Specifying tire pressures

[Jason]

I found this post on BITOG interesting, and thought it worth sharing. The author [CapriRacer] is a tire engineer. The post was in reply to somebody installing larger tires on his Corolla than what it came with (not me), but most of the rest of the post is interesting reading in general...



According to Tire Guides, in 2007 only the CE model of the Toyota Corolla came with P185/65R15's, but they came with 15X6" wheels. I'm assuming they are aftermarket wheels if they are 15X7"'s.

But if not, be aware that the 6" rim width is the smallest rim that a P215/60R15 fits on and may be part of the source of the center wear.

Another thought: THEE most important thing about tires is load carrying capacity - and inflation pressure is a major factor when it comes to that! When vehicle engineers size tires and select inflation pressures, it's where they start.

One of the lessons learned from the Ford / Firestone situation a few years ago is that the load carrying capacity of the tire's size / inflation pressure combination has to be larger than previously thought. In other words, there has to be some unused capacity.

The load carrying capacity of a P185/65R15 at 30 psi is 1088#. That means the load carrying capacity of the front tires was 2176. At the published GVW of 3585# and assuming a front / rear weight split of 60 / 40, the front load on the tires is 1971#, so the tires have about a 10% unused capacity - which seems a little small for my taste.

To carry the same load a P185/65R15 does at 30 psi, a P215/60R15 needs to have only 20 psi. (Actually the chart doesn't go that low, so I'm interpolating - plus I do not recommend anyone ever use less than 26 psi in passenger car tires, because that the pressure it is possible to unseat the bead when doing emergency manuvers.)

So using 39 to 35 psi is ...... well ..... way more load carrying capacity than the original tire size / pressure combination. As indicated, this may also contribute to the center wear.

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.

[Benny]

Interesting post you found, the last bit is why i tend to keep my tire pressure at or near what the sticker on my car says because if I do anything different, it could change it in a bad way. Everytime we look at a tire we think a tire is a tire, and yet, there's more to them than what you think, each tire has to be made to work exceptionally on a wide range of vehicles, and the car should drive like car engineer's designed it to.

[Jeepman]

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.

That's a very good point. A person might tend to over drive highly inflated tires.

[Christ]

Originally Posted by Jeepman

That's a very good point. A person might tend to over drive highly inflated tires.

Yeah, unless you don't drive like a jackass... then you shouldn't really have any problems. $0.02.

[shipo]

Originally Posted by Christ

Yeah, unless you don't drive like a jackass... then you shouldn't really have any problems. $0.02.

Even folks who don't drive that way occasionally need to make an emergency maneuver, in which case, properly inflated tires might mean the difference between life and death.

[Chrysler1924]

So, if even slightly over-inflating your tires as some people do for fuel-economy purposes say from 35-40 PSI causes the center of the tires to wear out as per the article states, then what about a properly inflated tire?

Example:

1996-2000 Chrysler minivans, the ones equipped with 16" wheels require 35PSI (I've never looked at the door sticker for a 15" model.) But as we all know, when that 35PSI tire gets warmed up, the tire pressure will increase to ABOVE the spec. 35PSI... does that mean the center of the tire is going to wear unevenly anyway?

[shipo]

Originally Posted by Chrysler1924

So, if even slightly over-inflating your tires as some people do for fuel-economy purposes say from 35-40 PSI causes the center of the tires to wear out as per the article states, then what about a properly inflated tire?

Example:

1996-2000 Chrysler minivans, the ones equipped with 16" wheels require 35PSI (I've never looked at the door sticker for a 15" model.) But as we all know, when that 35PSI tire gets warmed up, the tire pressure will increase to ABOVE the spec. 35PSI... does that mean the center of the tire is going to wear unevenly anyway?

No, they won't wear unevenly due to the fact that the specification was determined with the additional pressure due to tire warming in mind. If anything, if you could somehow keep your tires from warming up, 36 psi is too low for continuous operations.

[Jeepman]

Originally Posted by Chrysler1924

So, if even slightly over-inflating your tires as some people do for fuel-economy purposes say from 35-40 PSI causes the center of the tires to wear out as per the article states, then what about a properly inflated tire?

Example:

1996-2000 Chrysler minivans, the ones equipped with 16" wheels require 35PSI (I've never looked at the door sticker for a 15" model.) But as we all know, when that 35PSI tire gets warmed up, the tire pressure will increase to ABOVE the spec. 35PSI... does that mean the center of the tire is going to wear unevenly anyway?

A tire is designed for use on a wide selection of vehicles, hence the maximum psi on the sidewall will accomodate most applications where the specified pressures, per placard, are lower.
From a NHTSA web page on Tire Safety at http://www.nhtsa.dot.gov/cars/rules/.../brochure.html

Because tires are designed to be used on more than one type of vehicle, tire manufacturers list the "maximum permissible inflation pressure" on the tire sidewall. This number is the greatest amount of air pressure that should ever be put in the tire under normal driving conditions.

This site http://www.tirerack.com/tires/tirete....jsp?techid=72 has some interesting info about upping tire pressures to accomodate high speeds (118 to 168 mph) and the load reductions that apply, even when the max sidewall pressure is used, after 150 mph. It also says:

Never exceed the maximum cold inflation pressure branded on the tire's sidewall.

As already stated, the max inflation psi stated on the sidewall takes into account increases in air pressure during driving. In other words, if the max is 45 psi, then an increase to say 48 psi, or higher, during hot weather driving, is designed for and fine. There are safety factors built in. As to wear, one would need to monitor what's going on and if pressures need to be adjusted. Keep in mind that the pressures, per placard, are a compromise between wear and comfort, so a little more air should provide a little better wear and a little less comfort.

[Christ]

And yet, after years of running nearly ALL my tires at 50 PSI, I still haven't had any wear oddly... strange. I must be a pretty lucky guy when it comes to tires.

[Chrysler1924]

Okay so with all of this in mind, what do you make of this?

My 1996 has BF Goodrich Premier Touring tires tires sitting on its 16" rims. The tires are inflated at 35 PSI cold. When I look at the van from the side, the tires have an almost perfect sidewall margin all the way around the tire. Even the spot where the tire is on the ground isn't even a little more compacted under the weight of the van like you'd expect. Perhaps it is, but it is barely noticeable.

Shift your glace to the Chrysler Town & Country parked in front of it. This vehicle has brand new Michelin X-Radial tires sitting on its 16" rims. When these tires are inflated at 35 PSI cold: I look at the van from the side, the tires do NOT have a perfect sidewall margin all the way around the tires in the front. The spot where the tire is on the ground is pudged out A LOT in comparison to the 1996. It's not a small difference, it's big. The rear tires look the same on the 2000 as they do on the 1996. But the front tires on the 2000 look like they are going flat when inflated to the specified 35 PSI.

They've always done this regardless of what tire has been on the van. So... I've been worried about damaging the sidewalls of the tires because that's a great way to damage them. So I've been running all four of the tires at at LEAST 40 PSI on the 2000. This makes the front tires pudge out less. Since they are brand new tires, I don't want to wreck the side walls.

I realize the 3.8L engine is a slightly larger engine than the 3.3L but good lord... it can't weigh THAT much more. Plus most of the weight difference between my 1996 and 2000 isn't in the front, its in the back! So I'm confused on what to do with the tires.