Subject: Offset Wheels WAS:Legalities of change to 80 mm calipers

[donald w. miller]

>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
>- --

I think this is true but the situation can be improved.

Handling would be better with wheels at the four corners and no overhang.

The front wheels are fine now, with little overhang and not much weight
ahead of them.

The aft end is a different matter with a lot of overhang.

Worse still are those heavy objects located aft of the rear wheels.
Together there is well over 1,000 lbs. swinging around back there.

Generator set
Rear air
Propane tank
Fresh Water tank
Spare Tire
Flex Steel Rear Sofas

Moving rear wheels is too radical but reducing the weight back there is
possible.

While refurbishing, I am making an effort to reduce weight aft of the rear
wheels and to move it forward when possible.

By doing this I am hoping handling will be good enough after market fixes
will not be needed.

Don Miller
75 Glennbrook
Shenandoah Valley of Virginia

 

[patrick flowers]

>
> I think this is true but the situation can be improved.
>

>
> Moving rear wheels is too radical but reducing the weight back there is
> possible.

Don,

I didn't mean to imply that there is a design problem. Exactly the
opposite - if the rear suspension bushings are worn, the back end of the
coach acts as a pendulum with the front suspension being the pivot
point. The rear of the coach swings from side to side, and due to the
mass behind the front wheels, that moves the entire coach. From the
perspective of the driver it's hard to discern that the "tail is wagging
the dog".

Patrick
- --
Patrick Flowers
Mailtoatri63

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

 

[john dolan]

Rick... et al

> Further, that Cutlass had at least 2400 lbs riding on bearings about
> 1/4 the size of those in our GMCs, which carry about 4200 (spec) to
> 4500 (reality) in front. I think we have a little wiggle room on
> loading.

I agree with your numbers, which is to say that each side is carrying
about 2250 pounds static load. But consider that if you put on front
wheels offset enough to track with the back wheels. That is about 4
inches/per side more than the original design and that means that each
inner bearing will have an ~11,000 pound static load / per inner bearing
(because of the 4 additional inch lever arm per side)

Now put extra batteries and several grandkids up front. Be sure and have
one of the kids be the pothole and dipsie doodle spotter because the
dynamic loading is going increase dramatically...

I know there are people apparently getting away with this, but that
doesn't say the calculations are wrong. Hell, when someone says the
bearings are at their max loading with standard wheels, there is
obviously a safety margin designed in (otherwise the bearings would
fail as you let the offset wheel down off the jack). The point is, the
folks apparently getting away with it for some time are banking that they
designed in enough safety margin to cover their out of spec modification.

Someone here suggested (and I agree) that 100 - 200% is often designed in
for safety/etc. This offset modification increases the *static* loading
450% on inner bearings that are at their spec maximum to begin with.

- --
Regards,
John 74 Glacier near Washington, DC.

 

[john dolan]

> Having gone over a 40 foot cliff just north of Leadville, CO in l975 in
> a Travco with my wife and two kids due to a broken ball joint on the
> pitman arm,

Holy Smoke, Emery, you must have a strong heart. Mine would probably
have stopped in mid air. Yikes!

- --
Regards,
John 74 Glacier near Washington, DC.

 

[zachary zehnacker]

I have some problems with the 11,190 up and 11,190 down figures. I know
they were assuming the wheels would just fall off if someone tried to use
the "wrong" offset wheels , but I'm trying to figure out what is holding
the GMC up if the ups and downs cancel. Shouldn't there at least be a
total of 2,680 lbs more in the down direction to cancel the 2,680 lbs up
force from the wheel that causes the whole problem in the first place? Was
this just copied from the article incorrectly, is this a mistake in the
article, or is my thinking wrong?

Does anyone here have the exact measurements? We need to know how far off
the center of the bearing set the center of OEM wheel is. We also need to
know how far off the center of the bearing set the new offset wheel is.
The distance from the centers of the two bearings would also be needed for
accurate calcs. As far as I can tell, this is all that is needed to avoid
most major assumptions.

The bearing set inner-center to outer-center measurement seems to be the
most critical. Using 3" allows for the loads to be reasonable. Using 2"
makes the numbers 50% higher.

These calculations coupled with the fact that people have successfully used
these different offset wheels indicate that the bearings are definitely not
a weak link in the OEM design. Even if that is all that comes from this
discussion, it has still been worth it. These bearings, if properly
installed and non-defective, should never fail on a stock GMC. If they
have failed on a stock GMC, someone is at fault (improper installation/not
greased correctly/seal bad/etc) unless the bearings were defective right
from the start.

Zak

>Assuming 2680# per wheel... (Static load)
>
>Stock wheels:
>The geometry with stock wheels is such that the inner bearing carries 93%
>of the load and the outer carries 7%. 2490# and 190# (static loads)
>
>The geometry with the offset wheels (actually 'offset' is a misnomer, but
>it was the title of the thread when I joined) is such that each bearing
>carries an equal load of 11,190 The outer bearing has a 11,190 up
>vector and the inner bearing has a 11,190 down vector because the wheel
>load is cantilevered from the bearing set. That is where the 450%
>increase in bearing loads comes from, and that is just static loads.
>Then think about potholes, then think about hitting a pothole while
>braking trying to avoid it.

 

[john dolan]

Zak...

I think both the article and my post are correct. I admit I had a
problem with the words and could not really understand it till I drew it
all out on paper. Take a look at my drawing (in a preceeding or
suceeding post) and I think you will agree. I think they should have
drawn a picture in the article and saved the 1600 words they used to
describe it. I'll bet the way they produce the newsletter lets them
include photographs but not charts or diagrams.

- --
Regards,
John 74 Glacier near Washington, DC.

 

[john dolan]

> You are, of course, sacrificing some of your safety margin. But there
> is a little margin to work with, IMHO.

Rick, I hear you and I want to believe you, but these numbers concern me.
I'm not sure I would want to increase loads 450% of the original design
not knowing the total margin that was designed in. In addition, I'd say
the bearings are not the biggest danger (talk to Emery about going over a
cliff)

Robin...
This picture is the best I can do but it is better than my words...

Original
xxxx
* *
*
-------o-|* 4 1/4 inch inset
*
* *
xxxx centerline of tread is 0.4 inches inboard of bearing set
(causing inner bearing to take 93% of load) 2,490#


New Alcoa (light truck) Wheel (brings front out to rear wheel track)
xxxx
* *
*
--o-|* 1/4 inch inset
*
* * centerline of tread 4 inches outboard of bearing set
xxxx (causing inner bearing to support 11,190# down
vector and the outer bearning to support a 11,190# up
vector and remember that is just static loads)

That is where the 450% of original design loads
comes from.

x = tire tread
* = wheel structure
o = center line of bearing set
inset = distance from hub mounting face to center of rim
- --
Regards,
John 74 Glacier near Washington, DC.

 

[john dolan]

On my diagram if you print it out you can see the legend and the picture
at the same time making it a little easier to follow.

- --
Regards,
John 74 Glacier near Washington, DC.

 

[zachary zehnacker]

Obviously, my diagram came out like crap when all the tabs got dropped out.
I'm trying it again with spaces.

|11190 from bearing
|
|
_____________V______________
^ ^
| |
| |
11190 from bearing 2680 from wheel

Zak

 

[john dolan]

In order for the Wheel diagram to display properly you may need to switch
from proportional fonts to fixed fonts just to view it without space
distortion.
- --
Regards,
John 74 Glacier near Washington, DC.

 

[john dolan]

Zak...

We agree on the original wheel right?:
>This same idea is shown correctly with the loadings of the original
>wheel from the same post. One bearing has a 2490 down vector while the
>other has a 190 down vector. These total to 2680 down which is correct
>to cancel the 2680 up from the wheel. No problems here.

The difference is that on the light truck wheel the centerline of the
tire tread is outboard of the centerline of the bearing set, so the 2680
up vector from the centerline of the tire tread is multiplied by the
lever arm and exerts an up vector of 11,190 on the outer bearing. There
is an exactly opposite down vector pushing down on the inner bearing.
The reason that the total is so much higher is because of the lever arm
and the 11,190 reflects the torson in the bearing. The 2680 is still the
weight on the wheel but the lever multiples it and reflects the torson in
the bearing.

2680
4.014 lever arm from center of bearing set to center of tire tread
|o|......
^
^
xxxx
2680

Regards,
John 74 Glacier near Washington, DC.

 

[john dolan]

I like your diagram better... so here goes, but it is just what I was
trying to say in my post. I'm just adding to your diagram.

>Obviously, my diagram came out like crap when all the tabs got dropped

from coach 2680 | 11190 on (outer) bearing
exerted on B.center| |
| |
______v_____V______________
^ o ^
| 4.014 from centerline| tire to centerline bearing set
| |
11190 on (inner) bearing 2680 from wheel

- --
Regards,
John 74 Glacier near Washington, DC.

 

[zachary zehnacker]

John,

I'm sorry, but that doesn't do it for me.

What is that 2680 from the chassis applied to, the bearing spacer? It has
to be applied to one or both of the bearings. If it were applied equally
to both, then the loadings on the bearings would really be
- -11190+1340=-9850 lbs (down) and +11190+1340=12530 (up). These would
transmit forces of 9850 lbs (up) and 12530 (down) to the hub. Which would
be the 2680 different I said were needed to cancel the up force on the hub
from the wheel.

We are only looking at the loads applied to the hub. These loads include
the force from the inner bearing, the force from the outer bearing, and the
force from the wheel. There are no other forces on the hub. Anything
applied to the bearings (ie your load from the chassis) is already included
in the forces that the bearings apply to the hub and cannot be included
again. I am going off of basic statics laws. I do have a BS in mechanical
engineering, so I'm not making this stuff up.

Zak

>I like your diagram better... so here goes, but it is just what I was
>trying to say in my post. I'm just adding to your diagram.
>
>
>>Obviously, my diagram came out like crap when all the tabs got dropped
>
> from coach 2680 | 11190 on (outer) bearing
> exerted on B.center| |
> | |
> ______v_____V______________
> ^ o ^
> | 4.014 from centerline| tire to centerline bearing set
> | |
>11190 on (inner) bearing 2680 from wheel
>
>
>--
>Regards,
>John 74 Glacier near Washington, DC.
>

 

[john dolan]

> I'm sorry, but that doesn't do it for me.

>What is that 2680 from the chassis applied to, the bearing spacer?

As you probably know, the notion of a force being applied at the
centerpoint of the bearing set is only a convenience for explantion and
simplicity. In reality it is the intergration of forces being applied
along the length of both bearings.

I don't have the data to check whether the intergration was done
correctly... do you or anyone else following this have the dimensions?

As I see your argument, you appear to be arguing that in reality it is
really much safer than I said it was, because the increase in bearing
load over the original design is only 400% not 450%?

We all know these bearing are strong, but do you feel good about telling
folks to go ahead and make this change because there is always a safety
margin designed in? And further, I say the bearings may be the least of
the concerns, when one considers the increased load brought to bear on
other front end components, with this cantilevered wheel.

- --
Regards,
John 74 Glacier near Washington, DC.

 

[zachary zehnacker]

John,

I don't think you have to worry about everyone on the list going out and
trying these wheels just from what I have said. I give the GMCers here a
lot more credit than that. If any of them are like me, we just want to
know all the information from all the different sides. We value non-biased
opinions more than biased ones. We value opinions from some GMCers more
than others. In the end, we make the decisions that we think are best for
us. IMO, we can't do this without gathering and factoring in as much
information as we possibly can.

Forget it, I have now officially ended my quest for the correct results.
Little inconsistencies have always made me think that bigger problems might
be below the surface. Apparently it is not important. This is my last
posting on the subject. Since you seem to think it will make a difference,
here you go. NOBODY ON THIS LIST SHOULD EVER CONSIDER ANY CHANGES TO THE
OEM CONFIGURATION OF THEIR GMCs. THE 25 YEAR OLD GMC DESIGN CAN NOT BE
IMPROVED AT ALL.

Now in search of bias ply tires to put on our "safe" OEM non-radial steel
wheels,
Zak

 

[john dolan]

Zak...

>I don't think you have to worry about everyone on the list going out and
>trying these wheels just from what I have said. I give the GMCers here
>a lot more credit than that.

Well as *you* have said before, this has not been a total waste of time,
because your above statement is quite different that your previous
statement of a few days 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. From different reports,
>there are people that are using these wheels. I have not heard of any
>failure reports. In the case of this modification, I would think that
>any failure reports would be spread very rapidly throughout the GMC
>community.

As I said, I don't think most people that tried this experiment would
want to advertise how stupid they were, by reporting a failure. And I
cautioned you and anyone else considering this that there were serious
engineering arguments against undertaking this poor risk/reward
modification, and not just the bearings, and that as Thomas has said many
times, the static loading problems are the least of the real world
problems associated with this.

You have spent the last umpty upmth posts nitpicking the static load
equation while others have said yeah but there are even worse problems
even if you are right about the 1340 up and down vector component
associated with the weight of the coach.

I am going to contact the author (who was not Wes) and convey your
concerns about his findings. I bet we hear something like "the 11,190#
numbers are correct at the extreme ends of each bearing (one up and one
down) and that is the point where maximum compressive forces are exerted.

Regards,
John 74 Glacier near Washington, DC.

 

[ted schurman]

Zak, if its any consolation I agree with your point that the Y forces
should add to zero. I'm only embarrassed that I didn't catch the
mistake when I first read the article. Of course I had just installed
the normal Alcoa wheels so I wasn't all that interested in the other
wheels.
Ted Schurman
73 Glacier VA

>
> John,
>
> I don't think you have to worry about everyone on the list going out and
> trying these wheels just from what I have said. I give the GMCers here a
> lot more credit than that. If any of them are like me, we just want to