How Do I/Lighten My Car

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What is the significance of reducing rotating mass versus stationary mass, when lightening your Se7en?

Newton's Second Law of Motion states: "Acceleration is proportional to force and inversely proportional to mass."

Acceleration = Force divided by Mass

For a given force, a 'lighter' car will accelerate at a greater rate. "To go faster you make the car 'lighter'."
It takes Force to change the rate at which something is rotating in the same way it takes Force to change the speed an object is moving in a straight line.

As an example: if you remove 10kg mass from a Seven, say, the spare wheel and carrier, the car will be 2% lighter and thus, 2% quicker to accelerate. For example, the 0-60 time will decrease from 5.0 sec to 4.9 secs. That's optimistic for several reasons, but serves to illustrate the point.
The same 10 kg mass could be removed from the engine flywheel and it would have the same effect on the car being accelerated in a straight line as removing the spare wheel and carrier as above. But an addition affect would be that there is less mass for the engine to accelerate rotationally.
The flywheel gets spun from 1000rpm to 7000rpm, back down to 5000 and up to 7000rpm again every time you change up through the gears. Now consider a playground 'roundabout' where you push on it to get it going and the kids love it (quite old fashioned - not seen one for a few years). It takes several seconds of hard pushing to get it up to speed, and yet it's not actually going anywhere - it only has ROTATIONAL velocity and hence only rotational inertia. (Inertia is: "The opposition a body offers to a change in motion." A property of all bodies. Inertia is a 'quality', but measured in terms of mass, which is a quantity).

A 'lighter' equivalent, e.g. a spinning top, is easy to spin-up to speed because it has less mass. So back to our flywheel. I worked out once (long day at work) that from a normal to a lightened flywheel, saving about 3kg, would also give a 0.1 sec advantage 0-60. So I've lost a third of the mass, but got the same effect while accelerating the car in a straight line.
Magical other benefits are that I can change gear more quickly as the engine can drop from 7000 to 5000rpm quicker to let me get into 2nd to give it more "Welly" earlier. So I gain a bit more there.

Here's the science bit....... A concept necessary to predict the behaviour of a rotating mass is that of its "Moment of Inertia". This quantity not only expresses the amount of mass but also its distribution about the axis of rotation. Basically this takes into account that the weight on the edge has more effect than that near the middle. To demonstrate this principle; sit on your office chair, spin it round with your arms and legs straight out. Now pull them all in - you speed-up. Your "moment of inertia" went down as you had less mass away from the centre so you HAVE to rotate faster to keep your (angular) momentum the same.

CONCLUSION: Reducing a given rotating mass (wheels, tyres, engine flywheel, clutch pressure plate, gearbox, prop shaft, differential, drive shafts, wheel hubs and brake discs) is more beneficial to Se7en acceleration in a straight line than reducing the same 'fixed' mass. Reducing a given mass from the periphery of a rotating mass is more beneficial than the same mass removed from nearer the axis of rotation.

Dave Hooper

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