Re: DYNO RESULTS -- STOCK vs AFTERMARKET WHEELS

["Ian Overholt" <ioverholt@hotmail.com>]

Jason,

I think you answered your question about modulation with the 195's in the 
preceding paragraph.  The 195's have a shorter (but wider) contact patch.  
So that is giving the tire less rolling distance to work with when stopping. 
  That makes braking more off/on rather than incremental.

Ian Overholt
'88 16v
'78 Champagne Edition


>From: 16v Jason <jason@scirocco.org>

>As far as the effects on braking are concerned:  That's another story
>entirely.  It's very difficult to tell what kind of improvement or
>reduction there was in braking efficacy.  My gut feeling is that my car
>brakes better with the 185s.  It's doubtful (in my mind) that this has
>anything to do with the mass of the wheel/tire combo, as I can immediately
>lock up the 15s, meaning that the brakes themselves are capable of enough
>force to overcome the rotational inertia + grip almost
>immediately.  However, I attribute the perceived increases in braking
>distances to the *shape* of the contact patch rather than the weight of the
>wheel.  Why?  We had a debate a while back on this (I think it was on the
>MB list)-- but physics will prove time and again that the *area* of the
>contact patch is quite consistent regardless of what size tire you're
>wearing.  Despite me fighting tooth and nail about this, the engineers
>(including a tire engineer) on the list sat me down with pressure formulas
>and proved it to me.  So, at least in theory, the contact patch area stays
>constant.  So, I'll reduce that for real-world application and state that
>the contact patch stays "relatively consistent" in terms of area.
>
>What happens when you add width to the tire is that the contact patch
>becomes elongated (relatively speaking, of course) laterally, and shortened
>fore and aft.  It's my hypothesis that a laterally-elongated rectangular
>shaped contact patch would help with lateral forces, and diminish the
>tire's capability for longitudinal forces (i.e. braking or acceleration
>traction) slightly.  Likewise, using a narrow tire would help longitudinal
>forces by having a longitudinally-elongated rectangular contact patch, and
>thus would help with traction on acceleration in braking, and hence its use
>in snow and ice applications.
>
>Regardless of that debate (which I'm *still* not sold on), I find my 195s
>*much* more difficult to modulate than I did the 185s.  If my car had ABS,
>I would most certainly go and do before-and-after 60-0 deceleration runs
>with the G-Tech... but I don't have a few sets of tires to burn through to
>practice modulation with both types of tires to the point that I can
>perform reliably consistent braking.

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