The "correct" tire pressure --- NOT A SCIENCE!!!!

[y_p_w]

 Originally Posted By: pitzel

I jack my 32psi-rated tyres up to 40psi. Like the slightly stiffer ride, and I'm sure it gives me an extra few MPG. The door sticker is to protect the manufacturer from extreme conditions, ie: some dodo that drives in the +120degF heat, at maximum gross weight, and travelling 100+mph, from tyre failure.

You've got that backwards. Higher pressures would do that. Lower pressures are usually for a more compliant ride and perhaps even wear. Higher pressure means lower rolling resistance and less heat generated. I certain value CapriRacer's input, but his was not a blanket recommendation that the manufacturer's recommendation is the final word. Most people don't install OE tires, which changes the equation quite a bit.

 

[Junior]

 Originally Posted By: y_p_w

 Originally Posted By: pitzel

I jack my 32psi-rated tyres up to 40psi. Like the slightly stiffer ride, and I'm sure it gives me an extra few MPG. The door sticker is to protect the manufacturer from extreme conditions, ie: some dodo that drives in the +120degF heat, at maximum gross weight, and travelling 100+mph, from tyre failure.

You've got that backwards. Higher pressures would do that. Lower pressures are usually for a more compliant ride and perhaps even wear. Higher pressure means lower rolling resistance and less heat generated. I certain value CapriRacer's input, but his was not a blanket recommendation that the manufacturer's recommendation is the final word. Most people don't install OE tires, which changes the equation quite a bit.

CapriRacer is correct. I did an experiment with my car, and in the range of 4 psi, it went from an understeering dog to something that wants to skip the rear end around without a lot of advanced notice. Unless you push your car to the limits you probably wouldn't notice. I do however from time to time so I know how the car is going to react if and when I need to do an emergency move.

 

[IndyIan]

What car do you have and on which tires? I wish I could change the handling of my car that much with just tire pressure. I guess if you went up 4 psi on the front and down 4 on the back the car would feel a bit more loose. Most/all cars still can get oversteer and spin using momentum even with factory tire pressures though so maybe you were just feeling that?

 

[rpn453]

I managed to get a little lift-throttle oversteer at the end of a hard corner on a wet road with the ex-girlfriend's MX-6 at 35/32 psi one time. That's the only time I've induced oversteer on a FWD without snow, ice, or gravel.

 

[PandaBear]

I have a piece of chalk handy when I drive with new tires, and adjust the pressure with chalk mark across the trends and see at what pressure will the contact area be used completely. That's the best I can come up with.

 

[labman]

Sounds better than my idea of looking at the wear pattern.

 

[IndyIan]

 Originally Posted By: rpn453

I managed to get a little lift-throttle oversteer at the end of a hard corner on a wet road with the ex-girlfriend's MX-6 at 35/32 psi one time. That's the only time I've induced oversteer on a FWD without snow, ice, or gravel.

I'm pretty sure your current car could be coaxed sideways if you wanted, Mazda tends to make more neutral cars than most manufacturers. On an offramp I'm sure you could get it sideways with throttle lift and a sharp turn in, in the middle of the corner.

 

[Junior]

 Originally Posted By: IndyIan

What car do you have and on which tires? I wish I could change the handling of my car that much with just tire pressure. I guess if you went up 4 psi on the front and down 4 on the back the car would feel a bit more loose. Most/all cars still can get oversteer and spin using momentum even with factory tire pressures though so maybe you were just feeling that?

I may have been a little overly dramatic. The stiffer axle is going to slide first. So, with the upgraded rear sway bar and 36 PSI in the rear, I could feel the back trying to pass. Drop back town to 34 front, 32 rear gives in my opinion balance between ride and handling. I could still get the back to come around on a wet clover leaf but I expect it now. Like I said, you really have to try and you have to have more of a performance tire to begin with. I doubt you would notice with touring tires. This happened to be with an 04 accord with aftermarket wheels and summer Michelin tires. I have since switched to grand touring tires and the affect is muted.

 

[rpn453]

 Originally Posted By: IndyIan

I'm pretty sure your current car could be coaxed sideways if you wanted, Mazda tends to make more neutral cars than most manufacturers. On an offramp I'm sure you could get it sideways with throttle lift and a sharp turn in, in the middle of the corner.

Probably. I've actually never reached the cornering limits of this car, and I can't remember ever doing a mid-corner throttle lift. I usually go slow in and fast out on curves. It's the only truly safe way to do hard cornering on public roads, IMO. However, the situation I described occurred while barely beating the light at a T-intersection to join a lower-speed road from a freeway off-ramp, so it was an unusual (for me) fast-in corner but I went from the outside lane to the inside lane while using the concrete center divider as the apex so I had a lot of extra room if I needed it. No other cars were around me. I only eased up on the throttle at the end of the turn to keep from speeding excessively!

 

[IndyIan]

It's funny how on the street it feels like your car has tons of grip, because you never really push it that much(nor should you). I have average all seasons on my Neon at 38psi (they are 60 series) and an upgraded rear sway bar, I run the safe corners fairly hard and I feel like Schumacher. Those same tires on the autocross course are plain useless... Feels like no grip at all, even R comps feel like they have no grip if you are sliding away from the apex... It's all relative I guess. You should try hitting the brakes mid corner sometime just to get a feel for what happens, just in case you get surprised.

 

[ET16]

 Originally Posted By: pitzel

I jack my 32psi-rated tyres up to 40psi. Like the slightly stiffer ride, and I'm sure it gives me an extra few MPG.

I wouldn't do this.

 

[friendly_jacek]

 Originally Posted By: CapriRacer

BTW, the tire load tables are predicated on the deflection being the same for all points on the table. One of the fallouts of that is that the footprint size will be the same for all those points. This means that using proportionally more inflation pressure reduces the size of the contact patch. Plus, more inflation pressure raises the spring rate of the tire. Changing the pressure split would change the handling characteristics. So using a 38 / 35 spilt on a car calling for 30 / 30 is going to result in a car with less understeer. Plus using a higher pressure is going to increase what we tire engineers call "aligning torque". It's the force that pulls the tire back into position when the steering wheel is turned. But the inverse of that is that less slip angle develops for a given steering input - and that results in a more precise steering feel to the car - call it "crispness". So overall, the car is going to feel more balanced and precise. However, this has a down side. Understeering cars communicate their reaching the limit of adhesion by giving you plenty of warning. Increase the inflation pressure, and the limit of adhesion is approached more abruptly. Make the car have less understeering and the approach to the limit of adhesion is less noticeable. Overall, vehicle engineers consider this less safe, as the warnings that the limit of adhesion is being approached become less obvious. So that's some of the science behind the tire pressure thing. A word of caution: Vehicle engineers spend coutless hours driving their cars on their test tracks in order to find the little handling quirks. So be cautious about using an inflation pressure different than what is specified.

Very good read and very convincing. However, how do you explain that I get a lot of shoulder wear in front tires running stock pressures (30-35 PSI, sample of 3 different cars) disappearing when I use higher pressures (38-42 PSI; max cold is 44). Also, much higher pressures are recommended for high speed driving. I discovered that recently in an International service guide for corolla. For example, Australia 32 PSI under 140 km/h and 41 PSI over 140 km/h. Europe, 32/39 PSI for under/over 160 km/h: http://kre.unitra.sk/down/toyota/service_data_sheet/corolla_gasoline%202004.pdf How does it mesh with your explanation?

 

[y_p_w]

 Originally Posted By: friendly_jacek

Very good read and very convincing. However, how do you explain that I get a lot of shoulder wear in front tires running stock pressures (30-35 PSI, sample of 3 different cars) disappearing when I use higher pressures (38-42 PSI; max cold is 44). Also, much higher pressures are recommended for high speed driving. I discovered that recently in an International service guide for corolla. For example, Australia 32 PSI under 140 km/h and 41 PSI over 140 km/h. Europe, 32/39 PSI for under/over 160 km/h: http://kre.unitra.sk/down/toyota/service_data_sheet/corolla_gasoline%202004.pdf How does it mesh with your explanation?

I think the stock tires may be different. It rather an inexact science too. There's certainly a difference between the wear patterns with short trip driving compared to long trip driving where the tires reach higher sustained temps (and subsequently higher pressures). I have a feeling that this is much like oil viscosity. There's a wide range that actually works, even though only limited types are recommended by the manufacturer. Many are still convinced that American Honda's 5W-20 recommendation is for purposes other than maximum longevity. The same engines in Europe or other parts of the world are getting heavier oil weights in similar climates.

 

[CapriRacer]

 Originally Posted By: friendly_jacek

Very good read and very convincing. However, how do you explain that I get a lot of shoulder wear in front tires running stock pressures (30-35 PSI, sample of 3 different cars) disappearing when I use higher pressures (38-42 PSI; max cold is 44). .........

This is a tough question to answer as it is not clear what you are saying. But you should be aware that one of the tricks tire manufacturers use to get good rolling resistances is to make the tread shoulders such that they don't get as involved with the footprint - and one of the effects is that the shoulders tend to wear faster than tires that don't have this characteristic. Replacement market tires tend to be all about tire wear - so tire manufacturers tend not to do that. Also, higher pressures tend to slow down the wear rate, making it increasingly difficult to judge whether the tire is wearing evenly or not. But don't forget that wear is but one characteristic of many that are balanced.

 Originally Posted By: friendly_jacek

......... Also, much higher pressures are recommended for high speed driving. I discovered that recently in an International service guide for corolla. For example, Australia 32 PSI under 140 km/h and 41 PSI over 140 km/h. Europe, 32/39 PSI for under/over 160 km/h: http://kre.unitra.sk/down/toyota/service_data_sheet/corolla_gasoline%202004.pdf How does it mesh with your explanation?

Recognizing that there are several characterisitics that are being balanced here - and of course, this might vary according to locale. For example: In the US, speeds are restricted - and the vehicle placard reflects that. However, high speed usage in - say - Germany, would need to be accounted for. And that's exactly what that pdf page says. Rememeber this is the recommendation from the vehicle manufacturer - which is what everyone is saying isn't the "correct" answer. What I'm saying is that vehicle engineers do a lot of work - and that pdf file is proof.

 

[friendly_jacek]

 Originally Posted By: CapriRacer

This is a tough question to answer as it is not clear what you are saying. But you should be aware that one of the tricks tire manufacturers use to get good rolling resistances is to make the tread shoulders such that they don't get as involved with the footprint - and one of the effects is that the shoulders tend to wear faster than tires that don't have this characteristic. Replacement market tires tend to be all about tire wear - so tire manufacturers tend not to do that.

What I'm saying that I saw a lot of shoulder wear with stock pressures. Nothing more, nothing less. I saw it in both OEM and aftermarket tires. It's possible that city driving component and frequent cornering is responsible for that. Then it would imply that short trippers should use higher pressure? I didn't understand your comment about shoulders not included in the footprint. If so, why do they wear?

 Originally Posted By: CapriRacer

Recognizing that there are several characterisitics that are being balanced here - and of course, this might vary according to locale. For example: In the US, speeds are restricted - and the vehicle placard reflects that. However, high speed usage in - say - Germany, would need to be accounted for. And that's exactly what that pdf page says. Rememeber this is the recommendation from the vehicle manufacturer - which is what everyone is saying isn't the "correct" answer. What I'm saying is that vehicle engineers do a lot of work - and that pdf file is proof.

I simply asked why high speed requires higher pressure? it's contra intuitive, as I would assume that high speed would stress and heat tire enough to produce high pressure regardless. Or, does high pressure from the start prevent high temp buildup in the tires? But then, that would invalidate all that load calculations you elegantly presented. Thanks!

 

[CapriRacer]

 Originally Posted By: friendly_jacek

What I'm saying that I saw a lot of shoulder wear with stock pressures. Nothing more, nothing less. I saw it in both OEM and aftermarket tires. It's possible that city driving component and frequent cornering is responsible for that. Then it would imply that short trippers should use higher pressure? .......

As you've alluded to, there's a lot going on here. Let's do a thought experiment. If you took a car and drove in a large diameter circle - like a flat parking lot, would you expect the tires to wear evenly? Of course not! On the tires on the outside of the circle, you'd get more wear on the outside shoulders. But what about the inside tires? This involves the Akerman principle. When you turn the steering wheel, the left and right front tires turn different amounts - the inside tire turns more. This difference accounts for the smaller diameter the inside tires are traversing compared to the tires on the outside of the turn. The amount of the difference depends on the diameter of turn, the speed you are taking it, the wheelbase and track of the vehicle, as well as the amount of roll the vehicle body is experiencing - and quite a few other factors. HOWEVER, as you increase the speed of the turn - keeping the same turning diameter, the tires develop more slip angle, and the optimum difference needed between the front tires decreases. In other words, the amount of Akerman for optimal cornering decreases. At the same time, the suspension on the inside of the turn is being unloaded and the camber changes. On many cars, high speed cornering results in the inside tire riding on the inboard shoulder - and that portion of the tire wears more. The net effect is that both shoulders get more wear - which may lead some folks to think that they don't have enough inflation pressure. Interestingly, increasing the inflation pressure has the effect of reducing the steering angle needed for a given side force. This results in a decreased amount of Akerman seen by the vehicle for a given combination of turning diameter and speed. This technique is used a lot by Autocrossers and Road Racers when they are using basically stock or slightly modified cars. A beneficial side effect to this increased pressure is a more immediate response from the vehicle to steering input. (BTW, this is sometimes confused as generating more cornering force.) What I am trying to get at here is that the way you drive and the amount of cornering you do has a major influence on the amount and type of tire wear you experience.

 Originally Posted By: friendly_jacek

........ I didn't understand your comment about shoulders not included in the footprint. If so, why do they wear? ...........

If you'll look carefully, I said the shoulders were not as involved. Once you've understood that they are involved but to a lesser extent, I think it is easy to see that they might wear faster.

 Originally Posted By: friendly_jacek

........ I simply asked why high speed requires higher pressure? it's contra intuitive, as I would assume that high speed would stress and heat tire enough to produce high pressure regardless. Or, does high pressure from the start prevent high temp buildup in the tires? ........

Yes, increased starting pressure reduces the amount of heat buildup, which is why in high speed operation it is recommended to use higher pressures. Keep in mind that the other things that adversely affect tire performance are decreased - properties such as the risk of impacting an object.

 Originally Posted By: friendly_jacek

....... But then, that would invalidate all that load calculations you elegantly presented. Thanks!

The load calculations also include other factors that are part of the "Operating Conditions". A good example of this is tires for Mobile Cranes. In the "Off-the-Road" section of the Tire and Rim Association Yearbook - the one where the load table comes from, there are a series of similar load tables for various types of tires - Earthmover, loader/dozers, graders, .... and Mobile Cranes. Part of the table includes a page of notes. Within those notes are modifiers to the load carrying capacity. For example, at 10 mph, operation on a smooth floor allows the load capacity to increase 35% - while at 15 mph, the load is decreased 15% for off-road conditions, and increased 25% (over the stated load in the table) by 25%. This principle is true for all tires - it's just that passenger car tires have a fairly well defined set of operating conditions, as well as a need for simple and easy to use instructions.

 

[friendly_jacek]

Thanks CapriRacer for the in depth info. I will keep using my increased pressure, maybe modified some by chalk testing.

 

[y_p_w]

 Originally Posted By: CapriRacer

This principle is true for all tires - it's just that passenger car tires have a fairly well defined set of operating conditions, as well as a need for simple and easy to use instructions.

Why can't it get a little more complicated (other than some people will find it over their heads)? I remember the folks' Camry, which comes with different recommendations for basic loads and loads up to gross vehicle weight rating. I know some car companies have turned oil weights into a one-size fits-all recommendation - such as Honda with their 5W-20 across the board for all conditions and nearly all cars from the Fit to the Pilot. My WRX however has an US owner's manual that leaves it to the user to decide, with three recommended oil weights (5W-30, 10W-30, 10W-40) as well as high-speed/high-temp/towing recommendations for 30, 40, 20W-40, 20W-50. In other markets (like Europe) the same cars come standard with 5W-40. Why can't it be a little more complex than to just set it to 31/29 PSI front/rear?

 

[benjamming]

ypw, 31/29 psi front/rear is too complicated for many so 30 front/rear. Heck, any tire pressure is too complicated for many so now we have government mandated TPMS

 

[y_p_w]

 Originally Posted By: benjamming

ypw, 31/29 psi front/rear is too complicated for many so 30 front/rear. Heck, any tire pressure is too complicated for many so now we have government mandated TPMS

I took another look and it's actually 32/29 for my WRX. I've got aftermarket tires, so I bumped it up to 34/31 and it feels pretty good with decent wear and handling.