anyone know the benifits of lightweight wheels?

[fu_im_from_texas]

I've been thinking about this question for a few days. My gut reaction was: lighter wheels will make a significant improvement in accelleration and mpg. My calculations show the opposite. Maybe my logic is flawed? Here's what I've got so far.

Consider two cases:
1) Totally stock Yaris from 0-60mph
2) Yaris with lightweight rims from 0-60mph (NOTE: I'm assuming identical tires)

The easiest way to compare the two is from an energy standpoint.

Rotational Energy of a Wheel = .5*Mw*(Rw^2)*W^2

where:

Mw= Mass of the wheel
Rw= radius of the wheel
W= rotational speed (Radians/second)

For Case 1)
Mw= 15lbs. I base this on the wheel/tire combo weight of 33 lbs. tirerack.com says the tire weights 18lbs. 33-18=15
Rw = 7.5 inches. 15in diameter / 2
W = 92.15 (RAD/s). @ 60mph 1mile takes 60 sec. for stock tires tirerack.com says 880 rev/mile so 880/60 = 14.6Rev/second *2Pi= Rad/s

convert to metric

Mw= 6.8kg
Rw=.19m

Rotational Energy = .5*Mw*(Rw^2)*W^2 = .5*6.8*(.19^2)*92.15^2
=1042 watts = 1kWatt

For Case2)
Mw=10 lbs. = 4.5kg This seems like a reasonable lightweight 15 inch wheel
Rw=7.5 inches = .19m
W = 92.15 RAD/s

Rotational Energy = .5*Mw*(Rw^2)*W^2 = .5*4.5*(.19^2)*92.15^2
= 690 watts = .69 kWatt

1- (690/1042) = 33% less energy to accellerate the lighter wheel. This sounds like a lot untill you compare it with the total energy of the car @ 60mph.

Energy of Yaris = .5*M*(v^2)

Where:
M=mass of yaris=2200 lbs = 998kg
v=velocity=60 mph= 97 km/h

Energy of Yaris = .5*998*(97^2) = 4695 kWatts

so (Energy of Wheel) / (Energy of Yaris) @ 60mph is 1/4695 = .0002

.000002% of the total energy is insignificant.

Did I make a stupid error in my calc? i would have guessed lightweight wheels would make a big difference...but I proved the opposite. someone help? ...show me the error...

[fu_im_from_texas]

Originally Posted by fu_im_from_texas

...
so (Energy of Wheel) / (Energy of Yaris) @ 60mph is 1/4695 = .0002

.000002% of the total energy is insignificant.

...show me the error...

Woops, I moved the decimal point the wrong way...it should be .002%...which is still far less than 1%... anyone else see mathmatical errors?

[dallas]

Removing weight helps acceleration as it takes less energy to move the mass but beware of diameter too. If you go with a 17 wheel even if its lighter than the stock steel wheel (not many wheels like that) you have moved the mass 3inches further out from the hub (based on 14 wheels) so it will take more enery to turn the wheel. I have 205 50 15 tires on light weight 11lb wheels , the stock steel wheel and tire are still lighter.

[CaysE]

Originally Posted by dallas

Removing weight helps acceleration as it takes less energy to move the mass but beware of diameter too. If you go with a 17 wheel even if its lighter than the stock steel wheel (not many wheels like that) you have moved the mass 3inches further out from the hub (based on 14 wheels) so it will take more enery to turn the wheel. I have 205 50 15 tires on light weight 11lb wheels , the stock steel wheel and tire are still lighter.

This is not necessarily true, because you're not taking the tires into account. Tires can typically weigh between 15 and 20 lbs, and if you maintain the overall diameter of the wheel/tire combo, a lighter weight wheel with a larger diameter would require a tire with a shorter sidewall, thus a lighter tire. Depending on the actual weights and wheel design, the mass can even be moved inward.

[dallas]

I did consider the tires in my weight, and yes the shorter sidewall helps, but the mass of a wheels weight is not in the spokes so if you move it 3" further from the hub even thought the tire tread is the same diameter away as the 14wheel, you have added more mass further from the hub over a stock 14" wheel. I have yet to find a 17" wheel tire combo that I can afford that would weight less than the stock 14 wheel and tire combo.

Originally Posted by CaysE

This is not necessarily true, because you're not taking the tires into account. Tires can typically weigh between 15 and 20 lbs, and if you maintain the overall diameter of the wheel/tire combo, a lighter weight wheel with a larger diameter would require a tire with a shorter sidewall, thus a lighter tire. Depending on the actual weights and wheel design, the mass can even be moved inward.