Unsprung weight

driller

driller

El Presidente
Staff member
I'm sure most of us know the difference the unsprung weight of a vehicle has on performance. The most obvious change anyone can make to change their unsprung weight for the better is lighter wheels/tires. This brings about a couple of questions.

Does anyone have information on the weight of stock rims? You can find the weight of aftermarket wheels quite readily for comparison. In my case, would there be much of a difference between the chrome 10 spoke directional vs. the silver 20 spoke directional? Others may want to know how a set of snowflakes compare to knockoff Cobra R's they are considering buying.

How about tire weights? Are certain tires renowned for their light weight? Are there tires you may want to avoid because of their weight? Seems to me, only aftermarket strip tires are advertised with their weights(such as drag radials). For passenger car tires, in my case, how does a set of Michelin Pilots tip the scale compared to a set of Yokohama AVS DBs?

Lastly, are there any 'rules of thumb' or common track knowledge as far as tradeoffs go? For example, where is the point of diminishing returns for a larger tire patch for traction vs. the extra unsprung weight due to the larger rim to support the recommended bigger tire?
 
RE: Unsprung weight

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Does anyone have information on the weight of stock rims? You can find the weight of aftermarket wheels quite readily for comparison. In my case, would there be much of a difference between the chrome 10 spoke directional vs. the silver 20 spoke directional? Others may want to know how a set of snowflakes compare to knockoff Cobra R's they are considering buying.

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some info just a few post below:
http://www.lincolnsclub.org/dcforum/DCForumID10/21.html
 
RE: Unsprung weight



some info just a few post below:
http://www.lincolnsclub.org/dcforum/DCForumID10/21.html
Click to expand...

DOH!{Homer Simpson impression}

Steve! ... honest, I did a search! Please!... {j/k}

Well at least someone else was thinking along similar lines. Evidently I wasn't paying attention.:)

For the record, Calviroman posted:

Lacy spoke W/ OEM tires = 37.0 LBS

10 spoke chrome W/ OEM tires = 47.2 LBS

17X8" Chrome Bullitt's w/255/50R17 = 54.3lbs

That is a LOT of difference! For example, almost 10 pounds/wheel going from Lacy Spoke(snowflakes) to 10 spoke chrome directionals. Comparatively speaking, how much would this difference translate to, performance wise?
 
RE: Unsprung weight

I think the general rule of thumb is: unsprung weight slows you down about 3 times as much as normal weight on the car. so if 100# of weight in the trunk slows you down .1 second in the 1/4 mile, then 100#'s of unsprung weight will slow you down .3 seconds in the 1/4 mile.
 
RE: Unsprung weight

I think the general rule of thumb is: unsprung weight slows you down about 3 times as much as normal weight on the car. so if 100# of weight in the trunk slows you down .1 second in the 1/4 mile, then 100#'s of unsprung weight will slow you down .3 seconds in the 1/4 mile.
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Interesting. Put another way, 1 pound of unsprung weight would be roughly equivalent to 3 pounds of normal weight? If so, then using the above example of 37 pound snowflakes vs. 47 pound 10-spoke directionals for a difference of 10 pounds per wheel:

10 pounds per wheel X 4 wheels = 40 pounds unsprung weight
40 pounds unsprung weight X 3 = 120 pounds normal weight

Using a couple different estimated HP calculators, a 3900 pound vehicle at a fixed ET of 14.6 seconds at 96 MPH yields 248 to 269 HP. Adding 120 pounds to the vehicle weight for 4020 pounds yields 255 to 278 HP. A difference of 7-9 HP, or 2.8% to 3.3% difference in power. Roughly a 3% power difference for difference of 10 pounds per wheel!

A quick estimate of ET for a 3900 pound car with 250 RWHP is 15.472 seconds vs 15.312 seconds for 3780 pounds, a difference of 0.16 seconds in the 1/4 mile ET. This being true, lighter wheel/tire combinations are definitely nothing to sneer at.
 
RE: Unsprung weight

That's right, but there's something wrong in there. You're not talking about unsprung weight, you're talking about rotational inertia.

Unsprung weight is just that "unsprung". You don't have to do anything special to get that weight to the end of the track as fast as the rest of the weight. It does affect how fast your suspension can respond, though.

Anything that is spinning is a completely different story, because not only does the car have to accelerate that mass along the track, it has to accelerate it rotationally. Wheels are the biggest factor in this, because of their diameter. Since rotational inertia is determined as much by the distance the weight is from the axis of rotation as the weight itself, diameter makes a big difference. Since wheels are the largest diameter item in the driveline, wheels make the biggest difference.
 
RE: Unsprung weight

Wheels tires and brake parts are still unsprung weight, but are also rotating mass. That is why the unsprung wieght in question in THIS thread troughly takes 3 times longer to accelerate than normal "sprung" weight.

Here is a more in depth outline of the differnce that Dave was talking about.


"Unsprung" weight is a term used to describe the portion of a vehicle that is not supported by the suspension (i.e. wheels, tires, brakes, various suspension items) and therefore most susceptible to road shock and cornering forces. Unsprung weight is one of the most critical factors affecting a vehicle's road holding ability. Reducing unsprung weight provides more precise steering input and improved handling characteristics.

ROTATING WEIGHT
"Rotating" weight is mass. It is basically what the engine has to accelerate. Race engines are not only measured in power, but also the speed at which they accelerate. Sometimes, that is not always sheer horsepower being used at its best. One reason why, is rotating weight. Obviously, the amount of work an engine has to accomplish from one corner to the next depends on the weight of the parts involved. The lighter the parts it has to turn to get going, the quicker the car will get to the other end. That is why rotating weight is an issue. Rotating weight includes anything that turns -- not only the parts of the engine itself, but the flywheel, transmission, propeller shaft, wheels, and even the tires.
 
RE: Unsprung weight

well to nitpick further an engines crankshaft and camshaft would be considered rotational weight, but an engines pistions and valves also need to be lighter to rev faster and they are not rotating :) just being a dork. The new Mitsu Lancer Evos have hollow camshafts, just incase anyone is wondering.
 
RE: ROTATING WEIGHT

RE: ROTATING WEIGHT

Got it. In this context we are actually talking about rotating weight.

And as Dave was eluding to, wheel and tire size is more critical to rotating weight simply because of the radius of the largest rotating element. So would it be fair to say even with light rims, a lighter tire would benefit ETs more than the rims because the rotating weight of the tire concentrated at the larger radius? In other words, an increase in tire weight would be more detrimental than the same increase in rim weight.
 
RE: ROTATING WEIGHT

RE: ROTATING WEIGHT

I love it when you guys talk technical! This has been really informative, and I even understood some of it. :)
 
RE: ROTATING WEIGHT

RE: ROTATING WEIGHT

Join in any time!

I hope it has been informative. I've always believed it is not good enough to know something, you are better off to understand it. I knew some of the basics discussed here, but now feel I actually understand more of the intricate details.
 
RE: Unsprung weight

well to nitpick further an engines crankshaft and camshaft would be considered rotational weight, but an engines pistions and valves also need to be lighter to rev faster and they are not rotating :) just being a dork. The new Mitsu Lancer Evos have hollow camshafts, just incase anyone is wondering.
Click to expand...

The pistons and valves are not rotating weight, but reciprocating weight. The radius and weight distribution isn't important as it is in rotating weight, but the lighter these components are, the less force is required to accelerate them. If an engine is spinning at 6000 rpms, the pistons are traveling up and down 100 times per second. They have to accelerate extremely fast at the top and bottom of the stroke, which puts tremendous force on the connecting rods and crankshaft. Weight reductions on reciprocating components can have a huge effect on component life and how high you can rev the motor.
 
RE: ROTATING WEIGHT

RE: ROTATING WEIGHT

Rotating elements have "moment weight". It can be measured roughly by the same premise as torque is measured. It is in unit weight per radius (gm/cm, oz/in, or lb/ft, etc.).

Take a sledge hammer by the handle and swing it. Now stop it in full swing. Now, grab it by the head and do it again. Naturally, you'll swing it and stop it faster. You're swinging the same amount of weight, but not at the same radius.

The same principal goes for your tires/wheel. A wheel with a heavy steel center, but a lightweight rim and tire will accelerate and decelerate much quicker than an aluminum wheel with a heavy rim and tire even though the overall weight is exactly the same.

Anyone who has picked up a drag racing slick and a passenger car tire of the same size has noticed the incredible difference in weight for obvious reasons.

When your engine turns 5000 rpm in 3rd gear with 4.10 gears, the tires are rotating at 1220 rpm. Now imagine how much horsepower it takes to spin 4- 40lb tires/wheels to that speed in 12 seconds. Imagine how much you would improve your times if you reduced that moment weight by 10 lbs per wheel in the back and 25 lbs per tier in the front (skinnies).
 
RE: ROTATING WEIGHT

RE: ROTATING WEIGHT

Just as a side note.Our cams are hollow too.They're only a bout 10 years behind.LOL
 
RE: ROTATING WEIGHT

RE: ROTATING WEIGHT

unstoppable,

We all know that you can't run your car without slicks, so there's no way of telling how much the reduction in weight of your rear tires helps your ETs.

But, how much time are skinnies worth to you? Have you ever run with full size front tires?
 
RE: ROTATING WEIGHT

RE: ROTATING WEIGHT

Dave,you can call me Chris.And on average going from normal fronts to skinnies is a couple tenths in the quarter mile.Unless you have very heavy wheels like the 20s I had then it would be a hair more.
 
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