1 Northlander March 2013 - North Country Region

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re-express it in terms of weight ratios independent of the system of units used. Any conversion
factors between weight and mass, such as the in cancel out. (Actually, in the system
of units used here, one unit of pound-force (weight) is approximately equal to one unit of poundmass,
a convenience that can lead to great confusion.) Using this and Equation (6) we obtain
For a car the weight changes in the wheel assemblies are usually small compared to the
weight of the car, that is,
, so this term can be dropped in the denominator.
Equation (7) then simplifies to
This states that the equivalent increase in horsepower due to weight reduction in wheel assemblies
can be well approximated by 6 times the power-to-weight ratio of the unmodified vehicle times the
weight reduction of a single assembly. As an example, consider reducing the weight of each wheel
assembly by The effective weight reduction for all four assemblies is then ,
10 of which are attributable to rotational mass and 20 to inertial mass. From our Article 1 example
of a 2001 Boxster S,
and for the GT3 RSR
. With this results in and , respectively.
It is easy to see that modest performance enhancement can be obtained over stock assemblies by
using race-engineered wheel designs employing lightweight ceramic rotors ( 25% lighter than
iron), lightweight wheels ( 25% lighter), and lightweight tires ( 10%), all of which reduce the unsprung
weight as well. Next month we will consider another area of improvement, the lightweight
flywheel, which has some interesting effects on performance.
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March 2013
57 Northlander