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Old 06-01-2008, 06:37 AM   #1
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Post Tire pressure myths, facts, observations, and improvements (canned post)

Note: I posted this in another thread, but I realized it makes more sense to just post it here and link to it when it's needed in other threads. I plan to edit this as time goes on and I refine it.

Tire Air Pressure Myths and Facts
A canned post by theholycow
Originally posted at
http://www.gassavers.org/showthread.php?t=7712
Please include the link if you post/distribute this elsewhere.

Edit: Some decent numbers can be found at http://www.metrompg.com/posts/tire-p...resistance.htm

Increased tire pressure is a cure-all for problems, and generally an improvement even when there are not problems. I have been driving with greatly increased tire pressure for hundreds of thousands of miles, and have not suffered ANY consequences, though other drivers with other vehicles could. I have only benefitted from it.

There are limits, though. You should never significantly exceed the tire's maximum pressure rating, which is stamped on the side of the tire (often next to its maximum load rating, which requires the maximum pressure). If you add pressure and find that your handling has suffered or your ride is terribly harsh, you should reduce pressure until that issue is gone. Those things are very important limits. If you exceed them, you could possibly cause a tire failure (though I doubt it); however, real consequences may include center wear, possible liability in an accident, and sketchy handling that could cause an accident.

Within those limits, you can find the following advantages:
- Decreased rolling resistance which means increased gas mileage
- Better handling due to reduced sideways curling of the tire under the lateral load of a hard curve
- Better handling due to quicker response from less tire flex
- Better wear, unless you normally wear tires perfectly evenly and always get more than their treadwear warranty out of them

If that's all so great, why do car manufacturers not recommend the full pressure of the tire? It's because they recommend the lowest pressure that their legal department approves based on industry-standard load-inflation charts. For any amount of load on a tire, there's a minimum safe inflation which provides the softest ride. To sell cars, they want the ride as soft and compliant as possible when you test-drive it. More pressure isn't bad, it's just not traditional to try to sell a car that way.

Modern automotive tubeless radial tires are NOT subject to the same types of failure as the bicycle tires you remember popping as a child. They won't blow off the rim (unless you use way, way more than their maximum rated pressure). They won't pop when you hit a bump. They won't explode in your face. They almost always fail from underinflation (which causes overheating) or a break in the sidewall caused by an impact (which is less likely to happen if you have more pressure preventing the bump from squeezing the sidewall against the rim).

http://www.tirerack.com/tires/tirete...e.jsp?techid=1 has a section cautioning against overinflation which includes this:
Quote:
However, higher inflation pressures reduce rolling resistance slightly and typically provide a slight improvement in steering response and cornering stability. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures.
This site for police officers has an article about tire pressure:
http://www.officer.com/article/artic...on=19&id=27281
Here's selected pieces:
Quote:
Higher pressure results in better performance, decreased tire wear, and it lessens your chance of hydroplaning at a given speed. [...]

Many agencies maintain tire pressure at 35 psi since this is what is listed in the owner's manual and on the door placard. [...] You are not looking for a soft and cushy ride, you want performance. [...]

The tires will not balloon out creating a peak in the center portion of the tread when tire pressure is above 35 psi. There is a steel belt that prevents this from happening. Also, you are not overstressing the tire with higher pressure, and the tire will not be forced off the rim with higher pressure. [...]

If you were able to watch a tire as it travels across the ground at high speed, you would see that it deflects to one side during cornering. The faster you are going through a corner, the more tire deflection you get. As the tire deflects over onto the sidewall, you get less traction and more of a tendency to understeer or oversteer. [...] Higher pressure keeps the tire from deflecting onto the sidewall as much, which keeps more of the treaded portion on the road. A good demonstration for EVOC instructors is to have students drive a high-speed course in a vehicle with 32 to 35 psi. Then have them run the same course with 44 to 50 psi in the tires. The student will experience a marked difference in performance. [...]

Tire manufactures and the Association of Law Enforcement Emergency Response Trainers International (ALERT) have shown that tires have more of a tendency to hydroplane when pressure is low. [...]

In 1999 the San Jose Police Department realized a significant cost savings by increasing the pressure in the training fleet to 50 psi. They soon followed up by increasing the pressure in the patrol fleet to 44 psi. [...]
Two vehicles on which I've had much (but not all) of the aforementioned hundreds of thousands of miles experience:

I had a 1997 Pontiac Grand Am SE V6 with P205/55R16 tires. I bought some Bridgestone "Turanza T" tires, which lasted about 70,000 miles (that was less than 3 years for me) before the sidewalls were cracked and the tread was worn at the corners. I ran them at the pressure printed on the car's door post, which may have been 30 or 35 psi. I replaced them with the same, but inflated the new ones to the tire's maximum pressure, 44psi. Handling went from soft and sloppy to responsive and decent. Ride did not suffer. They wore perfectly evenly and had 80,000 miles on them when I sold the car -- and they were still good for more miles.

On my 2002 GMC Sierra 1500, I keep my tires inflated to their full 80 psi rating. Sometimes I run the rear tires a little lower, because the handling gets sketchy with the bed empty and the road surface slick. Other than that, I get long, even wear and decent handling.

6/18/2008: New data makes me question my tire width theories, but supports higher inflation pressure. A post by user slogfilet with a very interesting link shows that higher inflation does NOT reduce contact patch. I can't say for sure that the new data is correct, or even analyzed correctly, nor can I say if it's wrong. I hope to do some of my own testing.

It would also suggest that the theories about how increased inflation helps FE may be wrong, but that only affects the explanation behind the observations; hard data from users on this forum shows that increased inflation helps FE, and that hard data is undeniable.
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Old 06-02-2008, 09:44 AM   #2
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Here's some additional Tirerack articles and what they have to say. Tirerack barely addresses overinflation, but continually cautions against underinflation.
Air Pressure vs. Wet Performance
Quote:
Most drivers realize that tire load capacity is determined by tire size and inflation pressure. Larger tires and higher inflation pressures provide more load capacity, while smaller tires and lower tire pressures provide less.
Air Pressure, Temperature Fluctuations
Quote:
Your tires support the weight of your vehicle, right? Well they don't! It's the air pressure inside them that actually supports the weight.
Air Pressure/Load Adjustment for High Speed Driving
Quote:
Because of the weight they bear, pneumatic tires' sidewalls bulge and their treads flatten as they roll into contact with the road. This results in dimensional difference between the tire's "unloaded" radius (i.e., between the center of the axle and the top of the tire) and its "loaded" radius (between the center of the axle and the road). The engineer's call the difference between the two radii "deflection." Increasing vehicle speed will cause the tires to deflect quicker and increasing vehicle load will cause the tires to deflect farther (if tire pressure isn't increased).
Note that Tirerack reports the following about tire usage on the Autobahn:
Quote:
In order to accommodate higher speeds, the tire size and inflation pressure recommendations are tuned beyond what is branded on the tire's sidewalls.
That means that tires are safe when overinflated even beyond their stamped maximum, though I still don't support exceeding that number in the US for insurance/liability reasons.

Edit 2008-11-02: Found this large compilation of articles stressing the importance of never running underinflated for the trucking industry: http://www.retread.org/Inflation
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Old 06-09-2008, 05:47 PM   #3
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Wow I have been running my car tires at 32 Psi for years! but on my mountain bike I have them up to 44 to 50psi because the bike is faster!

Why have I not thought of this before! I will change my car tires this month as mines have perished, I will totally inflate them to the maximum levels on the side walls, thanks man.
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Old 06-09-2008, 05:55 PM   #4
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Just be cautious until you're sure the car still handles well with the tires at that pressure. Since you're going to be buying new tires, take a look at the other tire thread I posted the same day about tire width.
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Old 06-10-2008, 08:34 PM   #5
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The single most significant change to my mileage (so far) has been pumping the pressure up past the manufacturer's suggestion. The fact is that my tires are so cheap that the savings in gasoline far outweigh the six months to a year that I lose in tread life.

That said, I wouldn't recommend this if you're rolling on $200 Pirellis.

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Old 06-11-2008, 06:17 AM   #6
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I suppose every car and driver is different, but increased pressure has always increased tread life for me.
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Old 06-11-2008, 08:44 AM   #7
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Is it true that race cars and touring cars run high Psi in there tires?

Does this apply to trucks and buses?
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Old 06-11-2008, 08:59 AM   #8
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Well, in the first post in this thread I did quote a bit about autocross racers using higher pressure. Additionally, NASCAR teams (and I'm sure those in other motorsports too) are constantly adjusting their pressure to adjust traction, though I don't know exactly what pressure range they use.

It applies to trucks and buses (and RVs and trailers) so much that load-inflation charts for those tires are much easier to find, and many include text explaining that for higher speed all the pressures must be adjusted up.

I don't know if tire rolling resistance is as big a factor for trucks and buses, so I don't know if the FE improvement will be noticable. I do know that the wear advantages are just as useful, at least in my experience with trucks up to 15,000 pounds.
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Old 06-12-2008, 06:53 PM   #9
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I put my tire pressure up to 44Psi at the front and 40 at the back and the car felt stiffer but my goodness it can roll now! I can tell its faster which sounds stupid I know, the manual recommended 32Psi all round but I realised after reading THC first post that this was just legal cover, not what is best.


The steering feels a lighter but not dangerous, I would say it feels more direct on the road.

This is a great tip and I wish I knew about it years ago, thanks ever so much man!
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Old 06-13-2008, 06:12 AM   #10
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Glad to hear of your good results!
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