Re: Offset: + or - Followup

["David R Johnson" <johnsnd@domain.elided>]

In AD-551, Simon Favre wrote:

> I don't see what the confusion is. Offset is offset. The way the Tire
> Rack page defines it is correct. They would know. There's a real simple
> way to remember this. American muscle cars have large negative offsets.
> FWD (and most modern) cars have large positive offsets. There's no point
> of view about this. The tire and wheel industry has defined these terms
> consistently. People just get confused about it.

The confusion is simply that of which direction is considered positive, hub
surface outside wheel centerline, or inside it (relative to the car body).  I
am more familiar with the muscle-car definition (negative offset or deep-dish
wheels), which is in agreement with TireRack.  This may be because a lot of
muscle car owners I communicate with buy from them.

However, I have in my collection two chassis testbooks, "Chassis Engineering",
by Herb Adams, and "How to Make Your Car Handle", by Fred Puhn.  Both are
respected authors, and both define offset as opposite from the TireRack web
page.  That is, a deep dish wheel, with hub surface inboard of the wheel
centerline, is a positive offset.  Admittedly, the diagram in Herb Adam's book
is confusing, leading one to believe the opposite, but the text example agrees
with Puhn.

I am not trying to say that Simon's definition is wrong, just to reinforce the
fact that there is confusion within the industry.  As I stated before, I
suspect that there were once two terms, preferred by two separate groups of
people, which have since merged and become impossibly confused as a result.

> When fitting aftermarket wheels that are a different size than the
> original, both offset AND back space are important. Ideally, you want to
> keep the same offset so you don't change the suspension geometry, BUT
> in some cases, you have to compromise the offset because a rim of that
> width and offset has a back space that interferes with a ball joint or
> a fender lip.

Exactly correct.  Offset is important to steering geometry, helping to define
the scrub radius and therefore controlling bump steer characteristics.  The
only exception would be if the car was designed with a poor geometry (not as
uncommon as you might suppose, although probably more prevalent in 50's Chevys
than in Alfas), in which case a change to the offset might be desirable to
reduce bump steer.  And it is certainly true that to fit wider tires and wheels
the offset might have to be compromised due to the limit on backspacing,
although one wonders then whether the extra rubber is worth the cost.

Regards,

Dave J.
1982 GTV6

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End of alfa-digest V7 #554
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