Benefit of lightweight rims?
- Thread starter Brady
- Start date
I saw a test done at a local shop. They took a stock BMW Z4, and put it on the dyno. Then they changed the stock wheels out for some lightweight wheels. I don't know the exact weight difference, but the car made 15WHP more just by lightening the wheels.
I wouldn't have believed it if I didn't witness it with my own eyes.
I wouldn't have believed it if I didn't witness it with my own eyes.
Well, people talk about lightening the crank to lower rotating mass... and lighter driveshafts, ect. But all these things are close to the centerline of rotation... their weight has an effect, yes, but not that great of one compared to that which is further away; at a longer radial torque distance.
Imagine a teeter totter, with a big fat kid right at the end of his side; it's going to take a lot of force to get his bigass in the air. Now, if you move that kid towards the pivot point of the teeter totter, it will take progressively less force to lift his bigass. This is because you (your engine) will have a greater mechanical advantage on the big kid (the tire), relative to the pivot point (your axle center of rotation). As the big kid moves towards the centerline, you gain more and more mechanical advantage to lift him, and he gets less and less mechanical advantage to resist being lifted. Of course, the alternative is to swap out the big kid for a smaller kid, which is what I'm talking about with lighter rims and tires.
Now take that same principle and apply it to a wheel... the further the weight is from the pivot point in radial torque, the more torque (and therefore power, as that power is a function of torque), it takes to rotate that weight. It may seem counter intuitive for mechanical advantage, but if you go to a playground and try it, I'm fairly confident you'll see what I mean very quickly.
Get lighter wheels and tires, and it's akin to shedding an entire passenger and your spare tire... not only that, each weight saving is muiltiplied by 4: saving 3lbs per tire and 5lbs per rim makes a total savings of 8x4=32lbs all around. If the factor of 10 rule really is true, that's like dropping 320lbs of static mass. That's like ripping out your back seat, getting the wife to stay home, ripping out the A/C, and the spare tire... of course, the factor of 10 rule is just something I read, I'm unable to back it up... but you see where I'm going with it.
Imagine a teeter totter, with a big fat kid right at the end of his side; it's going to take a lot of force to get his bigass in the air. Now, if you move that kid towards the pivot point of the teeter totter, it will take progressively less force to lift his bigass. This is because you (your engine) will have a greater mechanical advantage on the big kid (the tire), relative to the pivot point (your axle center of rotation). As the big kid moves towards the centerline, you gain more and more mechanical advantage to lift him, and he gets less and less mechanical advantage to resist being lifted. Of course, the alternative is to swap out the big kid for a smaller kid, which is what I'm talking about with lighter rims and tires.
Now take that same principle and apply it to a wheel... the further the weight is from the pivot point in radial torque, the more torque (and therefore power, as that power is a function of torque), it takes to rotate that weight. It may seem counter intuitive for mechanical advantage, but if you go to a playground and try it, I'm fairly confident you'll see what I mean very quickly.
Get lighter wheels and tires, and it's akin to shedding an entire passenger and your spare tire... not only that, each weight saving is muiltiplied by 4: saving 3lbs per tire and 5lbs per rim makes a total savings of 8x4=32lbs all around. If the factor of 10 rule really is true, that's like dropping 320lbs of static mass. That's like ripping out your back seat, getting the wife to stay home, ripping out the A/C, and the spare tire... of course, the factor of 10 rule is just something I read, I'm unable to back it up... but you see where I'm going with it.
BorHor;972996 said:
you'll be VERY suprised... the "c" on the end means there the cheap ones... i found that out AFTER i got mine, not that i paid for them, but i was still disapointed... and yes 12lbs on the supra actually is awesome, considering how much weight can be shed from the elephant of a car...
but like i said, you have to consider the wheel AND tire together... some tires are heavy too. The 5-zigens i had were 17x8 +42 with 235/45/17
there a pretty small size for 5zigen... so i would almost bet borhor you dont save any weight with your setup. but it also depends on how much of a tire you run on them. IMO i would rather spend a crazy amount of money on my brakes and suspension then on my rims. especially the brakes.
Keros;973204 said:
lol, don't worry, I'm talking in imperial but all the math was done in metric. 60mph = 96.56kph, 17" rim = .215m radius, 23lb = 10.43kg, etc. Actually, I never converted kph to mps :nono:, so I'll concede and say I'm HALF a dumbass, but it doesn't change the ratio of rotating kinetic energy to translational kinetic energy. Still, I really hope I'm wrong, I always got the impression that the benefit was a ton more than that... it's disheartening, like learning there's no santa... :cry:
well if you live where the roads are all mest up and there are pices of the road missing, dont get them. light rims are made to be light, its hard to find light and strong rims. take in to mind you DONT HAVE A HONDA. this is not a skinny girl, she like a bull havey and strong.