Over inflation

[Mickey_M]

Razl

quote:

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However, 35 PSI does two things. It reduces heat build-up and increases the load capacity fo the tire.

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Actually the load capacity of the tire is a function of its size and design.

What it does do is allow you to carry the rated load without heat buildup.

The heat comes from contact with the road (friction), which increases with speed, and from the tire itself flexing. Increasing the pressure reduces the flexing.

 

[1sttruck]

All of the car tires (205-70/14 or 205-65/15) for either sedan typically had a 35 psi max, so for years I would run them at 32 to 33 psi. This is pretty much the door sticker recommendation on either car. The last couple of years I started running them at 35 psi with no apparent problems. Last year we got new tires for the older car, and since the tires had a 44 psi max I ran them at 38 psi. I still use that in the summer, but during the winter I'll run them at 35 psi as there seems to be a bit better traction with the lower setting.

 

[Mitch Alsup]

quote:

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If I recollect those correctly, you measure the tread temperature across the width. When you've got an even contact patch, you'll get the same temperature reading all the way across. It's the very fast version of my approach, which measures the tire wear over some months.

Who makes them and what do they sell for?

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You are correct in the general operation of these devices. However, you are looking for a linear progressing from the outside edge of the tire to the inside edge of the tire (about 10dF for street operations 20dF for track operation). The outside of the tires is cooled better by the air rushing by.

I got mine through LongAcre, however ost race catalog shops have these things. You can find these as low as $99, they last forever, and can save hundreds of dollars in tire wear.

The "proper" tire pressure results in a linear progression of tire temperature from the outside edge (cooler) towards the inside edge (hotter)with the center as an average of the inside and outsde temperatures.

If the outer edge is hotter, the vehicle needs more (negative) camber. If the inside is more than 20dF hotter, the car needs less camber.

 

[PandaBear]

The right way to set pressure, from what I heard, is to put chalks on tire and do donuts. Start with high pressure, then gradually lower until the chalk mark worn out completely over the trend pattern.

You will get maximum tire patch contact in cornering this way.

 

[CapriRacer]

A couple of comments:

"Actually the load capacity of the tire is a function of its size and design."

AND inflation pressure. There is a load curve for every tire size and it depends on inflation pressure. What is written on the sidewall is the maximum load from that curve. Use a lower inflation pressure and the load capacity is reduced. BTW, the pressure written on the sidewall is not always the pressure where the maximum load capacity occurs. Frequently the pressure written on the sidewall is the maximum safe pressure, so I caution folks who use what is written on the sidewall as a guide as that figure has more to do with how a regulation is interpreted than how the tire is constructed.

The problem with using chalk, or wet footprint tracks or pyrometers to set tire pressures is that it is only applicable for the way the condition of the vehicle at the moment the test is conducted (load, suspension settings, etc) and only deals with the footprint. Change the conditions and the test results will be different.

But there is another aspect that is completely discounted when using these methods - durability. Tire manufacturers design their tires to use the load curve (as mentioned above) to test for durability. So using a pressure that is too low may cause the tire to fail due to overloading. Since there is so much more to setting pressures for durability than just the static load, the vehicle manufacturers are required by law to place their pressure specification on the placard. It would be foolish to ignore this as they've spent a lot of time and effort testing the vehicle at this pressure.

Another consideration is that most tire wear occurs in the cornering mode, so any test that doesn't look at cornering conditions has a built-in deficiency.

Besides, there are a number of items that can affect the results of these tests that are independent of durability, such as rim width, camber, etc. and you may get results that compromise the durability.

My advice would be that if you want to conduct tests to set the inflation pressure - fine, just use the placard pressure as a lower limit. The results of overloaded tire failures can be pretty tragic.

 

[Mickey_M]

"'Actually the load capacity of the tire is a function of its size and design.'"

"AND inflation pressure."

That's a common misperception. A tire's load rating, or "Max Load," indicates the individual tire's safe maximum load-carrying capacity when inflated to its recommended pressure.

It's true that with a given load at a given speed the higher the inflation pressure, the less heat buildup due to tire flex takes place. But the actual load capacity of the tire, which is on the sidewall these days as a number and letter, is a function of size and design.

Here's a discussion of load ratings and pressure, although from an agricultural vehicle perspective, that lays out how load capacity, pressure, and use relate:

http://ianrpubs.unl.edu/farmpower/nf300.htm


.

 

[CapriRacer]

Perhaps Mickey and I are saying the same thing, but in different ways.

Yes, each tire size has a maximum load capacity and the size determines that. This maximum load capacity occurs that some inflation pressure. Use less than than that pressure and the actual load carrying capacity is reduced - something lower than what is written on the sidewall.

For example, a P215/70R15 has a maximum load capacity of 1620 # at 35 psi. If the usage inflation pressure is 32 psi, then the load capacity of the tire is 1554 #.

I wish I could point to a web page to illustrate this, but this is directly out of the Tire and Rim Association Yearbook and they don't publish this stuff on a webpage. But if you take a look at Figure 2 on the link that was provided, you'll find the same thing - a graph of load capacity vs inflation for various tire sizes.

But my main point of the earlier post was to use the placard inflation as a lower limit.

Hope this helps.

 

[slugsgomoo]

because of the variability in pressure between hot & cold, I find that setting the tire at the same pressure cold can produce different results when hot, depending on quite a few factors. As such, I don't set my tires cold anymore.

I know for a fact that I want the advans to run about 43.5psi hot, when i'm going to be agressive, and about 39lbs hot when i'm not going nutty.

Of course, that's a little bit more involved process than setting up a daily driver, and I don't bother to be that anal with my snow tires & whatnot.

Like many here, I go for the optimum tire footprint & wear, and have never trusted vehicle manufacturer specs. If people had inflated their firestones to the proper inflation, instead of ford's 15psi underinflated pressure recommendation, perhaps we would have had a lot fewer roll over accidents.

 

[sjlee]

Remember, the tire pressure that's printed on the tire is the maximum COLD tire pressure, not the recommended pressure by the tire manufacturer.

I don't think you'll have any problems going over the tire pressure that's listed on the driver's door by a few PSI.

 

[steveh]

My question is will running a higher psi cause increased wear in the suspension parts?

 

[tuong]

negative. is the answer to your question steveh