tire size & pressure relationships
 

Because of some earlier threads, I'm interested in building a table of what happens to tire size as a result of heat and pressure. If you want to help, I would like interested members to submit their info (using my own tires as an example - other parameters of interest welcomed too).

I have done this before, but didn't bother to keep the numbers....anyway....first I adjusted the pressure front to back to get the same static rolling radius. this was done by measuring from floor of garage to top edge of the wheel (not tire) at a cold setting (can't use top of tire as it expands with pressure changes, yet is not a function of rolling radius). Next, need to measure the same after driving and thus heating the tires.

As a follow-on, I intend to check the physical (vice static) rolling radius by marking the tires, and seeing how that matches up. Finally, I want to see what the difference is at the limits of turning radius. Why am I interested? I expect most of us have heeded the Subaru warning not to mix tires sizes front to back on AWD to avoid unduly stressing the transfer clutches/transaxle. I have interpreted that to really mean we should be matching the number of times a tire rotates (turns excepted??).

Opposing opinions solicited and most welcomed...I'd hate to be doing this then have a DUH! moment.

tire brand/type: Fulda Extremo, 225x50x16, Z-rated

static front cold: 20-1/2" @ 40 PSI
static rear cold: 20-1/2" @ 28 PSI

static front hot: 20-5/8" @ 42.5PSI
static rear hot: 20-9/16" @ 29PSI

10 rotations front = XX rotations rear (cold)
10 rotations front = XX rotations rear (hot)
@ Max turning, 10 rotations front, inner=XX rotations front, outer = YY rotations, rear (don't intend hot & cold repetitions)

edit: added first round of hot measurements. the PSI rating is what the tire pressure measures hot - not that I added any air.

Why is the cold inflation pressure 40/28 psig, f/r.
The recomended cold tire pressure is 33 psig/29 psig, f/r' which is 4 psi different. I can see running 5-6 psi difference, but not 12 psi.

Since the rear tires are slightly under inflated they will heat up more than the fronts for the same mileage.
Tires are elastomeric. They flex, change shape, expand and contract. The only way I can see that you can approximate hot diameters is to rig up some sort of very large calipers and measure the horiz. outside diameter (OD) of the tires acrross the center of the tire when cold and then when hot. I don't think that you can approximate meaningful expansion data from pressures, because the sidewalls bulge when hot. Good Luck.

40/28 seems horendously excessive....

I agree that if you're overinflating the fronts, the rears need to be overinflated to the same degree.

LarryIII & FifthElement.....
 

and of course other forum members...

I get compulsive about things from time to time. I began to wonder about the potential for difference between the OEM tires, with certain characteristics, and other brands. With what little I've done I feel pretty confident in saying that the aftermarket brand I have doesn't match well with the OEM recommendations.

For my tire brand/model, check the numbers I edited in above. The hot tire pressure and size numbers indicate I need a bit more pressure in the rear (or less in front), but I don't believe it will come anywhere near the OEM recommendations. I intend to go down to 10 PSI diff front to back and check again when hot.

That's why I'm asking others to measure. Is it just some brands of tires, or ???? Maybe you'll find out some surprising info on your own tires.

The 40f/28r tire pressure that Lee is using is almost the same ratio as the 60f/40r weight distribution of the car, so it makes sense that the front and rear tires should have the same loaded radius when at these pressures. I don't think his rear tires are going to run any hotter than the front tires.

I will be curious to see how the rolling circumfrence of Lee's front and rear tires compares at these pressures. Lee, I think you're going to need to roll the wheels through much more than 10 revolutions if you want to measure the difference in front and rear rolling circumfrence with any sort of accuracy. I think you're going to have to drive the car for about 1/2 mile.

I'm interested in seeing where this will go. Keep us posted.

Re: LarryIII & FifthElement.....
 

I don't understand what you're trying to say.

Mychailo,

I dont't think 1/2 mile is enough driving distance to heat up the tires I suggest 3-5 miles. I say this because the Shell station where I get air (it's still free) is only 1/2 mile from my house. I measured the same tire pressure at this station as I did in my garage after the car has been sitting all night.

The way I measure is to check the pressure for each tire early in the morning in my garage and write down the reading. The first thing that I do at the gas station is measure each tire again and write it down. I check for psi differences in each tire. Then I fill to the correct pressure making sure to adjust for the differences.

For example: If the right front tire measures 29 psig at home and 33 psig at the gas station air pump I still put in 4 psig and measure it at 37 psig when I'm done.

I don't have to do this anymore since I switched to the Shell station with the free air.

Hi Larry,

I agree with you on this. I was referring to the distance that Lee will probably need to drive his car so that he can see the difference (if any) in the rolling circumference between the front and rear tires. This is one of the things he's intending to do.

Re: Re: LarryIII & FifthElement.....
 

I'm sorry, being a tech geek, English has always been a shortcoming of mine. I'll try again.

The recommendation from Subaru to have the same size/type/condition tires front to back is (IMHO) because we need to minimize the difference in the amount of tire revolutions per mile traveled. Said another way, if you drove down a straight road for a mile, and the front tire rotated say 1000 times in that mile, you would want the rear tire to rotate 1000 times as well to minimize slipping and wear in the transfer case.

So, what I'm trying to find out is how various brands of tires react to temperature and pressure differences. If I put the OEM pressure (33/29) in my particular brand/type tires they have almost 1/4" difference in rolling radius. Which is equivalent to having two new and two 80% worn tires mounted. Is this enough to do damage in the long term...I have no idea, but it doesn't seem good to me.

BTW, Rolling radius is the critical parameter to measure if you want to see how the tire sizes compare while the car is actually in operation. I have done it statically (car at rest) and want to confirm this by actually driving the car too... as the EXACT point to determine rolling radius is not easily defined in the real world.

I understood. BTW, my "hot" temperature measurement is after driving about 35 miles at speeds of 50 to 65mph down hi-way A1A from work to home.

And now that you mention it, I can see that 10 revolutions wouldn't be enough. Fortunately, FLA has miles and miles of nearly arrow straight roads so turns won't affect the test.

Lee

How are you going to count the revolutions?

Lee,

How are you measuing rolling radius? In fact, how did you measure to get the 1/4" difference?

first Q: not sure as I initially thought I just might have an observer, but mbtoloczko is probably right, I would have to cover significant distance to check the diiference. If needed I would estimate from tire size and distance covered. I doubt I could get an observer to count/walk alongside for more than a block :)

second Q: from the mid point of the tire on my garage floor to the mid point of my hub cap. I suspect (but don't really have the foggiest, and haven't thought too much about it yet) the actual place to measure exact rolling radius might be the point where the tire is first fully in contact with the road - that's why I want to check tire differences while actually driving.

Lee,

I respectfully want to disagree with your method of computing the rolling radius. I think that if you measure from the hub to the front of the flat that the tire makes with your garage floor, you will get erroneous readings. The weight on th efront tires is much grater than that on the rear tires, so the flat on the front tires will be much longer than the one on rear tires. Also the flat on the front tire is not even about the hub. It is longer to the rear of the front axle centerline than it is in front of the axle CL. Conversersly, the flat on the rear tire is longer in front of the rear axle CL. This is because the CG of the car is between the axles. {By flat, I mean the length of the tire contact patch}

IMHO; You should take the diameter measurement horizontally across the tire from edge of thread to edge of thread. Take it across the hub. Do this on all 4 tires. Add the 2 measurements for the fronts and divide by 4(2 diameters divided by 4 = 1 radius). Do the same for the rears.
I think this will cancel out any anomalies in the tires and side to side in your car. I would also drive to some point take the measurements & write them down. Then drive back tou your garage and take a second set of measurements.
This is a good way to make sure that your measurments agree and you have'nt messed up. You can average the two readings.
Good Luck and keep us posted.