Date:         Tue, 17 Aug 2004 10:07:12 -0700
Reply-To:     Al Knoll <al_knoll@PACBELL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Al Knoll <al_knoll@PACBELL.NET>
Subject:      Re: how much of a performance hit with 215/75/15 tires?
In-Reply-To:  <200408171443.i7HEhOPQ006787@ylpvm02.prodigy.net>
Content-Type: text/plain; charset="iso-8859-1"

First of all the performance hit is both in "thrust" or acceleration AND
more importantly in braking performance.

The brakes act on the hubs and apply a torque to the hub depending upon how
hard you push on the pedal.

The drive train acts on the hub and apply torque depending upon the throttle
or accellerator position and what gear is selected.

In Dunedin NZ where it snows mightily, these may be backwards to accommodate
the coriolis forces and the fact that the cars don't travel on the right
side of the road.  No worries though merely swapping the plus and minus
signs will fix all that.

For a given torque on the axle hub assembly the distance from the centerline
of that assembly to the road will dictate how much force is applied where
the "rubber meets the road".

EG:  If your tire has a rolling radius of 10 inches now and you install
tires and wheels that have a rolling radius of 15 inches.  Both your braking
force and your accellerative force will be reduced by a factor of 15/10 or
1.5.  So if it takes x amount of pedal force to achieve a particular braking
force with the smaller tires it will take 1.5x with the larger tires.

Same for the "thrust" only with the direction of the torque reversed.  (NB
makes no difference in the calculation)

I run 215-75x15 AND the SA big brake kit on the front.  The Syncro Westie
weighs in at 5000 GVW or so and would be unacceptably braked with the stock
brakes.

YMMV, mine always does.

Horsepower is a farming term, torque is an engineering term, thrust does the
work.