Date:         Thu, 23 Feb 95 08:46:54 CST
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Joel Walker <JWALKER@ua1vm.ua.edu>
Subject:      Re: 15" wheels & brake fluid leak

On Wed, 22 Feb 95 19:48:54 CST Steve Wall said:
>Can anybody give me some guidelines/formulas to determine my options?
>I understand that a larger wheel radius will/could affect the accuracy
>of the speedo/odo, etc. - are there other issues? How can I tell what
>tire will/won't fit? If the tire is too big, will it scrape the
>wheelwell and/or limit my turning radius? Obviously, I need some help
>understanding how to size a tire for a Westy. Any help is appreciated.

well ... your bus is "normally" setup for 185/70R14 tires on 14" wheels.
so what you need to do is figure how much difference you'll have from some
other tire. it kinda works like this:

185/70R14 => 185mm tread width x 70 percent (sidewall height ratio) = 129.5mm
             which is ONE sidewall height.
             since we have TWO sidewalls (on all normal tires :),   = 259mm
             now you need to add the diameter of the wheel: 14" =  355.6mm
             which gives a total "perfectly round" diameter of  614.6mm or
             24.2 inches. this is the "perfect" diameter of your 'normal'
             tire and wheel. big deal. to figure out how it affects your
             speedo, you'll need the circumference (how far the tire rolls
             in one revolution). well, ok: pi * dia = circumference or
             76.0 inches. that's normal (which, of course, does NOT account
             for speedometer errors, and speedos are ALWAYS off a little bit).

now, if you put a bigger tire/wheel on the thing, lets say a 185/70R15 ...
go through the same gyrations, and we get a diameter of 640mm or 25.2 inches,
which is exactly what we'd expect: one inch bigger. but the circumference is
79.2 inches. that's 3.2 inches or 4.2 percent bigger than the 'normal' tire.

so what does that mean? it means that the bigger tire will turn FEWER
revolutions to cover the same amount of ground. assume you are doing 60 mph
or one mile per minute. at 76 inch diameter (OLD tire), you'll turn 833.7
revolutions per mile, and (because you're traveling at one mile per minute),
833.7 rev/minute.

if you put the new tire/wheel on, you get 800.0 revolutions per mile. so at
the same engine rpm, you're going FASTER.
  3600 rpm x 76 inches  is less than 3600 rpm x 79.2 inches. so in the same
minute, you've covered more ground. so the speedometer is showing you going
SLOWER than you really are.

anyway, the whole laborious point is that you can figure up the amount of
difference between the new tire/wheel and the 'normal' ones and get a good
idea of what it will do to your speeds. it's not much, usually ... 4 or 5
percent. but it also affects your Odometer, and it's a little harder for the
engine to turn that slightly-larger wheel, so your fuel economy is gonna
look wierd when you calculate it out.

as to what will FIT the wheel well, like Derek is always hunting for, in
germany they can run up to 16" wheels with mud grip tires on them, so i don't
think you would have tooooo much problem with most any 15" tire. EXCEPT if
turned REALLY sharp with a very very WIDE tire ... then you might get some
suspension parts rubbing on the front tires. or if you hit hard and bottomed
out the suspension. that might scrape the top of the wheel well.

one problem i had with a set of 195 tires, however, was hydro-planning: if
you get wider tires, be SURE to get some really good tread pattern, one that
will get rid of the water easily. the ones i got did NOT have a good pattern
and that little extra width on each tire was just enough for the weight of
the mostly-empty bus to allow it to start hydro-planning. just something
extra to think about.


more than i originally started to write. ;)
joel