why does unsprung weight affect performance of your car more negatively than sprung weight?

how much does non-rotating unsprung weight affect a car? (caliper weight vs rotor or wheel weight for instance)?

 

here you go, from Musclecarclub.com (no i don't read it, i did a search )
stan

UNSPRUNG WEIGHT

An important concept is "unsprung weight." This is weight that is not supported by the suspension of the car. This usually includes the the weight of the wheels and tires as well as a percentage of the weight of the suspension itself, including control arms, anti-roll bars, shocks, and struts. Reducing unsprung weight is the key to improving handling. The lower the unsprung weight, the less work the shocks and springs have to do to keep the tires in contact with the road over bumpy surfaces. An easy way to reduce unsprung weight and improve traction is to replace stock wheels and tires with special lightweight wheels. Note that as the wheel diameter or width increases, the weight of the overall wheel and tire package increases, thereby increasing unsprung weight.

 

hmm... can you physically describe what happens in a hard corner, or what the unsprung weight does in a corner? I still don't see how having heavier say control arms, calipers, etc make it harder on teh suspension since they dont bend or flex (relatively they dont).

and how about unsprung rotating mass (wheels) vs unsprung weight (say calipers)? whats the difference?

 

Okay, how 'bout a big example.

Ever notice how smooth your car rides when it's full of people? Well why is that? It's because you've made the part of the car sitting on the springs weigh much more in comparison with before, The weight on the springs has increased while the parts of the suspension that actually go up and down over the bumps (wheels, tires, hubs, brakes, and half the linkage), still weighs the same. You have increased the "Sprung to unsprung weight ratio".

So you can have a smooth ride by either having lots of heavy friends... OR, make the suspension components that ride over the bumps weigh less.

So what you say? Say you have two cars, one with a heavy straight rear axle (like a truck) and an independent rear suspension like the Miata. Both drive over the same 2" high bump. The bump in the road pushes up on the wheel, tire, and everything else that rides over the road surface. Now here's the secret. If this weight of "suspension stuff" that the bump is pushing upward weighs (in the Miata) ... say, 200lbs, and the weight that this suspension is holding up weighs... say, 1000lbs, then only one sixth (200lbs/1000+200lbs) will get transmitted to your butt... that is, the 2" bump your suspension went over will only transmit 2" / 6 = 0.33" into your seat.

Now take the case of the truck hitting the same 2" bump. The 500lbs rear axle rides over the bump, pushing upward on the light 1000lbs back end. So look at the difference. You as a passanger feel a much larger bump, 500lbs / (500 + 1000lbs), or for a 2" bump, you feel a lot of it! Your seat jumps upward 0.67".

As far as sport cars go, the reason you want a high "sprung to unsprung weight ratio" is that it allows the nimble tires/wheels/hubs, etc to closely follow the road, never losing contact. A heavy axle is much more likely to go over a bump, and have momentum make it keep right on going, actually lifting off the ground. Very bad for cornering. Be cause of the above ratio, it's also much more likely to cause the entire car to rise higher, raising the CG... A bad thing.

As far as heavy suspension parts go, the heavier they are in relation to the rest of the car, the worse the ride will be because as they become a larger and larger fraction of what the car weighs. An extreme example would be if your suspension weighed the same as the rest of the car. Hit a 2" bump and the car bounces upward 1". Each part weighs the same, so they both equally share the defection amount.

For handling you always want a light car, with very light suspension.




[Modified by kb58, 4:50 AM 9/10/2002]

 

ok, think of it this way.

It's a lot harder to push a 100lb rock than it is to push a 50lb rock, right?

and just the same, it's a lot harder to STOP a moving 100lb rock, than it is to stop a moving 50lb rock, right?

well, just think of it like that. you go over a bump, the bump has to move the unprung weight. your suspension then has to stop that unsprung weight, and rebound it. The less unsprung weight you have, the easier your suspension can stop the weight from moving up, and the easier it will be for it to rebound it, and keep your tires on the ground.

That's pretty much it.

 

lift a 10 lb weight 10 times as fast as possible.
lift a 20 lb weight 10 times, see the difference in time and effort taken?