Subject: Offset Wheels WAS:Legalities of change to 8 0 mm calipers

balcom robin s

New member
Aug 28, 1998
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All of this is interesting to me, and I'd be interested in hard numbers to
go along with the discussion. Numbers such as how far apart are the two
bearings, where is the weight of the coach applied relative to the two
bearings using stock wheels, and how much offset is there in the offset
wheels. Just for giggles, I made some assumptions about those numbers,
disregarded any kind of variables you would get from moving down the road
(such as variations in side loading, hitting bumps, crashing into things,
etc.), and did some calculations.
The assumptions I used are:
1) each front wheel sees 2500# of weight - this is approximate, perhaps a
little on the high side, perhaps not;
2) the bearings are spaced 3" apart - a quess, but close enough for me for
now, and a nice round number;
3) the design of all the components (tire, wheel, hub, etc.) makes the
point where the weight of the coach is applied to be exactly in the middle
of those bearings, or 1 1/2" from each bearing;
4) the offset from the offset wheels amounts to 4", so that the point where
the weight is applied is 4" out from where it is with the stock wheels, or 2
1/2" out from the outer bearing (4" - 1 1/2 = 2 1/2").
My calculations:
Stock wheels: both bearings see 1250# (2500# divided by 2).
Offset wheels: the inner bearing sees 2083#, and the outer bearing sees
4583# (the math for this is more complicated, but uses the 2500#, the 3"
bearing spacing, and the 2 1/2" dimension noted above. A diagram would be
needed to help explain things).
For me, I can see that the bearings are subjected to more of a load with the
offset wheels, but I don't believe that it's on the magnitude of what I've
seen posted previously. And not just one of the bearings sees a greater
load: they both do. As to whether the greater load exceeds the capacity of
the bearing or not, I don't believe that it does, but it does use up some of
the margin of safety.
Time to get back to work!
Robin

 
Hi Robin...

I see why you say what you say, and while a full explanation would take
pages, maybe I can help with a few quick facts using real numbers:

Robin said:
>My calculations:
>Stock wheels: both bearings see 1250# (2500# divided by 2).
>Offset wheels: the inner bearing sees 2083#, and the outer bearing sees
>4583# (the math for this is more complicated, but uses the 2500#, the 3"
>bearing spacing, and the 2 1/2" dimension noted above.

I forgot where I read about this non-intuitive situation, but Paul Bartz
referred me to the 9/98 GMCMN which is where I read about it.
The great news is that I just noticed Wes Caughlin didn't write it,
although it is in his newsletter. That may increase its credibility for
some here.

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.

Emory pointed out that beyond the bearing load problems, a more serious
problem is the increase in load on all steering and front end components
because of the canterlevered load. He was a test pilot in this area and
tested some of those components to destruction in SOB.

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