velohead66

OK, we all know that weight loss will make the car go quicker.

Whats the big deal with unsprung vs sprung mass ???

Say you have a curcuit car, is it better to lose weight in one area or the other ?? And Why ??

Ta.

911

The result of light wheels, light tyres, light brakes (etc) allows much better control of suspension movement and damping.
This is why, for instance, Porsche used Forged magnesiun alloy for wheels and even used Aluminium alloy wheel nuts on their road cars.
A racing slick tyre is abot 40% of the weight of an Toyo road tyre of the same size.

Realistically, to save unsprung weight you need to use small alloy wheels, ie 16'' Sti wheels and correct tyres size.
You will be amazed just how much heavier a set of 17/18 inch wheeels/tyres are to a set of 16's.
Further, such large wheels have a pronounced gyroscopic effect due to the mass being at the circumference of the wheel. It is this effect that tends to make a push bike stay stable, which is great if you are on 2 wheels!
However, on a big (over sized?) wheeled car, ie Impreza on 19's there will be a marked reluctance for the car to turn off the straight and narrow as the effect is trying to keep it all running straight.
Read a road report pn P1's. Many say the P1 with 17's is great compared to the same car on 18's for this reason.

911 (all in MHO)

AvalancheS8

Unsprung mass is the weight of the parts that moves up and down with the wheels when the suspension moves. One reason it is good to keep this low is that it allows the suspension to work more effectively, since there is less inertia in the system and the lighter wheel, wishbone etc can move quickly up and down to follow the road. This effect often allows a better ride and road holding without having to compromise the handling or body control. As the unsprung mass gets higher the ride will deteriorate as the wheels tend to crash into the bumps rather than moving up out of the way, and often heavier damping will be required to control the bouncing of the heavier wheels and so on as they move up and down, which also compromises the ride.

In the past some cars have moved the brakes inboard to the other end of the drive shafts to get them into the sprung rather than unsprung weight.

velohead66

Thanks Guys.

Nice to know the 'technicalities'.

patGT

This is true in general but not necessarily always the case. When seriously considered, wheels are often changed to improve handling AND to reduce unsprung mass.
For example, the 17" wheels Volk TE37 on my classic Turbo are definitely lighter than the stock 16"

Patrick