unsprung weight does affect horsepower--anybody know how much? Thx.

 

hp isn't the biggest problem. it adds more stress on the bearings. hurts braking and slows down transition speed. grab a 5lb weight and spin around, now try it with a 10lb weight. notice how much slower you're able to come to a stop or move once in motion?

 

Isnt 1 lb of rotational mass equal to 7 lbs of non?

 

I've heard it's around 6 pounds. But distance from the axis also needs to be factored in. The further away, the greater the impact.

 

if a disc was 4" in diameter and another disc was 8" in diamater but weighed the same amount would the power to turn those discs be the same?

 

Free spinning - yes. Kind of like pound of feathers and pound of lead. Add friction or any other force and game changes drastically. Also the larger wheel will build up more kinetic energy and will require greater force to change motion.

 

"power to turn" is a bit ambiguous, what we're interested is the power it takes to accelerate.

The amount of "rotational mass" an object has is called its "Moment of Inertia". Imagine if you're holding a weight attached to a string and start spinning it around in circles. If you loosen your grip on the string and let more out so that the weight is spinning a larger circle around you, then it becomes harder to spin the weight, even though the actual mass of it stayed the same. Its because you increased its "rotational mass", or "moment of inertia".

Moment of inertia depends on the weight, shape, size, and distribution of an object, and can be pretty complicated.

Here is a list of how to calculate it for a variety of shapes:
http://en.wikipedia.org/wiki/List_of_moments_of_inertia
For wheels, if we assume they're a giant metal ring (ignore the spokes and hub, since they weigh less than the outside anyway), then their moment of inertia is:

I = mr^2, where r is the radius of the wheel.

So, if there are (2) 20lb wheels, and one is 17" and one is 20", the 20" wheel, then:

17" wheel moment of inertia = 20*(8.5^2) = 1445
20" wheel moment of inertia = 20*(10^2) = 2000

So you can see the 17" wheel in effect has a third of the rotational mass that the 20" wheel does, even though they weigh the same.

Its obviously more complicated than that, but its a decent estimate. In real life the 17" wheel (of the same weight) probably would be more like ~20% less instead of ~30% less rotational mass. Also, the effect is somewhat balanced out, because smaller rims = bigger tires, so you always have the same radius of tire. But, ask any guy who has put 35"+ tires on their truck/SUV/jeep, and ask them how much MPG and power they lost because of it.

Also, I found a dyno test on the internet that showed that each pound of wheel weight added per wheel, took away about 1hp on the dyno. So a car with 40lb wheels/tires would have about 10hp less than the same car with 30lb wheels/tires. Since Dyno's test power a little differently than in real life, the difference is probably about half of that, .5hp per pound of wheel weight added per wheel.

Sorry for the long winded post. I kinda like physics

 

I've heard the one pound to six/seven pound rule of thumb as well.

So figure your new heavy wheels would be like you've strapped a grandma and a golden retriever to your roof.