Lighter Rims=More Wheel HP

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Old 03-19-2002, 06:57 PM
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Lighter Rims=More Wheel HP

I was wondering has anyone either done a run with new rims of the same size but weigh less, or dynoed their car with different rims. I am looking at moving up to an 18" polished aluminum, not so much for looks, but more for power gains. Any information on wheel HP gains with new rims whether it is for the TL or not would be great.
Old 03-19-2002, 11:10 PM
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Re: Lighter Rims=More Wheel HP

Originally posted by mbeebe
I was wondering has anyone either done a run with new rims of the same size but weigh less, or dynoed their car with different rims. I am looking at moving up to an 18" polished aluminum, not so much for looks, but more for power gains. Any information on wheel HP gains with new rims whether it is for the TL or not would be great.
I don't have a dyno, but you would probably get a slight increase in torque.
Old 03-19-2002, 11:47 PM
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Why would there be a increase in horsepower??? Lighter rims, less unsprung weight which gives less mass for available horsepower to pull around.
Old 03-19-2002, 11:49 PM
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hum... good question... I am not sure but here is what I know and what I think.

I know that if you upgrade from 16 to 19 you lose HP on the dyno. I think it's mainly because of the difference in size...

lighter rim will give you better performance, that's for sure... but will it give you a better number on the dyno... I think it will, too...
Old 03-20-2002, 06:09 AM
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Technically, you don't lose or gain horsepower when you don't change any aspects of the engine. An engine with 350 hp will still have 350 hp, no matter how much a car weighs. Now acceleration will be improved, as well as braking, but not engine hp. As for the dyno, to get a true reading of engine hp, you need to pull the engine out and set it up by itself, and measure hp that way.
Old 03-20-2002, 10:08 AM
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There are two ways changing rims will affect you car's performance:

First, it's not just the mass of the wheel (plus tires) that matters, but also its moment of inertia. remember that you have to expend energy to spin the wheel, and that energy comes out of the same bucket that pushes your car forward.

Second, changing the diameter of the wheel changes the moment arm at the axle, effectively changing your transmission's gear ratio. The larger your wheel diameter, the less force is available to push your car forward at the same torque output.

It's a linear relationship, so changing your wheels from say, a 18" diameter (including tires) to 20", for example, will change your radius from 9" to 10", so at the same torque setting you will get 9/10 = 90% of the original force applied to the ground.
Old 03-20-2002, 11:20 AM
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Originally posted by daverman
There are two ways changing rims will affect you car's performance:

First, it's not just the mass of the wheel (plus tires) that matters, but also its moment of inertia. remember that you have to expend energy to spin the wheel, and that energy comes out of the same bucket that pushes your car forward.

Second, changing the diameter of the wheel changes the moment arm at the axle, effectively changing your transmission's gear ratio. The larger your wheel diameter, the less force is available to push your car forward at the same torque output.

It's a linear relationship, so changing your wheels from say, a 18" diameter (including tires) to 20", for example, will change your radius from 9" to 10", so at the same torque setting you will get 9/10 = 90% of the original force applied to the ground.
I don't think he is asking about diameter. Your point is releated to total diameter. If he goes to 18" from 16"/17" but keeping the original total diameter with a smaller tire, how would we answer these two questions.

Two questions:

Let's think both 16", 18"are the same total diameter.

Would the 18" have more mass on the outside from the center of the rim, if yes how much of a difference would that make?

If each 18" is 6 pounds lighter than the 16" with tires, how much power would you gain. Total weight loss 24 pounds. This would increase hp/weight ratio right ?
Old 03-20-2002, 12:11 PM
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I hate to say it, but engine HP doesn't matter. It is wheel HP to weight ratio that makes the difference. If you have a 350hp engine that only gets 200hp to the wheels it is no better than a 300hp engine getting 200hp to the wheels. The deciding factor is the curve of the dyno and the weight. I read on a very distinguished site that going from a 28lb rim to a 15lb rim on a AWD Eclipse GS-T is a gain of 52hp to the wheels. This sounds to be about right. Meaning that a loss of 1lb at the drive wheels is equal to 1hp. Can anyone back this up?
Old 03-20-2002, 01:07 PM
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Don't hold me to this but I believe I read somewhere that 1 lb turns into 4 lbs when rotating. So if you change from a 28 lb rim to a 15 lb rim, you are essentially dropping 208 lbs of the total weight of the car when in motion. (13 lbs x 4 lbs x 4 wheels ???) There is a equation where you can convert weight reduction to whp gains. This sound right to anyone.
Old 03-20-2002, 02:14 PM
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Originally posted by Bitium
I don't think he is asking about diameter. Your point is releated to total diameter. If he goes to 18" from 16"/17" but keeping the original total diameter with a smaller tire, how would we answer these two questions.
OK, if the total diamater remains constant then the forward force should also remain the same.

The only difference now is the weight of the wheels and their moments of inertia. I don't know the typical moments of inertia for rims, so I have to plead ignorance in this case.

But typically, enlarging the rim diameter pushes mass to the outside of the wheel. In this case, the moment of inertia goes up significantly.

Two questions:

Let's think both 16", 18"are the same total diameter.

Would the 18" have more mass on the outside from the center of the rim, if yes how much of a difference would that make?

If each 18" is 6 pounds lighter than the 16" with tires, how much power would you gain. Total weight loss 24 pounds. This would increase hp/weight ratio right ?
Not necessarily. Again, the lower mass of the wheel helps lighten the car's weight, but any gains it has can be easily absorbed by an increased moment of inertia.

You'd almost have to measure the moment of inertia of each wheel to make a fair comparison. Wheel designs vary so much that I can't really predict the change with any accuracy.
Old 03-20-2002, 04:36 PM
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What does this mean in terms of wheel power increase and 1/4 mile times?
Old 03-20-2002, 07:42 PM
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If you are happy with your gear ratios now, choose a wheel and tire combo that are as light as possible but the same diameter as you have. If you are not having traction problems leaving the line or you want a lower final drive ratio ( higher numerically) buy a light combo that is of a smaller diameter. The Tire Rack can really help you with this part.
Old 03-20-2002, 09:05 PM
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Originally posted by daverman
There are two ways changing rims will affect you car's performance:

First, it's not just the mass of the wheel (plus tires) that matters, but also its moment of inertia. remember that you have to expend energy to spin the wheel, and that energy comes out of the same bucket that pushes your car forward.

Second, changing the diameter of the wheel changes the moment arm at the axle, effectively changing your transmission's gear ratio. The larger your wheel diameter, the less force is available to push your car forward at the same torque output.

It's a linear relationship, so changing your wheels from say, a 18" diameter (including tires) to 20", for example, will change your radius from 9" to 10", so at the same torque setting you will get 9/10 = 90% of the original force applied to the ground.



Just wondering, Dave are u a Phyics Major. I dreaded that I was gonna take phyics this year but I love that damn class. I have never missed or skipped that class.
Old 03-21-2002, 09:18 AM
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Originally posted by JeffyTL
Don't hold me to this but I believe I read somewhere that 1 lb turns into 4 lbs when rotating. So if you change from a 28 lb rim to a 15 lb rim, you are essentially dropping 208 lbs of the total weight of the car when in motion. (13 lbs x 4 lbs x 4 wheels ???) There is a equation where you can convert weight reduction to whp gains. This sound right to anyone.

that sounds about right. So while you are not increasing hp, you are reducing weight, more specifically the rotating mass, thus making your car faster down the 1/4..... Less weight is being pushed by the same hp.
Old 03-21-2002, 12:32 PM
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I know you are not so much creating more HP but freeing up some HP to the wheels. My question is, will buying new rims give me enough time in the 1/4 for it to be a worthwhile mod. I don't care about the looks, and if I want better handling I can just get tires, but would rims be worthwhile in dropping my ET.
Old 03-21-2002, 01:12 PM
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Originally posted by dnd2984
Just wondering, Dave are u a Phyics Major. I dreaded that I was gonna take phyics this year but I love that damn class. I have never missed or skipped that class.
No, I was an Engineering major. I love Physics too, but our professors were duller than ditchwater!
Old 03-21-2002, 02:33 PM
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Yes, I had a 2000 Auto Maxima, stock dyno was 174 hp. I put 17x8 Momo Arrows on my Max, dynoed later, and got 164 hp stock!!! lol!

But after ypipe and intake, It dynoed with the wheels 184hp. All the same dyno.
Old 03-21-2002, 10:36 PM
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Originally posted by mbeebe
I know you are not so much creating more HP but freeing up some HP to the wheels. My question is, will buying new rims give me enough time in the 1/4 for it to be a worthwhile mod. I don't care about the looks, and if I want better handling I can just get tires, but would rims be worthwhile in dropping my ET.
if you want a better ET, wheels will HELP... but the smaller/lighter the better... stick with 16s, and lighter wheels...

lower profile tires are harder to lauch.... and to catch (get grip off the line)
Old 03-22-2002, 11:14 AM
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Originally posted by johnnyb_s
Technically, you don't lose or gain horsepower when you don't change any aspects of the engine. An engine with 350 hp will still have 350 hp, no matter how much a car weighs. Now acceleration will be improved, as well as braking, but not engine hp. As for the dyno, to get a true reading of engine hp, you need to pull the engine out and set it up by itself, and measure hp that way.
why would you care about HP at the crank? we only care about HP to the wheels because that the amount of usable power.
Old 03-22-2002, 11:29 AM
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Originally posted by Turboara



that sounds about right. So while you are not increasing hp, you are reducing weight, more specifically the rotating mass, thus making your car faster down the 1/4..... Less weight is being pushed by the same hp.
My understanding too - that reducing unsprung weight will provide more acceleration (lateral too - handling) than an identical reduction in sprung weight.
Old 03-22-2002, 11:40 AM
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Originally posted by Speedy3.2CL-S
Yes, I had a 2000 Auto Maxima, stock dyno was 174 hp. I put 17x8 Momo Arrows on my Max, dynoed later, and got 164 hp stock!!! lol!

But after ypipe and intake, It dynoed with the wheels 184hp. All the same dyno.
that's because momo arrows suck and are more than 10 lbs more than stock rims. (about 30lbs)
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