Since I am finally sitting here with some momentum going in the automotive
email direction, I'd thought I'd comment on last week's discussion of tires.
As I recall, someone asked about BFG, Yokohama and Hoosier tires, and some
relative merits of each. Say, speaking of relatives, we drove past Hoosier's
plant in Indiana on our way to a family reunion!
Okay, sorry about that. What I was going to say had to do with weight. The
radial performance tires are heavy, compared to Hoosiers. On my Spitfire
autocross car, we were running 13 x 6 cast aluminum rims with 185/60 Yoko 8Rs.
Good tires, reasonable wear, fairly easy to exploit. A wheel and tire combo
weighed in at 30 pounds. With the same tire on a 7" steel rim from Fastwheels,
you had 36 pounds. Yuck.
Last year we got some 8" wide alloy race wheels, and some 225/45/13 Hoosier
bias ply tires. The total for that was 17 pounds, 10 for the tire, 7 for the
wheel. We were able to shed 13 pounds per corner on the car. Of course, most
vintage groups would not like to see a Spitfire with tread width wider than
the car is tall show up for vintage events, so one might save another pound or
two with a 13 x 6 wheel and 185/60 tires.
Remember, that weight lost in the wheel and tire gives you three benefits.
First is what one might call the static weight reduction, whereby the car
weighs 13 x 4 or 52 pounds less. And since we are aware of the F = ma
equation, a possibly significant increase in a, given the same force, can be
had by that reduction in m. In the case of the Spitfire, that amounted to
a roughly 3.5% increase in acceleration, without lifting the bonnet. Assuming
a 1 minute 30 second lap, half of it accelerating, that 3.5 percent translates
to about 1.5 seconds per lap. Rough estimates, mind you, just to give you an
idea, I don't feel like cracking open the calculus text. And when the F is a
negative number, as in braking, then a lighter car will brake more quickly
from a given speed.
Secondly, a significant reduction in wheel/tire weight gives you a reduction
in the rotational inertia needed to turn the four little flywheels on the
corner of the car. Less torque applied to just getting it going means more
torque applied to going forward, faster.
And the third point is the weight reduction can also be subtracted directly
from the unsprung weight column, allowing the suspension to react more quickly.
So weight difference among various tires may be a thing you want to consider
in any tire decision.
mjb.
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