| Re: [GMCnet] Hensley GMC Motorhome Airbag Solution [message #118313] |
Thu, 10 March 2011 10:20  |
Gary Casey
Messages: 448
Registered: September 2009
Karma:
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Senior Member |
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Excellent post from Rob. A few comments: He said "The max load a tire can
carry and the max pressure." More correctly it would read that it is the "max
load a tire can carry AT the max RATED pressure." So if you run a load range E
tire at 65 psi it has the same load "rating" as a load range D tire. For that
reason I see no incentive to go out of one's way to get a E-rated tire unless
the load was such that the pressure had to increased above 65.
Another thing - the "load rating" of a tire is the load that can be carried
resulting in the normal life and reliability of the tire. Overloads from
cornering, braking, etc. are already taken into account. Therefore, overloading
the tire for a brief period at modest speeds should not be a cause for
particular concern. However, loading it to 68% overload is a bit extreme. If I
needed to get to town and it wasn't more than maybe 20 miles away, and I could
keep my speed down and the road wasn't too rough, I suppose I would do it. Then
there is the stress on the suspension arm - that's a different question, but I
think the answer is the same. A 50% overload is likely tolerable, but all it
takes is one chuckhole with that load to perhaps bend the arm - it's not a
matter of durability. Handbrake turns not allowed.
I still don't know how the system physically carries the load. The air pressure
is not that far below the compressor pressure during normal operation - how do
you put maybe 50 to 70% more air pressure in the bag to carry the load? The
only way I can think of is to jack the coach to the max height, fill the bag
with the max pressure and then let it down. Is that the way it's done? If so,
the jack you presumably don't need because of the Q-bag is still required to
lift the coach.
And then there is the question of which tire is flat: If it is the rear, the
middle tire will carry considerably more than if were the middle that is flat.
I guess I'll stick to the factory bags at least for now and just keep the
insurance policy paid up. :)
Gary Casey
'74 23' lightweight
Fred,
I posted this data awhile back and no one commented.
Reference GMC Motorhome Operating Manual X-7521A (issued 1974)
GVWR (Gross Vehicle Weight Rating) represents the maximum permissible loaded
weight of the vehicle.
26 foot GMC GVWR = 11,700 lbs
GAWR (Gross Axle Weight Rating) the maximum weight that the axle can carry.
26 foot GMC GAWR
GAWR = 4,200 lbs Front Wheels
GAWR = 7,500 lbs Rear Wheels
Tire Load Range: The max load a tire can carry and the max pressure.
D load range max load @ 65 PSI = 2,235 pounds
E load range tire max load @ 80 psi = 2,680 pounds
The weight distribution per tire would be dependant on the how much pressure
the GMC driver applied manually to the air bag supporting the remaining tire
or the Powerlevel/Electrolevel systems applied automatically. For arguments
sake lets say it is what the two tires supported. Using the rear GAWR noted
above the load on the tire would double going from 1,875 pounds to 3,750
lbs.
The overload would be 1,515 pounds (68%) on a D range tire at 65 psi.
The overload would be 1,070 pounds (40%) on an E load range tire at 80 psi.
If the tires are not inflated to these pressures the overload would be even
higher.
It should be noted that using the 11,700 GVWR noted in the manual is
actually a low number. Ken Burton posted the average weights on each of the
wheels of the coaches that were weighed at the DuQuoin GMCMI Convention. I
added them together and came up with a weight of 12,563 pounds. If you would
like to see the message in which Ken posted that information it is message
#101370 on the Forum.
The bottom line is that if you drive your GMC with one tire removed (or one
air bag blown) you WILL be operating the GMC with the remaining rear tire
overloaded.
I am of the opinion that it is not safe to do that.
In one of the responses I sent in to this question I noted that if I was a
couple of miles out of town I'd probably do it. However, upon reflection I
don't think I would do it. The reason being I hadn't taken into
consideration that I could damage the remaining tire. I would proceed at
slow speed to the closest spot where I could effect repairs.
Regards,
Rob M.
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