How much tire air pressure is still safe?

[AMX]

With increasing tire pressure, how much traction loss is there? For example, the sticker on the door jam of my car says 30psi for the tires, but I have them around 38psi. They are 44psi max tires. Is there any significant amount of traction loss with doing this? For example, if it was raining hard, and I had to slam on my brakes.

[EH3]

the general consensus is that traction is usually improved with higher pressures

this article supports it
http://www.officer.com/article/artic...on=19&id=27281

[GasSavers_RoadWarrior]

To a point, I've got great traction at 40 psi and it's rain and hydroplaning safe, but it goes to crap much beyond that. These are tires marked 35 psi. So I'd say that once you go beyond sidewall recommendation, test carefully.

[1993CivicVX]

my 35psi front tires are at 60 and it is veritably unsafe to run them that high. I just drove back on I-90 in the rain and I can tell the cornering is just very unsafe. Accelerating out of the toll booth the front tires spun. And remember, I'm trying to drive to save gas, not spin my tires! I think I am going to bring them down to 50. They have no traction at 60. Even at 50 it's unsafe. Your cornering traction will be compromised too, not just braking, but braking will be compromised as well. I'd say the loss of handling would be small, maybe even negligible, at 45PSI. But you should find an empty parking lot on a rainy day and test different PSI settings--starting at 60 and incrementally letting the air out.

@EH3: but I think that's referring to dry surfaces only. And not sure if there is a cut off point for that. Does that still hold true at 100psi?

[kamesama980]

the manufacturers number on the sidewall is COLD inflation (have to account for long summer highway runs when the asphault will burn skin) and accounts for a LOT of abuse (potholes, debris, crazy people overinflating them) which is called safety margin. given the number of people on this forum who have technically overinflated tires and the number who have had problems because of it, I'd say it's not exactly russian roulette odds.

[molinee]

Thanks to all for great comments. I just checked my 2001 Jeep Cherokee and see they have Michelin LTX M/S P245/70R16 Radial X, and they have less than 8K on them. The sidewall says 35 psi. I see no indicator of other pressures like sidewall pressure. I went into Michelin's web site and found the exact tire but again no indication of sidewall pressure. Where can I find out this info? Any thoughts as to how high I could inflate these tires for every day around town use and next to none for extended highway? Living in Colorado at 5,280 elevation.
-TIA

[EH3]

sidewall says 35. there you go.


100psi is just absurd unless it's a loaded semi or something. inflate to the max psi on the sidewall and you'll never have to worry. the difference in FE above that is probably not even measureable.

[molinee]

Originally Posted by EH3

sidewall says 35. there you go.


inflate to the max psi on the sidewall and you'll never have to worry. the difference in FE above that is probably not even measureable.

Do you really think that this is true? Mileage would be no better at 45 psi than it would at 35 psi? How is this possible.... Just wondering.

[landspeed]

My tyres, which I bought brand new, have a sidewall pressure of 35 PSI. I won't be buying them again!. At 30PSI you could feel the car slowing down due to the rolling resistance. Inflating to 38PSI made a huge difference in reducing the RR. My tyres on Bluey I were inflated to 55PSI with a sidewall pressure of 52PSI IIRC. Coasting was so much easier!

[theholycow]

Ah, I see I'm late to this thread, lots of catching up to do. I'm pretty well studied and experienced with tire pressure; I've put on hundreds of thousands of miles on tires inflated way beyond the vehicle's recommendation in a few very different vehicles, as well as paying close attention to how pressure affected things before I experimented with higher pressures. Here's what I have to offer:

Originally Posted by 1993CivicVX

I visited the local tire store and they were very adamant that over inflating the tires is a bad idea.

For liability and insurance purposes, if they are not completely stupid they'll tell you to always follow the car manufacturer's instructions. If they say anything else they expose themselves to HUGE liability.

I heard on Car Talk that up to 200psi the tire won't burst. But the guy at the tire story was telling me at 60psi I was very lucky that I hadn't had an accident yet and that I had a very short fuse so to speak....

Modern automotive tubless radials are NOT like the bicycle tires you remember popping as a child. They're very different in so many ways. Their design and construction is such that more air tends to be better, and they are way way way overbuilt because of the abuses they're subjected to and liability concerns. What all that adds up to is that you have a lot of room to work with when experimenting with pressures.

Personally, I don't exceed the maximum pressure that's stamped on the sidewall for insurance coverage purposes. In case of an accident, I don't want my insurance company to deny the claim based on my inflating the tire past its maximum rating.

they also said "the max psi" on the sidewall is not the max recommended tire pressure, but referring to something else and that most tires should be inflated between 25-34psi with 34 really being the upper limit.

They're talking about the tire's maximum rating compared to the car's recommended pressure. The 25-34 number is common if you look at the recommendations printed on the car. I think that was pretty reckless of them, though, unless they were talking specifically about your car; many cars come with a recommendation of 35 psi, for example, and I've rarely seen one that recommends 25.

I suppose running firmer tires would make them more susceptible to lacerations from nails and glass and what have you.

Intuitively, you might think this. In practice, higher pressure seems to prevent lacerations and intrusions. In practice AND (if you have any experience with this type of failure) intuitively, higher pressure definitely helps against potholes and curbs. Potholes and curbs destroy tires by compressing them against the rim so they get cut on the edge of the rim or pothole/curb; higher pressure fights that compression.

Originally Posted by RoadWarrior

By the way, did anyone know, that tires are certified for their maximum rated load, ONLY at the max sidewall pressure!

So if you replace some 35 PSI max tires on a Jimmy or something, where GM says tire pressures should be 28, with 44 PSI tires, of the same load rating, and inflate them to 28, per GM, then you've probably only got 1/2 the safe load carrying capacity

That's not quite how it works. Load capacity is a combination of tire size and pressure. You're correct that the max load does require the max pressure, but when you get a same-size tire with a higher pressure rating, the max load rating is higher. There's actually some decently-universal standards in the tire industry for pressure-load charts.

Originally Posted by R.I.D.E.

When I drove my Scion XB home from the dealer the tires were inflated to 65 PSI! The max sidewall pressure was 45.

I was once told by a dealer that all tires on foreign-built cars are inflated to 65 psi at the factory before being shipped to make sure no damage happens in transit, and that the dealer then has to let air out before selling the car. I don't know if that's true.

Originally Posted by AMX

With increasing tire pressure, how much traction loss is there?

You lose some straight-line traction but usually gain cornering traction (from the stiffer sidewalls) and hydroplaning resistance (from the smaller contact patch). In my experience, I've never managed to lose enough straight-line traction to affect my braking distance, though 80psi on the rears on my empty pickup (with an open differential) does leave me spinning a tire on wet roads often.

Originally Posted by 1993CivicVX

my 35psi front tires are at 60 and it is veritably unsafe to run them that high.

The question of traction isn't how far past the tire's rating you are, but how far past the car's recommendation, which is generally the minimum safe pressure for the stock tire size carrying the car's weight. They use the minimum to give a slightly more comfortable ride when test-driving a new car.

@EH3: but I think that's referring to dry surfaces only. And not sure if there is a cut off point for that. Does that still hold true at 100psi?

The article he linked mentions increased resistance to hydroplaning too. There is a cutoff point, and I could probably figure it out with a bunch of math but the easy way is for each driver to experiment.

Here's another one: http://www.tirerack.com/tires/tirete...e.jsp?techid=1 cautions against overinflation Even so, here's how the part cautioning against overinflation ends:
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.

Originally Posted by molinee

Thanks to all for great comments. I just checked my 2001 Jeep Cherokee and see they have Michelin LTX M/S P245/70R16 Radial X, and they have less than 8K on them. The sidewall says 35 psi. I see no indicator of other pressures like sidewall pressure.

The sidewall says 35psi. That's the maximum that Michelin rated for those tires. I personally do not recommend exceeding that, for liability reasons I described above.

Your 2001 Cherokee reportedly has a curb weight of around 3200 pounds (thanks, google), not very heavy at all, especially for big 245/70R16 tires. If your tires were rated for higher pressure, I might want to try up to 55 psi, but even that high would probably not handle well for you. Keep in mind that one end of the vehicle may be heavier than the other end; if so, pressure in that end should be higher than the other end.