Gross Axle Weight Percentage Tire Inflation Procedure

[CapriRacer]

Originally Posted by RidingOnRailz

Well, according to tirepressure . com’s old vs new tire calculator, when I put in “P225/55R17” and Standard Load for existing tire, and Metric 225/50R17, standard load for replacement tire, the converter calculated a cold pressure for the replacement tires 1psi higher than for the P-metric. 33 vs 32.

Their website, their calculator, not mine. Same happened for my wife’s tires. 31psi for metric vs 30psi for OEM P-metric. Indeed, a “slight difference”, as you yourself stated. But a difference, nonetheless.

Even in your examples on your own website, going from a P-metric tire to a Metric resulted in a higher tire pressure to carry the same load as before.

Grass is green and sky is blue. I report what I see and what I experience

First, you changed sizes: 55 vs 50. That's not the same as going from P metric to metric in the same size.

Second, there is a problem with the calculator. It assumes that each way of doing the calculation is correct - and that can not be. The physics of the situation is that, physically a P225/55R17 is the same as a 225/55R17. The difference is in what label is applied to the tire - and the labels are different. From there would be testing to assure that the tire passes the test.

Put another way, if I gave you a tire without anything written on the sidewall, and TOLD you it was a P metric, you should test at the P metric conditions.

If I gave you the same tire and told you it was a metric tire, you should test it at metric conditions. In both cases, you'll get the same result - the tire fails in the same manner and under the same conditions.

Ya' see, each standard is trying to describe what is going on with the tire, but the way each standard is done is different, so you get slightly different answers because each way is imperfect in its description (the formula!)

Yup, the math comes up with a different pressure, but that's because the math is imprecise.

Want a further complication? There is a third standard - JATMA (Japanese Automobile Tire Manufacturers Association) - and it's standard (the formula) is different still. This standard has sizes that look exactly like the metric standard - 225/55R17 (no "P") - and some people have a different word to describe what is published by tirepressure.com - Euro-metric - where they call what is published by JATMA - hard metric. BTW, what tirepressure.com is using is from ETRTO (European Tyre and Rim Technical Organization).

Ya' see, this is all very imprecise. As I said before, each standard is trying to describe what is going on and each standard isn't very precise when they do that. That's because each of these standards was created BEFORE there were digital computers capable of doing more detailed analyses.

Just as an example of this imprecision: tirepressure.com lists their metric load tables using English units. The standard is actually done in metric units - BUT - they did the load OK, but the pressure is off. What they list as 36 psi is really 250 kPa (36.3 psi). Again, imprecision.

But in the big scheme of things, it doesn't matter. It's close enough.

And remember where this discussion started: using 51 psi as the rating pressure. And clearly none of the standards do that.

[RidingOnRailz]

Originally Posted by CapriRacer

First, you changed sizes: 55 vs 50. That's not the same as going from P metric to metric in the same size.

Second, there is a problem with the calculator. It assumes that each way of doing the calculation is correct - and that can not be. The physics of the situation is that, physically a P225/55R17 is the same as a 225/55R17. The difference is in what label is applied to the tire - and the labels are different. From there would be testing to assure that the tire passes the test.

Put another way, if I gave you a tire without anything written on the sidewall, and TOLD you it was a P metric, you should test at the P metric conditions.

If I gave you the same tire and told you it was a metric tire, you should test it at metric conditions. In both cases, you'll get the same result - the tire fails in the same manner and under the same conditions.

Ya' see, each standard is trying to describe what is going on with the tire, but the way each standard is done is different, so you get slightly different answers because each way is imperfect in its description (the formula!)

Yup, the math comes up with a different pressure, but that's because the math is imprecise.

Want a further complication? There is a third standard - JATMA (Japanese Automobile Tire Manufacturers Association) - and it's standard (the formula) is different still. This standard has sizes that look exactly like the metric standard - 225/55R17 (no "P") - and some people have a different word to describe what is published by tirepressure.com - Euro-metric - where they call what is published by JATMA - hard metric. BTW, what tirepressure.com is using is from ETRTO (European Tyre and Rim Technical Organization).

Ya' see, this is all very imprecise. As I said before, each standard is trying to describe what is going on and each standard isn't very precise when they do that. That's because each of these standards was created BEFORE there were digital computers capable of doing more detailed analyses.

Just as an example of this imprecision: tirepressure.com lists their metric load tables using English units. The standard is actually done in metric units - BUT - they did the load OK, but the pressure is off. What they list as 36 psi is really 250 kPa (36.3 psi). Again, imprecision.

But in the big scheme of things, it doesn't matter. It's close enough.

And remember where this discussion started: using 51 psi as the rating pressure. And clearly none of the standards do that.

������

Typo. Sorry. Size of original and replacement is 225/50R17.

The OEM tire had “P” in front, the replacement, just the metric.

I corrected it, so awaiting your further input. We should be having this conversation via something with a real keyboard, or better yet, over coffee or tea - not via phones or tablets with fake keyboards! So it’s natural for mistakes like that to occur.

225/50R17 period. Sorry for muddying the waters

[CapriRacer]

Sorry. It's not like me to miss that. What's worse is I even did a comparison using the correct size, noticed the 2 psi difference, and didn't connect the dots! I totally blew it.

OK, back on track. Let's go back to your first post:

You have a 2010 Honda with GAWR's 2403#/2061#

It originally had P225/50R17 93V's at 32 psi. That's a load carrying capacity of 1367#.

That means the tires are loaded to 88% /75%. That's just about what I would expect for the fronts and, of course, the rears use the same pressure for simplicity as well as to prevent the consumer from having an underinflated tire when the tires are rotated.

I think this illustrates how those pressures were selected by the vehicle manufacturer.

And the calculation you used resulted in 41 psi/35 psi. Clearly not in the same ball park.

You test drove the car at those pressures and got what I would expect: A much harsher ride, but quicker steering response.

On to the Toyota:

You didn't indicate what tire size it has, so I couldn't do the math. How's 'bout you do the calculation and see what percentage you get. I'll bet the fronts are in the same vicinity, and the rears are lower because of the GAWR's.

[RidingOnRailz]

Originally Posted by CapriRacer

Sorry. It's not like me to miss that. What's worse is I even did a comparison using the correct size, noticed the 2 psi difference, and didn't connect the dots! I totally blew it.

OK, back on track. Let's go back to your first post:

You have a 2010 Honda with GAWR's 2403#/2061#

It originally had P225/50R17 93V's at 32 psi. That's a load carrying capacity of 1367#.

That means the tires are loaded to 88% /75%. That's just about what I would expect for the fronts and, of course, the rears use the same pressure for simplicity as well as to prevent the consumer from having an underinflated tire when the tires are rotated.

I think this illustrates how those pressures were selected by the vehicle manufacturer.

And the calculation you used resulted in 41 psi/35 psi. Clearly not in the same ball park.

You test drove the car at those pressures and got what I would expect: A much harsher ride, but quicker steering response.

On to the Toyota:

You didn't indicate what tire size it has, so I couldn't do the math. How's 'bout you do the calculation and see what percentage you get. I'll bet the fronts are in the same vicinity, and the rears are lower because of the GAWR's.

Re: Honda OEM tires: Correct, Capri. Now it wears “225/50R17 94V” - drop the “P” and up the load. Based on those considerations, tirepressure.com ‘s calculator returned a cold psi recommendation of 33psi, versus the 32psi recommended for the OEM 93V tires listed on the placard. Makes sense: Higher load index, higher required pressure to carry same load.

Re: Wife’s Corolla: In my original post, I explained how I used the same protocol, as I did for the Accord. I didn’t include all the figures and calculations to keep an already miles-long post a little briefer. Toward the end, I posted the calculated new pressures of 35psi front, and 32psi rear, for the Toyota. An increase pretty much in line with the increase for the bigger Honda.

Re "88/75%" of... max load for the tire?

[CapriRacer]

Originally Posted by RidingOnRailz

Re: Honda OEM tires: Correct, Capri. Now it wears “225/50R17 94V” - drop the “P” and up the load. Based on those considerations, tirepressure.com ‘s calculator returned a cold psi recommendation of 33psi, versus the 32psi recommended for the OEM 93V tires listed on the placard. Makes sense: Higher load index, higher required pressure to carry same load.

Re: Wife’s Corolla: In my original post, I explained how I used the same protocol, as I did for the Accord. I didn’t include all the figures and calculations to keep an already miles-long post a little briefer. Toward the end, I posted the calculated new pressures of 35psi front, and 32psi rear, for the Toyota. An increase pretty much in line with the increase for the bigger Honda.

Re "88/75%" of... max load for the tire?

First, I was hoping you'd do the same calculation I did: GAWR divided by Load carrying capacity for the original tire size at the specified pressure.

But your last paragraph indicates you didn't understand what those percentages were.

What that gives you is an indication of how the vehicle manufacturer views what is appropriate tire loading. My opinion is that tires shouldn't be loaded to more than 85% of their load carrying capacity at the pressures specified.

I'll tell you why in the next post, after we have the calculation for your Toyota.

[RidingOnRailz]

Originally Posted by CapriRacer

First, I was hoping you'd do the same calculation I did: GAWR divided by Load carrying capacity for the original tire size at the specified pressure.

But your last paragraph indicates you didn't understand what
those percentages were.

What that gives you is an indication of how the vehicle manufacturer views what is appropriate tire loading. My opinion is that tires shouldn't be loaded to
more than 85% of their load carrying capacity at the pressures
specified.

I'll tell you why in the next post, after we have the
calculation for your Toyota.

What I did for the Toyota was exactly what I did for the Honda:

GAWR as a percentage of tire max load at max pressure as are indicated on the tire.

In the Corolla’s case:

Front GAWR per tire = front GAWR/2=

1885/2 = 943(rounded up from 942.5)

Max load on her ProContacts is 1,356lbs.

943 is 69% of 1356.

51psi max cold * .69 = 35.4psi. I rounded off to 35

Repeat for the rears, to get 32 & change cold.

Now, if I used, as you suggested, the “rated” pressure - 36psi?,

I’d get a front cold pressure of .69 * 36 = 24.8 or 25psi! WAAAY too low, even for a Corolla. Or, if “rated” pressure was 41psi: .69 * 41 = 28.2psi for the front tires. Again, still below Toyota’s own recommendation of 30psi cold all around.

So please tell me which “rated” cold pressure I should be using in my calculations. Because using the max cold pressure in my calculations still gives me numbers that make *relative* sense: slightly higher than door frame values, plus front/rear split.

Would I use the pressures I derived for long term? Probably not, especially on the lower-profile shod Accord(50 series vs Corolla 65). Perhaps during winter I might do 36 in the front of the Accord, and 34 in the rears, and then run 33 all around in summer. Dispelling the myth of running lower cold pressures in winter - I prefer not to drive in snow or ice on my sidewalls! lol

[CapriRacer]

Originally Posted by RidingOnRailz

....So please tell me which “rated” cold pressure I should be using in my calculations. ......

You shouldn't be doing this calculation at all. You should be using the load table for the original tire size. So let me do that:

2004 Toyota Corolla Front GAWR = 1885# You didn't give me the rear GAWR, but it doesn't matter. The front one is the one that is important.

Original Tire Size and Inflation: You only gave me the pressure = 30 psi, but you did tell me that the replacement tire is a ContiProContact 1356#, 51 psi and it's a 65 series tire.

So I pulled out my copy of Tire Guides, a publication that lists all the placard information for cars available in the US and there are 2 tire sizes listed for a 2004 Toyota Corolla: P185/65R15 86S and P195/65R15 89S - both at 30 psi.

So I went to Tire Rack and they have a list of ContiProContacts and at 1356#, 51 psi they list a 195/65R15 91H SL with those specs.

I am now satisfied I have all the relevant information and there aren't any contradictions.

So the load carrying capacity of a P195/65R15 89 SL is (from the load table as published by tirepressure.com) is 1190#. (You have to interpolate!)

So Toyota specified a 1190# tire for an 1885# Front GAWR - and the math comes out that the tire is loaded to 79% of its capacity at the specified pressure.

Previously I calculated the same statistic for the Honda = 88%. So Toyota is being quite a bit more conservative.

Allow me to go back and calculate what the pressures would be if both Honda and Toyota used the front GAWR without factoring in a safety margin: Honda = 23 psi, Toyota = 17 psi.

Please note: The previous paragraph has a calculation that isn't kosher. The minimum inflation pressure listed on the table is 26 psi, and the values calculated are using an inaccurate extrapolation technique. That was done for illustration purposes only. The formula for the load curve doesn't do that.

Watcha think?

[RidingOnRailz]

Originally Posted by CapriRacer

You shouldn't be doing this calculation at all. You should be using the load table for the original tire size. So let me do that:

2004 Toyota Corolla Front GAWR = 1885# You didn't give me the rear GAWR, but it doesn't matter. The front one is the one that is important.

Original Tire Size and Inflation: You only gave me the pressure = 30 psi, but you did tell me that the replacement tire is a ContiProContact 1356#, 51 psi and it's a 65 series tire.

So I pulled out my copy of Tire Guides, a publication that lists all the placard information for cars available in the US and there are 2 tire sizes listed for a 2004 Toyota Corolla: P185/65R15 86S and P195/65R15 89S - both at 30 psi.

So I went to Tire Rack and they have a list of ContiProContacts and at 1356#, 51 psi they list a 195/65R15 91H SL with those specs.

I am now satisfied I have all the relevant information and there aren't any contradictions.

So the load carrying capacity of a P195/65R15 89 SL is (from the load table as published by tirepressure.com) is 1190#. (You have to interpolate!)

So Toyota specified a 1190# tire for an 1885# Front GAWR - and the math comes out that the tire is loaded to 79% of its capacity at the specified pressure.

Previously I calculated the same statistic for the Honda = 88%. So Toyota is being quite a bit more conservative.

Allow me to go back and calculate what the pressures would be if both Honda and Toyota used the
front GAWR without factoring in a safety margin: Honda = 23 psi, Toyota = 17 psi.

Please note: The previous paragraph has a calculation that isn't kosher. The minimum inflation
pressure listed on the table is 26 psi, and the values calculated are using an inaccurate
extrapolation technique. That was done for illustration purposes only. The formula for
the load curve doesn't do that.

Watcha think?

Whoahh!

Those values you arrived at - 23 & 17psi - resemble reasonable bias ply pressures from back in the day. Is Nixon still in office? Are the Beatles still together? lol!

Can you show the formula you used, based on the GAWRs I listed for my cars?

I think the methodology I used, in posts #1 & #21, yield far more reaaonable results, if one must run cold tire pressures significantly higher than those listed on the vehicle door frame, and more representative of front-to-rear offset due to the strong front end bias in front wheel drive family cars such as Accords, Corollas, Cruzes, Sentras, etc. A typical operator of such cars might just pick pressures out of the blue - 35psi for a 30psi vehicle, or 40psi when 32-33psi is shown on the door frame sticker.

I took the time and did actual calculations based on loads and percentages using the maximum cold pressure published on my tires' sidewalls, as unorthodox as such method might have seemed. And while the calculated pressures are higher than those recommended by the veh. mfgs. themselves, especially for the heavier front ends, they are only reasonably higher, not just arbitrarily jacked up 10psi or such.

[CapriRacer]

At this point, I think I should show you how car manufacturers decide what size/inflation pressure to use. (or at least MY version. I am sure there are a number of different methods.)

Let's use your Toyota.

Front GAWR = 1885#. That 943# per tire.

Good engineering says to use a larger value. You don't want to specify the minimum. So let's use 15% more = 1084#.

Now I want a 15" tire (brake clearance), and I've found that a 65 series tire at 30 psi gives a good compromise between ride and handling. Since this car is destined for the US, I want to use a P metric tire, so off to the load tables I go and:

A P185/65R15 96 SL at 30 psi has a load carrying capacity of 1087#. That's almost perfect. (That's what Tire Guides shows as the base tire.)

Since I want to simplify things, I am going to specify the same tire size and pressure on the rear, even though the GAWR is smaller. As manager of this project, I'll have the ride engineers select rear springs, shocks, and sway bars to get the amount of understeer needed to make the car stable.

In the process of designing the car, the designers think that tire size looks too small, but as manager of the project, I am trying to keep the cost low, so to satisfy them I add an upsize option for the upgraded versions: P195/65R15 - and I keep the 30 psi because the spring rate of a tire is closely matched to the inflation pressure. If I use the same pressure, I don't have to change the springs, shocks, and sway bars. And, of course, I will instruct the ride engineers to design those such that there is acceptable ride and handling from both tires with the same suspension components, paying particular attention to the rear. I don't want too much understeer and oversteer needs to be avoided at all cost. My experience with the ride engineers is that they have done this before, so it shouldn't be a problem this time.

So there you have it: a 2004 Toyota Corolla with 2 tire options at the same pressure.

Notice that the max pressure is nowhere in that sequence of events.

[RidingOnRailz]

CapriRacer wrote: "Since I want to simplify things, I am going to
specify the same tire size and pressure on the rear, even though the
GAWR is smaller. As manager of this project, I'll have the ride engineers
select rear springs, shocks, and sway bars to get the amount of
understeer needed to make the car stable."

Thanks for explaining this process as you did.

So rather than using a percentage of the maximum pressure, you select a tire size that would support its share of the GAWR of the vehicle, plus a fifteen percent safety margin. Got it.

Now, regarding what I quoted above:

Can different thicknesses of spring - not coil diameter, but thickness of the metal section - be used to allow for the same cold tire pressure front and rear?

I measured the thickness of the metal in the front and rear springs in both our Honda and Toyota. The front coils in the Honda were approx. two mm thicker than the rears, and in the Toyota, the front coils were approx one mm thicker than the rears.

So do I take it that both our carmakers already designed their suspensions to perform optimally on equal front and rear tirepressures? Not to mention optimize handling dynamics on unequal weight axles

[CapriRacer]

Not only can the wire diameter affect spring rate, but the number of coils, too!

Amongst my books is a cheat sheet that tells what you to do to fix handling problems with a racecar. Needless to say, inflation pressure is on the list, but so are a bunch of other things. That's where ride engineers earn their money.

So, Yes! Car manufacturers design the spring, shocks, and sway bars so that if they specify the same inflation pressure for both front and rear tires, the vehicle is balanced the way they want. Usually that means quite a bit of understeer, because oversteer is difficult for the average consumer to deal with.

Does that mean that the tires will wear evenly? No - and that is the purpose of rotating tires.

- AND -

Some vehicle manufacturers - I'm looking at you, BMW - have so much camber that it is nearly impossible to get the tires to wear evenly. But their cars handle well!

[RidingOnRailz]

Originally Posted by CapriRacer

Not only can the wire diameter affect spring rate, but the number of coils, too!

Amongst my books is a cheat sheet that tells what you to do to fix handling problems with a racecar. Needless to say, inflation pressure is on the list, but so are a bunch of other things. That's where ride engineers earn their money.

So, Yes! Car manufacturers design the spring, shocks, and sway bars so that if they specify the same inflation pressure for both front and rear tires, the vehicle is balanced the way they want. Usually that means quite a bit of understeer, because oversteer is difficult for the average consumer to deal with.

Does that mean that the tires will wear evenly? No - and that is the purpose of rotating tires.

- AND -

Some vehicle manufacturers - I'm looking at you, BMW - have so much
camber that it is nearly impossible to get the tires to wear evenly.
But their cars handle well!

Speaking of BMW - and Mercedes Benz, and some models from Subaru:

Different front and rear cold tire pressures are specified on many models of the above makes. Would you speculate that the same coil thickness is used on both axles, and instead the vehicle mfg. controls spring rate, handling balance via the tire pressure instead?

More generally, I will say that both of my family sedans, with all four cold tire pressures set at, or no more than 2psi above, the recommendation on the door frames, handle the most predictably and the most securely. And probably would handle the same under sudden/emergency maneuvers

I guess I can say my period of cold tire pressure calculation and experimentation with, are drawing to a close. I hope this has been education dialogue for others wondering what tire pressures to run on their vehicles

[CapriRacer]

I can not imagine why a vehicle manufacturer would deliberately size the front and rear springs the same as even cost savings would be pretty minimal.

[RidingOnRailz]

Originally Posted by CapriRacer

I can not imagine why a vehicle manufacturer would deliberately size the front
and rear springs the same as even cost savings would be pretty minimal.

Cost didn't even cross my mind.

Most BMWs, for instance, have been either rear or all-wheel drive, hence, an intrinsically more balanced package in the first place. Never more than a 52/48 weight split front-to-rear or vice versa, so different gauge coils not as critical. More focus on tire pressures, and of course things like sway bars, etc.

In a front-drive econo-scenario, spring gauge assumes a more prominent role in balancing fore & aft handling and stability attributes. So in econo-box design, front and rear suspension rates are varied so as to allow the same cold tire pressures to be used at all four corners - easy enough for typical consumers of such cars to set pressures. In a well-balanced 'ultimate driving machine', similar suspension compliance is designed into all four corners, while a lower cold tire pressure is specified either for just the front or rear axle, and the door pillar of such an enthusiast-oriented make is decaled accordingly