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07-10-2007, 06:49 PM | #19 | |
Chillin' out by the Bay
Drives: 2007 Yaris Liftback Join Date: Nov 2006
Location: The back streets of San Fran
Posts: 151
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Faster suspension response (up and down) due to less weight. The shocks don't have to absorb the heavier mass; and, faster steering respone for the same reason. "Quick" is the word when lighter wheels are installed, as in, "dayum, dis steerin' sho is smooooth' (spoken in inner-city ebonics for emphasis). And....the lower the profile tire, the greater the risk of damage. Typically, its best to use a 15" wheel with a 60 series tire to absorb the punishment of road hazzards. You get super quickness and ride quality at a slight penalty in handling. I'm going 15's...
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Gettin' all the way down. Meteorite '07 HB STILL completely stock; but.... checkin' out da other ones to see what's what. |
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07-10-2007, 08:03 PM | #20 | |
Parallel Parking Pro
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You see some racing cars run bigger wheels simply because they need to fit bigger brakes inside the wheels, for lower stiffer sidewalls, and because they have power to spare to turn monster 18 inch wheels. Which is why I can't see the point of anything bigger than 16s on a Yaris. The car isn't fast as stock, you're making it slower by throwing wagon wheels on it.
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Leong's NCP91 Toyota Yaris E Hankook V12 Evo 195/55/15 on Buddy Club P1 replicas 15x7 ET +38 Stebel horns, Philips Silvervision turn indicator bulbs TTE Lowering Springs, Camber bolts TRD Blue brake pads Fujitsubo Super Wagolis axleback exhaust DC Sports header Last edited by cleong; 07-11-2007 at 06:17 AM. Reason: Grammatiks |
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07-10-2007, 10:36 PM | #21 | |
Chillin' out by the Bay
Drives: 2007 Yaris Liftback Join Date: Nov 2006
Location: The back streets of San Fran
Posts: 151
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Quote:
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Gettin' all the way down. Meteorite '07 HB STILL completely stock; but.... checkin' out da other ones to see what's what. |
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07-11-2007, 01:29 AM | #22 |
Drives: Scion FRS Join Date: Aug 2006
Location: Arizona
Posts: 179
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the real question is how much do the steelies weigh so we can have something to compare xD (without tires)
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07-11-2007, 02:12 AM | #23 |
The Numbers
I've been thinking about this question for a few days. My gut reaction was: lighter wheels will make a significant improvement in accelleration and mpg. My calculations show the opposite. Maybe my logic is flawed? Here's what I've got so far.
Consider two cases: 1) Totally stock Yaris from 0-60mph 2) Yaris with lightweight rims from 0-60mph (NOTE: I'm assuming identical tires) The easiest way to compare the two is from an energy standpoint. Rotational Energy of a Wheel = .5*Mw*(Rw^2)*W^2 where: Mw= Mass of the wheel Rw= radius of the wheel W= rotational speed (Radians/second) For Case 1) Mw= 15lbs. I base this on the wheel/tire combo weight of 33 lbs. tirerack.com says the tire weights 18lbs. 33-18=15 Rw = 7.5 inches. 15in diameter / 2 W = 92.15 (RAD/s). @ 60mph 1mile takes 60 sec. for stock tires tirerack.com says 880 rev/mile so 880/60 = 14.6Rev/second *2Pi= Rad/s convert to metric Mw= 6.8kg Rw=.19m Rotational Energy = .5*Mw*(Rw^2)*W^2 = .5*6.8*(.19^2)*92.15^2 =1042 watts = 1kWatt For Case2) Mw=10 lbs. = 4.5kg This seems like a reasonable lightweight 15 inch wheel Rw=7.5 inches = .19m W = 92.15 RAD/s Rotational Energy = .5*Mw*(Rw^2)*W^2 = .5*4.5*(.19^2)*92.15^2 = 690 watts = .69 kWatt 1- (690/1042) = 33% less energy to accellerate the lighter wheel. This sounds like a lot untill you compare it with the total energy of the car @ 60mph. Energy of Yaris = .5*M*(v^2) Where: M=mass of yaris=2200 lbs = 998kg v=velocity=60 mph= 97 km/h Energy of Yaris = .5*998*(97^2) = 4695 kWatts so (Energy of Wheel) / (Energy of Yaris) @ 60mph is 1/4695 = .0002 .000002% of the total energy is insignificant. Did I make a stupid error in my calc? i would have guessed lightweight wheels would make a big difference...but I proved the opposite. someone help? ...show me the error... |
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07-11-2007, 06:22 AM | #24 |
Parallel Parking Pro
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I couldn't really understand the calculations you have done, and I am not a physics major, but would that be an instantaneous value (0.000002%) requiring multiplication by time factor?
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Leong's NCP91 Toyota Yaris E Hankook V12 Evo 195/55/15 on Buddy Club P1 replicas 15x7 ET +38 Stebel horns, Philips Silvervision turn indicator bulbs TTE Lowering Springs, Camber bolts TRD Blue brake pads Fujitsubo Super Wagolis axleback exhaust DC Sports header |
07-11-2007, 08:42 AM | #25 |
That Focus Guy
Drives: 2004 Focus RSVT Join Date: Jul 2007
Location: USA
Posts: 51
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fu, the weights of the wheels themselves could've told you the same thing without going through all that calculation. 10 lb wheels are 33% lighter than 15lb wheels, thus requiring 33% less energy to move them.
The second half of your equation is only comparing the total weight differences of the car with the two different wheels. The two front wheels are attached to the engine, so you'd also eventually conclude that you have more power to the ground due to less total weight of the moving drivetrain. Cliffs notes: you're only looking at half the picture. |
07-11-2007, 12:02 PM | #26 |
Drives: '07 3 Door Yaris of Doom! Join Date: Dec 2006
Location: Miller Motorsports Park
Posts: 88
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Yep!
You are comparing the amount of energy while driving at 60 mph, not the amount of energy it took to actually accelerate the extra 20 pounds of wheels. Also you didn't factor the rotational weight effects. You just treated the wheels as a non independent part of a mass that is the car.
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07-11-2007, 12:57 PM | #27 | |||
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Can you please provide a calculation for the other half of the picture? Quote:
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One error I do see is I compaired the energy of 1 wheel, when there are 4 on a car....multipling the energy of 1 wheel x 4 still nets a neglegable ammount of energy... Can anyone provide an alternate calculation? |
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07-11-2007, 01:05 PM | #28 | |
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The time factor would come into account if we were looking for the power required to accellerate to 60mph. Energy/time = power. I simplified the problem by only concidering the energy required. I believe this is a valid way to model this situation...but there may be an error in my calc... |
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07-11-2007, 01:15 PM | #29 |
Woops, I moved the decimal point the wrong way...it should be .002%...which is still far less than 1%... anyone else see mathmatical errors?
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07-11-2007, 01:46 PM | #30 |
Drives: Yaris Hatch /Landrover D2 Join Date: Jun 2007
Location: Canada
Posts: 242
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Removing weight helps acceleration as it takes less energy to move the mass but beware of diameter too. If you go with a 17 wheel even if its lighter than the stock steel wheel (not many wheels like that) you have moved the mass 3inches further out from the hub (based on 14 wheels) so it will take more enery to turn the wheel. I have 205 50 15 tires on light weight 11lb wheels , the stock steel wheel and tire are still lighter.
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07-11-2007, 01:53 PM | #31 |
That Focus Guy
Drives: 2004 Focus RSVT Join Date: Jul 2007
Location: USA
Posts: 51
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I can't calculate the other half of the picture, because it's more complicated.... dependant on what gear your in, gear ratios, the power output of the motor, and the weight of the moving parts of the drivetrain.
You can roughly determine the increase in HP to the ground by guessing the percentage of weight that is taken away from the total weight of the moving drivetrain with two lighter weight wheels, and comparing this to given HP numbers from the flywheel and the ground. Like I said, you only have half the picture, which is that there's a .08% decrease in the required energy to move the car. The other half is that you have more power available to output 4695 kWatts faster. The percentage decrease in the required energy to move the drivetrain is much larger than .08%. |
07-11-2007, 01:58 PM | #32 | |
That Focus Guy
Drives: 2004 Focus RSVT Join Date: Jul 2007
Location: USA
Posts: 51
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Quote:
Last edited by CaysE; 07-11-2007 at 02:14 PM. |
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07-11-2007, 02:05 PM | #33 | |
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please provide a calculation for the other half... |
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07-11-2007, 02:20 PM | #34 |
That Focus Guy
Drives: 2004 Focus RSVT Join Date: Jul 2007
Location: USA
Posts: 51
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I CAN'T provide a calculation. I have no idea what the weight of the drivetrain is, the given flywheel HP and BHP numbers are, or the gear ratios. You need to take into account what gear you're in and the ratio, because the number will be different for every gear. I can go 60mph in 2nd gear or 6th gear in my car... the difference in power output is significant.
If you get those numbers, feel free to calculate it yourself. I find it hard to see a reason for these calculations aside from getting empirical evidence to back up the fact that lightweight wheels do improve braking performance, suspension response, and acceleration. You don't need these numbers to understand what's going on. |
07-11-2007, 05:24 PM | #35 | |
Drives: '07 3 Door Yaris of Doom! Join Date: Dec 2006
Location: Miller Motorsports Park
Posts: 88
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Quote:
What you need to do is calculate "force", mass multiplied by acceleration. And of course it will take less energy to accelerate a lighter object. The formula to calculate rotational acceleration is a bit more complex, but this is actually what you are looking for here. Try google...
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Last edited by Biggie™; 07-11-2007 at 11:11 PM. Reason: typo |
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07-11-2007, 07:56 PM | #36 |
vroom vroom
Drives: lil red 5-door Join Date: Sep 2006
Location: Bangkok, Thailand
Posts: 7,744
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Man, I need to find my old physics textbooks
Back on the original topic, I find it easier to explain these phenomena with basic everyday examples... Effect of total car weight Imagine you're at the supermarket. How much easier is it to push the trolley around when it is empty compared to when it is full? It's a lot easier to speed up, stop, and turn when you have less weight. Effect of wheel weight (or pulley, or any rotating mass) Imagine you have two tops (spinning tops, what kids used to play with long ago...), a heavy one and a lighter one. Think of how much extra twisting force you have to give the heavy one to get it to spin at the same speed as the lighter one. It will also keep spinning longer, and if you try to stop it by hand, you will need more stength to do so. Also, imagine you are riding a bicycle, and you want to go up a low curb or something similar. You'd usually slow down, because when you go fast, the inertia of the wheel (the same force that lets your bike stay upright instead of falling over, when you are moving), will want to drive the bike straight through the curb, instead of up and over it. In car terms, the lighter the wheel, the less inertia it has, and the easier the suspension can react to bumps at higher speeds. In reality there is always a compromise somewhere... smaller lighter wheels will help with response, but that means running a taller sidewall which can flex and make the handling less precise. Also, something I've been pondering, the Yaris is quite tall and kinda top-heavy, so heavier wheels may be an advantage in stabilizing the car, shifting the center of gravity down a bit, keeping the car a bit more planted at speed? This would only be really useful on a highway, but just something that might be worth to consider. Anyway, I hope that helped, for the less technically-minded out there
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