can someone explain wheel geometry/offset theory ?

[dave silva]

I can convert the front hubs to GMC 8-lug wheels

In the rear I have spindles on a heavy drop axle. The spindles are apparently quite unique and not compatable with any other, more common wheel/hub combinations.

So i'm exploring some alternative spindle, like from a GM one-ton.

But I don't understand offset.

The Revcon rims have some negative offset but not as much as the GMC as they are NOT dually rims. In the front some of the offset is countered by the wheel adapter that takes it from Toro 5-lug to a much bigger five lug. So, ultimately the front rims are close to being over the center of the hub face.

So, what would be the issues with the front end if you ran zero offset rims on a GMC? Assuming they lined up correctly with the rear wheels?
--
Dave & Ellen Silva


Check out the website:

http://www.oldrv.net

 

[dave11]

Hi Dave

I was thinking the front hubs from a GMC/Chevy 1 ton van might work. They
appear to use the same bearings as the GMC rear wheels. I am not sure if
the bearings are spaced the same distance apart or if the brake rotor will
clear the bogie arm. I was thinking a searching for a hub/rotor the next
time I get a chance to visit an appropriate mall.

It would be nice to be able to use the lower offset wheels from the
mid-ninties SRW GMC HD pickups.

http://www.rockauto.com/ has a catalog that shows the bearing sizes for a
lot of the GMC HD vehicles.

My research indicates the GMC bearings are:
Inside bearing OD = 2.891 ID = 1.625
Outside bearing OD = 2.4375 ID = 1.030

Dave

> -----Original Message-----
> Subject: [GMCnet] can someone explain wheel geometry/offset theory ?
>
> I can convert the front hubs to GMC 8-lug wheels
>
> In the rear I have spindles on a heavy drop axle. The spindles are
apparently
> quite unique and not compatable with any other, more common wheel/hub
> combinations.
>
> So i'm exploring some alternative spindle, like from a GM one-ton.
>
> But I don't understand offset.
>
> The Revcon rims have some negative offset but not as much as the GMC as
> they are NOT dually rims. In the front some of the offset is countered by
the
> wheel adapter that takes it from Toro 5-lug to a much bigger five lug.
So,
> ultimately the front rims are close to being over the center of the hub
face.
>
> So, what would be the issues with the front end if you ran zero offset
rims on
> a GMC? Assuming they lined up correctly with the rear wheels?
> --
> Dave & Ellen Silva
>

 

[Craig Lechowicz]

Dave, I've forgotten most of what little I knew on this subject. S=
omeone like Bob deKruyff would probably be able to add a lot to the topic=
. But, the combination of wheel offset, hub offset, and the angle between =
the upper and lower ball joints (on the front) changes what is called scrub=
radius. If a more or less vertical line through the ball joints interse=
cts the center of the tire, that would be zero scrub radius. Less tire wea=
r and steering effort when lots of slow speed turns are made. I think posi=
tive scrub radius (center of tire outboard of line through ball joints [use=
d to be called king pin inclination if you're old enough . . .]) creates ov=
ersteer, and negative scrub radius creates understeer, but I could have tha=
t backwards, as it's been a long time. The other issue with offset is, i=
f you get the center of the tire very far away from the center of the wheel=
bearings, (or single bearing in the rear) it starts to drive an awful lot =
of bearing load and possible wear. Not an answer, but some thoughts on iss=
ues you might want to hunt down with someone smarter than me. -- Crai=
g Lechowicz '77 Kingsley, Waterford, MI

 

[bob de kruyff]

orgotten most of what little I knew on this subject. Someone like Bob deK=
ruyff would probably be able to add a lot to the topic. But, the combinat=
ion of wheel offset, hub offset, and the angle between the upper and lower =
ball joints (on the front) changes what is called scrub radius. If a more=
or less vertical line through the ball joints intersects the center of th=
e tire, that would be zero scrub radius. Less tire wear and steering effor=
t when lots of slow speed turns are made. I think positive scrub radius (c=
enter of tire outboard of line through ball joints [used to be called king =
pin inclination if you're old enough . . .]) creates oversteer, and negativ=
e scrub radius creates understeer, but I could have that backwards, as it's=
been a long time. The other issue with offset is, if you get the center=
of the tire very far away from the center of the wheel bearings, (or singl=
e bearing in the rear) it starts to drive an awful lot of bearing load and =
possible wear. Not an answer, but some thoughts on issues you might want t=
o hunt down with someone smarter than me. Pretty good Craig!! My=
brain cells are waking up and here are some thoughts. Generally over or un=
dersteer is controlled by compliance and roll geometry. In this case I thin=
k we are talking about the rear and the only effects on our coaches will ce=
nter around bearing loads. Now the front end is a very different story =
-- Bob de Kruyff 78 Eleganza Chandler, AZ

 

[gord h]

Hi Dave,

Wonderful inexpensive book explaining steering and chassis geometry - car based but it still would apply and great for grasping the goings on when in motion.

http://www.amazon.com/How-Make-Your-Car-Handle/dp/0912656468

Gord
--
Scruffy 74 Canyonlands Use the forum - it's easy! http://gmc.mybirdfeeder.net/GMCforum/index.php?t=thread&frm_id=1