> Reply-To:     Derek Drew <derekdrew@RCN.COM>
>
> Among the issues that has come up is the limited amount of horsepower that
> the 2.1 litre motor has to push the vehicle, and how much of this
> horsepower gets eaten up driving larger tires. I have seen reference in
> various places to "increased rolling resistance" of larger tires, and the
> consequent apparent decrease of power to the driver of using larger wheels.

Tire drag figures measure the linear coefficient in a speed to drag
curve, and is quantified in rolling resistance per vehicle weight unit,
usually the figures are in kg drag/1000kg vehicle weight.

Tire makers calculate these figures, and they are unique to a particular
tire design. Unfortunatley they are hard to come by. When I tried some
years ago I only found it from two or three tires. Strikingly they
vary by a factor two between similar size tires.

Luckily for us total drag (ie the combined drag from air and tire resistance)
is more relevant to compare to available engine power. This can be quite
easily measured for a particular tire vehicle by s set of timed coasting
trials. Similarly available rear wheel power and torque can be calculated
from acceleration tests. I outlined the testing procedure and suggested
to collect data from a variety of Vanagon/Westy/Synchro/various tires
combinations. Unfortunately nobody (other than myself) performed the tests
and sent me data. My drag and power curves are below. (Sorry I don't have
time to redo this now. New job...)

This is for 2WD Westy with Futura metric 205/70/R14 at 44psi

http://cs-www.cs.yale.edu/homes/jag/vw/engine/diesel1.9/WestyDrag_12_98.gif

Power: 1.9l TD relatively stock, some minor intake and exhaust mods,
injection timing very carefully set, but no fueling quantity mods or
intercooler. This is rear wheel power.

http://www.cs.rochester.edu/u/jag/vw/engine/diesel1.9/Diesel1.9l_12_98.gif

Given these tow curves one designs transmission ratios to make sure
that at each speed/rpm available engine power exeeds total drag by some
margin.

Cheers,
Martin