Tire air pressure question

The tires on my 2001 PT call for 34 psi cold tire pressure.
Since mpg and tire wear are based on the amount or tire tread* that
contacts the pavement, wouldn't it make sense for the front tires to
have a higher psi to compensate for the weight of the engine?
Looking at the tires makes it quite clear that more tire surface of the
front tires contact the road than the rear tires.

* I know that other things such as driving habits, tire sidewall flex,
etc.. also effects the mpg and tread wear, but my question is just
about the air pressure.

 

Not necessarily. I'm not a tire engineer, so I don't know the answer to
your question for sure, but here's something to think about. The
question is whether it is more important to have the same pressure on
the road from all of the tires or the same area on the road. I'm
guessing it is better to have uniform pressure at each tire and thus the
reason that the same pressure is specified for each tire. If you
increase the tire pressure to make the contact patch size the same front
and rear, then you are also increasing the pressure on the road exerted
by the front tire as compared to the rear.

I suspect having different contact pressures is probably worse for
handling, etc., than is having different contact areas.

If you know this, why do you state above that MPG and tread wear are
based on the amount of tread that contacts the road?


Matt

 

Lots of flexing and high rate of flexing generates heat ( = absorbs
energy that has to be made up by the power source/engine/additional fuel).

From a rolling resistance/fuel mileage standpoint, the best tire would
be a rigid metal tire. You would essentially have an infinitly small
contact point (line) and no flexing (compared to what you have with a
rubber/jello tire). Wouldn't have very good traction, but if the *only*
priority was fuel mileage, we would have metal tires.

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

 

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Subject: Re: Tire Pressure
Date: Mon, 26 Jul 2004 21:00:55 -0400
From: MoPar Man <>
Newsgroups: alt.autos.dodge, rec.autos.makers.chrysler

If you're a tire, you don't really care what car you're on. You care
if you're mounted on the correct-sized rim, and then you care about
how many pounds you're being asked to carry.

It's actually a pretty dumb thing for car makers to put the tire PSI
on the door jamb. Ok, well, they do know what tires they put on the
car from the factory, so that's really the only *correct* situation
where the door-jamb PSI spec is valid. Other than that, once you
start putting on different tires (and different sizes) then the tire
makers should have specs as to what a given tire should be pressurized
to for a given weight to obtain the correct rolling profile.

You need enough pressure so that the tire doesn't deform a lot
(flatten-out) as it turns. That's a function of the weight of the car
(and all cars are different). Too much air results in too little
contact patch surface (and a hard ride, and too much center wear, but
probably great fuel economy).

Most passenger car tires know they're going to be carrying (3700 / 4 =
) 925 lbs, so you'd think that instead of the ridiculous "max
pressure" rating on a tire that there'd be *the correct freeking PSI
rating* for 1000 lbs load.

What ever happened between Ford and Firestone's SUV tires? Was it
proven that Ford's door-jamb rating was not correct for the particular
tires that were blowing out on the highway?

---------------

Subject: Re: Tire Pressure
Date: Thu, 29 Jul 2004 23:03:02 -0400
From: MoPar Man <>
Newsgroups: alt.autos.dodge, rec.autos.makers.chrysler

I've never seen a sidewall that has the "recommended" pressure
stamped on it. They usually have the "max pressure".

Tire makers seem reluctant to stamp a recommended pressure on tires.
That seems odd. I'm sure they have a chart for every tire showing PSI
on the Y axis and load (weight) on the X axis.

The max load rating of a tire is proportional to the volume of air
contained within the tire. The wider and taller a tire is, the higher
the MAX PSI is (and the higher the MAX load is).

http://www.goodyear.com/truck/pdf/databook/5113_Lsec_V.pdf
http://www.tirerack.com/tires/tiretech/tirespecskey.jsp#maxload
http://www.dodge.com/bodybuilder/2004/docs/intro/tirecharts.pdf

RV forums sometimes have tire PSI discussions - such as in this link:

http://www.rv.net/forums/index.cfm/fuseaction/thread/CFB/1/TID/246059.cfm

The correct strategy here is to "(1) weigh the rig and (2) consult the
tire manufacturers load/ pressure charts to find the right (air
pressure) value."

The PSI spec found on the door jam sticker of passenger cars assume
that all tires have the same load/pressure chart (you tell me if
that's a realistic assumption). It would be interesting to look at
the tire inflation specs from 3 different 300M cars (one with 16"
tires from the PHP package, one with the standard 17" wheels, and the
third with 18" wheels from the 300M Special package). Are the PSI
values different?

Clearly, the PSI value on the door sticker has some over-all car
weight in mind. If it's the GVWR (max gross weight which is a fully
loaded car) then if you drive around with only yourself (and not 4
other adult passengers) then inflating your tires to the spec value is
technically over-inflation for what you're doing (it's safe, and it's
probably going to give you good milage, but it's going to give you a
harsh ride - and depending on the conditions of your roads your tire
will experience more internal "injury" with higher PSI's).

The goodyear link (above) has a chart which also indicates that up to
5 extra PSI (for car tires) and 10 PSI (truck tires) are recommended
for speeds between 65 and 75 MPH. The extra pressure at high speeds
is desired to reduce the amount of flexing within the tire as it
turns. There is energy (heat) generated within the tires as they
turn, and the tires can only dissapate so much heat per unit time. At
high speeds, you want the tire to generate less internal heat per
revolution - so you give it more internal air pressure.

--------------

Subject: Re: tire pressure?
Date: Sun, 09 Oct 2005 11:14:15 -0400
From: MoPar Man <>
Newsgroups: rec.autos.makers.chrysler

For some reason, automakers feel compelled to post tire PSI numbers on
the car (door jab) or in the operating booklet. Maybe it's the law
(however quaint).

Technically the only time these numbers are right are when they
pertain to the oem tire.

The PSI you should put into any given tire will depend on the size of
the tire (the VOLUME of space inside the tire), the load you will put
on the tire, the width of the tire (which plays a role in the first
item), the construction of the tire (thickness of sidewall), etc.

Tire makers are notorious for not providing PSI vs load charts for
their tires. They only indicate the max load and max pressure.

No doubt there are liability fears over making the correct information
more available or correctly posted/printed in appropriate locations.
Give as little information as possible, and make it as vague and
un-specific as possible. In the case of tire makers - if you don't
have to provide it, then don't provide it.

Vehicle makers would like to see you ride with tires that are more
under-inflated than over-inflated. Under-inflated tires will absorb
more road bumps and reduce the work of the suspension system to handle
pot-holes and such. Automakers can essentially let under-inflated
tires "eat" road problems and don't have to put extra cost and
engineering into the suspension system to do this (more "protection"
for aluminum alloy wheels for example if the tires absorb punnishing
city potholes).

Under-inflated tires also make it harder to roll larger vehicles
(SUV's) so again they'd rather see an under inflation rather than over
inflated. They may even purposely post low PSI numnbers on the door
jamb decal to insure this (Ford vs Firestone for example).

If the tires fail or wear prematurely, the owners will blame the tire
(brand and maker) so the auto maker has nothing to lose by posting low
numbers (ie it doesn't reflect poorly on them or have negative
consequences unless you really dig into the issue, which few do).

Ideally, you want a PSI that prevents (as much as possible) the tire
flattening out as it rolls. Shape change for the tire equals internal
heating and wear - which you want to avoid. Then again, you can't
inflate tires to 100 psi either. Unless the roads you drive on are
really bad, you're better off with 35 PSI vs 30 PSI for a typical
passenger car tire (215/60/16 or 225/55/17). If you consistently car
pool or load your vehicle with more than just the driver, then you are
definately better off with 35 psi.

For summer highway driving, the fact that the PSI goes up because the
air inside gets warm is a good thing because

1) highways are generally smoother than city/urban roads, so there are
likely to be fewer dammaging or irritating bumps or other
irregularites (over-inflated tires _might_ result in reduced road
noise vs under inflated tires).

2) fewer stops and starts on a highway also puts reduced load on the
tires, very little lateral or shear forces because you're usually
going in a straight line (curves are banked, etc).

3) higher speeds means more rotations per second or per minute for the
tires. Each rotation equals tire distortion or flattening, which
equals internal heat generation and wear. Increase the PSI and you
reduce the extent of tire deformation pre rotation, which reduces
internal wear and heat generation (not to mention reduced rolling
friction, and better fuel economy). When you get off the highway,
internal air temperature will go down, so will PSI, and you shift the
operating point of the tire to city/urban roads, where road handling
(road contact, bouncing, cornering, etc) become more important.

 

Hi Bill...

Perhaps not solid metal; how about solid metal with a
non-inflated rubber surface instead?

Solid metal hitting even a smallish stone on the road would
mean that the stone would have to be either crushed or
climbed over... which would cost energy

Take care.

Ken

 

Heh heh! Maybe, but I think that solid metal would still be more
efficient. Of course it is a moot point as neither would be used in the
real world. Trains of course are what we're talking about - but they of
course don't have the rocky surface to ride on like a road. Hmmm -
that's the answer - steel wheels and steel roads!

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

 

Ignore the tires. You need to look in your owners manual at the inflation
instructions. That'll answer your question.

 

I can't believe all the windy answers to this question. Nobody seems to
notice that you read that off the tire. The manufacturer of the tire does
not know which tire is on the front and which is on the back. For pete's
sake. The directions from Chrysler will be in or on the vehicle. Read that.
It may well be higher in front.