tire pressure bias. If there was to be more stuff in the trunk, then raise all pressures by 2psi to 32/30.

    One other way I prototyped the tire pressure deal was to turn the front wheels side to side while parked, then roll the car back and look at the resulting rubber on the concrete. With 30psi in H78-14 tires on a 5.5" width rim, there was no light spot in the middle of the tire's print. With 28psi, just a bit, but 30psi made one black print on the concrete.

    Popular Science magazine, circa 1968 I believe, had an article on tire pressure and how the car handled. They ran a test with the tires at 24psi, cold. On the highway, they did a lane change maneuver. They couinted the number of weight transfers the car did as a 2 lane passing maneuver was executed. Something like EIGHT weight transfers! Of course, most of these were due to the softness of the tires at the lower inflation pressure.

    When they re-ran the test with 30 or 32psi, the car was more solid as the tires were stiffer with more air in them. Less steering input and less weight transfer activity.

    So, I started with the highway speed recommendation 28psi and increased the front pressure to get the front's heavier weight supported equally well as the rear, which was 30psi. Having the inflation pressure customized to how much weight the tires were supporting also resulted in the same tire wear on both ends of the car! Another benefit of my tire pressure bias situation!

    If the tires were over-inflated for the weight they were carrying, then there would be more wear on the center treads and less on the outer tread ribs. With "under-inflation" for the weight being carried, then the outer tread ribs were where the tread wear was, as the center section of the tread would "buckle upward". There were many illustrations of this in car maintenance documents back then . . . might check the Chrysler MasterTech courses for those things.

    So, with the OEM bias ply tires, the inflation pressures I ran resulted in great tire wear and better steering response, with a firmer ride. It all worked well.

    Why the desire for the lower front tire pressures by the factory? A driver will usually slow down when the front tires squal in a turn, as they slip before the rear tires, which is safer. If the rear slips first, that's oversteer and can become dangerous for the normal driver.

    And then, on the '66 C-body wagons, they went with a 22psi/32psi f/r pressure recommendation (for full rated load). More pressure in the back for heavier loads, less in the front so you wouldn't try to "road race", I suspect.

    The original radial tires ran the same pressure as the bias ply or later belted bias ply tires did. The modern P-metric radials came around in the earlier '80s, modified to not ride "hard" with the higher 35psi inflation pressure desired for better fuel economy.

    When the Chrysler LH cars appeared, they had P225/60R-16 as the standard tire size. No options, except for the later Performance Handling Package. These tires were mounted on 7" wide rims. DEFINITLY "performance territory" compared ot the 5.5" wide rims of prior times! Recommended tire pressure was 30/30. At that pressure, everything worked, although sharper handling could be had if the pressure was 34/30. Most fwd cars' weight distribution was 60/40, so 4psi more in the front than the rear. Those P225/60R-16 on 7" wide rims came to be the standard tire/wheel combination on almost every normal car back then, including most GM and Fords.

    With radials and belted bias ply tires, the outer belts keep the tread more "flat" on the road, which means they don't need air pressure to do that (as the bias ply tires do). BUT sidewall stiffness due to increased inflation pressure (in addition to any internal bead area stiffeners in the tire itself) result in crisper and more responsive steering inputs.

    When I got my '80 Newport, it had the stock 15x5.5" wheels and P215/75R-15 radials on it. I scored several sets of the Magnum GT 15x7 wheels/rings/center caps/bolts when Chrysler had then on "closeout". I put one set on the Newport with some cheaper P215/75R-15 whitewalls from Sam's Club. What happened there was that with the wider rims with tires designed for 6" wide rims, the sidewalls were more vertical, with decreased "radial bulge", at any inflation pressure. With the 32/30 pressure bias, that car acted like it was on rails when cornering. A joy to drive with the factory "load carrying" HD suspension (no rear sway bar). And THAT's a trick that can work on any car, even with non-performance tires. Kinda glad I didn't put the wider and heavier 70 series tires on it as the lighter tires ride better and handle just as well.

    Radials were around by about '67 or '68. Chrysler didn't offer any as optional equipment until about '70. My '70 Monaco owner's manual mentions using them and such. '74 was probably when Chrysler really started using them, with "rear sway bar" suspensions, too. Ford would put Michelin X tires on almost any car they sold. GM used UniRoyal Tiger Paw radials. Main concerns were with ride harshness, which would peak at about 45mph, then "get quiet" at highway speeds for that "radial feel" ride.

    My experiences. Proceed at your own risk. Key thing is to read ALL of the OEM tire pressure recommendations rather than just one part of them.

    CBODY67
     
  13. 413

    413 Member

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    Well it would be best to keep that to yourself!
     
  14. commando1

    commando1 Mr. Normal FCBO Gold Member

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    Not to worry. I fixed it.