Tire pressure

[bjwirth]

My father-in-law has this notion that tires should be filled to the "Max pressure" listed on the tire. (when he saw our forester this weekend, he said "whoa, your tires are too low." I told him I filled them to 32 psi to which he said "no, too low, my tires have 44 psi...") He has no idea why he does he fills his tires up that high, he just does it. But then I realized that I have no idea why I fill my tires to the "recomended" tire pressure either.

 

so could someone please tell me what the recomended presures are based on (weight of car? I guess I always assumed tire pressure was a function of "load per tire" but I have no idea where I heard that from)

 

what are the advantages/disadvantages of increasing the tire pressure? I've heard of people who lower pressure for a "comfortable" ride but that's about it.

 

Is there ever a reason to use the "max pressure"? It is listed on the tire so there should be a reason right?

 

Thanks

BJ

[WANTONSOUPGUY]

I don't know about the Max Pressure thing...maybe you would increase for heavy loads (ie trailer...???).

 

In your owners manual or in the door jamb there should be a sticker that tells you the correct pressure for the front and rear. My Legacy takes 29 for pressure...and the tires look like they need air, but they don't. I think your dad must wear his tires out prematurely, as overinflated tires wear much faster in the center of the tread. On the other hand underinflated (IE 20psi) will also wear irregularly. Ultimately, following the standard spec will improve your tires rate of wear, and your vehicles handling and gas mileage.

IF I were you, i'd just inflate my tires to the recommended spec. and not tell my dad. Go for it! Be a rebel!

[MorganM]

You run what pressure the TIRE says. The pressure you run depends on the performance you are searching fore. MAX pressure the tire will yield better gas milage and less wear (especially the side walls). Running lower than max will average gas milage, a more 'soft' ride and should wear evenly. Airing them way down will give you maximum traction, very poor gas milage, and a lot more wear on the tire.

 

He prolly airs them to the max suggested by the tire cuz he does a lot of freeway and in town driving. This is fine and I'm sure he enjoys it. Running what the car manufacture suggests usually is an even balance. Good milage, smooth ride, average wear on the tire are what they are aiming for. In the case of offroading you air down. This gives you excellent traction at the expense of the tire. When you are done you air back up and head home

[RallyKeith]

I don't know about the Max Pressure thing...maybe you would increase for heavy loads (ie trailer...???).

 

In your owners manual or in the door jamb there should be a sticker that tells you the correct pressure for the front and rear. My Legacy takes 29 for pressure...and the tires look like they need air, but they don't. I think your dad must wear his tires out prematurely, as overinflated tires wear much faster in the center of the tread. On the other hand underinflated (IE 20psi) will also wear irregularly. Ultimately, following the standard spec will improve your tires rate of wear, and your vehicles handling and gas mileage.

IF I were you, i'd just inflate my tires to the recommended spec. and not tell my dad. Go for it! Be a rebel!

 

Exactly what he said. You car exerts a certian force on the tire. The tire pressure must then be set, based on that force, to the correct pressure to give the tire the correct contact patch for max performance and tread life.

 

Keith

[bjwirth]

thanks for the info. from what I'm hearing a higher pressure is necessary for towing or a more comfortable highway ride. are there any disadvantages?

[applegump]

Isn't the max pressure written on the tire, the maximum pressure that the tire is certified and safe (from blowing up etc) to inflate to. It has nothing to do with the car and the correct inflation for it. I use what the owners manual states as the minimum and go from there. Personally, I'm running 38 psi.

[SevenSisters]

Better overinflated than underinflated (ask a Ford Explorer owner) but:

 

Faster outside tire wear.

Poorer braking performance (dry, wet, don't go out in the snow).

Less resistance to impact damage.

Less steering control.

Less suspension control.

Less comfortable ride.

 

The list could continue. Inflate to the Subaru recommendation or slightly higher. Never MAX unless you have the load to justify it and in a pascar, I doubt if you ever will.

The reason they say MAX is because it's maximum (WARNING WILL ROBINSON). Not the recommended like on the door jam. Tire manufacturers don't set recommended pressures. The vehicle manufacturers do.

[99obw]

I have been airing to:

 

Summer

35 PSI front

34 PSI rear

 

Winter

29 PSI front

28 PSI rear

 

with very good results. Very even wear and good traction. YMMV

[Ranger83]

Increasing pressure will make the steering lighter and decrease the slip angle, making handling more responsive. The tire will also run cooler on the highway, stop better under most driving conditions, have increased load carrying ability, and hydroplane at a higher speed.

 

The primary downside is a lumpy ride. I've driven my old car (which I raced) on the street with up to 45 psi for weeks, with no ill effect other than hitting my head on the roof after going over some railroad tracks.

 

The bias between front and rear does affect handling, and starting with that front/rear pressure differential is a good idea. Inflating the fronts to max recommended cold pressure, and then backing them off a little if you find the ride too hard is a good approach. Underinflation diminishes handling, braking, and tire life so much that it should be avoided at all costs unless you're traveling in loose sand or snow. I have a good tire gage in all my cars and check them regularly. a 3-4 psi bias on the front wheels will make the car veer slightly to the right or left on the highway.

 

Do a search for traction circle or tire traction circle if you want to understand more about how tires work. Modern tires are extraordinarily good.

[MorganM]

Increasing pressure will make the steering lighter and decrease the slip angle, making handling more responsive. The tire will also run cooler on the highway, stop better under most driving conditions, have increased load carrying ability, and hydroplane at a higher speed.

 

The primary downside is a lumpy ride. I've driven my old car (which I raced) on the street with up to 45 psi for weeks, with no ill effect other than hitting my head on the roof after going over some railroad tracks.

 

The bias between front and rear does affect handling, and starting with that front/rear pressure differential is a good idea. Inflating the fronts to max recommended cold pressure, and then backing them off a little if you find the ride too hard is a good approach. Underinflation diminishes handling, braking, and tire life so much that it should be avoided at all costs unless you're traveling in loose sand or snow. I have a good tire gage in all my cars and check them regularly. a 3-4 psi bias on the front wheels will make the car veer slightly to the right or left on the highway.

 

Do a search for traction circle or tire traction circle if you want to understand more about how tires work. Modern tires are extraordinarily good.

Ranger is on point here

[blitz]

What Ranger said!

 

If you live in an area with really bad roads (MICHIGAN) the nice, light, crisp steering afforded by higher pressures can unfortunately be offset by the tendency to patter, skitter, & annoyingly false-trigger the ABS in corners, so if that's the case, you gotta compromise and drop the pressure a bit to keep the contact patch planted a little better.

 

So many variables.

[Setright]

The wear pattern is more likely caused by suspension geometry. A Radial tyre has a very stable contact patch, and distributes force fairly evenly across the tread.

 

Cross ply tyres - rare these days - will wear shoulders when underinflated and centres when overinflated.

 

 

Oh, I run 195/60R15, presently at 36 front 31 rear. This compensates for the lack of a rear anti roll bar on my cheapo Impreza, and allows for some tail-out fun :-)

 

Even so, the OUTSIDE shoulders on all my tyres are what wear out first....there was a list of tyre related stuff in my Legacy manual, and this type of tyre wear could be caused by "Excessive speed in turns."

 

 

YOU BET!

[bjwirth]

I'm hearing contradictory things here....

 

"Poorer braking performance (dry, wet, don't go out in the snow)." vs "stop better under most driving conditions"

 

"Less steering control." vs "making handling more responsive"

 

"overinflated tires wear much faster in the center of the tread" vs "Faster outside tire wear"

 

I tend to think (I have no data or experience to back this), if steering is lighter and braking is worse, handling should also suffer- right? I always thought (street) drag racers would lower the pressure in their tires for a race to get more traction, then fill them up before they drove home.

 

Otherwise the consensus is:

stiffer ride

better gas mileage

higher load carrying ability

[Ranger83]

I tend to think (I have no data or experience to back this), if steering is lighter and braking is worse, handling should also suffer- right? I always thought (street) drag racers would lower the pressure in their tires for a race to get more traction, then fill them up before they drove home.

You're not a drag racer - you don't have an immensely powerful car and you don't only drive in straight lines.

 

Braking isn't worse with higher pressure (within normal operating range) than lower pressure. Handling isn't worse, because the higher pressure prevents the contact patch from rolling over. Rather than be argumentative, I simply suggested that you search more websites for a comprehensive view....

 

Try this. Take your Subaru to an autocross - it's a handling competition set up at an airport or parking lot, laid out with pylons. Or just spend some time cornering hard on your local roads - obeying all traffic laws, of course.....

 

Look at the front tire sidewalls. You'll see scuff marks well up the sidewall of the tire. Increasing the pressure will stiffen the tire, keeping the tread on the ground. If you're the precise type, you can even take a white tire market and mark the sidewalls so you can see just how far up you go. This is also useful when buying a used performance car when you want to see how hard it has been driven. Most SUV's, especially those with beam axles, cannot generate enough cornering power to do this - they roll over, instead....

 

Any fundamental racing text will cover this in detail.

 

http://auto.howstuffworks.com/tire4.htm

 

http://www.geocities.com/motorcity/downs/3837/autoxfun.html

 

http://www.miata.net/sport/Physics/10-GripAngle.html

[bjwirth]

thanks for the links. I gotta admit that higher pressure=better handling seemed anti-intuitive until I read the stuff in the links. it's a lot clearer now.

 

 

But what about the MAX PRESSURE setting? I can now understand the benefits of HIGHER pressures, especially for special circumstances. I'm not talking about a performance car or anything carrying a heavy load- just a regular minivan filled to 44 psi. This has gotta translate to a smaller contact patch and with no great force generated on any one wheel from hard handling.

 

And this question will seem dumb, but if you fill your tire to the max pressure (say when the tire is cold), then drive around for a while and the tire heats up, will the tire "pop" or do anything when the pressure is exceeded?

[MorganM]

44 PSI seems pretty high for any regular passanger car radial tire I've seen. All the one's I've had were between 30 and 35 PSI for MAX rating on the tire. Were these some fancy tires on that minivan or something?

[Ranger83]

But what about the MAX PRESSURE setting? I can now understand the benefits of HIGHER pressures, especially for special circumstances. I'm not talking about a performance car or anything carrying a heavy load- just a regular minivan filled to 44 psi. This has gotta translate to a smaller contact patch and with no great force generated on any one wheel from hard handling.

Depends on the car, tire size, and tire. I don't get into religious arguments about it - I experiment and stick with what works. He's not likely to be pushing the cornering limits of the tire in a minivan, and other than a very hard ride he's probably not doing much. If he's showing excessive wear in the center tread, that's too high, for sure.

 

And this question will seem dumb, but if you fill your tire to the max pressure (say when the tire is cold), then drive around for a while and the tire heats up, will the tire "pop" or do anything when the pressure is exceeded?

No. As witnessed by your realtive who has overinflated his tires by 25%.

These pressures aren't all that high - the D load rated tires on our cargo van are rated for something like 70 PSI. It takes a good 2,000 lbs to make the ride reasonable. We usually run them around 40 psi. Big trucks are even higher - as much as 100 psi.

 

As they say, your mileage may vary, so max out your tires, drive down your favorite curvy, bumpy road, then let out two psi and try again until the ride/handling mix feels good.

[LosDiosDeVerde86]

You run what pressure the TIRE says. The pressure you run depends on the performance you are searching fore. MAX pressure the tire will yield better gas milage and less wear (especially the side walls). Running lower than max will average gas milage, a more 'soft' ride and should wear evenly. Airing them way down will give you maximum traction, very poor gas milage, and a lot more wear on the tire.

 

He prolly airs them to the max suggested by the tire cuz he does a lot of freeway and in town driving. This is fine and I'm sure he enjoys it. Running what the car manufacture suggests usually is an even balance. Good milage, smooth ride, average wear on the tire are what they are aiming for. In the case of offroading you air down. This gives you excellent traction at the expense of the tire. When you are done you air back up and head home

 

whoa whoa.....it says in my chilton's manual, and i know from working at a garage, too much pressure (44psi) will cause center wear. too little will cause shoulder wear. use either the recommended psi or 35psi seems to work great. never go full psi unless you're carrying enough weight to flatten the tire out on the road.

[LosDiosDeVerde86]

so basically, too much psi make your tire contact the road like this: \_/

too little makes the tire contact the road like this: \_/^\_/

 

 

get it.......?

[sprintman]

2000 Outback, 38psi Nitrogen all round. Specialist tyre mechanic who looks after my vehicles says "perfect" 40/38 Nitrogen front/rear in my Mazda turbo from same guy. As he told me and motoring mags doing tyre tests say "let the tyre manufacturer pick the pressure as the vehicle vendor is purely intereested in ride comfort".

[86subaru]

also here is something , when you get new tires put onto a car ot truck any make alot of times they will put less air into the tires so you will wear the tires down faster and come back and buy other , just my 2 cents

[arracado]

2000 Outback, 38psi Nitrogen all round. Specialist tyre mechanic who looks after my vehicles says "perfect" 40/38 Nitrogen front/rear in my Mazda turbo from same guy. As he told me and motoring mags doing tyre tests say "let the tyre manufacturer pick the pressure as the vehicle vendor is purely intereested in ride comfort".

I would think that the car manufacturer might also be interested in gas mileage, safety, and performance too, not just ride comfort....

[LosDiosDeVerde86]

also here is something , when you get new tires put onto a car ot truck any make alot of times they will put less air into the tires so you will wear the tires down faster and come back and buy other , just my 2 cents

if you go somewhere that does that, you need to find a better garage

[outbackfan]

Here's a Quote from Michelin's USA website:

 

Always inflate your tires to the recommended pressure listed by your vehicle's manufacturer. This information can be found in the owner's manual and often on a placard located in the vehicle's door jamb, inside the fuel hatch, or on the glove compartment door.

 

 

Note it says nothing about printing on the tire. Follow the recommendations on the door sill.