# Car guys: Why is unsprung weight so bad?

#### raptor13

##### Golden Member
Unsprung weight on a car is horrible for performance because for every additional pound of unsprung weight a car has, that corresponds to an equivelant gain of 3 pounds (or is it 6?) of normal, sprung weight. So even though I know that, why is that the case? It seems to me that the motor has to move the mass of the car regardless of where that mass is located. I guess it could mess with the handling more than sprung weight but I was under the impression it affected acceleration and top speed as well.

Why?

#### Zenmervolt

##### Elite member
Actually, what you're talking about is rotating mass, not unsprung weight. Rotating mass must be accelerated/decelerated every time the speed changes, and it's harder to accelerate the rotating mass. Since most rotating mass (aside from the engine and the tranny) is unsprung weight (half-shafts, wheels, brake rotors), a reduction in unsprung weight generally entails a reduction in rotating mass, which helps acceleration. It is rotating mass that is roughly 3 times harder to accelerate than normal weight. Plain old unsprung weight is not any harder to accelerate. Basically, rotating mass must be accelerated twice every time the car speeds up, it must be accelerated linearly (in the direction of the vehicle's travel) and rotationally (in the direction of its spin).

As for a reduction in unsprung weight independant from rotating mass, this generally improves the suspension's ability to work properly. Unsprung weight contributes to inertia in the suspension, which in turn affects the geometry assumed by the suspension.

ZV

Unsprung mass has the same effect as sprung mass when it comes to acceleration as long as it is not a rotating part in the driveline.
If it is a rotating part (Such as a tire/rim combo),It will take more hp to accelerate a heavy rotating mass to a specific velocity than a lighter rotating mass.The same holds true for deceleration.

Edit

Zenmervolt beat me to it

#### Zenmervolt

##### Elite member
Words cannot express how happy I am that Roger didn't find something wrong with what I'd said. I feel special to have gotten it right according to one of the experts here.

Oh, Roger, love the Westy. Someday I'm gonna have one too. That damn 914 of mine has me hooked on the air-cooled flat-4 engines. When were you in the White mountains? I was there early August this year, visited a friend at Dartmouth and a friend working for the AMC. Absolutely beautiful area. The Westy looks right at home on Mt. Washington too.

ZV

#### raptor13

##### Golden Member
I was aware that rotating mass was even more of a killer than unsprung mass so that doesn't surprise me. I suppose if almost all the unsprung weight is part of the drivetrain (i.e. rotating) then that would explain why I assumed it was ALL unsprung mass that was bad. But, like I said, non-rotating unsprung mass affecting acceleration never really made sense to me. But it does now! Thanks guys!

#### StageLeft

##### No Lifer
I thought I had a good understanding of physics but let me get this straight: Are you saying that:

Car 1) Car weighs 3300 pounds, 300 of which is in the wheels (75 pounds each)
Car 2) Car weighs 3300 pounds, 100 of which is in the wheels (25 pounds each)

Car 2 will accelerate faster? It seems like energy is getting lost somehow somewhere, but at the same time I can appreciate that if you put a tire on a skateboard and kicked the skateboard the tire would move ahead with a certain acceleration (glue the tire to the skateboard, so both are accelerating), and if you did the same thing but also wanted that tire spinning suspended over the skate board you'd need not only the forward/backwards force, but also an additional force to spin the tire.

Hmm...

#### Bulk Beef

##### Diamond Member
Car 1) Car weighs 3300 pounds, 300 of which is in the wheels (75 pounds each)
Car 2) Car weighs 3300 pounds, 100 of which is in the wheels (25 pounds each)

Car 2 will accelerate faster?
Yes.

Zenmervolt

YHPM

#### trmiv

##### Lifer
In racing we try to minmize unsprung weight because it hurts handling. When the weight isn't supported by the springs, the shocks and tires control that weight. On bumpier tracks, a large amount of unsprung weight can become difficult to control, and reduce tire contact to the racing surface, which will reduce grip. Also, since the tires are controlling the unspung weight it can, in certain situations, cause heat build up, affecting tire perfomance. Also, really light race cars (mine is 2500 pounds, so it's considered light), are very sensitive to unsprung weight, so we attempt to minimize it whenever possible.

#### Viper GTS

##### Lifer
Originally posted by: Skoorb
I thought I had a good understanding of physics but let me get this straight: Are you saying that:

Car 1) Car weighs 3300 pounds, 300 of which is in the wheels (75 pounds each)
Car 2) Car weighs 3300 pounds, 100 of which is in the wheels (25 pounds each)

Car 2 will accelerate faster? It seems like energy is getting lost somehow somewhere, but at the same time I can appreciate that if you put a tire on a skateboard and kicked the skateboard the tire would move ahead with a certain acceleration (glue the tire to the skateboard, so both are accelerating), and if you did the same thing but also wanted that tire spinning suspended over the skate board you'd need not only the forward/backwards force, but also an additional force to spin the tire.

Hmm...

Turn a bike upside down, take the back tire/tube off & put the bare rim back on.

Turn the pedals.

Now put the tire & tube back on, & do the same thing.

See how much harder it was to turn the pedals?

Viper GTS

#### StageLeft

##### No Lifer
Originally posted by: Roger
Zenmervolt

YHPM
How come I never HPM?
Cool; I'd never really thought of it enough before ;0

#### Bulk Beef

##### Diamond Member
Cool; I'd never really thought of it enough before ;0
Are you being sarcastic or something, cause if you are, I might have to break bad on your ass and send an Arby's coupon book to MrsSkoorb or something.

#### Fausto

##### Elite Member
Originally posted by: Viper GTS
Originally posted by: Skoorb
I thought I had a good understanding of physics but let me get this straight: Are you saying that:

Car 1) Car weighs 3300 pounds, 300 of which is in the wheels (75 pounds each)
Car 2) Car weighs 3300 pounds, 100 of which is in the wheels (25 pounds each)

Car 2 will accelerate faster? It seems like energy is getting lost somehow somewhere, but at the same time I can appreciate that if you put a tire on a skateboard and kicked the skateboard the tire would move ahead with a certain acceleration (glue the tire to the skateboard, so both are accelerating), and if you did the same thing but also wanted that tire spinning suspended over the skate board you'd need not only the forward/backwards force, but also an additional force to spin the tire.

Hmm...

Turn a bike upside down, take the back tire/tube off & put the bare rim back on.

Turn the pedals.

Now put the tire & tube back on, & do the same thing.

See how much harder it was to turn the pedals?

Viper GTS
This is why bike racers typically have a "race-only" wheelset. You train on durable (but heavy) wheels with fat tires and then swap them out for the light wheels prior to a race. With the lower moment of inertia of the lighter wheels you suddenly feel like you're flying since they're so much easier to accelerate.

A really good discussion of this very topic.

#### StageLeft

##### No Lifer
Originally posted by: sward666
Cool; I'd never really thought of it enough before ;0
Are you being sarcastic or something, cause if you are, I might have to break bad on your ass and send an Arby's coupon book to MrsSkoorb or something.
No I really hadn't. A hick friend of mine used to complain about how his car was slower in the winter because of metal studs in the tires and I was like "stfu they can't wait more than a few pounds", but in actual fact he was right - although I'm sure a few pounds of metal in the tire is not going to make a noticable difference. Viper's example is good too. I always just assumed that mass to move to a certain velocity would require a certain force regardless of how the mass was distributed, but in this case that tire/wheel has to move in the direction of car and maintain a momentum circularly. I thought they were just somehow tied in together but I guess they aren't.

#### B00ne

##### Platinum Member
also the unsprung weight (tires, suspensenion rods...) is supposed to react fast to the bumps in the road. The lighter these parts are the less force is required to move them and the faster they can react to the bumps. Resulting in a safer and smoother ride. Additionally u need to consider the rotational energy(or rot. inertia? Physics is long ago. anyway, both are connected) off the wheels. The heavier they are, the harder it is to change the axis of rotation (angle, up/down).

#### teddymines

##### Senior member
Originally posted by: Skoorb
No I really hadn't. A hick friend of mine used to complain about how his car was slower in the winter because of metal studs in the tires and I was like "stfu they can't wait more than a few pounds", but in actual fact he was right - although I'm sure a few pounds of metal in the tire is not going to make a noticable difference.
Also, studs add a significant amount of friction. Somewhere around 25% of a car's horsepower is used to overcome rolling friction on plain tires, and studs add to that. Try coasting down from 40mph on studded tires on a dry road and you'll feel significant drag.

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