tire pressure?

Hi...

Have one more question, if I may?

Driver side front tire was leaking down; took it into Walmart and
asked them to fix it up. (mistake?)

They did, but both on the bill and verified with a pressure guage
they've pumped up both tires to 35 psi.

Tires say max 35 psi; the door panel says 30 psi.

Ride now is harsh, so harsh that body noises are obvious. On the other
hand fuel is expensive (but my lifestyle is such that it doesn't
matter much) And it might feel harsh because of my lifetime of
soft boatlike GM ride.

Which do you folks recommend?


Thanks, and take care.

Ken

 

35 psi might be a bit high. Try 32 psi cold. But remember, as the temp drops
over the next month or so a 35 psi tire will become a 32 psi tire .

Richard.

 

Well, this topic was beat to death fairly recently, so you may want to
search the ng. Basically, their are a range of opinions. I fun my
tires at the max pressure that the tire maker allows. I don't find the
ride all that harsh on my minivans at 35 PSI. I find it gives the best
and most even tire wear and maximizes MPG. However, if ride is more
important to you, then use the car maker's recommendation. It is meant
to be a decent compromise of ride, mileage and tire wear.


Matt

 

Would not the tire size, make and vehicle gross weight have anything to
do with all of this? It's all a bit strange. I thought I had
over-inflated the Voyager's 14" 205x70 and was all set to let air out.
But it was actually under-inflated. So there went my advanced
perceptions of tire pressure.

Pressure was around 30 psi. The placard has it officially at 32 psi. I
try to keep the tires at 39 psi [cold pressure, tires rated to max 44
psi cold]. Nice comfy ride. Still can't figure out why it felt
over-inflated when under-inflated. Usually under-inflated is a mooshy,
soft but quiet ride. But it explained the mpg loss until the oxygen
sensor started to act up instead.

Just when I thought I was getting ahead...

 

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.

 

I'm surprised they aren't required to put microprocessors in the tires
to adjust the air pressure for all operating conditions. I saw a
commercial on TV this morning for a freakin' toothbrush with a
microprocessor built into it! Idiots!

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

 

The popular Sonicare toothbrush has a microcontroller of some sort in it.
This is how they provide a less vigorous initial mode, time the cycle and
provide the points to switch quadrants during the brushing process.

Technology overtakes us all, Bill. Then there is the microprocessor
controlled toilet in Japan.

Ken

 

Heh heh! The one I saw was an Oral B product. So I guess there are at
least two microprocessor controlled toothbrushes on the market now.

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

 

Suggest you over-inflate the tires before the snow flies. Once it does, the
air pumps in gas stations freeze up and stop working. If your tires are
low, it's tough to get them pumped up in the dead of winter, especially a
Canadian one.

Happy Thanksgiving to my Canadian neighbours.

Arthur

 

Eh?

There's this $15 little 12 volt auto pump, eventually does the job.
Need to get one now. They get sold out during the summers with all the
air mattresses and beach balls that need filling. Might be handy when
things get messy.

 

Hi...

Not too pretty much fun to use at 40 below, though, eh?

If you're picking one up at Wallmart, look carefully. The
gauges read up to 200 psi, so all of the "tire pressure" range
is compressed into a tiny portion of the bottom of the scale.

There are (or were) two that look identical side by side on the
shelf. They look identical, but aren't; one of them has far, far
easier to read gauge. (at least to my old eyes)

Take care.

Ken

 

I just looked at one. Even distribution of numbers from 0 to 300 I
think. Not the biggest numbers in the world but could make out 20 40 60
which would be enough to work with. Not worried about 200 psi at the
moment. Now if I'm trying for 37 psi, that's a problem since a small
mark between 35 and 40 but doable.

You bring up a good point, although a cold one. What if the pump does
not work well in 40 below? I don't know. It has a short air hose, very
short, but a long electrical connector but that's not important. I
guess one could try and connect a long air hose so the unit is inside
the vehicle working with warmed air. Another hassle. Might work though.
Need to ask those in your neighborhood.

So another suggestion. Those cans of inflatable air, like Fix-A-Flat. I
keep them under the seat. The cheap one-shot cans as well as the large
reuseable cans. Since it's in the usually warmed vehicle, it's at a
useable 70 F or whatever it needs to work. If not, I put them right
under the heater to get them nice and toasty but not exploding. The new
formulations may not be explosive as the old dangerous, nasty ones
were. In any case, if it's used, it's nice to warn any tire worker that
the stuff is inside the tire and can come flying out if one unmounts a
tire. It turns into a liquid as I recall and sloshes around inside the
tire. It does work and seals slow leaks well. One just drives a mile or
two for the stuff to really expand and swell and seal a tire. Now does
it expand and seal in 40 below? Don't know. If I mess up screwing it
onto the valve, it's like filling a tire with shaving cream. Oh joy!

At this point, I think I would just call up those in the business and
ask what do you do at 40 below 0 F with an underinflated tire? Is this
McMurdo Base? Ah, what do you guys do when your tires go pffft at 100
below? Liquid nitrogen? Just kidding. Probably freeze the rubber into
smithereens. On second thought.