Darrell,
Excellently stated. Simple for those who need to get the measurement done
and detailed enough for those that what to know why. I used to frequent
Caldwell up there on Colorado Blvd. In my case it wasn't Ansens, but rather
their somewhat infamous recaps or used tires. I used to enjoy watching them
true tires with their tire lathe. At the time ( I was 18-21) I always
wondered whether you got more tread life because the tire was round in
relation to the wheel, or it was a gimick that pre-wore the tire for you!
As a side note I had a friend who had put Caldwell recaps (C - 50 - 13")
on his Datsun 510 about 1980. The car sat on those recaps (mostly flat) for
about 21 years. Who knew how much older the casings were! In 2003 when I had
to take the car to the DMV to get VIN verification I put air in those recaps
and all be it slowly and bumpy, drove it there.
This whole discussing about what means what reminds me of the time I went
into Alhambra Wheel & Hobby and told the owner Bill Mason (who had a voice
like Chet Huntley) that I wanted a left pedal for my bike. He questioned me
"Left?" And of course I said "right." Well, I think you can see where that
went (he strung me along for about two minutes). Thanks again, I'll print
your reply out and slip it into Phun's book so I don't forget.
Tom Witt B9470101
----- Original Message -----
From: Sharon Mountjoy <mtjoy@telis.org>
To: tiger list <tigers@autox.team.net>
Sent: Wednesday, March 16, 2005 6:22 AM
Subject: offset vs. backspace
> Okay, I may as well throw my hat into the ring on this one. Buckle up; it
may
> be a long ride.
>
> Back when I was a kid in '67, Ansen Sprint mags and "Firebird" (Verduro)
Green
> had just hit the market place. I NEEDED those Sprint mags to compliment
that
> new color I'd sprayed my '57 T-Bird. So I went to Caldwell Racing Treads
in
> Pasadena, CA. (Still there, BTW), to get a set of the Ansens. When they
> started talking about offsets for my car my face glazed over so they
explained
> what it was to me.
>
> Visualize a wheel. Now turn it sideways so the outside is on one side and
the
> inside is on the other. Draw an imaginary vertical line down the middle
on
> the wheel. This plane is the wheel's "centerline", half way between the
> outside and inside of the wheel. This is the base of reference for
offset.
> Now, with your sideways wheel and your centerline in place, picture the
> "mounting surface" of the wheel, a parallel plane to the centerline, where
the
> wheel mounts to the vehicle.
>
> The distance between these two planes is referred to as offset. If the
> distance >from< the wheel's centerline >to< its mounting surface goes
toward
> the wheel's outside it is measured as "positive" offset. This outboard
> mounting surface moves the wheel toward the middle of the car.
Conversely, if
> the distance >from< the centerline >to< the mounting surface goes toward
the
> wheel's inside then it is measured as "negative" offset, This inboard
> mounting surface moves the wheel toward the outside of the car.
>
> The problem with "offset" is that it is difficult to measure in the real
> world. When I was having PS Engineering make the kidney port wheels for
my
> Tiger I discovered the miracle of ""backspace". It's very tangible and
easy
> to work with. Take your wheel and set it face down. Place a straight
edge
> across the wheel from lip to lip. Measure perpendicularly down from the
> straight edge to the mounting surface. That's your backspace. That's it!
> Simple and it gives you all the dimensions, along with wheel diameter, to
> orient and measure for wheel fitment.
>
> We could get into tire specs, which would be the missing link to actually
> determining if a wheel/tire combination would fit the space we have
available
> in our wheel arches, but that's another story...
>
> Darrell
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