tire pressure myth?

[corning_d3]

Some people just can't grasp the concept... Just hope your not behind or meeting them on the highway when they pop a tire.

[KiwiBacon]

Originally Posted by corning_d3

Some people just can't grasp the concept... Just hope your not behind or meeting them on the highway when they pop a tire.

Or when they spin out in a corner after hitting a small ripple that wouldn't upset a tyre at normal pressure.

[corning_d3]

I've been there. I was toying with the pressures on my monte, and noticed too much pressure in the back was almost as bad as a defective strut in rippled corners! I now run 40 up front(it's heavy), and 34 in back for best all around wear and mileage..

[2.2 Straight six]

Originally Posted by GreyGoose006

i'm sorry, but if the sidewall of my tire says 44psi. (it does), i'm gonna inflate it to 44psi. if it says 15, i'll happily put 15psi. in it.

think about it, the tyres on YOUR car are not made solely for YOUR car (that model) they are put on OTHER cars, which will have DIFFERENT weight.

now, contact patch is in proportion to vehicle weight and tyre inflation pressure. if your car weighs 3,000lb and you run it at 44psi then your contact patch will be different to a 3,500lb car running 44psi.

you see, it doesn't matter about the tyre's max in this case, the manufacturer uses these tyres and test them on the vehicle, they then look at ride comfort, contact patch etc and THEY determine what pressure should be run, to give a good balance in those areas.

hence the reasoning that you should not inflate a tyre to the max pressure (as listed on the sidewall, since they're used on different cars, with different inflation pressures the tyre manufacturer can't say what pressure YOU should run, they can only tell you the max pressure the tyre can hold for its laod rating) and that you should follow what the car manufacturer reccomends.

but if you want to over-inflate them, then get killed because your contact patch isn't sufficient then by all means do so. but don't tell people you should run them at the max when it's not true for everday driving.

[redstang423]

Here are a few things that have been overlooked.

Nitrogen DOES!!! increase pressure with temperature. All gases (not fuel type gases) ideally follow the Ideal Gas Law (granted, all depart this law to some degree, but follow it more or less). It is an equation defined by PV=nRT. P is the pressure of the gas. V is volume of the gas - basically unchanging in our idea of a tire. n is the number of moles of the gas (technical way of saying how much) - again, this is unchangning. R is the universal gas constant, again unchanged. T is temperature. So what the equation tells you is as the temperature increases pressure INCREASES. It does NOT remain the same as long as the tire remains sealed closed.

A bike (at least a motorcycle) is intended to be turned by leaning, rather than turning the handlebars. This causes all the forces of cornering to be directed directly though the vertical plane of the bike, from the rider's head to feet. It is not like a car where when you turn, you get thrown to the side and the tires must provide the friction in the lateral direction. Because of this, tires and their pressures are calcuated differently.

The "maximum" pressure is not truly the maximum pressure the tire can run. All engineers build in a factor of safety to nearly everything they do. Take a folding chair for instance, with a weight limit of 250 lbs. You WILL have some person that weighs 275 sit in it. It will not break. These safety factors are everywhere in engineering designs. They may be very low for something like a Formula Car, but very high for every day products that anyone can use. For the tires, there would be a reasonably large factor of safety, because idiots will inflate it beyond the "max" pressure. The tire can't explode on someone even if they slightly overload it. Additionally, most people don't understand pressure will increase when the tire (and air inside) gets hotter after use. Otherwise, someone could fill several PSI under the "max" when the tire is cold, and experience failure after they use it for a few minutes.

-Jeff

[GreyGoose006]

which is why i inflate to the pressure on the sidewall.
that pressure was calculated to be safe on the average car driving down the road. why would you inflate to a lower pressure to increase the contact patch for regular driving?
unless your drive to work sees lots of drivin at 150+ for extended time, lots of HARD acceleration, and high speed cornering at the limit of traction, the number on the sidewall is perfectly safe fo everyday driving.

that being said,
if i had the tires from my car on a miata (about half the weight), i WOULD take into consideration the fact that 44psi might be too much.

if i had my tires on an F-350 and was going to be towing a trailer full of horses, i might inflate to a different pressure.

thing is, tires come w/ a load rating.
this is the load that they were designed to carry safely.
i bought tires that were slightly above the weight of my car.
this way, i can happily inflate to the pressure on the sidewall, and be done with it.

redstang basically said what i was thinking but couldnt verbalize.
tires wont "explode" as many have said, under normal conditions.
there is a safety net built in by all engineers, and it happens to be quite large in the case of tires.


also, for those people who put nitrogen in their tires thinking it helps...
you are getting ripped off.
assuming that nitrogen helped with anything in the first place, tires would have to be filled exclusively w/ nitrogen. no air.
that means that all the normal air must be vacuumed out prior to filling w/ nitrogen.
also, the main reason for using nitrogen is that it expands LESS. no gas remains at the same volume regardless of heat... its called PHYSICS. these laws have been in place for cenuries.

lastly,
you do realize that this inferior stuff called air, contiains 78.084% nitrogen by mass?

[redstang423]

Greygoose, you should still inflate tires, regardless of any weight classifactions (assuming appropriate for your vehicle), to the car manufacturer's recommended pressures. Pressure is equal to force over area. The force would be the weight of the car seen by each wheel, leaving pressure to control the area, or contact patch of the tire. Automakers do significant testing to figure out what the best settings for that particular vehicle would be, factoring in everything from comfort to safety (proper amount of contact area) to tread wear of the stock sized (in terms of dimensions) tire. While you are free to fill to whatever pressure suits your fancy, departing too much from the recommendations can throw off the handling and safety characteristics of your vehicle. Its very similar to why when Escalades first came out with its small (on a relative scale) wheels and people started putting 22" wheels on it, Cadillac discouraged it since it would alter the inteded handling characteristics of the vehicle, possibly making it unsafe.

[Moppie]

Originally Posted by GreyGoose006

which is why i inflate to the pressure on the sidewall.
that pressure was calculated to be safe on the average car driving down the road. why would you inflate to a lower pressure to increase the contact patch for regular driving?
unless your drive to work sees lots of drivin at 150+ for extended time, lots of HARD acceleration, and high speed cornering at the limit of traction, the number on the sidewall is perfectly safe fo everyday driving.

So when your driving down the road at your sedate old mans pace, and someone pulls out in front of you, and you stand on the brakes, only to find that because you've over inflated your tyres instead of stopping you drive straight into them, what are you going to do?

For the final time, the number written on your tyres side wall is a max pressure for max loading, for that tyre.
It is not a recomended pressure for all cars. It is only there as a tested maximum pressure for that tyre, assuming its carrying a very heavy load.

[GreyGoose006]

Originally Posted by Moppie

So when your driving down the road at your sedate old mans pace, and someone pulls out in front of you, and you stand on the brakes, only to find that because you've over inflated your tyres instead of stopping you drive straight into them, what are you going to do?

For the final time, the number written on your tyres side wall is a max pressure for max loading, for that tyre.
It is not a recomended pressure for all cars. It is only there as a tested maximum pressure for that tyre, assuming its carrying a very heavy load.

hey moppie, you are EXACTLY right. EXACTLY RIGHT.
there is no problem with what you are saying... that is if i were using the standard tires on my car. guess what, IM NOT!!!
see, there are these things called "HIGH PRESSURE TIRES". they are meant to operate at HIGHER PRESSURES than standard tires. the reason for these tires is that it decreases rolling resistance, and allows for more precise handling and decreased tire weight.

next time your ecectricity goes out due to a tree falling on a line, go look around for those Govt. vehicles that show up.
on the sidewall of these tires, it says "90 PSI"
that means that you are to inflate the tires to 90 psi.
one of these govt F-250s dosent weigh significantly more than a standard F-250, so why the high pressure???

TIRE WEAR...
at 90 psi, the tires wear less and need to be replaced less often.
granted, they are $200 ea. but to the govt, it is worth it.

i am POSITIVE that if you went and bought 90 psi tires, and only inflated them to 25 psi, they would look nearly half way flat, decrease your mileage SIGNIFICANTLY, like cut it in half, and wear the tires out in about two weeks.
my tires are called "HIGH PRESSURE CAR TIRES" and are meant to be inflated to 44 psi.

you know, tire technology HAS advanced since the 70's and as a result, the pressure ratings on sidewalls is becoming the deciding factor when inflating your tires. any time you change wheel size tire size tire profile wheel width, lower your car, or even buy now tires, the door jamb is now DEFUNCT.

i agree that on a stock vehicle, you should go by the doorjamb, or the owners manual... STOCK.
MY TIRES ARENT STOCK!!! and my doorjamb is 23 years old. it makes a difference.

oh, and as for the grandfatherly pace i drive at,
im 19.

[UncleBob]

I guess we can continue to repeat ourselves into infinity. I'm too lazy to quote what I've already said.

This is going nowhere fast.

[Steel]

Actually, you'd be hard pressed to tell the difference between 40 psi and 90 psi in a 10-ply rated E load tire by just looking at it if the truck is unloaded.

Anyway i can guarantee you that the pressure rating written on the side of your tire is the max COLD pressure. And if you want to fill it to that point, that's fine. You're right in that it decreases rolling resistance, and the other guys are right in saying that it also decreases your traction and ride quality. But if you need to squeeze 3 really fat peaple in with a couple dead bodies in the back and some... lead pipes... you'll be all set for the load. I fill my civic tires to 37psi all around because i do a LOT (~23k miles a year) of highway driving and the highways are in generally good condition. If i know it's going to be snowy then i lower them a bit.

People always worry aobut overfilling tires, but that's really tough to do. I've seen customer cars with 50-55 psi in all the tires, 10psi over max and no bad side effects. USually the worst that happens is when they hit a pothole with tight tires like that is the the tire will bubble on the sidewall and need to be replaced. Usually saves the rim from damage too (unless its low profile... but i dont need to worry aobut taht with 185-70-13's ) The big problem is UNDERFILLING them and having blowouts. You see people riding damn near on the rim all the time, and THAT scares me.

Another think - the ~100% nitrogen filled tires = waste of money. The air you breathe is 78% nitrogen. Though it is true that pressure will increase even with 100 N2 tires, its generally not as much as ROA (regular 'ol air). That and don't forget that auto manufactures AND tire manufacturers take into account the fact that the tires heat up and the pressure rises, so now you're working against them. So in conclusion, there is no advantage to N2 filled tires, unless you're the guy that came up with the idea.

[UncleBob]

I see overinflation damage all the time at the shop. Not nearly as often as I see underinflation damage and lack-of-rotation damage....frankly, the average car owner just doesn't pay any attention to their tires. I would take a guess that the AVERAGE car I see is at least 1 quart low on oil also for the same reason, and I see at least two cars a week that are 3+ quarts low on oil a week. Finding abusive owners is definitely not hard to do.

But the point is, its not some type of mystical rare thing.

[2.2 Straight six]

Originally Posted by UncleBob

I see overinflation damage all the time at the shop. Not nearly as often as I see underinflation damage and lack-of-rotation damage....frankly, the average car owner just doesn't pay any attention to their tires. I would take a guess that the AVERAGE car I see is at least 1 quart low on oil also for the same reason, and I see at least two cars a week that are 3+ quarts low on oil a week. Finding abusive owners is definitely not hard to do.

But the point is, its not some type of mystical rare thing.

stuff like that cracks me up, when i talk to people about seized engines (lack of oil) and they say "but why haven't they made engines that don't seize?"

umm....they do, you just have to put oil in them...

[UncleBob]

Originally Posted by 2.2 Straight six

stuff like that cracks me up, when i talk to people about seized engines (lack of oil) and they say "but why haven't they made engines that don't seize?"

umm....they do, you just have to put oil in them...

I tell yah, I'm simply stupified by the level of engineering these days. I really don't have any idea how a plain bearing motor can live for 15K+ miles running on the same oil, and half of that is under half its oil capacity. I can't tell you how many times I pulled less than 1/2 a quart out of the oil pan. The reason they brought the car in, was because the "oil change" light was coming on when they went around corners. That being the oil PRESSURE light.

Makes you appreciate a well designed engine. If a motor can live through that and still make it to 200K miles. Then if you actually MAINTAINED the car in a proper manor, I'm sure it would see a million miles without a hickup

[GreyGoose006]

the million mile mark is not all that much anyway.
tons of volvo 240's and such make it to the million mile mark.
its all in the maintenance, and of course engine design
old pinto's were said to be able to run for five minutes at WOT w/ little or no oil in them.
i know someone who emptied the oil out and drained the radiator, then drove at 80 on the highway in an attempt to blow the engine. He went for 2-3 minutes before the thing started clacking so bad and lost power.
still ran tho lol.