RE: [ihc] Tire & Wheel Questions

["Mac @ TRIAD" <mac@xxxxxxxxxxxx>]

## >> 12.5 width vs 15.5 width? It really depends on your rim
## >> width. I run 33
## >> (tall) x 15 (rim size) by 12.5 (tread width) on one of the scouts. The
## >> offset of the rim is not part of the tire size, but rather
## >> determined by
## >> where the mounting surface is found in relation to the
## >> inner/outer surface
## >> of the rim. My wheel mounting surface is half way between the
## >> inner and
## >> outer, and my rims are 10 inches wide.

tread width.  always a question.  and never quite what the manufacturers
tell you it is, either.  as specified in more detail below, our TravelAll
runs tires listed by the factory as 33"x16.5"x12.5", and the 800A runs a set
labeled as 31"x15"x10.5".  the actual measurements of these tires, however,
differ from what they are labeled as.  the Scout's tires in reality measure
only 29.75" tall and have 9" treadwidth (the width of the sidewalls at full
inflation is 10.25", with 10.5" at the sidewall bulge where the tire rests
on the ground).  the TravelAll's tires are 31.5" tall and have a treadwidth
of 11.25", with a sidewall width at full inflation of just over 12 inches,
and a width of the sidewall bulge at the ground of 12.5".  when mounted on a
wheel and at full inflation, laying flat on a table or spinning freely, i
have no doubt these tires measure what the factory says they should, at
least for height.  when mounted on a vehicle and that vehicle's weight
resting on them, the measurements fall quite short.  the spare tire on the
TravelAll, unmounted on a rim and laying flat on the roof, has about 20%
tread left on it, and measures 32" diameter.

to be honest, i don't know much about wheel offset, either.  i've got an
excellent old '70s offroad handbook put out by a guy named Bob Waar
("Offroad Handbook with back country travel tips" by Bob Waar, copyright
1975, H.P. Books, ISBN 0-912656-15-8), and it has a lot of really excellent
information in it.  according to this book, wheel offset is measured
positively or negatively from the wheel mounting surface.  zero offset
places the wheel mounting surface exactly centre of the wheel, positive
offset is with more wheel outboard than inboard, and negative offset is with
more wheel inboard than outboard.  the book also says that you should get
the measurements of the original wheels the vehicle came from the factory
with, and recommends that when acquiring new wheels, one should get wheels
with as close an offset to stock as possible.  i'm not absolutely positive,
but i'm fairly sure that most IHC products with the smaller 5-lug wheels
came with zero offset.

## >> This got discussed here a few years back, and I don't
## >> remember whether the
## >> "zero" offset is measured from the center, inner, or outer
## >> rim surface.  I
## >> prefer to have about a 20-25% narrower rim than tread, but no
## >> less than that
## >> because I worry about losing the bead at low tire pressure. I
## >> don't know if
## >> that's justified, but I worry anyway, usually out loud so
## >> that my wife will
## >> have something on which to focus while I worry about other things.

we run 16.5x10.5" 8-lug mag wheels on our TravelAll, with 33x12.5" tires.
they're your basic all-terrain type tires, decent lugs for offroad
performance but not so big that you get a lot of highway noise, either.  i
rather like them, they handle well, have adequate traction even in rain,
etc.  they're some kind of relabeled Cooper Discoverers (they're actually
identical except for the raised lettering on the sides to the set of Cooper
Discoverers they replaced).  on 10.5" rim width, these tires look just about
right for tread width versus wheel width.  i haven't tried any other tires
on this truck, so this might be something specific to the Discoverers, and
other tires may not be right for this wheel width.

our Scout has 15x8" wheels up front and 15x10" wheels on back, these are
what it came to us with.  it's running 31x10.5" tires all the way around.
these tires appear to have exactly the right tread to rim width on the
front...  but on the back, they look positively dangerous for offroad use.
the rim overhangs the tread on these wheels by at least an inch on each
side, which to my thinking puts WAY too much sidewall at risk for puncture
in rough terrain.  if we decide to take this thing out anywhere offroad,
i'll be replacing those rear tires with something wider.  many critics will
say it would be easier, and probably better, to just replace the rear wheels
with a set that match the front width and have the same width of wheels.
and they would probably be right.  however, this truck has a set of matched
wheels on it, source unknown, that look particularly smashing (and they
appear to be a fairly uncommon pattern).  since this is Becca's truck, and
she doesn't want her wheels replaced, i kind of have to stick with her
demands, you know?  given a choice, i'd rather change out the fronts to 10"
wide and then get 31"x12.5" tires all the way around instead.  she's still
mad at me for the impromptu body redesign i performed on it two years ago,
so her patience with me and "modifying" her truck is at an alltime low.

as far as it goes, i haven't seen any really good guidelines published
anywhere, though that doesn't mean there aren't any.  my rule of thumb is to
run wheels about two to three inches skinnier than the tire being mounted on
them.  10.5" tires on 8" wheels, 12.5" tires on 9.5 to 10.5" wheels, and so
forth.  it's hard to get wheels in a width that meets this criteria for
tires wider than 12.5", though.  i don't think i've seen very many 15"
wheels wider than 10".  since i'm not running anything bigger than
33"x12.5", this is not likely to be a problem for me.  eventually i'd like
to run a set of tires and wheels from a HMMWV (there's a guy locally here
that sells these used--about 70% to 80% tread left--from the military for
cheap).  HMMWV wheels are 16.5" on 8 lug, in exactly the same pattern as
what my TravelAll uses.  the tires themselves vary from 36" to 38", and are
invariably Goodyears.  i don't remember the width, but i *think* they're too
wide for the mags i've currently got, hence the change to HMMWV wheels as
well.  i'll need about 2" of lift front and rear, and about an inch all the
way around the rear fenders trimmed to fit 36", and i'm not keen to do this
anytime soon, so it's going to be a while before i try it.  but this is one
of the possible future plans, especially if we wind up staying in this place
for a long time.  the tires are cheap enough to justify the work required to
mount them, i think.  especially since i wanted to be able to run 36" tires
on this beast anyway.

--Mac

---------------------------------------------------------------
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Krystof "Mac" MacBryghde
TyrGothi

http://master.triad.ath.cx/