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Power loss with 20's?

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Old 11-28-2006, 06:17 PM
  #21  
CrazyAl
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Default RE: Power loss with 20's? here we go again!


ORIGINAL: Sleeper05


crazyal--while weight does have an large affect on power loss, the moment of inertia of the wheel is mass * radius^2...so the radius has an exponentially greater effect on power loss. granted, however, radii do not change much (a couple inches max), while weight might increase 2-fold with big rims.

Believe me, I understand this well. I have a Mechanical Engineering degree!

The formula you quoted has to be integrated over the entire wheel. It is not such a simple calculation in the end. Your calculation is making a lot of assumtions that aren't necissarily true, though I agree with the net conclusion.

Becasue the weight difference of the 20 is concentrated towards the outside of the wheel, it's moment of intertia is significantly higher...then again, I did't want to get into this technical of a discussion here. There's plenty of people who already don't get it and now we're dragging math into the picture....
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Old 11-28-2006, 06:30 PM
  #22  
06GT4RAD
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Default RE: Power loss with 20's? here we go again!

Ok i have 20's and can say that you will notice a small loss of power but it is nothing a tune and CAI and a set of gears cannot get back plus some. If your not drag racing the car it isn't going to matter. If however you are using your car for racing mostly then stay away from 20's But if your car is used a fair weather car for weekend drives and showing at car shows then get the 20's. Or if you just want the car to look good and racing is of no concern expect once in a whiel. Then keep the stockers and race with them.

R

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Old 11-28-2006, 06:45 PM
  #23  
Sleeper05
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Default RE: Power loss with 20's? here we go again!


ORIGINAL: CrazyAl


ORIGINAL: Sleeper05


crazyal--while weight does have an large affect on power loss, the moment of inertia of the wheel is mass * radius^2...so the radius has an exponentially greater effect on power loss. granted, however, radii do not change much (a couple inches max), while weight might increase 2-fold with big rims.

Believe me, I understand this well. I have a Mechanical Engineering degree!

The formula you quoted has to be integrated over the entire wheel. It is not such a simple calculation in the end. Your calculation is making a lot of assumtions that aren't necissarily true, though I agree with the net conclusion.

Becasue the weight difference of the 20 is concentrated towards the outside of the wheel, it's moment of intertia is significantly higher...then again, I did't want to get into this technical of a discussion here. There's plenty of people who already don't get it and now we're dragging math into the picture....
sweet! 3rd yr ME here. I was assuming that the wheel was a disk that was of uniform density and thickness everywhere, and thin. im workin on my dynamics hw, so i felt obligated to post, but didn have the time to do all the math out right. lol.

the point was that you DO lose power with bigger wheels, even if they weigh the same, and the loss can be significant in many circumstances.
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Old 11-28-2006, 06:48 PM
  #24  
androdz
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Default RE: Power loss with 20's? here we go again!

ORIGINAL: 05SDI

every lb you add in rotational weight is = to 4x non-rotational weight

Well, it changes with speed = inertia

I believe that you could have seen a change in the dyno numbers. But assuming the tire sizes are correct the 235/50/17 Pirelli was not the stock tire the stock tire was 235/55/17 and they were 27.1” tall. When you changed to 255/40/17 Hankooks you tire height dropped to about 25” tall. What did you change on either the dyno set up or calibration to make up for the 8% difference in the two tire sizes? (if your 235/50/17 tire size was correct then it is closer to 5% difference in tire size) Now the HP numbers you have given are within 3% and that can be the difference in either weather or a clean air filter.
Now IF you had stock 18” wheels and you mistakenly typed 235/50/17 instead of 235/50/18 and you lost HP when you bolted on a set of 17” wheels, this would be quite humorous to me!

I have always said dynos don’t win races!! They are just a tool to inform you of one aspect of how a vehicle could perform.

Sorry wrote the #'s wrong I had the stock crapellis and I put in 255/50/17 hankooks k104 if I am not mistaken. Airfilter was clean since both pulls were done 3 days apart, and the weather doesn't really change here in Puerto Rico. The tires just weigh more.

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Old 11-28-2006, 06:49 PM
  #25  
05SDI
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Default RE: Power loss with 20's? here we go again!


ORIGINAL: CrazyAl

now we're dragging math into the picture....
I like your sense of humor!



ORIGINAL: Sleeper05

the 20in rim will require 38.4% more power to accelerate at the same rate.
Sleeper05,

Quite simply; NO IT WILL NOT!!

You are on track with your logic, but your numbers are.. well quite... er ah... fuzzy!

For example based on your numbers;
Stock GT making 300 hp and wearing 20" rims (300hp x 61.6%) Equals the equivalent of 184.8hp! (using your numbers) Therefore a stock V6 will not only beat a GT wearing 20's in the quarter mile but will CRUSH a GT, based on those numbers! And this is NOT true as it will not happen!


Please check your math or try and give a practical example of how drastic you are making this sound by spreading the same amount of weight 1.5” out in your example of the same weight of 2 rims, one 17” and one 20”.

05SDI
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Old 11-28-2006, 06:56 PM
  #26  
androdz
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Default RE: Power loss with 20's? here we go again!

ORIGINAL: CrazyAl


ORIGINAL: 05SDI
I could confuse the issue by saying even in drag racing if I showed up with either of our cars and ran passes where the only thing I changed was rear tires, for example 215/65/16 (stock on some V6 Mustangs) and made passes with a set of 285/30/20 that the traction I would gain at the launch would never be over came by the hp I would be robbing as the car accelerates down the track!
Now this IS speculation on my part and next summer because my wife’s car isn’t banned from the track (lol) I will practice this theory!
I think you're probably right. All else being equal, if you have good tires on those 20's you would probably see that it would do better than the other car with the stock tires.

HOWEVER, let's say that instead of 20's with 285/30-20, you had something like 18s with 285/40-18. That would most likely be even better.
+1 it is all about sidewall. Like CrazyAl said most 20's are made for looks and not performance. I would say more than 90% of 20' rims are made for looks and not with performance in mind. You can find those made for performance, but if you are really looking for perfomance gains you usually go with a small tire. I would go for 17's wheels If my gt500 Brembo's didn't require 18' wheels or larger.

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Old 11-28-2006, 06:58 PM
  #27  
dotsonian
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Default RE: Power loss with 20's? here we go again!


ORIGINAL: CrazyAl


ORIGINAL: Sleeper05


crazyal--while weight does have an large affect on power loss, the moment of inertia of the wheel is mass * radius^2...so the radius has an exponentially greater effect on power loss. granted, however, radii do not change much (a couple inches max), while weight might increase 2-fold with big rims.

Believe me, I understand this well. I have a Mechanical Engineering degree!

The formula you quoted has to be integrated over the entire wheel. It is not such a simple calculation in the end. Your calculation is making a lot of assumtions that aren't necissarily true, though I agree with the net conclusion.

Becasue the weight difference of the 20 is concentrated towards the outside of the wheel, it's moment of intertia is significantly higher...then again, I did't want to get into this technical of a discussion here. There's plenty of people who already don't get it and now we're dragging math into the picture....
I agree that it is not as simple as the calculation as presented. If you consider that the diameter of the tire/wheel combo of both setups are the same, the difference as a percentage would probably be about half of what was presented.
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Old 11-28-2006, 07:01 PM
  #28  
Fast Toys
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Default RE: Power loss with 20's? here we go again!

Increasing wheel diameter improves cornerning performance only to a certain point and then anything above that actually hurts performance. That is assuming the weights of all the wheels stay the same which they do not. This has been proven time and time again. Look at what auto-x and road racing people use. You are not going to see 20" wheels there.

For drag use, you don't want a heavy, large diameter wheel with a small sidewalled tire. You want a small, light wheel and a tire with a huge sidewall. Most people run 15 and 16 inch rims.

MASS is the enemy of performance and reciprocating mass is even worse.

On a 300 HP car, I highly doubt most people's but-o-meter is going to notice a 10 HP loss hence a lot of people's comments.


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Old 11-28-2006, 07:04 PM
  #29  
dotsonian
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Default RE: Power loss with 20's? here we go again!

For example based on your numbers;
Stock GT making 300 hp and wearing 20" rims (300hp x 61.6%) Equals the equivalent of 184.8hp! (using your numbers)

I think he is saying that it will take 61.6% more power to turn the 20. so, if it took 10hp to turn the stock it will take 16.16hp to turn the 20.


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Old 11-28-2006, 07:05 PM
  #30  
Sleeper05
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Default RE: Power loss with 20's? here we go again!

ORIGINAL: 05SDI


ORIGINAL: CrazyAl

now we're dragging math into the picture....
I like your sense of humor!



ORIGINAL: Sleeper05

the 20in rim will require 38.4% more power to accelerate at the same rate.
Sleeper05,

Quite simply; NO IT WILL NOT!!

You are on track with your logic, but your numbers are.. well quite... er ah... fuzzy!

For example based on your numbers;
Stock GT making 300 hp and wearing 20" rims (300hp x 61.6%) Equals the equivalent of 184.8hp! (using your numbers) Therefore a stock V6 will not only beat a GT wearing 20's in the quarter mile but will CRUSH a GT, based on those numbers! And this is NOT true as it will not happen!


Please check your math or try and give a practical example of how drastic you are making this sound by spreading the same amount of weight 1.5” out in your example of the same weight of 2 rims, one 17” and one 20”.

05SDI
05SDI, i clearly stated that it would require that much more power to accelerate the RIMS at the same rate. aka, if the 17wheels have a parasitic loss of 20hp on their own, the 20s of the same weight would have a parasitic loss of 27.68hp by my number. This means essentially a loss of 7-8rwhp in that situation with those numbers, not 115! Read first, then attack.

then i went on to explain that the actual number would likely be 80%+, meaning 20*1.8 = 36=20+16...essentially being 15-20rwhp loss

[8D]no offense taken tho!
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