Lighter Weight Wheels --- Worth It?

[justinschmidt1]

Thats not that much weight...I would think the difference would be minimal

[bgd73]

Originally Posted by The Atomic Ass

Personally, I vote for HEAVIER rims, but my opinion doesn't count because I mean in-wheel electric motors, which would certainly mean better economy.

I notice the same thought for 3 main boxers. A true balance is everything out to wheels, the gain is momentum keepers, and of course round wheels taking real balance. heavy welcome in my world, in fact it was a mircale adding 10 pounds a wheel on a 90hp boxer.
for all other setups, I check car weight.
heavy car long stroke, heavy wheel,
long stroke light car, heavy wheel,
little stroke, light car, light wheel <- these are almost extinct.

and then there is ratings of tires. if the car needs a t rated and above. (AWD anything) and sporty, lightweight wheels aren't gonna last. trial and error for me from pickups to 4000 pound sedans to chevettes and ford escorts. The subaru personally is the most bizarre example of needing something found on a corvette.

ignore this post if it is in this thread already. my net is absolutely terrible. I do not exist sometimes. I could guess this is my third time (at least) answering this thread. why i disappear is as strange as 5 mains in a boxer and calling it good.

[Christ]

it's actually quite a bit of weight, when you're dealing with unsprung weight.

Each pound off a wheel, or the flywheel, or the crank, or drive shafts, axles, hubs, etc, equals 7 lbs of equivalent weight loss of the rest of the vehicle.

Each 100 lbs lost of "basic" weight = .10 faster 1/4 mile times, on average.

[RandomFact314]

I was thinking about getting light weight rims until I realized that with the MPG gain (which I don't think will be much) would never pay off what you paid for the rims. Once I had new tires and rims that weighed 36 pounds extra all together and I saw no real change in my MPG, they were 44psi and the others were 35psi so I think that small difference canceled out the weight gain problem. But if the rims are super cheap I would go for it.

[Who]

*redacted*

[Christ]

Originally Posted by Who

*redacted*

Thanks.. we solved that issue back before this thread was resurrected.

[alohaspirit]

Originally Posted by Christ

it's actually quite a bit of weight, when you're dealing with unsprung weight.

Each pound off a wheel, or the flywheel, or the crank, or drive shafts, axles, hubs, etc, equals 7 lbs of equivalent weight loss of the rest of the vehicle.

Each 100 lbs lost of "basic" weight = .10 faster 1/4 mile times, on average.

So 8 lbs off your rim weight = 56 lbs of equivalent basic weight

-.05 faster 1/4 mile time


Dont you wish there was a formula for mpg too

[Christ]

Originally Posted by alohaspirit

So 8 lbs off your rim weight = 56 lbs of equivalent basic weight

-.05 faster 1/4 mile time


Dont you wish there was a formula for mpg too

There is.. I can't find it.

Basically, figure out (for the original weight) how much more HP you'd have to add to shave .1 off your 1/4 mile time, which would give you the HP/Weight equivalent (gaining X horsepower = losing N weight)

You can then compare the weight loss to the horsepower necessity, and since you know that 1 HP = about 750w, you can compare the 750w figure to the actual kW figure of the car that you're theorizing about.

i.e. = Lets assume his car in OEM form weighs 2000 lb, and has 100 HP (75kW)

We'll also assume (incorrectly, but for the sake of example) that he can either drop his weight 100 lbs to lose .1, or gain 10 HP to lose .1

This means that (10HP = -100LBS), or for every 10 HP you add, you will get the same effect as losing 100lbs, and vice versa.

So, based on the above, 10HP = 7.5kW, where the car had 75kW.

We can then assume that losing 100lbs makes for a reduction in necessary HP by about 10%, for this exercise. (Keep in mind, these figures aren't correct, but for the sake of display, they'll have to do.)

So if the car required 35 HP to accelerate from 0-60, it would now only require approx 31.5HP, or 3.5 HP less.

(I actually wonder how far off the numbers are... it doesn't seem outrageous to think that you could use 10% less HP by removing 100 lbs...)

Anyone care to chime in?

EDIT: I think that formula only applies for when you're accelerating though... I don't think it makes that much of a difference when you're just cruising.

[MechEngVT]

Originally Posted by Christ

(I actually wonder how far off the numbers are... it doesn't seem outrageous to think that you could use 10% less HP by removing 100 lbs...)

Anyone care to chime in?

EDIT: I think that formula only applies for when you're accelerating though... I don't think it makes that much of a difference when you're just cruising.

I would agree you're roughly in the ballpark. Those "equations" are ballparkin' it anyway and are based more on empirical trending than mathematical/physics theory. I thought that it was more common to say it takes X% more power than you have to shave Y seconds off your ET, as that would be more accurate even at very high power/low ET ends of the scale, and would therefore be more accurate at the opposite end where most ecomodders would play.

I agree also with your edit. Changes in mass, whether rotational/unsprung or static/sprung, will only make notable differences during changes in speed. That said, there are probably changes that would be very difficult to quantify due to effects of cornering and bumps where lighter unsprung mass (and lighter sprung mass to a lesser degree) would be an improvement. If companies can develop shock absorbers that contribute significant electrical charge energy then there is a non-negligible amount of energy used to move the wheels up and down to cover road surface irregularities. Lighter wheel/tire packages will waste less of this energy, even if it isn't recaptured by a regenerative shock absorber.

[jabb]

In my opinion MPG gain from lightweight rims might be small compared with purchase price however if rims do not get damaged there is only minimal loss in value in time. When you get rid of your car you can put your old rims on and sell lightweight ones for roughly the same price you bought them (if you're lucky).
Of course some investments are not there for making profit, some are just to make you feel better and that's not a bad thing either