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Re: Bolt pattern and offset

To: triumphs@autox.team.net
Subject: Re: Bolt pattern and offset
From: Mark J Bradakis <mjb>
Date: Tue, 7 Mar 1995 16:51:01 -0700
The big TRs use a 4 on 4.5" pattern, don't remember offhand what the offset
is on the various wheels.  I knew it when I bought a batch of 15 x 8.5"
wheels, though.  Still have a set, mounted with some Hoosier autocross tires,
maybe I'll get my 250 motor redone this summer and get a chance to use them.

Popular cars with the 4 on 4.5 pattern are Datsun 240/260 Z, along with some
Datsun sedans like the 510, 200SX and such.   Some mid 60s, early 70s Ford
products, Corvairs, geez, here's a list:

4 on 4-1/2
        Acura - Legend '86-'89
        Buick - Special '61-'63
        Chevrolet
            Corvair all 4-bolt
            Chevy II  all 4-bolt
            Sprint '85-'87
            Nova ('61-'70)
        Datsun/Nissan - most models to '89
        Dodge Colt '79-'88
        Dodge Colt (early)
        Ford
            Mustang all '65-'68 4-bolt
            Falcon, Maverick, Granada all 4-bolt
        Mazda - 626, RX7 '83-'87  ????
        MGA, MGB, MGC
        Olds - F-85 '61-'63
        Plymouth - Arrow, Sapporo '79-'88
        SAAB all      ???
        SAAB - 900 to '87
        Toyota - most exc. MR2 and P.U. to '89
        Triumph TR 2..6




But be careful, a lot of these wheels are 13 or 14 inch diameter, or have
odd offsets, and won't clear the front calipers properly.  Hey, wait a
minute, I remember ( insert sounds of disk access here) yep, here it is:


Okay, here's a wheel and tire.  Well, a quick rendition of a wheel and tire:


         |            |
         |  tire      |   Outside of car
         |            |
         o-----\/-----o
                |
                |
   Suspension   |
      stuff     |
                |
                <- a ->
                |
         <- b ->|
                |
         <--- w ------>
                |
         o-----/\-----o
         |            |
         |            |
         |            |


Now, for some dimensions:

w: nominal width of whell, i.e. the 8 in 15x8


a: frontspacing??  Usually ignored for the most part, as the backspace
                   dimension is more critical.
                   

b: backspacing - the distance from the edge of the wheel to the mounting
                 face on the wheel center.

Offset:  If a = b = w/2, then the wheel has zero offset.
         If b > a, then b - w/2 = offset, in POSITIVE direction.  Many front
         wheel drive cars and Volvos have lots of positive offset.

         If b < a, w/2 - b = negative offset.

For example, I have some 15x7 American Racing rims on my TR.  They have a
backspacing of 4 inches.  So, b = 4, w = 7

                     4 - 7/2 = 0.5" positive offset

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