Can someone explain "UnSprung Weight" for dummies
 

I was at an auto-x this weekend and at the track was an old Jaguar on jackstands. I was pointing out to my friend that the rear rotors are inboard and near the differential. Then I heard someone mention that those rotors were a great benefit for "unsprung weight".

I'm just wondering how this works - mostly for my own curiosity.

Unsprung weight is all weight from components that are Not mounted to places that are attached to the suspension springs. The benefit is that the suspension is not moving as much weight so it reacts faster and with less energy to try to keep it oscillating. Example: Batteriies, steering rack, engine are all unsprung (not attached to the end of the springs); Wheels, tires, brakes are sprung weight, they are attached to the end of the spring.

Unsprung Weight.
 

It's all right here:

Unsprung mass - Wikipedia, the free encyclopedia

I think that's backwards, no? If the weight acts on the springs, it's sprung (body and everything in it), everything on the other side of the spring is unsprung (tires, wheels, brakes).. and now that I looked it up - "Unsprung weight includes the mass of components such as the wheel axles, wheel bearings, wheel hubs, tires, and a portion of the weight of driveshafts, springs, shock absorbers, and suspension links. Even if the vehicle's brakes are mounted outboard (i.e., within the wheel), their weight is still considered part of the unsprung weight."

Other way around.

ooops... i did that backward. dammit, been a long day.

No one has it quite right yet.

On the car, some weight is unsprung, some is sprung, and some is some proportion of both. Here is what Fred Puhn has to say:

"The chassis supports the engine, body and occupants. It rests on springs which insulate the chassis from the road irregularities, and from the driver's point of view the chassis bounces up and down on the springs. The weight of the chassis and all parts mounted on the chassis is considered to be sprung weight.

The tires, wheels and suspension parts that move up and down with the wheels are effectively underneath the springs and are not insulated from bumps in the road by the cushioning effect of the springs. The moving suspension parts are considered unsprung weight.

Some parts are attached to the frame at one end and to the wheel at the other. The frame end is sprung and the wheel end is unsprung."

"Some parts" include axles, control arms, springs, and shocks. Axles and control arms would be 50% sprung and unsprung. Shocks and springs are different. For a shock, the weight of the parts that don't move with the wheel are sprung.

Scott

Nice.

Now for us super dummies.. Why does it matter? how does it impact handling? is it more theoretical or can you really tell the difference when driving?

This is why. He describes it fairly well. Except for his unsprung/sprung terms being reversed. It is more noticeable if there is more of it (unsprung weight). It also depends on how and where you drive. And your sensitivity to that kind of thing. The commuter going to work in the Camry, probably not. If you're asking the question, probably not as well. To a skilled driver on a track, especially bumpy, most likely.

Think of inertia. The less mass in the moving suspension components the easier time the shock and spring will have keeping the tire in contact with the road-and that is what the suspension is for after all.

Edit-this includes the brakes/wheel as well of course.

This is actually the right question to be asking. What does it matter, and what can anyone do about it?

Easiest thing to do is to make sure your wheels are as light as possible. The difference between a Fuchs forged alloy wheel and just about any other standard cast wheel is significant - and this translates into more unsprung mass. Drilling your brake rotors could also help, although I think this is usually done to improve heat dispersal. The Jag's inboard brake rotors in effect took all the rotor mass and turned it into sprung weight (well, at least a portion of it anyways). Yes, you can feel the difference in handling when there is less unsprung weight. It's actually pretty amazing.

Less unsprung weight will show up in a few ways:
1. the suspension's ability to absorb bumps and settle down quickly is reduced the more unsprung weight there is. this means that if the car suspension has to deal with a bump and is unsettled during a corner entry, or mid corner, it won't be as compliant and you will not be able to go as fast as you could with more compliant suspension.
2. less unsprung weight = quicker direction change. The greater the rotating mass of a wheel the greater its gyroscopic effect, which is what keeps it wanting to continue going in a straight line. Reducing the rotating mass makes it easier to turn, making for 'better handling', lighter steering, quicker direction changes.

There are probably some other variables that unsprung weight affects as well like how hard your suspension has to work, and therefore how long it will last. And how much heat it will generate, which effects how well it works. These are secondary, imho.

So-called 'mag' wheels were a big deal back in the day because they were made of magnesium alloy which was significantly lighter than the then-standard steel wheels. I don't know if carbon fiber is being used for car wheels - it is for motorcycles. It takes unsprung weight to the next level.

You mean something like this?

http://3.bp.blogspot.com/-gMubM-EjjX...0/DSC_6425.JPG

As seen here: danumatic: Dymag carbon fiber wheels

I read that some auto racing groups had outlawed or frowned upon the use of carbon fibre wheels. I guess when they get damaged, they fracture badly it's often with quite catastrophic results. Or maybe I misunderstood.

I know they are used in motorcycles regularly. In this, having less unsprung weight is quite important. A race bike weight of 350 lbs isn't abnormal, so even saving a few ounces makes a difference in top levels of racing.

Thanks! You may have explained it backwards (as others have said), but the explanation makes perfect sense. Now I get it.

There are aftermarket full carbon fiber motorcycle wheels (not just rims, they are monoblock, hollow spoke). I think they are made in South Africa. There was an article in Sport Rider a while back.

BTW, the Formula SAE team used carbon fiber wheels for a bit. They ended up breaking and they are now back to minilites.

For those that might get confused, he is talking about the student race team at his college, not a professional Formula racing team.

One big effect I did nod red yet:

The less rotating mass, the less energy needed to accelerate/brake. This effect you can really feel on a motorcycle with lighter wheels or on a bicycle.

I read that some auto racing groups had outlawed or frowned upon the use of carbon fibre wheels.
http://www.hostbest.net/3.jpg

Correct. I know the guy quite well. He makes wheels for several motorcycle racing teams on the international circuit. Impressive stuff, especially his destructive testing methods!

That is an attribute of the dynamics of a vehicle but it is a different subject then unsprung weight. Every property of a vehicle effects every other one which is why vehicle dynamics is one of the more headache inspiring endless pursuits.

The mass of the wheels, rotors, and the axles (50%) do take a 'triple hit' as they are unsprung weight, they need to be spun up to get them moving, and they need to be accelerated forward like the normal dead weight of the car.

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To really get a clear mental picture of the importance of unsprung weight a good mental exercise is to imagine the cornering interaction of the suspension with a washboarded road. Most everyone has had the experiance going around a hairpin corner on a gravel road where it is heavily washboarded. You notice that the car just doesn't want to corner right after you notice that all your fillings just fell out...

We talk about slip angle a lot and it is pretty obvious that if you push sideways with a great quantity of force then the rubber carcass of the tire is going to flex. The opposite is true too. Unless the tire is flexed then it isn't going to generate the cornering force. As you go between the ridges of the washboarding the flex of the tire is going to relax. When it hits the next ridge it isn't going to create cornering force until the tire gets flexed again.
Say for example that your tire is flexed about 1/2" when it is creating your maximum cornering force on this surface... That means that once the tire comes in contact with the next ridge it still isn't going to create its maximum force until it moves another 1/2" laterally. In other words each time the tire becomes unstuck you have a double whammy from at first not being in contact and second from waiting for the cornering force creating flex to build back up.

If you had a magical suspension where the moving parts had zero mass the tire would be able to follow the contours of the ground exactly so the preloaded flex of the tire would be constant and you would get a similar cornering force on a rough surface as you would get on a perfectly smooth road. (Computerized "active suspensions" basically attempt to replicate this by trying to force the tire to exactly follow the contours of the road.)

One of the details of why unsprung is so important has to do with how a conventionally suspended car performs in practice. Say you have a reasonably smooth road and a car with a nice low unsprung weight and a good suspension that keeps the force pressed on the contact patch in the range of 95% to 105% of the cars weight. Look back at the interaction of the tire with the washboarded road. The same thing will happen here but what you will see is that when you get to a 95% loaded moment your slip angle will release so you are generating 95% cornering. The gotcha is when you hit a compression a 105% downward pressure you don't get 105% cornering. You have to wait till the slip angle builds again.
The end result is you won't get more then the 95% cornering of your lower adhesion limit.

Think about all this the next time you think about putting Big Reds on your early car that has never had its brakes get near to overheating. :D