wheels/tyres

["Dave Eaton" <Dave.Eaton@clear.net.nz>]

we had a planning session last night with a few buddies about the
international rally coming to nz in a couple of months.  it looks better
than ever this year, certainly with more works teams (8 confirmed) and a
rationalisation of stages making it easier for those of us travelling over
600kms to view it.

anyway, after a few cold ones, the talk got around to cars, as you do...  i
made a throw away comment about most wheel/tyre upgrades being a waste of
time.  after that, debate raged for quite a length of time.  afterwards, it
struck me that there is a heck of a lot of mis-understanding on this topic,
not in the least helped by marketing "spin" by tyre/wheel suppliers.

for example, who knows that the contact patch of a +1 or +2 (+whatever)
type/wheel upgrade, while changing in shape, stays the *same* in size?  for
example, your 245/40 17 wheel is providing just the same footprint size as
your 215/50 15?  generally speaking it will just narrow your longitudinal
contact patch (at the penalty of traction and braking) and increase your
contact patch width (a potential cornering improvement).

or that, by changing the contact shape, the steering axis can also change?
and the natural self-aligning torque produced by the tyres to aid stability
and steering gets changed.  or that the hubs are put under more stress.  or
that the unsprung weight will increase making transient response, turn-in
and bump worse?

the only guaranteed sure-fire win is looks.  mind you, that's only some of
the time, looking at some of the monstrosities going around the roads where
i live...

i keep coming back to the same thing, if the chassis designers designed for
a particular type/wheel combination, you are better off with that, unless
you really know what you're doing.

clearly many on this list have btdt with "positive" results.  what am i
missing?

dave
'95 rs2
'90 ur-q
'61 mb fintail