8 replies to this topic

[Wes]

Try as I may, I can't get a grasp on unsprung weight. As I undertsand it, it is more important (for example) to lose a pound in the brakes as opposed to losing a pound from the engine block. Why is that? The engine has to propel the same amount of weight, regardless of wether or it's in-board or out-board on the suspension system, right? What am I missing? Any answer would be appreciated, the more technical the better!

Thanks in advance.

[desmo]

Here's a very non-technical description of unsprung weight:

Unsprung weight which is the weight of the car not carried by the springs is in practical terms the weight of the tires, wheels and brakes, and to a lesser extent the suspension components. This weight or mass is undesirable because it is mass whose inertia must be overcome for the suspension to react. The ratio of sprung to unsprung mass is a critical factor in suspension compliance. You want as much of he car's weight as possible carried by the springs.

To give some idea of how critical this is, Ferrari tested Be alloy brake calipers and found the reduction in unsprung mass worth around a half second per lap.

[swoopp]

First of all, the job of the suspension is to keep the tires on the roadwhen going over bumps.
When wheels and brakes are heavy, the wheel stays in the air longer(when the wheel is in the air, you dont have control of that corner of the car) and the suspension has to work harder(shocks get hot).

Hopefully thats simple enough.

[Sudsbouy]

I also understand that unsprung weight becomes more important as its percentage of total weight increases. Therefore, unsprung weight is a more critical factor for a formula car than a standard sedan.

Also, when you're talking about the brakes, there are rotational mass considerations that must also be dealt with.

Thank you.

[BT52]

Originally posted by swoopp

When wheels and brakes are heavy, the wheel stays in the air longer

Can you explain this??

[davo]

Imagine for a moment the unsprung weight of the car is actually massless. Running over kerbs the CHASSIS (sprung weight) will deflect according to how the spring damper is set, however on the trailing side of the kerb the massless wheel will happily track down - not bounce in the air as we see on the German GP super slow mos.

A great quote (from Carrol Smith I think) "A tyre in the air generates no grip".

A thought provoker - with inboard suspension (push or pull rod activated) is the spring/damper mass...

A unsprung
B sprung
C actually contributing to both

[westendorf]

Wes, [my dad's name] where you got shortcircuited in your thought process was thinking only in forward speed of the car. Unsprung weight has to do with the handling of the vehicle. Think of this, if the road was perfectly smooth and flat [imagine glass here] unsprung weight would have little to do with grip. Ciao, GFW

[AndersF1]

See it this way:
If you're throwing a tennisball, or something similar, vertically up into the air, then the change in our orbit around the sun would hardly be noticed, but if you would stand on earth and throw Mars out of the solar system, then the change in our orbit would be significant.

The same thing applies to unsrung weight(wheel, brake-discs, etc.), that gets a vertical momentum (mass*velocity) when you're running over a bump. You(your springs) need to apply a force to the unsprung weight to change its vertical momentum, in order to force the wheel to follow the surface of the track/road, in the same way as you apply a force to the tennisball in order to change its vertical momentum. So, the higher your cars relative weight is, compared to the unsprung weight, the lesser its "orbit" will change.

[tak]

The best way to appreciate the significance of unsprung weight is to drive a rear wheel drive car with an axle, and a rear wheel drive car with independant suspension around a bumpy corner. The axle has much higher unsprung weight, and the rear end will hop and skitter across the bumps. The Independant rear car soak up the bumps and get through the corner quite a bit quicker. Really, borrow some cars and go try this. I'd recommend comparing a 1st generation Rx-7, camaro, or mustang against a BMW, supra, or 944.