GMC Tire pressures

claude brousson

New member
Jan 20, 1999
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Well I am going to launch in and add some more opinion, fact and likely
myth to the ongoing tire "wars" which we all seem to rather enjoy.

It is sometimes said that our suspension system is unique and the
dynamic loading is different than other vehicles. Perhaps that was
somewhat true some years ago, but today there are many motor homes and
also trucks which ride on air springs at each wheel.If one has a gauge
monitoring the air bag pressures one will see the needle going up and
down slightly as the bag is compressed and released. I think the tire
would experience the same thing. What is different for most of us is we
have one air bag for two wheels, whereas others have one for each wheel.

Any vehicle which has tandem wheels one in front of the other as we do
will have lateral forces on the tires. Recently I watched a large
unloaded truck with a trailer do a full tight turn in the loading yard.
His rear middle tires literally scrapped totally sideways on the
pavement as he turned with the rear/ rear, ones serving as the fulcrum
point. He laid down a good mark of black rubber from the turn. This was
all done with no load.Our vehicles are similar except the wheel arms
twist a bit so the wheels look as if they are going to come off almost.
Of course large trucks have more than E range tires!!

I would like to see one of those who preach that one should have 80 lb.
in the tires, regardless of what ones vehicle weight is,contact each of
the tire companies
and ask them the question. I would like to wager , that they will not
support that concept unless of course ones vehicle weight is up close to
that requirement. Doubtless some with stretched vehicles, bigger tires,
and loads of other stuff may be up near the 2680 lb. limit especially on
the front. I spoke with one company Michelin- and they do not suggest a
much higher pressure than ones load.Likely it is prudent to have a
little more pressure than ones exact loading due to changing loads, and
likely most are not checking their tire pressures every week.So really
the greater risk is running some time with not enough air due to lack of
checking pressures. For the person who hates to check, they should be
sure they keep enough margin of safety.

While it is true, an E range tire can not empirically carry any more
weight than a D range at the same weight, I would think that be virtue
of its greater strength it will likely in the end stand up better. For
example, a large motor and a small motor may be going down the pike at
the same speed and be using the same amount of gas, but when push comes
to shove, one can tromp on the gas and make the large motor go a lot
faster, because it has a lot more reserve power and strength which the
smaller doesn't have.Likwise the E range has more residual toughness to
take more and heavier punishment than the D range even though both have
the same weight carrying ability.
Punishment for punishment, the E will stand up longer than the D.

Incidentally, Michelin no longer makes the D range 8.75 and the 9.50R
16.5 is also scheduled to be eliminated in D range, so for those who
love their D's it is going to get harder to get them.

Soooooooooo-what do I do. My rear wheels have about 1700 lb. weight on
each one, so theoretically I could run at 45 lb. in each tire. Front
right can be as high as 2200- 2300 lb.. so that means 65 lb. pretty
well. For simplicity,easy to remember,
gas mileage, tire wear etc., I keep 65 lb. all round.

Now for the "80 lb." guys who don't have any where near 2680 lb. in
their rear tires, lets have some back up facts from the tire companies.
What do you say fellows.

Whoops wife just called for breakfast got to go.

Claude.

Perhaps one could then argue that if one is loading up the tires way
over what the load requires, one is actually stressing the tires and
punishing them more than necessary.Who knows.Why doesn't someone ask the
tire companies.
 
Hi Claude,

Some months ago I swore off this whole topic. But here goes one last time
anyway.

>It is sometimes said that our suspension system is unique and the
>dynamic loading is different than other vehicles. Perhaps that was
>somewhat true some years ago, but today there are many motor homes and
>also trucks which ride on air springs at each wheel.

The uniqueness is not air springs. It is in the entire geometry. The GMC
has a leading arm and trailing arm suspension that is coupled using the air
spring. Other suspensions are separate axles with separate shocks and air
springs. Different geometry, different forces, different consequences.

>I would like to see one of those who preach that one should have 80 lb.
>in the tires, regardless of what ones vehicle weight is,contact each of
>the tire companies
>and ask them the question. I would like to wager , that they will not
>support that concept unless of course ones vehicle weight is up close to
>that requirement.

Well you lose your bet. Goodyear said "inflate the tires to the pressure
recommended by the coach manufacturer." GM says LRE at 80 PSI.

>Soooooooooo-what do I do. My rear wheels have about 1700 lb. weight on
>each one, so theoretically I could run at 45 lb. in each tire. Front
>right can be as high as 2200- 2300 lb.. so that means 65 lb. pretty
>well. For simplicity,easy to remember,
> gas mileage, tire wear etc., I keep 65 lb. all round.
>
>Now for the "80 lb." guys who don't have any where near 2680 lb. in
>their rear tires, lets have some back up facts from the tire companies.
>What do you say fellows.

I say we've been over this ground many times before. If I thought that the
facts really mattered to anyone running at lower pressures I'd spend more
time helping them to understand. BUT, uninformed opinion seems to hold more
sway than the manufacturer's recommended practice. The simple fact is that
GM (first through GM dealers and now through their sole licensee Cinnabar)
states that the tires must be steel sidewall LRE at 80PSI.

Henry
 
Judge Judy would say, "put a period, and move on."

> ----------
> From: casady[SMTP:casady]
> Reply To: gmcmotorhome
> Sent: Tuesday, March 28, 2000 12:56 PM
> To: gmcmotorhome
> Subject: Re: GMC: GMC Tire pressures
>
> How hot or cool your tires run has nothing to do with it. You probably
> have the right amount of air (whatever That is) in them. If you run
> with less air the E's will be hotter maybe still cool but none the less
> hotter, than the D's. Also it is not just the sidewall but the belts
> and tread as well, they flex too. You also will have less heat if your
> tread is almost gone, maybe that is why racers shave the tread on new
> tires.
> As for sidewalls, bias ply (does anybody use those anymore) have much
> stiffer sidewalls than any grade radial.
>
> Gee now that I think about it maybe we should run new worn out D tires.
>
> The real point of my post was that tires are more complicated than
> load range and air pressure.
>
> >
> > Boy where did this one get started. E range all steel tires have stiffer
> > sidewalls than D range no doubt about it, and they flex less not more.
> I
> > can tell you at 80PSI that my Michelin XPS ribs were very cool to the
> touch
> > after travelling at 70-75MPH on the way to Florida. I always like to
> check
> > both the sidewalls of each tire and the hub of each wheel when stopping
> for gas.
> >
> >
> >
> >

> > >

> > >
> > > range
> > >tires are stiffer because of the extra material so they generate more
> > >heat when they flex. Therefore you need more air in an E than a D tire
> > >carrying the same weight or the E will fail sooner from excess heat.
> > >The E tire of course can take more air and carry more weight, but only
> > >if fully inflated.
> > > >>
> > >
> > >Sorry but I don't believe this.
> > >
> > >Emery Stora
> > >77 Kingsley
> > >Santa Fe, NM
> > >
> > >
> >
> >
> >
>