effect of 16" vs 17" rims on 1/4 mile time?
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effect of 16" vs 17" rims on 1/4 mile time?
hey guys. i just got a set of 17's, which i was pretty aprehensive about getting in the first place because i don't want to sacrifice any straight-line power. well now i have the chance to go back to 16's and was wondering if it would make that much of a difference.
is there any way to calculate how much power i'll be missing because of the larger rotational mass of the 17" wheel? or does anyone have any real life experience with this dilemma? any help is greatly appreciated
-marc
is there any way to calculate how much power i'll be missing because of the larger rotational mass of the 17" wheel? or does anyone have any real life experience with this dilemma? any help is greatly appreciated
-marc
#2
Im no math expert, but from my experience with 16s and 17s, the weight of the wheel is more of a factor than the size.
I used to have 20lb+ 17" wheels and the car felt VERY sluggish. I went back to the stock wrx wheels with snow tires during last winter, and the car felt so much better -- Faster acceleration, smoother up/down shifting and better braking.
If your 17's are lighter than the stock 16.5lb wheels, I doubt you will feel a difference. But if your 17's are heavier than stock, you may very well feel a difference. If I had the money, I would most definatly buy 17's as long as they were lighter than stock.
I used to have 20lb+ 17" wheels and the car felt VERY sluggish. I went back to the stock wrx wheels with snow tires during last winter, and the car felt so much better -- Faster acceleration, smoother up/down shifting and better braking.
If your 17's are lighter than the stock 16.5lb wheels, I doubt you will feel a difference. But if your 17's are heavier than stock, you may very well feel a difference. If I had the money, I would most definatly buy 17's as long as they were lighter than stock.
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The size and weight both matter. If you have 17s and 16s that are exactly the same weight, the car will accelerate faster with the 16s. Because the 16s have less rotational inertia.
That's a hell of a trap speed
That's a hell of a trap speed
Last edited by MVWRX; 03-24-2006 at 02:18 PM.
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Originally Posted by impreza25rstn
i have 17" konigs. and i have 15" steel wheels. I run about 2-3 tenths better with the 15's on. faster acceleration, lower trap speed.
12.222 @ 107.85!!!!!
12.222 @ 107.85!!!!!
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Originally Posted by diz
hmm these are the results i was expecting. 2-3 tenths is quite a difference..
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This is just a quick and rough calculation about rotating mass/inertia that could affect a 1/4 mile time...I wrote it about 17 to 18 but it gives the idea of what goes on...
If you go from 17 to 18 inch wheels, you're effectively moving the weight 1 inch outward from center. This means a 5.88% increase in diameter. Since the inertia increases as the square of the diameter, you're increasing the inertia ~ 34.57%. So you'd have to decrease the weight of the wheel ~30% or so to get the same inertia. If your 17w/tires is 39lbs, that means you'd need an 18w/tires that weighs roughly 27lbs...hard to find in an 18...
This is an approximation that doesn't take into account a few things, but it shows you how drastically moving a small amount of weight away from the center of rotation can affect the inertia of the wheel. It also shows that a weight decrease of ~2lbs probably won't make up the inertial loses of a larger wheel. But if the grip and sidewall stiffness make up for it on the track, this is all pointless...
If you go from 17 to 18 inch wheels, you're effectively moving the weight 1 inch outward from center. This means a 5.88% increase in diameter. Since the inertia increases as the square of the diameter, you're increasing the inertia ~ 34.57%. So you'd have to decrease the weight of the wheel ~30% or so to get the same inertia. If your 17w/tires is 39lbs, that means you'd need an 18w/tires that weighs roughly 27lbs...hard to find in an 18...
This is an approximation that doesn't take into account a few things, but it shows you how drastically moving a small amount of weight away from the center of rotation can affect the inertia of the wheel. It also shows that a weight decrease of ~2lbs probably won't make up the inertial loses of a larger wheel. But if the grip and sidewall stiffness make up for it on the track, this is all pointless...
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for linear motion, if we wanted to know how much power we needed to accelarate a car down a drag strip at a certain accelaration, we needed only this:
F= MA
there is a cousin for rotational accelaration:
T = Ia
where a(alpha) is our rotational accelaration, T(tau) is the torque or rotational force, and I is the Moment of Inertia(rotational mass).
let's simplify our wheel into just a hoop of metal of radius r(where most of the weight is anyway). In this case, the rotation inertia for any wheel would be I=mr^2 or:
I=(mass of wheel)(radius of wheel)^2
What we are concerned about is the accelaration obviously...
a = T/I
So if you want to figure out how fast a given wheel will accelarate(wheel mind you, not car) you can use this:
a = T/( (mass of wheel)(radius of wheel)^2 )
this equation explains what many already know intuitively. If you power(T), you accelarate faster(duh). Adding mass to the wheel slows you down. What I find interesting is how much more radius will slow you down!!!!
A simple example with some bogus and entirely made up numbers
Wheel 1: 10kg, 16", 50ftlbs (wrx-ish)
a = 50/ ( 10 * 8^2 )
a = .078 imaginary accelaration units
Wheel 2: 10kg, 17", 50ftlbs
a = 50/ ( 10 * 8.5^2 )
a = .069 imaginary accelaration units
That's a 13% decrease in angular accelaration with just a 1" jump in wheel size, and probably a BETTER wheel since it weighed as much as the 16" one. Looks like adding 1" of wheel is equivilent to adding about 2kg of weight. Yuck.
Now note that I don't claim a car with 16s is 13% faster that a car with 17s. Rotational inertia is only one force we have to fight to get power down, but my best guess is that it is responsible for about 5% or so of power loss. On a dyno maybe you will see a .5% reduction in torque? Over 1/4 mile, that is certainly a couple of tenths.
Bottom line, plus sizing sucks. this is only in a straight line. If I had more time I would get into what it means to increase your unsprung mass, and why lo-pro tires don't grip.
Chris Lock
F= MA
there is a cousin for rotational accelaration:
T = Ia
where a(alpha) is our rotational accelaration, T(tau) is the torque or rotational force, and I is the Moment of Inertia(rotational mass).
let's simplify our wheel into just a hoop of metal of radius r(where most of the weight is anyway). In this case, the rotation inertia for any wheel would be I=mr^2 or:
I=(mass of wheel)(radius of wheel)^2
What we are concerned about is the accelaration obviously...
a = T/I
So if you want to figure out how fast a given wheel will accelarate(wheel mind you, not car) you can use this:
a = T/( (mass of wheel)(radius of wheel)^2 )
this equation explains what many already know intuitively. If you power(T), you accelarate faster(duh). Adding mass to the wheel slows you down. What I find interesting is how much more radius will slow you down!!!!
A simple example with some bogus and entirely made up numbers
Wheel 1: 10kg, 16", 50ftlbs (wrx-ish)
a = 50/ ( 10 * 8^2 )
a = .078 imaginary accelaration units
Wheel 2: 10kg, 17", 50ftlbs
a = 50/ ( 10 * 8.5^2 )
a = .069 imaginary accelaration units
That's a 13% decrease in angular accelaration with just a 1" jump in wheel size, and probably a BETTER wheel since it weighed as much as the 16" one. Looks like adding 1" of wheel is equivilent to adding about 2kg of weight. Yuck.
Now note that I don't claim a car with 16s is 13% faster that a car with 17s. Rotational inertia is only one force we have to fight to get power down, but my best guess is that it is responsible for about 5% or so of power loss. On a dyno maybe you will see a .5% reduction in torque? Over 1/4 mile, that is certainly a couple of tenths.
Bottom line, plus sizing sucks. this is only in a straight line. If I had more time I would get into what it means to increase your unsprung mass, and why lo-pro tires don't grip.
Chris Lock
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Thanks Chris, I was trying to explain that as best I could. But I don't have time to go into the detail you did. I knew that a bigger wheel was way worse than anyone was admiting in terms of acceleration....
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