If on a classic car that came from the factory with bias ply tires, and has no up grades to suspension or anything, except it now has radial tires in a proper size, should the factory recommended pressure still be used, or do radials need more or less pressure due to the different sidewall construction???
tire pressure bias vs. radial
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Answer is variable, IMO start at factory pressure.
I read about this very thing in a Hot Rod (I think) magazine several years ago. If you lay down a short patch* and check the darkness of what you left behind, it should show you if the contact patch is maximized - that is what you are looking for. Either under or overinflated will lessen the patch and you will see a lighter area either at the edges (overinflated) or the center(underinflated).
*where legal.
I read about this very thing in a Hot Rod (I think) magazine several years ago. If you lay down a short patch* and check the darkness of what you left behind, it should show you if the contact patch is maximized - that is what you are looking for. Either under or overinflated will lessen the patch and you will see a lighter area either at the edges (overinflated) or the center(underinflated).
*where legal.
My car won't burn rubber, it is a 6 cylinder automatic. I am mostly concerned about ride, handling, and tire wear. Think I will just try it at different pressures, and see what I think. Guess maybe it was a silly question, but thought maybe there was a simple formula for radial vs. bias pressures.
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Somewhere was a picture of the traction patch of a 1960's Impala vs a modern day one. The 60's tire was way skinnier and had more weight.
I'd start at the higher end of pressures with your testing, so the tire can take the heat and load better.
IDK where the "bias tuned suspension" comes into play though-- except that there is such a thing.
I'd start at the higher end of pressures with your testing, so the tire can take the heat and load better.
IDK where the "bias tuned suspension" comes into play though-- except that there is such a thing.
Assuming the radial tire isn't one of those low profile jobs, the PSI should be about the same whether radial or bias. My first set of radial tires were put on a 66 Bug. The handling was greatly improved over the bias plies.
Originally Posted By: old1
My car won't burn rubber, it is a 6 cylinder automatic. I am mostly concerned about ride, handling, and tire wear. Think I will just try it at different pressures, and see what I think. Guess maybe it was a silly question, but thought maybe there was a simple formula for radial vs. bias pressures.
Ah well...but maybe in reverse?
No formula I'm aware of. But your plan sounds good; driving around for 1000 miles and checking wear on the fronts should give a good indication too.
My car won't burn rubber, it is a 6 cylinder automatic. I am mostly concerned about ride, handling, and tire wear. Think I will just try it at different pressures, and see what I think. Guess maybe it was a silly question, but thought maybe there was a simple formula for radial vs. bias pressures.
Ah well...but maybe in reverse?
No formula I'm aware of. But your plan sounds good; driving around for 1000 miles and checking wear on the fronts should give a good indication too.
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Why not look at the tire? Max inflation pressure will be on the sidewall.
There was a article in a hot rod type magazine that said to be careful putting radials on the old bias ply type rims because of rim metal thickness??? I would agree 30lbs should be good to go for you though.
Maybe call Wheel Vintiques and see what info they would have.
Maybe call Wheel Vintiques and see what info they would have.
Originally Posted By: Traction
Not knowing the size and weight of the car, 30 psi pretty much covers everything.
Beat me to it. Good advice.
Wayne
Not knowing the size and weight of the car, 30 psi pretty much covers everything.
Beat me to it. Good advice.
Wayne
Man, oh man, oh man. Lots of really [censored] advice.
First, we don't even know what the car is, nor what the original tires were on it, nor what is on there now - and people are giving advice without that info? You just can't do it!
So let's get that first.
But be aware that I'll bet the original tire size isn't what is on there now! So whatever was specified back then isn't applicable now unless they are equivalent - and we do not know that!
Plus, the way tire sizing is done now is not the way it was done back then.
Also, laying a patch, chalking the tires, masking tape, whatever, is NOT the way to figure out if you have enough pressure. Tires are NOT necessarily designed to wear evenly (or even have good contact patches for traction). One way might (for tires for longevity purposes) is with the pressure build up. You don't want more than 10%.
So, let's get some particulars and THEN give advice!
First, we don't even know what the car is, nor what the original tires were on it, nor what is on there now - and people are giving advice without that info? You just can't do it!
So let's get that first.
But be aware that I'll bet the original tire size isn't what is on there now! So whatever was specified back then isn't applicable now unless they are equivalent - and we do not know that!
Plus, the way tire sizing is done now is not the way it was done back then.
Also, laying a patch, chalking the tires, masking tape, whatever, is NOT the way to figure out if you have enough pressure. Tires are NOT necessarily designed to wear evenly (or even have good contact patches for traction). One way might (for tires for longevity purposes) is with the pressure build up. You don't want more than 10%.
So, let's get some particulars and THEN give advice!
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Tire pressure advice is based on load on the tire and speed and camber angle.
If its an oldtimer with originally bias tires, the maximum speed will not be above 160km/99m/h and camber angle ( wheels like this on the (imaginary) axle /-\) will also not be the case here.
Then if you can determine the axle loads in your use and give the specifications of tire, like maximum load and kind of tire ( probably P-tire) and sises and speedcode ( all can be found on sidewall) , then I can calculate a save lowest pressure . Above that you can play for handling reasons, but not go lower.
Bias ply tires are calculated in their advice pressure with a different formula then P-tires radial. In 1928 they introduced the universal formula for calculating loadcapacity for a sertain pressure and it was for Bias ply tires with a power in it of 0.585. About 1970 they chanched that formula for radial tires in America with power 0.5 to 0.65 for P-tires and in Europe for all kind of radial tires ( from normal car tire to truck tire) to power 0.8 . the higher the power the lower loadcapacity it gives for a sertain pressure.
As late as 2006 American TRA swiched over to the European ETRTO system for P-tires and XL/reinforced/extraload only .
For Light-truck ( LT) tires in C-load and up still old power of 0.7 is used in America.
Power 1 is the same as part of pressure for maximum load= part of maximum loadcapacity of tire, as most think is logical.
If its an oldtimer with originally bias tires, the maximum speed will not be above 160km/99m/h and camber angle ( wheels like this on the (imaginary) axle /-\) will also not be the case here.
Then if you can determine the axle loads in your use and give the specifications of tire, like maximum load and kind of tire ( probably P-tire) and sises and speedcode ( all can be found on sidewall) , then I can calculate a save lowest pressure . Above that you can play for handling reasons, but not go lower.
Bias ply tires are calculated in their advice pressure with a different formula then P-tires radial. In 1928 they introduced the universal formula for calculating loadcapacity for a sertain pressure and it was for Bias ply tires with a power in it of 0.585. About 1970 they chanched that formula for radial tires in America with power 0.5 to 0.65 for P-tires and in Europe for all kind of radial tires ( from normal car tire to truck tire) to power 0.8 . the higher the power the lower loadcapacity it gives for a sertain pressure.
As late as 2006 American TRA swiched over to the European ETRTO system for P-tires and XL/reinforced/extraload only .
For Light-truck ( LT) tires in C-load and up still old power of 0.7 is used in America.
Power 1 is the same as part of pressure for maximum load= part of maximum loadcapacity of tire, as most think is logical.
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Having owned numerous classic cars for years, I have pretty much stayed with 30PSI in all four radial tires. This PSI always seemed to give me the best feel in all my classic/muscle vehicles.
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Thanks to all, I think I have a handle on it now.
Easy way, draw a couple chalk lines across the tread and drive forward 20 feet and look at the line on the tire, if it's scrubbed off evenly across the tread your pretty close to the right pressure. If it's scrubbed more in the center, drop down a few pounds and try again.
Originally Posted By: old1
Thanks to all, I think I have a handle on it now.
Wait just a minute. You haven't told us what vehicle you have, what size the bias tires that were on it originally, and what tire size you have on it now. How can you have a handle on it based on what is in this thread? Nobody knows what the particulars are!
Thanks to all, I think I have a handle on it now.
Wait just a minute. You haven't told us what vehicle you have, what size the bias tires that were on it originally, and what tire size you have on it now. How can you have a handle on it based on what is in this thread? Nobody knows what the particulars are!
It is a 65 mustang coupe 6 cyl automatic, with 195-75-14 tires, which are suppose to be the modern equivilant of the original size which I don't remember of the top of my head, and the original owners manual calls for 28 to 30 psi, and I simply wondered if with the modern tires if I needed less, or the same or more psi. I am just gonna try 30 and see how it works. No sweat!
Mayby the specifications of the old bias tires is written somewhere on the car or in the manual, for instance on the same plate as your advice pressures. If you can read that and then especially the maximum load. The pressure needed for that then still is a question, the system for Bias tires was a bit different. for your radial tires the pressure needed for the maximum load( of tire) up to 160km/99m/h is most likely 35psi/2.4 bar in American system.
Mayby the GAWR's and GVWR ( Gross axle/vehicle weight rating) can also be found on your Mustang. Then we can use that for fully loaded pressure advice as nowadays is also given on modern cars only.
then the maximum speed of your Mustang can be higher then that 160km/h I think.
But for the tire pressure advice the maximum speed you ever use is only important.
So that is not so difficult to keep yourselfes at the speedlimit of 99m/h, even if you sometimes go over the speedlimit that is allowed in America.
but the trying out you now want to do is a bit dangerous, can be you have a comfortable ride and handling is OK, but still the sidewalls deflect to much and you get tire damage and blowing tire at a sertain moment with all the missery that goes with it.
Als the chalk test dont go for radial tires, the widht of the treath stays on the ground in a large range of deflection. So still to much deflection so it gives damage to tire can happen when chalk wears of evenly.
If you want to play with the chalk test , begin much to high , and stop when chalk wears of evenly, Then you will have the least deflection of tire that still gives even wear of your treath.
Mayby the GAWR's and GVWR ( Gross axle/vehicle weight rating) can also be found on your Mustang. Then we can use that for fully loaded pressure advice as nowadays is also given on modern cars only.
then the maximum speed of your Mustang can be higher then that 160km/h I think.
But for the tire pressure advice the maximum speed you ever use is only important.
So that is not so difficult to keep yourselfes at the speedlimit of 99m/h, even if you sometimes go over the speedlimit that is allowed in America.
but the trying out you now want to do is a bit dangerous, can be you have a comfortable ride and handling is OK, but still the sidewalls deflect to much and you get tire damage and blowing tire at a sertain moment with all the missery that goes with it.
Als the chalk test dont go for radial tires, the widht of the treath stays on the ground in a large range of deflection. So still to much deflection so it gives damage to tire can happen when chalk wears of evenly.
If you want to play with the chalk test , begin much to high , and stop when chalk wears of evenly, Then you will have the least deflection of tire that still gives even wear of your treath.
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Originally Posted By: old1
It is a 65 mustang coupe 6 cyl automatic, with 195-75-14 tires, which are suppose to be the modern equivilant of the original size which I don't remember of the top of my head, and the original owners manual calls for 28 to 30 psi, and I simply wondered if with the modern tires if I needed less, or the same or more psi. I am just gonna try 30 and see how it works. No sweat!
Thank you, thank you, thank you.
According to Tire Guides, a 1965 Ford Mustang originally came with 6.94-14's inflated to 24 psi front and rear.
That means Ford specified a load carrying of 1050# for each tire.
I wrote up an equivalency chart and published it here:
Barry's Tire Tech - Vintage Tire Exchange
And I think a 185/75R14 is the closest modern equivalent. But if you are using a 195/75R14, then you should be aware that the stock rim was 5" wide and a 195/75R14 calls for a minimum of 5 1/2". That is going to cause the tire to arch across the tread face and wear in the center. It will also cause the vehicle to be sensitive to crosswinds and pavement irregularities, such as grooves and ruts.
Also, the 195's are 1/2 wider, even compensating for the rim width difference - and while I doubt there is a clearance issue (those vehicle had a ton of clearance), it might be good to check - keeping in mind that the worst condition is the suspension fully compressed and the steering wheel fully turned.
But if I do the math, the equivalent inflation pressure would be ...... drumroll ...... 19 psi or 5 psi less! I don't like that answer.
First, it was pretty common back in those days for car manufacturers to specify marginal tire sizes. Nowadays car manufacturers do a better job. So if we add what we've learned about tire sizing over the years, I get 32 psi.
OK, so that is not much different than your takeaway, but at least we have a technical basis behind the answer.
It is a 65 mustang coupe 6 cyl automatic, with 195-75-14 tires, which are suppose to be the modern equivilant of the original size which I don't remember of the top of my head, and the original owners manual calls for 28 to 30 psi, and I simply wondered if with the modern tires if I needed less, or the same or more psi. I am just gonna try 30 and see how it works. No sweat!
Thank you, thank you, thank you.
According to Tire Guides, a 1965 Ford Mustang originally came with 6.94-14's inflated to 24 psi front and rear.
That means Ford specified a load carrying of 1050# for each tire.
I wrote up an equivalency chart and published it here:
Barry's Tire Tech - Vintage Tire Exchange
And I think a 185/75R14 is the closest modern equivalent. But if you are using a 195/75R14, then you should be aware that the stock rim was 5" wide and a 195/75R14 calls for a minimum of 5 1/2". That is going to cause the tire to arch across the tread face and wear in the center. It will also cause the vehicle to be sensitive to crosswinds and pavement irregularities, such as grooves and ruts.
Also, the 195's are 1/2 wider, even compensating for the rim width difference - and while I doubt there is a clearance issue (those vehicle had a ton of clearance), it might be good to check - keeping in mind that the worst condition is the suspension fully compressed and the steering wheel fully turned.
But if I do the math, the equivalent inflation pressure would be ...... drumroll ...... 19 psi or 5 psi less! I don't like that answer.
First, it was pretty common back in those days for car manufacturers to specify marginal tire sizes. Nowadays car manufacturers do a better job. So if we add what we've learned about tire sizing over the years, I get 32 psi.
OK, so that is not much different than your takeaway, but at least we have a technical basis behind the answer.
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