Welcome! Log In Create A New Profile Recent Messages

Advanced

Tire Pressure?

Posted by wodcutr 
February 28, 2009 03:01PM
Hey everyone - I have a question about what the proper inflation value is supposed to be for any given set of tires. Are you supposed to run at the maximum pressure stated on the sidewall or something less than that? I always thought that the max pressure was the proper inflation value, but every time I go into a tire store they always say my tires are overinflated. Are they stupid or am I? My max pressure on my tires is 51psi and they are always putting only 35psi in. It drives me nuts! What say you? confused smiley
February 28, 2009 04:01PM
The numbers on the sidewall are maximums. BMW recommends 29 front and 32 rear in the book that came with my '87 325, so that's what I use. That seems to work OK for just normal around town driving, which is about all I do. It does look like the front tires wear a little faster at the outer edges, which indicates a little under-inflation, but it isn't bad.

John
February 28, 2009 05:47PM
Quote
John Yust
The numbers on the sidewall are maximums. BMW recommends 29 front and 32 rear in the book that came with my '87 325, so that's what I use.

If memory serves me (and it doesn't often) that is really close to what my '89 325i says.

Quote

It does look like the front tires wear a little faster at the outer edges, which indicates a little under-inflation, but it isn't bad.
John

Or it indicates you corner harder than the average grandmother, you bad boy you winking smiley
February 28, 2009 07:42PM
Quote
John Yust
The numbers on the sidewall are maximums. BMW recommends 29 front and 32 rear in the book that came with my '87 325, so that's what I use. That seems to work OK for just normal around town driving, which is about all I do. It does look like the front tires wear a little faster at the outer edges, which indicates a little under-inflation, but it isn't bad.

John

Well then what is the purpose of buying a high performance tire? Generally the more expensive the rubber the higher the inflation pressures. If you are going to under inflate the tire why spend the money? I have always thought that higher inflation pressure keeps the tires tread more on the road surface when cornering hard etc. If I were to go around a corner at say 50mph and the front tires had only 29psi, I think the forces on the tire would deform it so bad you would be riding on the sidewall if not blow the bead and be flat. Then again, if I only inflated the tires to maximum pressure when I thought I was going to do some spirited driving then I would always be pissed cause my tires were low and I couldn't drift around my favorite turn. I am still confused! :confused:
sdp
February 28, 2009 07:53PM
I always went with the recommended tire pressure on the sticker that is usually on the inside of the driver door and then added a couple of pounds.

If you fill to max pressure when cold, what happens when the you are driving and the tires are hot?.. You are probably over the max at that point.

I also believe you are correct in stating that if you are a more aggressive driver then you need more pressure to keep the tires from deforming.
February 28, 2009 08:04PM
Quote
wodcutr
Hey everyone - I have a question about what the proper inflation value is supposed to be for any given set of tires. Are you supposed to run at the maximum pressure stated on the sidewall or something less than that? I always thought that the max pressure was the proper inflation value, but every time I go into a tire store they always say my tires are overinflated. Are they stupid or am I? My max pressure on my tires is 51psi and they are always putting only 35psi in. It drives me nuts! What say you? confused smiley

I'm not an aggressive driver so I take the maximum pressure stated on the tires and the recommended pressure on the door sticker and average them. I don't differentiate between front and rear and so far I'm getting the best mileage for both fuel consumption and tire wear.

If I were concerned with ultimate handling, then my choices would likely be a lot different.
February 28, 2009 08:06PM
Quote
wodcutr
Well then what is the purpose of buying a high performance tire? Generally the more expensive the rubber the higher the inflation pressures. If you are going to under inflate the tire why spend the money?

Let me preface this by saying I am not a tire expert. Heck, I'm not even a tire novice.
My guess would be that a high performance car does things that a normal car doesn't (I know, duh, but stick with me).
Those things that a normal car wouldn't do probably produces two scenarios:
1. The tire pressure increase. Heat, a hard bump, cornering, etc.
2. The car is in positions where a tire failure would be an incredibly negative experience, especially when compared to a normal driving flow out. A higher tires pressure rating might provide a higher level of insurance when abusing a tire.

But this is all fire side philosophizing, and is based on nothing but my immediate musings on the subject smiling smiley

~Tyler


ps
Third option, it's of some importance, but more a marketing thing than anything else.
February 28, 2009 09:00PM
Quote
sdp

If you fill to max pressure when cold, what happens when the you are driving and the tires are hot?.. You are probably over the max at that point.

All tires are supposed to be filled when cold and yes the pressure does increase when they get warm, but they are not overinflated because the manufacturer expects that to happen!
February 28, 2009 09:04PM
Quote
Earendil
My guess would be that a high performance car does things that a normal car doesn't

Is not the E30 a high performance car? I think that is why we have to buy H rated tires because we can go fast! smiling bouncing smiley
February 28, 2009 09:07PM
Quote
Archeo-peteriX

If I were concerned with ultimate handling, then my choices would likely be a lot different.

Please elaborate!!! B)-
February 28, 2009 09:41PM
Quote
wodcutr
Quote
Archeo-peteriX

If I were concerned with ultimate handling, then my choices would likely be a lot different.

Please elaborate!!! B)-

Ok...if I were to be doing auto-x, I might follow the advice of Ove Kvam and use tire pressures suited to my car, tires and the usage; if I were running on a track, I would probably take tire pressure advice from Rolf Christiansen.

What I pump my tires up to on the street and for maximum fuel economy would probably have the car handling like crap in those situations sad smiley
February 28, 2009 10:49PM
i run at around 38-40, but it is probably on the highside.


March 01, 2009 08:21AM
OK. I'm late into the long fray here (long day of karting yesterday, woohoo! grinning smiley), but I will jump in here. There is no one "perfect" tire pressure. The best tire pressure to use depends on both the kind of tire you're using and what your primary goal is: comfort? handling? fuel mileage?

That said, the maximum tire pressure written on the sidewall of the tire is just that: a maximum, above which you risk damaging the tire. 51 psi is very, very high -- much too high for any goal you might have. At high speed in hot weather your tires are probably nearing 60 psi, which is actually dangerous. When tires are overinflated the tread sort of balloons out, so that you are mostly riding on just the center portion of the tread. Not only does this give you significantly less grip, but your tires will wear out sooner than expected, and the center of the tread will be worn out while the edges are still good (because you're not riding on the edges).

The pressures recommended by the manufacturer and stamped on the doorframe are much closer to what you should be using. However, those pressures are almost always biased on the low side for a soft, comfortable ride. Many people, like me, are more interested in more grip around corners, so will increase those figures by a few psi. Generally I run about 32-35 psi all around. The higher side will give you slightly better handling and fuel mileage, while the lower side will be a little softer and more comfortable.

On the track, tire pressures make a huge difference in performance, and people spend a lot of time and effort tweaking pressures and figuring out exactly which pressures give the best lap times. For the tires I run on my track car (Toyo R888), I have learned that 40 psi hot is ideal. Lower than that, and the sidewalls flex too much and the steering response suffers. Higher than that, and I lose overall grip and my speed through the turns suffers. To reach 40 psi hot, I start out with 36 psi cold, and after several laps come into the pits and check the pressures immediately while the tires are still hot, and adjust as necessary to get to 40 all around.

It is very rare to find any situation anywhere where hot tire pressures above about 42 would be considered beneficial.

__________
Dave
'91 325iX
March 01, 2009 10:05AM
Quote
Dave_G
Generally I run about 32-35 psi all around. The higher side will give you slightly better handling and fuel mileage, while the lower side will be a little softer and more comfortable.

I like a stiff ride but I guess I will try the pressure at 35 and see how it feels. I wonder if that is why I get such good mpg...I will keep an eye on that too!
March 01, 2009 01:56PM
What dave g writes is quite correct.

That being said, I run the lowest pressure that gives me max grip without 'rolling' over in tight corners at speed. That has given me 34 lbs on the front and 32 on the rears on Goodrich Traction T/As. generally, with the different tires that I have run on my E30, the pressures have run between 32-36 in front and 30-34 in the rear. Overinflating does not give more grip, just resistance to deformation and better mileage.

Salut, Bob P.
March 01, 2009 04:51PM
Quote
Bob P 325is 88
What dave g writes is quite correct.

That being said, I run the lowest pressure that gives me max grip without 'rolling' over in tight corners at speed. That has given me 34 lbs on the front and 32 on the rears on Goodrich Traction T/As. generally, with the different tires that I have run on my E30, the pressures have run between 32-36 in front and 30-34 in the rear. Overinflating does not give more grip, just resistance to deformation and better mileage.

Salut, Bob P.

That is 'sound' advice grinning smiley
March 01, 2009 06:56PM
Quote
wodcutr
I wonder if that is why I get such good mpg...I will keep an eye on that too!
Very likely yes. When pumped up to maximum inflation pressure, the tires will have significantly less rolling resistance than if they were under-inflated.

There are a good series of tech-articles concerning proper inflation pressure at TireRack.com.
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=1

As Dave_G explained well, inflation pressure affects the shape of the tire contact patch and how the pressure across that contact patch is applied to the road. An under-inflated tire will put more load onto the shoulder edges of the contact patch, supporting the load more with the sidewalls of the tire, and will bulge the sidewalls out. An over-inflated tire will bulge the tire up, lifting the shoulders of the contact patch off the road, reducing the size of the contact patch and making the tire ride more along the centreline of the contact patch, less on the shoulders.

Ideally we would all use a pyrometer to measure our tire temperatures at the outer edge, centreline, and inner edge in order to always determine the optimum tire pressure to give an even tire temperature across the full width of the contact patch. An under-inflated tire will be hotter along the shoulders. An over-inflated tire will run hotter along the centreline of the patch. Run the tires long enough, either over or under-inflated, and eventually you'll be able to see either the centreline, or shoulders, wearing out faster.

Under-inflated tires will overheat faster and feel like mush because of the way they flex under side loads. Over-inflated tires will feel like they give crisper handling because they respond more quickly to steering inputs, but the smaller contact patch gives less grip particularly under braking. The optimum inflation pressure lies somewhere between those extremes and will depend on the type of driving you intend to do.
rkj
March 01, 2009 07:00PM
Quote
wodcutr
Hey everyone - I have a question about what the proper inflation value is supposed to be for any given set of tires. Are you supposed to run at the maximum pressure stated on the sidewall or something less than that? I always thought that the max pressure was the proper inflation value, but every time I go into a tire store they always say my tires are overinflated. Are they stupid or am I? My max pressure on my tires is 51psi and they are always putting only 35psi in. It drives me nuts! What say you? confused smiley

So you think max pressure is a good way to run your tires? smiling bouncing smiley thats funny!
March 02, 2009 10:48AM
Quote
rkj

So you think max pressure is a good way to run your tires? smiling bouncing smiley thats funny!

OK so I'm an idiot...I never had a father to tell me otherwise. I always thought that the pressure stamped in the side of the tire was the recommended pressure for that tire. I have now been given knowledge that I can pass onto my own son! I guess it's a good thing I don't buy high performance tires for all of my vehicles. They have mostly been stamped at 35 psi. :embarrassed:
March 02, 2009 11:55AM
Quote
wodcutr
OK so I'm an idiot...I never had a father to tell me otherwise.
We never get too old to learn new stuff.

I make it my goal to learn something new every day. Some days I might even accidentally learn two new things. But then I get to take the next day off.
March 02, 2009 05:23PM
Quote
Ferdinand
Quote
wodcutr
OK so I'm an idiot...I never had a father to tell me otherwise.
We never get too old to learn new stuff.

I make it my goal to learn something new every day. Some days I might even accidentally learn two new things. But then I get to take the next day off.

Exactly! Which is why when I graduate from Uni with a B.S. in CompSci here in a few months, I'm taking the next 40 damn years off.

That's the way it works, right? smiling smiley
rkj
March 02, 2009 06:34PM
Quote
wodcutr
Quote
rkj

So you think max pressure is a good way to run your tires? smiling bouncing smiley thats funny!

OK so I'm an idiot...I never had a father to tell me otherwise. I always thought that the pressure stamped in the side of the tire was the recommended pressure for that tire. I have now been given knowledge that I can pass onto my own son! I guess it's a good thing I don't buy high performance tires for all of my vehicles. They have mostly been stamped at 35 psi. :embarrassed:

Hey, I didn't mean to make you feel like an idiot winking smiley but it was funny. There is a reason for door jam pressures... smileys with beer
March 02, 2009 07:06PM
Quote
Earendil

Exactly! Which is why when I graduate from Uni with a B.S. in CompSci here in a few months, I'm taking the next 40 damn years off.

That's the way it works, right? smiling smiley

No, but if your career goes like mine, you'll get plenty of time of that you didn't expect. :rolleyes:

John
Sorry, only registered users may post in this forum.

Click here to login

Online Users

Guests: 4
Record Number of Users: 3 on September 29, 2015
Record Number of Guests: 116 on November 11, 2017