Unsprung Weight

[JB]

Just been reading an article on the Ferrari 360 Stradale where they're on about the reduction in unsprung weight.

What is unsprung weight? Is it different from the actual weight of a vehicle? Why is it so improtant to reduce the unsprung weight?

[Jezz_S13]

i though it was the weight of the stuff hanging off the car below the springs. IE stuff that the springs dont support like brakes axles etc.

[Algie]

I'm no expert on this but I was talking to a guy at an alloy wheel stand at a show and the jsit of it was.....

The unsprung weight of the car is all the bits which are under the springs on the suspension eg wheels, brakes etc.

He was also saying that if you save 1kg weight on a new lighter alloy wheel it equates to a 5kg saving of sprung weight. It seems to have a far greater effect on acceleration. 4 new alloys saves 20kgs!

Trouble is with a porker like me at the wheel thats a bit of a drop in the ocean!!

Alg;)

[Martin T]

Its not acceleration that reducing unsprung weight helps so much its hadnling and grip on bumpy roads. If a car has has no unsprung weight, then there will allways exactily the same force that a tyre exerts on the road, whether it has just hit a bump or if the road is really smooth, while if you have lots of unsprung weight, then th point as the tyre hits a bump, it momentarily has more grip, then immediately after, it has a lot less, or even none at all if the tyre leaves the ground.

edit: I am also no expert on this its just based on my knowledge of physics.

[Jon]

Everybody's right here (except the acceleration bit)

The suspension can control light wheels, tyres and brake calipers much easier than heavy ones.

[Algie]

That's probably what the guy meant and I miss understood.

He was trying to sell me a set of wheels at the time!

Alg;)

[Duff Man]

Surely lighter wheels would aid acceleration due to them having a lower rotational mass and thus requiring less effort (or torque) to turn them.

BUT!!! it probably wouldnt make an awful lot of difference, unless your existing ones are made of iron or something :D You would need to consider the maths behind it since as with lightening flywheels (from what i have heard), its better to remove mass from the outside edges.

Therefore I'd have thought that larger wheels with more metal towards the outside (since the rolling radius is considered to be roughly constant) and less tyre would have a negative effect on acceleration (assuming a tyres density < that of the tyre).

Im sure someone else with more time and knowledge could correct my arguement to make it more plausible/scientific (its been a few years since ive studied moments of inertia).