Re: Tire width: will they fit? (was Wheel Help...Please?)

[DeWitt Harrison <de@aztek-eng.com>]

On Wed, 23 Dec 1998 12:01:57 -0500, Eric Fahlgren wrote:

>I wrote a little
>spreadsheet on this windoze thing I have at home (mail me if you
>want the xls), that takes a nominal collection of wheel width,
>wheel offset and tire width, then computes "rub a dub" factors, [ ... ]
>
>   FR = 25.4*(nww-WW)/4 - (nwo-WO) + (ntw-TW)/2
>   TR = 25.4*(nww-WW)/4 + (nwo-WO) + (ntw-TW)/2
>
>where
>
>   FR = Fender Rub factor in mm (how much more the tire will
>        stick out towards the fender)
>   TR = Tie rod Rub factor in mm (how much closer to the tie
>        rod end and springs the inside of the tire will be)
>   WW = Wheel Width in inches
>   WO = Wheel Offset in mm
>   TW = Tire Width in mm
>   nXX = New values for WW, WO and TW
>
>Assumptions:
>[ ... worth reading but snipped for brevity ... ]

This approach is sound and could save folks a lot of anguish. It
is somewhat conservative since tire width and wheel width changes
don't usually add so directly. The effect of wheel width changes are
clearest when the tire width and brand/model all remain unchanged in
which case 25.4*(nww-WW)/4 is a good approximation. But when,
for example, both the tire and the wheel are increased by, say, 20 mm,
the (nww-WW) term effectively counts the tire width increase twice
(actually 1.5 times) because there is a built-in presumption that the
wider tires are to be run on wider wheels. Tire data tables often give
you baseline wheel widths that can be used to determine how to 
deal with this. If the data tables show the nominal wheel width changing
about as much as the proposed change, this factor can be dropped.
If not, the difference between nominal change and proposed
change (divided by 4 as shown) would be a good number to use.

In practice, tire style and manufacturing variations will generate more
uncertainty than this overestimation. Then too, since it is common to be
changing wheel diameter simultaneously with width, tire height will
change a few percent also which affects rubbing clearances. Taller
tires will rub sooner as a rule. Lowing the car will also tend to
exacerbate clearance problems. There will be many apples-to-oranges
factors which make an exact computation difficult as Eric's list
of assumptions suggests. In the end, it may be wisest to use
Eric's formulae as they are. At the very least, they can be used
to separate the "maybe" fitments from the "not-a-chance" ones.
Their use would certainly cut down on the number of 35mm
offset wheels sold to owners of older Audis.

DeWitt Harrison
Boulder, CO
88 5kcstq