GMC Rear tires

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claude brousson

New member
Jan 20, 1999
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Wellllll Guys-

Nobody read my post- The question was , and of course it was purely a
theoretical, mythological, hypothetical one--where is the fulcrum point
at the rear when the vehicle turns and so in which position would the
worst tires have the best chance of not having a blow out due to stress.

It seems that so many GMCers have had blow outs that we are now so
psychologically stressed out and sensitive about this that we
imeadiately rush to report our blow outs and start to conduct surveys,
and forget the question.Hopefully this will have a substantial cathartic
effect on our psychees and there will be less of a need too grip the
wheel with white knuckles.

Sorry guys but that's why I was asking the engineers as I lack in that
area,because the grid I view the world through is the psychological one.

But just to be fair to Roger Black - I also sent him my report of a blow
out. And it is really interesting to get a picture of many experiences.
But where is the fulcrum point, I'm still wondering?

Claude in Victoria.
 
Claude's question, I believe, was the beginning of the survey. Which tire
had the most stress. Also asked was where to install a marginal tire. The
two most significant, although unscientific by any stretch of imagination,
is that the most blowouts from any cause have occurred on the left and right
middle, and the right rear. Age also shows to be a factor and the change em
at the 5 year mark may be off by a year. There are many other factors to be
considered but were not included in the survey, such as mileage, type of
roads traveled, air pressures, etc. The survey was to see if there was any
pattern on blowouts and what wheel position would it be.

I'll maintain this for a while, you people tell me when it's time to quit
recording. Each of you has to take the information and use it according to
your experience. I will make no judgments as to right or wrong, only the
patterns I see emerging.

You make the call.

Roger
 
In a turn the coach puts equal stress on the tires of the middle and rear
bogies. In a severe stop the most stress is put on the middle bogies.

>Wellllll Guys-
>
>Nobody read my post- The question was , and of course it was purely a
>theoretical, mythological, hypothetical one--where is the fulcrum point
>at the rear when the vehicle turns and so in which position would the
>worst tires have the best chance of not having a blow out due to stress.
>
>It seems that so many GMCers have had blow outs that we are now so
>psychologically stressed out and sensitive about this that we
>imeadiately rush to report our blow outs and start to conduct surveys,
>and forget the question.Hopefully this will have a substantial cathartic
>effect on our psychees and there will be less of a need too grip the
>wheel with white knuckles.
>
>Sorry guys but that's why I was asking the engineers as I lack in that
>area,because the grid I view the world through is the psychological one.
>
>But just to be fair to Roger Black - I also sent him my report of a blow
>out. And it is really interesting to get a picture of many experiences.
>But where is the fulcrum point, I'm still wondering?
>
>Claude in Victoria.
>
>
 
On Wed, 10 Nov 1999 21:20:01 -0800 Claude Brousson
writes:
>Wellllll Guys-
>
>Nobody read my post- The question was , and of course it was purely a
>theoretical, mythological, hypothetical one--where is the fulcrum
>point
>at the rear when the vehicle turns and so in which position would the
>worst tires have the best chance of not having a blow out due to
>stress.
>
Speaking for myself, I read it but didn't understand the point and didn't
know how to respond.

I wouldn't want anything on my coach that could be described as my worse
tire. After a couple of hundred thousand miles of experience with the GMC
there is no doubt in my mind that the forward tandem takes a beating and
the front tires also are stressed. That is all I need to know. But I
change all tires at the same time and I use the best tires money can buy,
suitable to my application!

Dave Greenberg
GMC MOTORHOME REGISTRY
Seagate Towers 200 MacFarlane Dr PH4
Delray Beach, FL 33483-6829
 
Claude -
My guess is that its exactly midway between the footprints of the two tires,
so the two tires see the same side forces, but in opposite directions.
Robin

> ----------
> From: Claude Brousson[SMTP:cbrousson]
> Reply To: gmcmotorhome
> Sent: Thursday, November 11, 1999 12:20 AM
> To: gmcmotorhome
> Subject: GMC: GMC Rear tires
>
> Wellllll Guys-
>
> Nobody read my post- The question was , and of course it was purely a
> theoretical, mythological, hypothetical one--where is the fulcrum point
> at the rear when the vehicle turns and so in which position would the
> worst tires have the best chance of not having a blow out due to stress.
>
> It seems that so many GMCers have had blow outs that we are now so
> psychologically stressed out and sensitive about this that we
> imeadiately rush to report our blow outs and start to conduct surveys,
> and forget the question.Hopefully this will have a substantial cathartic
> effect on our psychees and there will be less of a need too grip the
> wheel with white knuckles.
>
> Sorry guys but that's why I was asking the engineers as I lack in that
> area,because the grid I view the world through is the psychological one.
>
> But just to be fair to Roger Black - I also sent him my report of a blow
> out. And it is really interesting to get a picture of many experiences.
> But where is the fulcrum point, I'm still wondering?
>
> Claude in Victoria.
>
 
>>
>>Wellllll Guys-
>>
>>Nobody read my post- The question was , and of course
>>it was purely a
>>theoretical, mythological, hypothetical one--where is
>>the fulcrum point
>>at the rear when the vehicle turns and so in which
>>position would the
>>worst tires have the best chance of not having a blow
>>out due to stress.
>>>But where is the fulcrum point, I'm still wondering?
>>
>>Claude in Victoria.
>>

The fulcrum points in the rear are where the pins are located.

But your question really relates to dynamic loads on the tires which is
something else. A while back I wrote a post about treating the center of
mass as the defining element for simplistic weight transfer from one side
to the other when cornering or riding on a crowned road. Of course it isn't
quite that simple. But for getting a ballpark feeling for the dynamics of
the GMC it isn't bad.

I haven't given up on modeling the GMC suspension, but I know it's pretty
complex (and will need to wait for a dry spell in business that I hope I
don't get).

The airbag action will tend to keep the loads on the rear tires equally
balanced with some system response time related to the air bag spring rate.
For sidewall flexure due to turning, the rear tires should have equal
flexure in opposite directions since the torque around the centerpoint of
the coach (at a point equidistant between the wheel centers) will transfer
equally to each set of tires. Note that this will not be the case when
rolling down a road that has bumps and dips - the dynamic transfer is more
complex than this but is still close.

My own belief is that from a strict flexure standpoint any wheel is a good
wheel and there should be no blow out difference. However, from my past
experience diving busses and other vehicles with tandem and more axles, I
expect the curb side tires to have the worst of it as folks run off the
pavement accidentally. Likwise, I expect the front bogie to catch the brunt
of "too tight turn" bumps. With the weight of the GMC I expect the vehicle
to move sideways when a curb is hit during a turn (that's my one experience
with hitting a curb). Keep in mind that tire rotation will skew any
statistics that we get because a tire may be damaged when in one postion,
but only fail later in another after it has been rotated.

Henry
PS I haven;t seen many (any) low speed (
 
Thanks all of you who responded to my hypothetical rear tire question.
There were some points I certainly hadn't thought of, and we even got
some data on failures.
Now of course everyone realizes that this was all only theoretical and
of course in real practice i would only think of putting on the very
best of new, E range--like all the rest do!!

Claude in Victoria.
 
and where did you come up with the conclusion that there is a difference in
blow out rates between the two systems?

>>>>>>>>>>>>>>>>>>>>
>In any situation except a very low speed turn, the steering angle is very
>slight,
>and any deflection of the side walls is probably not perceptible. In my
>opinion, the significant variables are inflation pressure, rear alignment,=
and
>loading.
>As they say, that's my 2=A2. I hope this helps. Scott Shean
>>>>>>>>>>>>>>>>>>>>
>
>Let me add to this... the deflection of the side wall may be more
significant if
>the inflation pressure is lower than it should be. But I am talking about
>deflection of the side wall from an up and down motion. Side wall flexing=
may
>be a better term for what I am talking about. That theory could explain=
the
>data that shows the leading bogies fail most often. Why should they fail=
most
>often? Here is my theory or hypothesis... The leading bogie wheel hit a=
bump
>and the side wall flexes. Milliseconds later, the trailing bogie hit the=
same
>bump, and with a two bag system, the leading bogie flexes again with the
>reflection of the same bump. This doesn't happen with a 4 bag system, as=
they
>are all independent. This could explain why many D rated tires on leading
>bogie, 4 bag systems do not blow, but 2 baggers do... just my .01
>
>Regards... John 74 Glacier (now Custom) near Washington, DC
>
>
 
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