Tire inflation?

[PoorManQ45]

Ok guys, i'm having a little bit of a problem.

I can't understand why everyone keeps recommending that I inflate your tires to the vehicle manufature's recommended pressure.

Well, doesn't that recommendation apply only to the OEM tire?

What happens if you change to a different tire?

Don't different tires have different load ratings at different pressures?

The reason I bring this up is that I purchased four 205/70 15 Sumitomo HTRs for my '92 buick century station wagon.

Well, these are 51psi maximum tires and are notorious for a "soft" sidewall.

Buick recommends that I fill the tires to 32psi.

Well I tried this and then drove a little bit. Then stopped and did the basic hand check. Well, the outer edges were warm and not the middle. So it was obviously underinflated.

I could tell by the excessive bulge that they were under-inflated too. But I figured I'd just test my theory anyways.

Well then I tried filling them up to 40psi. I waited a few hours for them to cool off of course. Well this resulted in only th emiddle being warm. So they were over-inflated.

Then I dropped this pressure to 38~39psi. This was still a little too much.

Then I went to 37psi and this was perfect. Even heat throughout.

So whos recommendation do you follow?

I remember I found a tire manufactures manual awhile back that showed the load rating vs tire pressure. This to me would be the appropriate thing to follow. Right?>

I mean, i'd rather listen to someone that made the tires less then a year ago then buick which made my car over 14 years ago.

So, what is the right way. Besides using the hand method?

[Defiant]

Ok guys, i'm having a little bit of a problem.

Lot of that going around, it seems.

Quote »I can't understand why everyone keeps recommending that I inflate your tires to the vehicle manufature's recommended pressure.[/quote]They're richer than us, and own cars from this century.

Quote »Well, doesn't that recommendation apply only to the OEM tire?[/quote]Not only OEM tire, but OEM model of tire running with new bushings on the original wheels. While this sort of consideration was nearly trivial in the eighties, it became more of a case-by-case situation as the cars and what was going on between the contact patch and the top of the seat upholstery became much, much more thoroughly engineered.

Quote »What happens if you change to a different tire?[/quote]Bingo. As you've found, things change.

Quote »Don't different tires have different load ratings at different pressures?[/quote]Yes, but a tire's overall rating also specifies the inflation pressure- which is also very close to the ground pressure in the contact patch.

Quote »The reason I bring this up is that I purchased four 205/70 15 Sumitomo HTRs for my '92 buick century station wagon.[/quote]Oh. Well. Personnel carriers, tuna trawlers and landing barges aren't really quite like cars.

Quote »Well, these are 51psi maximum tires and are notorious for a "soft" sidewall.

Buick recommends that I fill the tires to 32psi.[/quote]That was probably for Good Year Skidwells, whose primary function was to keep the rims from leaving scratches on the showroom's floor paint.

Quote »Well I tried this and then drove a little bit. Then stopped and did the basic hand check. Well, the outer edges were warm and not the middle. So it was obviously underinflated.

I could tell by the excessive bulge that they were under-inflated too. But I figured I'd just test my theory anyways.

Well then I tried filling them up to 40psi. I waited a few hours for them to cool off of course. Well this resulted in only th emiddle being warm. So they were over-inflated.

Then I dropped this pressure to 38~39psi. This was still a little too much.

Then I went to 37psi and this was perfect. Even heat throughout.[/quote]There's not three drivers in five-hundred who know to feel the tread, nor to let the inflation air cool between trials.

Quote »So whos recommendation do you follow?[/quote]As you've found, the car's.

Quote »I remember I found a tire manufactures manual awhile back that showed the load rating vs tire pressure. This to me would be the appropriate thing to follow. Right?>[/quote]Yes, until it's outdated by fresher technology. Which is usually between when the writer hits the last period, and the staples are run through the first printing.

Quote »I mean, i'd rather listen to someone that made the tires less then a year ago then buick which made my car over 14 years ago.[/quote]Or, GM at any time.

Quote »So, what is the right way. Besides using the hand method?[/quote]Infra-red pyrometers with real-time digital output to a laptop, two per tire as monitored over a 48-hour driving stint by assistants in lab coats. Even better if they're chicks.

Most of the people who get on the internaweb to talk about cars are already pretty sharp, but I'd venture that very few of them really check their tire pressure on a monthly basis, let alone weekly. You're far ahead of the game already. Taking the step to feel the tread temps puts you in the top percentile. I'll lay-off the Buick wagon.

It should at least be a Vista Cruiser.

[Eddie]

Don't different tires have different load ratings at different pressures?.....Well, these are 51psi maximum tires and are notorious for a "soft" sidewall.....Buick recommends that I fill the tires to 32psi.

Most poeple don't pay much attention to the change in load index when getting different size tires. (I will try to get a load index chart posted up on this forum that shows all tire pressures from 1-35 psi with the corresponding load indexes showing all the different load carrying capacities at different pressures.)

I know how to find your correct pressure algebraiclly.(STAANDAR LOAD tires reach their maximum load carrying cappacity at 35psi even though the max pressure may say 51psi. The max load carrying capacity is indicated on the sidewall and it corresponds to the load index number)

EXAPMPLE 1995 Nissan 240sx se: OE (205/55/16 89v) pressure: 29psi New (225/50/16 91v) pressure: ?

Max Load for index: 89= 1279lbs @35psi 91= 1356lbs @35psi---------------------------------------------------------------------------------------------------I.) For 89 load index determine the weight the factory intended to support with the factory pressure.

35/1279=29/w ===> (29)(1279)=35w ===> (29)(1279)/35=w w=1059.74lbs (weight that 29 psi is intended to cary @ Load index 89)

-----------------------------------------------------------------------------------------------------------II. With this weight the new tire pressure will be determined in a similar fasion using the new load index of 91.

35/1356=p/1059.7 ===> (35)(1059.74)=1356p ==> (35)(1059.74)/1356=p

p=27.353 ~ 27 (new tire pressure that will carry the same weight)

-----------------------------------------------------------------------------------------------------So the new pressure with a change in load index from 89 to 91 will be 27psi.

Hope this helps.


Modified by Eddie at 4:55 PM 8/29/2006