[Rolen]

I have a question about the running the higher tire pressures and I am curious about other peoples findings.

I am running at 44/42 with the tire cold (~75F ambient). I did notice the other day when I went to check the pressures that my left front tire, which was being exposed to the sun for about an hour, was at 46psi. The other tires were fine since they were still in the shade. I am using a very accurate pressure gauge so my concern wasn't from inaccuracy. I decided to check the pressures when the tire were warm. I drove about 100 miles in ~95F heat then parked and check them. The pressures now were 54 in the fronts. My tire are the originals and they have a max of 44psi. I am now concerned of what would happen if you were to have a blowout at those pressures. My guess is that a new fender would be needed, if not more.

Does anybody have any similar findings.

 

[jeff_kash]

I have the same experience, so I set the psi in the tires lower to accommodate the higher temps in the afternoon. I use 40 and 38. Seems to be safer.

 

[Bob Allen]

Tom and Ray Magliozzi in their Friday newscolumn addressed this question . A lady questioned her oil change mechanic who upped the tire pressure to about 40 or so; she wanted it to be left at the recommended settings posted on the inside of the driver's door. The mechanic said it was standard practice at his facility. Tom and Ray agree with her.
According to the brothers Magliozzi (Klick and Klack, the Tappit Brothers on NPR), the maximum posted limit of the tire, printed on the tire itself, is an absolute. Inflating a tire to a few pounds less than that is dangerous and will cause undue wear on the tires. The reason it's dangerous is because the tire cannot grip the road adequately enough to provide the necessary traction, and the additional pressure causes the car to bounce more.
Some folks on this and other chatlines are inflating their tires to 40 and above (the max on the original equipent tires is 44 psi) in order to get better gas mileage in the belief that Toyota has purposely understated the recommended pressure in order to be more conservative. I don't know if the latter is true or not, but I would be surprised if Toyota would purposely understate the tire pressure if better mileage could be SAFELY obtained at a higher pressure.
Tom and Ray also mentioned your concern about heat. 44psi is the cold temp max inflation, and tires are always inflated and measured when cold regardless of the desired psi, so the tire is designed to safely accomodate the higher pressure generated by heat, BUT, the above caveats about road handling and tire wear do apply.
If you manage to increase your mileage by 2mpg, say, from 45 to 47 mpg, you would save about a tenth of a gallon of gas over a 100 mile trip.
Not enough of a savings for me to justify jacking pressures up.
That said, it's probably ok to increase the pressure by one or two pounds to make the ride stiffer. The recommended pressures for the original tires are 35psi front, and 33 psi rear. Increasing that to, say, 37front and 35rear would probably not be too much.

 

[patrickg]

I used to virtually religiously follow the brothers M. They usually get it right (not always.)

The maximum inflation pressures as you state are intended to be measured "cold", not after a drive of more than a few blocks. Normal environmental conditions, including ambient temps and increase of tire temp due to use, hot roads, sunshine, etc are factored in. Increasing the cold inflation pressure a bit, within the limits on the sidewall, is not inherently dangerous or destructive to the tire. In some situations a couple PSI can actually lower the "in-use" temps by reducing tire flexure.

A fair guess at the "correct" inflation pressure can be had when other reliable info isn't available. Note the max load the tire is rated to carry at its max inflation. Also note the actual weight carried by that tire. It is a straight forward ratio and proportion problem. If you are carrying 80% of the max load then run 80% of the max PSI. If you really want tire pressure RIGHT you will adjust your pressure to accomodat changing loading. Driver only vs a FULL car requires different pressures to be "correct."

Within the rated cold inflation pressure, it is better to be a pound or two over than a pound or two under as underinflation leads to premature wear and tire damage must faster than overinflation. Underinflation and over inflation both cause uneven wear but underinflation generates excessive heat as well.

For those among us who are not innumerate:

The PSI of each tire multiplied times the area of its footprint in square inches and then summed over all 4 tires is equal to the weight of the car in pounds. With the tires likely to be used on this car there is virtually no appreciable contribution to supporting the car provided by sidewall strength/stiffness.

Many professional drivers exceed the auto manufacturer's recommended inflation by a few PSI. I know of NONE who underinflate except in certain drastic short term poor traction situations such as getting unstuck in sand or mud or...

Pat

 

[AgentElrond]

Doh! Just got back from a round trip to Epcot; was already gone this morning when this topic went up. Next trip, will have to check what effect such a long drive (approx. 1 hour) has on my tire pressure and report back.

 

[Bob Allen]

For those among us who are not innumerate:

The PSI of each tire multiplied times the area of its footprint in square inches and then summed over all 4 tires is equal to the weight of the car in pounds. With the tires likely to be used on this car there is virtually no appreciable contribution to supporting the car provided by sidewall strength/stiffness.

Many professional drivers exceed the auto manufacturer's recommended inflation by a few PSI. I know of NONE who underinflate except in certain drastic short term poor traction situations such as getting unstuck in sand or mud or...

Pat

Patrick: Your "innumeration" doesn't make sense. I measured the footprint of my car's tires: each tire is 6 inches wide and the footprint is approximately 6 inches front to back, for a total footprint of 36 sq in.
OK? So now I multiply that by the psi in each tire: 36 psi for the front tires and 33 psi for the rears. That's 36psi times 36sqin for a total of 1296 for each front tire, or front tire total of 2592. The rear total is 2376. The total for all four tires is 4968. You stated that this should be equal to the weight of the car, but actually, it's 171 percent of the stated gross weight of 2900lbs. Is this, instead, the maximum permissible weight the tires could support?
I might have measured the footprint incorrectly, but I think it would have to be substantially smaller than my numbers for the total to equal the weight of the Prius.
Bob

 

[CaPriusLover]

I have never adjusted my tire preasure, but I have checked it. 40 psi all around. This is how Toyota delivered it to me.

 

[jeff_kash]

It is my understanding, that the front tires should be 2 psi higher than the rear, no matter what psi you run.

 

[patrickg]

The total for all four tires is 4968. You stated that this should be equal to the weight of the car, but actually, it's 171 percent of the stated gross weight of 2900lbs. Is this, instead, the maximum permissible weight the tires could support?
I might have measured the footprint incorrectly, but I think it would have to be substantially smaller than my numbers for the total to equal the weight of the Prius.
Bob

Bob, Yes you most likely did make a considerable error in measuring the footprints. The algorythm is NOT for max permissable weight the tires could support which is easily found by reading the value written on the sidewall and multiplying by 4. It is simple physics and not just automobile related. The area in contact with the pavement times the pressure in the tire should be almost exactly the weight supported by that tire. The contribution from the sidewall with tires of this kind is minimal. If in fact the sidewalls were supporting a significant porton of the load then the contact patch would be smaller than predicted with the algorythm I gave.

If the pressure in PSI times the area of the contact patch (ignoring the sidewalls minor assistance) isn't equal to the load on that wheel then the "system" isn't in equlibrium and the car would be in vertical motion until they were equal.

Just sitting there parked, the load on the tire has to be the same as the "support force" or the car would be moving up or down depending on which force was greater, load or support force. If the car isn't moving up or down then the two forces are equal. The only way for the forces to be equal (again ignoring the small contribution from the sidewall) is for the tire pressure in PSI times the area of the contact patch to equal the load.

Like Scotty put it, "Captain, Ya cannow violate the laws of physics!"

Given my great confidence in your tire pressure measuring and in your multiplication (I don't think you to be innumerate), I'm thinking the most likely error to be in contact patch size.

Pat

 

[CaPriusLover]

Pat, I didn't understand your equation or what it meant at first, but think I get it now. If the tire preasure goes down, the footprint gets bigger (from deflation of the tire). As the preasure goes up, the footpring gets smaller. When these changes occure, the result of the equation would remain the same: the amount of support you are providing to "hold" the car up.

Am I close?

 

[patrickg]

Close? NOPE!

You are dead on the money, buddy! Exactly correct!

This topic used to always get laughs from those who "got it" at the expense of those who didn't or who fell into the HUGE GROUP (with a bad idea) who think the correct tire pressure is the one listed on the sidewall (max cold inflation pressure.)

There is a way to make the max allowed pressure the correct pressure... just add weight to the car till the tires are loaded to their max allowed load. Your mileage may not only varry but SUFFER!!

A close approximation is that you'd inflate the tires to the same percentage of max inflation as the percent of max load the tire is supporting.

Pat

 

[Daniel]

CaPriusLover: If they delivered your car with 40 psi all around, then the service dept at your dealer goofed.

Toyota says (and all the experts on the discussion boards agree) that the front tires must have 2 psi more than the rear. 35/33 is what Toyota recommends. A large crowd advocates 42/40, with some going as high as 44/42, some styaing with 35/33, and some in between. But regardless, 2 psi more in front. Has to do with handling.

As for measuring the tire's footprint, seems like a tricky matter. You'd have to jack up the car, sprinkle the ground with something, chalk perhaps, lower the car, jack it up again, and measure.

But here's a question:

If the tire had insignificant thickness the physics would be simple enough. But I wonder if the real-world thickness of the tire affects the experiment? I.e., is there a portion of the footprint, as measured in the chalk, which is not supporting much weight, due to the thickness of the tire, therefore giving a larger footprint than the formula predicts? Think of gluing a large piece of cardboard to the bottom of your shoe. You now have a big footprint, but most of it is not actually supporting any weight.

 

[patrickg]

Daniel, I won't belabor the point but if the tire stuck out beyond its support then that part would as you say not support the car and it would not wear the same either. Most tires when properly inflated wear relatively equally across the footprint. The distrubution of support may not be perfectly linear or equal but in aggregate has to sum to the load on that tire or that corner of the car would move up (or down) till it did.

Sorry, my friend, but I can't agree with your analogy. It just isn't applicable.



Pat

 

[Daniel]

I was not making an assertion, just asking a question. Your comment about tread wear (that it wears evenly across the tire, given proper inflation) answers my question. Thank you.

(It still seems to me as though actually measuring the footprint would be more bother than it's worth. -- Though if you had 4 jacks, some chalk, 4 scales with adequate capacity, and a ruler, you wouldn't actually need a tire pressure gauge!)

 

[patrickg]

Daniel, Peace man, we are in agreement. You are correct. Mostly the Hooke's law stuff relating to the tire structure turns out to be a minor effect with typical passenger car tires and certainly is so with our low rolling resistance ones.

Although theoretically correct and doable with some effort it is unlikely many tire gauges will be replaced due to this exchange. I just thought a few folks would be interested in this line of thought.

Actually, a neat way to determine footprint is to drive onto a strong transparent view port and observe it from the underside. Failing that, lowering the tire onto flour or similar would work. You don't need four jacks as you can do the tires ad seriatum. If the inflation is the same left to right and the car isn't loaded lopsided you could probably do just fine measuring one front and one rear tire and assuming that both fronts are the same as each other and likewise in the rear.

Pat

 

[vincent1449p]

Another way to measure the "footprint" is by shoving pieces of cardboard against the base of each wheel.

http://www.exploratorium.edu/snacks/tired_weight/

Vincent

 

[patrickg]

Vincent, A most excellent link. I was blissfully unaware of that site. Yet another way to get the width of the tire's contact patch is to drive over a piece of cardboard and note the tread marks. I suppose you could drive onto a piece of cardboard that is lying on flat cement (at a moderate speed) and then "LOCK UP" the brakes. This would abrade the cardboard over the area of the contact patch.

Thanks again,

Pat

 

[CaPriusLover]

Vincent,
Fantastic web site!! Thanks for sharing it.