How important is unsprung weight?

[Phil Offord]

Can someone tell me how important the unsprung weight of a car is. I am looking at some new wheels, but have been told that the ones i like are heavy 13.5kg in an 18", or 14.5kg in a 19"? I thought that so long as the circumference of the wheel/tyre combination is the same or very close to the stock set up then there would not be any performance difference! I have seen some other wheels that in an 18" rim weight 10.5kg.......would i notice a difference when driving do you think, or is it all complete bo**ocks?

 

Many thanks for help given

 

Phil. O

[Adam W]

It makes a huge difference to the handling of the car, and you're talking about a 50% difference in weight. I couldn't tell you why though, except to say that F1 teams don't use incredibly light, incredibly expensive wheels and suspension components for show!

 

Some passing expert or other will be along in a moment to expand on this . .. .

[Mark Ayling]

No point putting flash alloys on a "family car" Phil....

[Phil Offord]

Cheaky little sod!!!!!!!

 

Give me time......the mods are starting, so i will get there in the end......revenge will be very sweet....all be it in the wet weather

[Doughie]

Increasing unsprung weight (i.e. wheels / tyres / brakes / hubs etc) means that there is more weight for the suspension to control when you hit a bump etc.

Job of the suspension in a car is to keep all 4 wheels in contact with the road as much as possible. If a wheel weighs a lot it is more weight to be accelerated up and then down again, and the springs and dampers just can't do as good a job of controlling it as it would with a lighter wheel.

 

Also if the additional unsprung weight is contained in the rotating bits ie. wheel and tyre, then it will hurt acceleration and deceleration MORE than the actual additional weight would, if it was just a lump of lead stuck in the car (i think).

This is because the wheel/tyre will have a greater rotational moment of inertia which makes it "harder" for the wheel to be spun up (acceleration) and spun down (deceleration).

 

 

Chris Wilson could probably give a far more eloquent description though. (Earth calling Chris ??)

[Chris Wilson]

Originally posted by Doughie

Increasing unsprung weight (i.e. wheels / tyres / brakes / hubs etc) means that there is more weight for the suspension to control when you hit a bump etc.

Job of the suspension in a car is to keep all 4 wheels in contact with the road as much as possible. If a wheel weighs a lot it is more weight to be accelerated up and then down again, and the springs and dampers just can't do as good a job of controlling it as it would with a lighter wheel.

 

Also if the additional unsprung weight is contained in the rotating bits ie. wheel and tyre, then it will hurt acceleration and deceleration MORE than the actual additional weight would, if it was just a lump of lead stuck in the car (i think).

This is because the wheel/tyre will have a greater rotational moment of inertia which makes it "harder" for the wheel to be spun up (acceleration) and spun down (deceleration).

 

 

Chris Wilson could probably give a far more eloquent description though. (Earth calling Chris ??)

 

Gyroscopic precession, the natural reluctance of a spinning disc on an axis to change direction. Coupled with carefully designed compliance in the stock suspension bushes an increase in gyroscopic precession from heavy rims van have some very odd effects on the handling. Stuart also cites very valid points. It is ALWAYS worth buying the lightest rims you can, it is not for pure greed that stock rims are so expensive, they are top quaility, in both material and execution, finding something as good in the aftermarket can be very hard without spending a fortune. I think Silverline Wheels and Tyres may be the best bet, they do some proper rims.