More tire size stuff

[colin cohen <102621.2256@compuserve.com>]

    28-Nov-95 11:13 CST
Sb: Cooper in the trunk Fm: INTERNET:PDQSHIP@aol.com <<Sure, however my point
was in reference to the float caused by the higher sidewall not the narrower
tread, this is chassis to tire float reference not a tire to crud
reference..... In fact, my post referenced the scrocco as having too much float
on a "wide" 185 tread tire, so you must have missed my float reference........
  And the post was a 195/65 vs a 205/60, and for the 3 tenths of an inch, you
will notice little, if any difference in crud float, but you will notice a big
difference in sidewall float....>>

I think what you are saying is that there is a critical trade off between
weight and float. On lighter cars, the weight vs the sidewall stiffness is so
low that the aquaplane effects cannot be offset by the weight of the car. So it
behooves one to go to a narrower tire. This does not effect stability or
response to the same extent as say an Audi, because the absolute speeds are
lower, the CoG is usually lower and one needs that weight to get tread action
vs the distortion in the heavier car.  I think you are also saying that changes
of the order of 195/65 to 205/60 are clearly going to be beneficial. However if
one went to say 245/45 (with a wheel change, and assuming clearance), there
might be a loss of wet/winter traction for the same reasons that would effect a
much smaller change on our Coopers. Do I read you right?


<<I refer to this as time delay syndrome.....  If you take your summer tires
and go 60 mph, and do lane jumps quickly, you can eventually get the tire
behind the steering input.....  If you do this with a higher sidewall summer
tire you will notice that the tire falls almost a whole input behind the
steering, that is the definition of tire float on sidewall......  A stiffer
sidewall can "help" but not eliminate the problem, cuz it's just rubber and
flexing steel belts after all, you have a height to weight problem.....>>

I tried this exact exercise on a skid pan with the Coopers many moons ago using
various sizes of Dunlop SP Rally tires, and you are quite right...in the dry!
Once we got the surface wet, the narrower sections did better than the wider
tyres after a very small change. I dont remember how much they changed, but we
went from 6" wheels to 7" and tested 4 sections on both 10" and 12" rims. In
fact on the widest wheel/tyre combo, we were aquaplaning at 50 mph, and unable
to do emergency brake turns any faster than 30 mph.

So bottom line, and back to Audi (so as not to bore the lurkers - G) I dont
think that one can generalize about this, but i would not recommend wide
section low profile snow tyres for quattros, irrespective of their fat chops!
Going from 195 to 205 wont hurt in all but the most marginal conditions, but
any wider and I think that at slow speeds, you are compromising the quattro
qualities.

I once drove my 4000 CSQ from cleveland to Buffalo in a snow storm, up I90,
using the unplowed left lane to duck out of the slow traffic in the plowed
lane. My speeds varied between 30 and 60 mph. I suggest that the tread pressure
generated by my Goodyear Eagle M+S 185 section tyres benefitted me far more
than I was hurt by sidewall float at some higher (later) speeds.

Enuff!  Regards

A Minicooper or my 1988lb scirocco can go with very narrow tires cuz they are
very light cars, and are much more sensitive to width than height, not enough
weight to crud push, sidewall stiffness is plenty for the weight...........
 The audis OTOH, are big pigs with too small a wheel/tire combo from the
factory......  They tend to be sensitive to the sidewall more than the tread
width (and we are talking tenths here) becuz of weight, a 225 tire on a 3500lb
car can prolly push the same amount of crud as a 165 on the mini, but the
3500lb teetering on a 70series tire is asking for control trouble for the audi,
and little to  none for the cooper....

You can eliminate some of the sidewall problem with wider wheels/narrow tire
combo, but the sensitivity to height is still there.....  And in fact, I know
that there are some stiff narrow tires on the market, but the height still
gives you a control disadvantage, and it's before the car reaches it's 10/10
point in any conditions.....  You want the tires to follow your steering input
as accurately as possible, low traction almost more than hi traction
conditions, cuz low traction makes the steering and tire outputs crutial for
maintaining the contorl of the vehicle.....  You are comparing the apples to
oranges with the pocket rocket.....  Portly is the nice word for the
audis.......   So, your argument is well founded for a 195/60 vs a 205/60, but
given the MPS (that's miss piggy syndrome) of the audis, I doubt you'll notice
any difference unless driving 10/10.....

Scott





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