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

[The Murphy-Fahlgren Family <efhome@master.adams.com>]

Ok, here's a sciuntifik answer to the perennial question, "Will
these fat tahrs rub?"

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,
i.e., a guess as to how much more a tire will protrude when you
change from the nominal dimensions.  Here are the formulae
followed by a few examples.

   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:
   1) The "crown" of the sidewall is halfway between the
      tread and bead, so I'm dividing the effect of
      wheel width increase by 4.  That is, it only increases
      the width of the tire at the "rub point" by half of
      the wheel width.

   2) All tires have the same sidewall protrusion beyond
      their nominal tread width (the aforementioned "sidewall
      crown").  If the new tires differ by having a bigger
      cantilever, then the potential for rubadub is even
      worse than shown.

   3) "Sidewall crown" is at the same diameter on all tires,
      which, of course, it is not.  If you change out 14"
      rims for 17" you obviously move the tie rod rub point
      out by a bunch (maybe good, maybe bad, y'all gotta
      look at *yer* car).

Here's an example (with holiday wishes that the table
isn't mangled):

                WW	WO	TW	FR	TR
Nominal         7	45	205	 0.00	 0.00
Less offset	7	38	205	 7.00	-7.00
Wider wheel	7.5	45	205	 3.18	 3.18
Wider tire	7	45	225	10.00	10.00
Rub a dub.	7.5	38	225	20.18	 6.18

Negative values are "good;" they indicate more clearance
rather than less.  Further interpretation is left to the
reader.

Eric Fahlgren
86 5kcsTq 215/60HR15 ET38 sleipoins

DeWitt said:
> On Mon, 21 Dec 1998 18:11:49 -0500, Harrison Sapir wrote:
> >They will bolt up at least.
> >
> >Scott Sierakowski wrote:
> >> I have a set of TSW Blades 4X108, offset 35mm with 225/50/16's off my 95
> >> 90Q Sport.  Does anyone know if these will fit on a 86 5000S?
> >>
> 
> Sorry, Scott. Rub a dub dub. In fact, the rubber will extend well
> beyond the sheet metal in the rear.
> 
> DeWitt Harrison
> Boulder, CO
> 88 5kcstq

-- 
The Murphy-Fahlgren Family            Try to take over the world.
efhome@adams.com                                  Canton, MI, USA