Offset Wheels WAS:Legalities of change to 80 mm calipers

Z

zachary zehnacker

New member
Oct 3, 1997
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Paul,

Thanks for the info. That is good to hear. I'm glad there is some real
high mileage experience with the wheels. Do you know if this person has
done anything to avoid the non-hub centered problem? Are they using an
adaptor, cone lugs, or something else? Since truck ruts are really the
only problem, I don't see us making any changes like this. It is an
interesting experiment to follow though. I wonder how much of the truck
rut problem is eliminated with this change. I suspect that it would make
some difference, but I don't think that the problem could be totally cured.

Thanks,
Zak

>Zak:
>
>I'm aware of one individual who I'm told has around 80,000 miles running the
>wide offset front wheels.
>
>Last summer he did experience a serious failure. However, it may have been
>of his making. The individual who helped him repair the damage, which
>included a broken drive shaft (he thinks because of excessive heat
>build-up), told me that the owner pumped about 15 shots of regular grease
>into his zerk equipped knuckle. That apparently blew out the grease seal
>and allowed the grease to leak out, leading to the failure.
>
>After being repaired, the offset wheel were put back on.
>
> Paul Bartz
 
Yesterday I dissassembled the right wheel, bearing and hub of my coach that
70K miles on it. All of the paint is still on the control arms just like
new and the ball joints seem fine. When I pulled the bearings however the
grease looked more like tar, and probably has never been changed. The
bearing came off very hard also.

I have not driven my coach many miles yet, maybe 1500 miles but it does not
track the ruts as many have described and drives very nice. could it be
that the coaches having these problems have problems either in the air
suspension, front end, or bogies that is causing this, and only need to be
put back to factory specs?

One thing that concerns me is the idea of many to change the design of some
parts of the GMC thinking that it is designed incorrectly. As an engineer I
have faced this problem many many times before on other products and
witnessed first hand the disasters that can happen when self taught
engineers attempt to redesign a product thinking that they know more than
the designers. maybe they do and maybe they don't. People do not understand
the many design compromises, and complicated mathematical calculations that
are involved in designing for instance the rear bogie wheels. No design is
perfect and of course can be made better in one area at the detriment of
something else.

bottom line in my opinion is that if you do not have access to the original
design criteria for critical areas like the suspension and wheels, and in
addition do not have the engineering background to evaluate this data and
take it into consideration before making these changes I would leave it
alone. Remember any change in the bogie geometry changes the front end also
and hence the drivability etc. In the case of the offset front wheels, I
would not touch it with a 10' pole. Be cautious, remember your life and the
lives of your passengers are at stake. this is not a 4,000# car we are
dealing with but a 12,000 motorhome.

Off he soap box for the rest of the day.
tom

>>I'm aware of one individual who I'm told has around 80,000 miles running
>>the wide offset front wheels. Last summer he did experience a serious
>>failure. However, it may have been of his making. The individual who
>>helped him repair the damage, which included a broken drive shaft (he
>>thinks because of excessive heat build-up), told me that the owner
>>pumped about 15 shots of regular grease into his zerk equipped knuckle.
>>That apparently blew out the grease seal and allowed the grease to leak
>>out, leading to the failure.
>
>>>Thanks for the info. That is good to hear. I'm glad there is some
>>>real high mileage experience with the wheels. Do you know if this
>>>person has done anything to avoid the non-hub centered problem?
>
>Zak...
>
>The guy I was referring to, posted about 5 months ago and said he had
>about 5 or 10K miles on the offset wheels and would let us know how it
>worked out. Do you really think anyone would be in their right mind
>making a decision to try the above based on one or two pieces of 2nd or
>3rd hand anecdotal data? (in the face of engineering calulations of
>around 500% overload) Or do you have more data?
>
>I am trying to track down the reference to the exact calculations (thanks
>Gene for the pointer to "sister coach" on your site but I turned up
>nothing. Is that what you said Sister Coach?)
>
>--
>Regards,
>John 74 Glacier near Washington, DC.
>
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach
 
John:

The source of the offset wheel article is Cinnabar's October 98 newsletter.

Another individual I'm aware of who has been running offset wheels for
several years is Jack Burke of Mentone IN and I was told that an individual
from California, who attended the GMCMI Las Vegas Convention was running the
wheels also.

Paul Bartz

From: John Dolan [mailto:jdolan]
Sent: Thursday, February 04, 1999 12:35 PM
Subject: Re: GMC: Offset Wheels WAS:Legalities of change to 80 mm
calipers

I'm aware of one individual who I'm told has around 80,000 miles running the
wide offset front wheels. Last summer he did experience a serious failure.
However, it may have been of his making. The individual who helped him
repair the damage, which included a broken drive shaft (he thinks because of
excessive heat build-up), told me that the owner pumped about 15 shots of
regular grease into his zerk equipped knuckle. That apparently blew out the
grease seal and allowed the grease to leak out, leading to the failure.

>>Thanks for the info. That is good to hear. I'm glad there is some real
high mileage experience with the wheels. Do you know if this person has
done anything to avoid the non-hub centered problem?

Zak...

The guy I was referring to, posted about 5 months ago and said he had about
5 or 10K miles on the offset wheels and would let us know how it worked out.
Do you really think anyone would be in their right mind making a decision to
try the above based on one or two pieces of 2nd or 3rd hand anecdotal data?
(in the face of engineering calulations of around 500% overload) Or do you
have more data?

I am trying to track down the reference to the exact calculations (thanks
Gene for the pointer to "sister coach" on your site but I turned up nothing.
Is that what you said Sister Coach?)
 
>
> I honest dont know if the
> bogies have anything to do with this problem. But from
> what I have seen and experienced I do have an opinion.

I think it was Chuck Stoddard talked about this at Marion. If you think
about the relative positions of the front and rear wheels and the amount
of coach(both length and weight) that's behind the rear wheels and
relate that to an inch(or more) of play in the bogies, you begin to
realize how much an out-of-spec rear suspension could make the coach
sway. It's the tail wagging the dog. From your perspective in the
driver's seat, you don't see the back end moving around - all you know
is that the darn coach is all over the road. Chuck suggested you get
someone else to drive your coach on the highway and follow behind,
observing how the coach is tracking.

Patrick
- --
Patrick Flowers
Mailto:patri63

The GMC Motorhome Page
http://www.gmcmotorhome.com
 
John,

I actually think that this is one case where the lack of data and
information speaks volumes. With 4-5 times overloading, I would expect
failures to happen quickly and reports of the failures spread rapidly
throughout the GMC community. It seems that this hasn't been the case.
Someone has 80,000 miles on the wheels and only had problems because they
did something else that they shouldn't have. Even the person you mentioned
had 5-10,000 miles without failure.

I'm not making any decisions about anything. As I said, I don't really
think the truck rut problem is bad enough to warrant a change. I merely
think it is good that there are people out there having success with these
wheels. I think it is good to have these options/ideas being tested and
explored by members of the GMC community. Without people willing to do
things like this, we would never be able to make our GMCs as good as they
could be. It is the only way we can really know what works and what
doesn't.

The common sense/mechanical side of me wonders just how much of a
difference 1-2" offset could make. I've been surprised before though.

Zak

PS - I'm thinking that Timken is only going to rate their bearings
according to what GM requests so their liability is at a minimum. That
doesn't mean that the same bearings could not have been rated much higher
if GM had requested it. It is just a thought, but it could explain why
there have not been rapid failures on the GMCs that have attempted this
change. This is surely not something one could count on, however, because
Timken could always change the design to closer meet the ratings which
could cause problems.

>
>Zak...
>
>The guy I was referring to, posted about 5 months ago and said he had
>about 5 or 10K miles on the offset wheels and would let us know how it
>worked out. Do you really think anyone would be in their right mind
>making a decision to try the above based on one or two pieces of 2nd or
>3rd hand anecdotal data? (in the face of engineering calulations of
>around 500% overload) Or do you have more data?
>
>I am trying to track down the reference to the exact calculations (thanks
>Gene for the pointer to "sister coach" on your site but I turned up
>nothing. Is that what you said Sister Coach?)
>
>--
>Regards,
>John 74 Glacier near Washington, DC.
>
 
Tom,

The truck rut problem is pretty rare for us, but when we hit certain ruts,
we move around quite a bit. These are usually ruts that you can see are
very deep. These are ones that you can actually feel the GMC going into
and coming out of if you move around in the lane. It feels a little bit
like changing lanes on a highway where one lane has just been paved and the
other lane hasn't yet and is lower. I suspect you'll eventually experience
the problem as you put more miles on your GMC and travel on different roads.

The truck rut problem really depends on the area you are driving. The NJ
Turnpike has some really bad sections. Other states have no bad sections
because they keep up with their road maintenance. A big part of it seems
to be where you want to be in the lane. If you have a spot that you try to
keep it in, it seems to cause trouble. If you slowly let the GMC decide
where it wants to be in the lane, it usually seems to be ok. In most cases
just slowing down to 50-55 MPH seems to significantly reduce the affect.

Just about all our experience is towing the trailer, so I don't really know
how our GMC handles without the trailer. We probably only put maybe 1,000
miles on it total without the trailer. Other GMCers' experiences might be
different.

Zak

>I have not driven my coach many miles yet, maybe 1500 miles but it does not
>track the ruts as many have described and drives very nice. could it be
>that the coaches having these problems have problems either in the air
>suspension, front end, or bogies that is causing this, and only need to be
>put back to factory specs?
 
>I'm aware of one individual who I'm told has around 80,000 miles running
>the wide offset front wheels. Last summer he did experience a serious
>failure. However, it may have been of his making. The individual who
>helped him repair the damage, which included a broken drive shaft (he
>thinks because of excessive heat build-up), told me that the owner
>pumped about 15 shots of regular grease into his zerk equipped knuckle.
>That apparently blew out the grease seal and allowed the grease to leak
>out, leading to the failure.

>>Thanks for the info. That is good to hear. I'm glad there is some
>>real high mileage experience with the wheels. Do you know if this
>>person has done anything to avoid the non-hub centered problem?

Zak...

The guy I was referring to, posted about 5 months ago and said he had
about 5 or 10K miles on the offset wheels and would let us know how it
worked out. Do you really think anyone would be in their right mind
making a decision to try the above based on one or two pieces of 2nd or
3rd hand anecdotal data? (in the face of engineering calulations of
around 500% overload) Or do you have more data?

I am trying to track down the reference to the exact calculations (thanks
Gene for the pointer to "sister coach" on your site but I turned up
nothing. Is that what you said Sister Coach?)

- --
Regards,
John 74 Glacier near Washington, DC.
 
>
>The common sense/mechanical side of me wonders just how much of a
>difference 1-2" offset could make. I've been surprised before though.

I haven't done the math, but you are talking about a lever system. So, a
small shift can dramatically increase the loads created by the levers.

>
>Zak
>
>PS - I'm thinking that Timken is only going to rate their bearings
>according to what GM requests so their liability is at a minimum. That
>doesn't mean that the same bearings could not have been rated much higher
>if GM had requested it. It is just a thought, but it could explain why
>there have not been rapid failures on the GMCs that have attempted this
>change. This is surely not something one could count on, however, because
>Timken could always change the design to closer meet the ratings which
>could cause problems.
>

There are a few other reasons why we may not see bearing failures yet.

1. Timkin and most manufacturers derate their mechanical parts by various
factors to provide engineering safety margins. I don't know Timkin's
formulas, but using a 2x factor is not uncommon since it skews the failure
rate to make in-life failures unlikely.

2. There are relatively few coaches running the offset. The statistics
haven't caught up yet (or maybe they did with the one known failure).

3. The combination of GM safety factor, Timkin safety factor, improved
maintenace may make the bearings be on the edge of real load capability.

Henry
Henry Davis Consulting, Inc / new product consulting
PO Box 1270 / product readiness reviews
Soquel, Ca 95073 / IP reviews
ph: (408) 462-5199 / full service marketing
fax: (408) 462-5198
http://www.henry-davis.com/ http://www.henry-davis.com
 
Paul, Tom, Zak, et al...

>The source of the offset wheel article is Cinnabar's October 98

Thanks Paul, you saved me a lot of time, but I think you meant Sept 98,
right? A couple of things I need to correct: From memory I referred to a
7 inch offset, but the real number is more like 4 inches. The static
load increase on bearings are 450% (on bearings that are already at their
maximum load capacity.)

There is another anecdote about a real dualee set on the front (4 wheel
drive all on the front) used in Mexico at speeds not to exceed 5 or 10
miles per hour, to increase traction on muddy roads uphill.
- --
Regards,
John 74 Glacier near Washington, DC.
 
Zak...

I was just trying to discourage you and others from taking tah risk based
on your statement of day or so ago,

>I don't know if everyone agrees that the offset wheels on the front are
>as big a problem as they are made out to be.

I think it is pretty clear the risk/reward ratio is poor on this one.

With regard to the lack of failure data speaking volumes: I would say you
should consider that most people hesitate to advertise how foolish they
were.

- --
Regards,
John 74 Glacier near Washington, DC.
 
I appreciated this offset wheel thread and have learned from it. I would
like to hear more on this subject.

It would be interesting for those with the desire, experience and ability,
to explore further, including suspension dynamic loads.

It also would be interesting to hear some discussion beyond the analysis of
bearing loads.

The "truck rut" problem is not unique to the GMCMH.

Do offset wheels reduce the "truck rut" problem slightly or significantly ?

Offset wheels use a wider tire tread which might effect "truck rut"
behavior.

It would be interesting to separate the "wider tire" influence from the
"same track" influence.

If having the same front and rear track does substantially improve handling,
the suggestion to move the "A" arms outward and extend the drive axles would
be another interesting area to explore.

Obviously it would be more involved than bolting on new wheels but might be
a better alternative.

Some observations gained while spending a portion of my early life in R&D
labs:

1. Those with inquisitive minds will lead those with less inquiring minds
into the future

2. Without exploration of ideas there is generally no advancement

3. Critical analysis and heated discussion are usually part of the research
process

4. The smartest man can learn something from one less smart if he opens his
mind and pays attention.

A nice thing about this group has been the ability to hear from people with
a wide variety of experience and interests in different areas.

This provides insight and scrutiny of ideas from some unique perspectives.

It also brings about a fine banquet of ideas for those who are interested.

Cheers,

Don Miller
75 Glennbrook
Shenandoah Valley of Virginia
 
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