Why is unsprung weight bad

obi_waynne

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Just writing an article and would appreciate some input from the guys who know stuff, mainly to confirm my understanding and to see if I've missed anything.

What issues are caused by unsprung weight?
I have noted an inability to change direction and speed, and mentioned the gyroscope effect etc...

What dynamics are involved with unsprung weight and rough surfaces? Does it mean a loss of energy?
 
It seriously messes the ride comfort as well. Less unsprung weight means that softer springs and dampers can be used without compromising handling
 
More weight, or in this case more mass equate more inertia. If you wanna instant steering or instant return on road surface springs and dumpers must overcome that inertia so, as HDI fun said, they need to be stiffer.
Everything is about weight/power ratio. More unsprung weight needs more power to compensate and as weight increases, the smoothness of operation decreases.


(Just in my humble opinion!)
 
Unsprung weight is that portion of the total weight of the vehicle which is not supported by the suspension springs. It is comprised of the wheels, tyres, hubs, hub carriers and brakes (if mounted outboard) plus appoximately 50% of the weight of the suspension links, drive shafts, springs and dampers (if mounted outboard). Since this unsprung weight is what the dampers and springs must attempt to control - in the bump direction - in order to keep the tyres in contact with the road, the less of it there is, the better it can be controlled.

On older cars with solid axles the unsprung weight will include all of the drive shafts plus the axle casing and diff plus appoximatley 50% of the propshaft (depending on its design).
 
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So there is more skipping and bouncing OG and lighter wheels are easier to control and keep in contact with the road surface?

Does unsprung weight have much of a bearing on turning the wheel?
 
So there is more skipping and bouncing OG and lighter wheels are easier to control and keep in contact with the road surface?

Does unsprung weight have much of a bearing on turning the wheel?

Absolutely it does. Why else do you think that many manufacturers mount suspension components and steering components on an independent subframe?
 
So there is more skipping and bouncing OG and lighter wheels are easier to control and keep in contact with the road surface?

Does unsprung weight have much of a bearing on turning the wheel?

Absolutely it does. Why else do you think that many manufacturers mount suspension components and steering components on an independent subframe?

This is why alloy wheels have a far bigger part to play than just aesthetic appeal. And why else do tyre manufacturers seek to reduce the total mass of tyres?

It's not all just for fun.
 
I don't think it makes much difference. The force required to turn the wheels is used to overcome the friction of the tyres on road and the gyroscopic effect of spinning wheels. I don't think the weight of the wheel will make much difference, especially as the percentage difference in weight between a standard wheel and a lightweight version of the same dimensions isn't that much.

The quest for less unsprung weight is in order to improve handling and reduce suspension component weight, I don't think it is to make steering easier - power steering pumps already do that.


But I could be wrong :)
 
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There are 2 Natural oscillation frequencies that apply to car suspension systems.
One is the sprung mass frequency which defines what frequency the car body will oscillate at once set in motion. This rate determines the ride quality perception of passenger.
The second is the unsprung mass frequency which affects the wheel motion against the damper.
Reducing the Unsprung mass has effect of using stiffer springs it changing the effect of the damper so you need less force the achieve critical damping. (The condition where spring deflection is equal to the damper bump resistance).
 
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