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ET phone home

In a message dated 95-11-15 11:56:53 EST, you write:

>Can you tell me what positive offset means? I am confuse! :( 

Sure, offset can be one of three measures:

1) zero = hub face is centered at the centerline of the wheel.....  a 6" rim
with zero offset means that the centerline and the hub centers are exactly 3"
from either of the outsides of the rim

2) Negative offset =  hub face is toward the fender (away from the car)....
 a 6" rim with 25.4 negative offset means that the hub face is 1" toward the
outside of the car from the 3" centerline of the wheel.......  Remember the
centerline is just rim width divided by 2, and won't change regardless of
offset.....  Lots of "leaf spring" cars will run negative offset to put wide
tires inside the fenders

3)  Positive offset =  hub face is toward the center of the car from the
centerline of the wheel.....  So a 6" rim with a 25.4 positive offset means
that the hub face is 1" toward the car from the 3" centerline of the wheel.
 Most front drivers and quattros use positive because the mcpherson strut
prevents much negative offset without hitting something, and 2) a car with
positive offset is more stable when you have a flat, becuz it has less
tendency to put the wheel under the car (you have the weight to the outside
of the centerline on negative, so the wheel wants to dive under the car, not

Where the confusion arises is when you want to add rim width.....  Let's say
you go to a 7" wheel.....  If you had a zero offset, you would add a 1/2 inch
to either side of the rim....  The same holds true if you want to MAINTAIN
your current NEGATIVE OR POSITIVE offset.  If you have a negative offset of
25.4 and you want to go to a 7"  rim, you would add 1/2" to the outside rim
to centerline and inside rim to centerline to maintain the 25.4mm of negative
offset.  If you have a 7" rim on a POSITIVE offset of 25.4, again you would
add a 1/2 inch to the outside to centerline and inside to centerline

PROBLEM:   I can't add 1/2 inch to both the outside and the inside of the
rim, cuz the rim hits the steering arm, what do I do?   Change the
offset.....  So you have a 6" wheel that basically just misses hitting the
steering arm with a 25.4mm POSITIVE offset....  Well if you go to a 7" wheel
with a 37.1mm POSITIVE offset, the inside rim will be positioned exactly
where the 6" wheel inside rim is with a 25.4 offset (25.4 + 12.7 =
37.1mm)....  You have effectively added the width of one inch to the outside
of the rim and left the inside rim in the same location as your old
wheel......  Make sense?

You can easily tell if your increase in rim width will require an offset
change....  Just take a wheel (tire on, this could be the prollum more than
the rim) and look closely how tight the gap is between the wheel/tire and the
first obstruction (it's usually REAL tight in there, like a 1/2 inch) measure
that, that is how much width you can add to the inside to centerline distance
when going wider.....  Let's say its a 1/2 inch clearance with a 6 inch rim,
25.4mm pos offset, on a 205/60 tire, and you want to go to an 8in rim with
the same tire (keep it simple scott)......  So basically, you can add 1/2
inch to the  inside rim, and will need to add 1 1/2 inches to the outside
rim.......  Your offset is the same as above, 37.1, but now you need to watch
your fender clearance, cuz you pushed the rim 1 1/2 inches toward the outside
of the car instead of 1/2 in the 7" rim example above.....

Confused?  Draw a rim 6" and one 7" above it and one 8" above that, draw the
centerline so that they all match.  Then put in your offset (most audis are
35-45mm positive) line, then look what you need to do to that offset when you
try to get the inside of the rims to line up.....  Maybe easier to understand
this way.