One for the Guru's: effects of Unsprung Mass
 

This isn't a Subaru, but I'm sure the same basic theories apply:

What affect would an increase in wheel/tyre weight make on the ride quality, comfort and handling of a car without changing the suspension?

And how much increase would be seen as marginal or excessive to the extent that it doesn't affect the current (stock) suspension?

Say for example: A car with stock wheels and tyres weighing 20kg is replaced with wheels and tyres weighing 25kg (5kg increase), and the car had relatively soft springs, but moderately stiff shocks - what differences in ride/comfort and handling would I be most likely to see?

TIA :)

So I'm not the only who hasn't the foggiest idea (thought as much) :D :lol1:

I did this in reverse on my old VW Corrado when I switched from some heavy alloys to OZ 'Legerra's. The lighter alloys improved the handling considerably. To make sure it wasn't just placebo I sent my partner out in it without warning her why and she immediately commented that it felt more agile and responsive.

I'm no "guru", but I imagine it as holding a bucket full of water at arms length, swinging it up and down and trying to stop it accurately. Repeat the exercise with no water in the bucket and it's much easier. Substitute the bucket for wheels and your arms for the suspension assembly.

Corrado's 'bucket' analogy is really good.
The other effect of the weight of the wheel/tyre is the inertial nad the gyroscopic effect of the mass.
If the wheel/tyre/dics/caliper etc is heavey ( x 4 also) the engine has to accellerate this mass. If the sum of the above components is reduced then the car needs to do less work to accellerate.
The mass of the assembly unsprung also can act as a gyro which does not want to change direction, just like a bikes wheel.

If you drive an Impreza with 16's and low profile tyres and then drive the same car with 18's and tyres this effect will be quite noticable when driving quickly along twisty country lanes. The car will feel better though on Motorways/Dual carriageways where the gyro effect keeps the wheels in the dead ahead position.

Maybe that helps a bit.

Graham.

Ta, that bit in terms of braking/aceleration/turning makes sense.

But what about when the car hits a bump or dip (up and down movement of the wheel/brakes/hub) ? Does the weight of the wheel affect the inertia transfered to the shock/springs? - My thinking goes that in an extreme case it makes the suspension less sensitive to small bumps, but large bumps which do force the wheel upwards causes the wheel to deflect more as the suspension cannot overcome the upwards inertia of the wheel (thus bottoming out in extreme cases). Like wise on a dip in the road (or the rebound stroke after hitting a big bump) the suspension may have difficulty pushing the wheel back in contact with the ground (although gravity and vehicle weight will naturally counteract this - but it should make body-flex more noticable ) Poisbbly giving a "jacking down" effect on bumpy roads and thus reduced upward suspension travel.

Am I near the mark or way off? :D

Next task is working out how many Kilograms difference would have a negligble or excessive effect.

Think of the car thios way.
Car 1
Big Jaguar. Sprung weight = 1.5 tons (body/engine trans etc
Un sprung weight = 100Kg (say)
Car drives over a bump, the weight of the shell is such that the wheel will move over the bump (down and back up) and the shell barely 'notices' the movement = Jags rides well as the mass of the wheels etc is so small in comparison to the shell.
Change the Jag's wheels to 20'' etc and the unsprung will increase, but the shell is still much greater mass.

Car 2
Westfield with bike engine: weight abot 600Kg all up
Wheels etc still about 100Kg (say)

Drive over the same bump at the same speed and the shell will definitely feel the movement as it's mass is close to the unsprung weight

Result: car rides badly over bumps.

Car 3
Dream car, shell weighs 100kg
Wheels still @ 100kg

Over same bump etc and the car will jarr everywhere and actually be undriveable at any reasonable speed!

So to the Scooby. Get the unsprung weight down and the shell gains more composure, control and better ride = comfortable driver = faster car.

Wheew, worn out now..:norty:

Graham.