[SteveO (Steveo)]

I bought a set of Aluminum wheels to mount my drag radials on. I desided to weigh the steel tire/wheel combo and the Alum tire/wheel combo.
Wow! What a huge difference. The steel combo weighed 55 lbs. each, the Alum combo weighs 37 lbs. each. That's a total of 18 pounds per tire/wheel, times 2 equals 36 POUNDS OF ROTATING MASS!
Do you have any idea the difference that may make in horsepower gain at the rear wheels?
SteveO

[Christer N (Christer)]

No horsepower gain, but you get access to your horsepower quicker, which results in lower e.t.

[David Robson (Admin)]

I tried to figure it out but ran out of fingers and toes. Sorry!

[Christer N (Christer)]

Sorry, if my answer was a bit hard to understand but that is a way I have been explained that a light-weight flywheel acts contra a heavy flywheel. Maybe this case differs a bit so I will give the question another try.

I have done some calculation and I have ended up with the conclusion that a 36 ponds weight decrease on a 3750 pound car means that if you from the beginning had a 350 hp engine, you only need a 346.6 hp engine to get the same acceleration. The horsepower gain in this case is 3.4 hp.
I have not considerd that the mass is rotating but I don´t think that this fact has a great impact.
I have used the formula F (Force)=M (Mass)*A(Acceleration)

I hope this is a better explenation. If you give me the weight and horsepower-figures of your car I can give you a more accurate answer. I am unused to use Ibs and pounds-units but I hope I got it correct.

[Ciampone (Ciampone)]

I think Your evaluation is good, to do a better one, You need to know many more things....
And the results will change very little, I assure You, I have done some similar evaluation for trains.
The concept is right : less mass is like more torque : You have more acceleration.

I hope to bore no one with this explanation...

In your count You have no consideration that the wheels are rotating mass!
Reduce weight in rotating mass is much better than reduce the same weight in non-rotating mass, for example with fiberglass fenders or lexan windows.
Think that: why in race engines they search to have the lightest crank as possible?

All the rotating mass have a moment of inertia (lb*ft*ft or kg*m*m), for example, for a point, a little ball of lead (an object very small with mass), his moment is :
his mass for his distance from the center of rotation (distance squared)
For a solid, full, disk with constant thickness
J = M*r*r/2
There is a similar formula in this case :
T(torque)= J(inertia's Moment)* A(Angular Acceleration)
To have a rotating acceleration on a flywheel, alone, with no car, You have to put a torque
To have more acceleration You have to reduce the moment of inertia or increase torque.

In a car, or any other vehicle, there are rotating and non-rotating mass, so the rotating
mass counts 2 times, 1 for total mass, 1 for moment.
To do some counts, You can use the same formula you wrote but You must insert the equivalent mass, that usually is 10% more than the mass of the car.

At this point, I'm sure You say :" And what *#@* is the Equivalent Mass??"
EM = M + J1*(w1*w1)/(v*v) + J2*(w2*w2)/(v*v)+......
w is the rotating speed or angular speed of something that rotate (rad/s)
v is the speed of the car (m/s)
As You see, to do an accurate count You must know the moment of inertia of :
crank, gearbox, driveshaft, axles, wheels (steel and alluminium, with the tyres on)......
And finally you'll discover that.... Your weight reduction is 36 pound on the E.M. is about
1,1 * 36 = 39,4 pound...

[Daver (Daver)]

So in total the reduction in wheel mass could amount to anything from a 0.05sec to a 0.1 sec reduction in quarter mile time. That is a significant amount.

I told you guys this was more complicated than I could handle! Thank you Ciampone.

I might add that the main advantage of lighter wheels is the reduction in unsprung weight.

This allows the suspension to react quicker and the car handles better as a result.

[Henry (Henry)]

Which in turn will improve traction.

[Daver (Daver)]

Yes because the wheel follows the imperfections in the road surface better.

[Ciampone (Ciampone)]

Yes, this is the real target of lighter wheels!
The sospensions works better with the max rate possile : total mass / unsprung mass
To have the best, You'd need to have a wheel and tyre and hub that have no weight.... (not possible, of course...)

[Christer N (Christer)]

Thanks for the lesson, Ciampone.

I will try to do a kind of conclusion (with simple words):
Due to the gear ratio in the transmission, the engine is revving a lot more than the wheels, as you all know. Of course, it takes more power to increase the rotation with 5000rpm, than 1500rpm. That is why it is more costly (in terms of acceleration) to have unnesessecary weight in crankshaft and flywheel for instance, than in wheels.

Another thing to have in mind: Driving down the strip with your brute Challenger, you rev the engine up to max rpm four times (if you got a 833), but the wheels only rev up to max rpm once!!!
(...and the wheels max rpm is only about 1500)

I hope you understand what I am getting at. Otherwise I am sure that Dave gladly will continue with the explenation...

[Daver (Daver)]

I'm keeping quite. You guys are all too well educated for the likes of me!

It's not my fault I was thrown out of maths class!

On second thoughts...it was my fault. But the teacher was smelly!

[Ciampone (Ciampone)]

Christer, you are right! Your conclusion are ok.

The weight was lost in the wheels and then the difference in acceleration was very little. But if You could save the same weight on the engine crank and reduce the crank's inertia.....

phisic eplenation of your conclusion...
At any speed, the wheels (and also the axles) rotate slowly because they have a big radius :
w = v/r, and then the rate (w*w)/(v*v) is low...., at the same speed the engine and the driveshaft rotate at least 3 times more and the ratio (w*w)/(v*v) is at least 9 times more.... It depends by what's your final ratio (3.23, 3.55, 4,1 ecc.) and in which speed you are (only for engine).
For Dave...
I am (going to be) a mechanical engineer, but I'm not pracical as you with american cars, I think I will never be....
What's "smelly" Dave??
Hi everybody, I hope to has been clearly.
Ciampone

[Ciampone (Ciampone)]

Old remembering....
When I was 14-15 years old, here in Italy there was many "piaggio Vespa" 50cc motorbikes for 14 and up child, modified by the owners with many many special parts as cilinder of 130cc and CARBON FIBER flywheel to reduce inertia's moment of the crank....

[Daver (Daver)]

'Smelly' means she did not wash herself often.

[Ciampone (Ciampone)]

OOOHHpps.....
Anyone has his little (???) defect....
Excuse me but my English is not very good yet..

[Daver (Daver)]

It is as good as mine mate! You should hear my accent. You would not think I was talking English at all!

[Daniel (Hemi6pack)]

A message to Ciampone . Each time I have been to Italy I have only ever seen small compact cars , are there many challengers there

[Ciampone (Ciampone)]

To Daniel :
not at all... In italy there are only few american cars and less muscle cars....
The mopars are ignored to exist by the most part of people, even car's fanatic... They prefer modify small compact cars...
For that I know, there are 3-4 Challengers, most of them imported from Swiss. For our lucky the Swiss is not far away...
Hi!

[christer (Christer)]

Guess what I have found. A test comparing a lightweight wheel with a heavy one.

Intrested about the results? First they outfitted a Chevelle with a set of steel Corvette 15x8-inch ralley wheels. With this setup they made eight dragstrip passes and averaged the time of the three quickest runs. Then they removed the steel wheels in favor of 15x8½-inch aluminum Super-lite Weld Wheels. The same size front and rear tires were used for this test session; nothing else was changed.

With the steel wheels their Chevelle was able to run an average of 14.93 seconds at 97.20 mph. When tested with the lightweight Weld Wheels, on the other hand, they knocked out an averaged 14.55 seconds att 99.58 mph. That factors out to an improvement of .38 second and 2.38 mph attributed to a simple wheel swap.

The difference in weight-per-wheel figured out to an overwhelming 19 pounds. That´s 19 pounds at each corner for a total of 76 pounds removed from the car.

Oddly enough, testing the same wheels on an ´85 Monte Carlo SS didn´t generate the same performance gain. The Monte only picked up .03 second and .83 mph with the lightweight wheels. This is probably due to the Monte´s slower performance, since the best it could generate was a 15.94/87.94 mph time. (Everything from Car Craft, May ´85)