[V8] wheel offset and wheel bearing death

[Jeremy Ward]

No, mine haven't died *that* quickly...  ;-)

I asked lister Justin Lewis (automotive engineer) to clear up my
confusion about how offset is related to premature wheel bearing death,
to which he replied the following:

'A wheel offset that places the center of the wheel at the same position
as the center of the bearing would impose the least amount of load on
the wheel bearing.  If the center of the wheel is moved in either
direction from the center axis of the bearing then a bending moment is
produced and is seen as additional load on the bearing.  This is true
regardless of wheel shape, size, or orientation.  Wheel camber and
cornering forces may dictate that the actual position of least load is a
few mm in either direction but you get the idea.  
The best way to think through these types of problems is to imagine the
extremes.  Imagine a wheel with such deep dish that the tire was 3 feet
outboard of the hub.  There would need to be an immense amount of force
(actually moment, which is like torque) exerted by the narrow wheel
bearings to keep the rear of the car from looking like this //--\\.  The
moment induced forces would be in addition of the load imposed by the
weight of the vehicle.  As I understand it, positive offset or a zero
offset would necessarily mean that the center of the wheel is outside of
the center of the bearing.  As mentioned above this is not ideal.'



Reminder - Offset is defined as the distance (in mm) from the mounting
face of the hub to the centerline of the wheel.  On our V8's with a 7.5"
wide wheel, the offset should be ET35 (stock).  In other words, the
centerline of the wheel sits 'pushed in' 35mm behind the hub face, I am
assuming somewhere directly over the center line of the wheel bearing.
(Can someone confirm how far back the centerline of the wheel bearing
sits in relation to the hub face?)

For the sake of argument, let's say that the centerline of the bearing
*is* 35mm behind the hub face.  A 7.5" wide wheel with an ET35 offset
would cause the least amount of unwanted forces to the wheel bearing.
The same wheel with an offset of ET30 or ET40 would have a small amount
(yet equal amount at ET35 +/- 5mm) of unwanted force applied to the
wheel bearing.  And in my case at ET20, that unwanted force is even
greater (this would be the same amount as an offset of ET50; ET35 +/-
15mm).

Keep in mind that the ET35 number I have been throwing around is only
for a 7.5" wide wheel.  When you change the width of the wheel, the
offset 'magic number' changes...

http://toy4two.home.mindspring.com/offset.html

http://www.tirerack.com/wheels/tech/offset.jsp

http://www.fikse.com/fitment.html


Hope this helps to clear up the discussion we had last week.  

Anyone want to start a pool for the death date and time of my wheel
bearings?  ;-)

Cheers,

- Jeremy
http://198.107.18.114/V8Q