Gearing / Power Physics - Kawasaki ZX-10R.net
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post #1 of 26 Old 04-12-2008, 03:28 AM Thread Starter
 
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Gearing / Power Physics

Ok this is simple enough but has always puzzled me...

You have X bhp at the crank

You have X (less) bhp at wheel

If you shorten gearing, say -1 tooth front do you increase measured bhp at rear wheel.

From googling about I think you do, and if say I dyno 161bhp at wheel then shorten gearing 5% I make 5% more power at wheel ? (ie - 169). I'm thinking this because by reducing the gearing you lessen the power loss between the crank and wheel as less work involved for engine. Maybe this works to the point where the rear wheel bhp would almost match crank bhp though at that point you wouldn't be traveling very fast at all.




edit: feel free to point out the flaws in my thinking lol, im really not very bright :)


edit again: ok reading about it some more it seems to only effect torque, which makes sense, lower gearing more torque.. power remains at a constant.


edit again again: looks like both remain constant.. quote

'Horsepower is a function of Torque and RPMs that the engine puts out.

Hp(T,R)=R*T/5250 where R=rpms and T=ft-lbs

Gearing does not change the ammount of torque that your engine produces or the rpms that it produces it at, therefore the power is also unchanged. The effect that changing the gearing has is to use the same availble power but with a modified linear relationship between the speed of your engine and the speed of your tire. The more revolutions the tire does for each revolution of the engine, the slower the engine can accelerate the tire between fixed speeds (within the powerband for example) because it can only do a certian ammount of work/time. if that same work/time is applied to a lower ratio of tire revolutions to engine revolutions, the tire will accelerate to a faster speed. Stunters change their sprockets for this advantage, more acceleration within gears with the downside of shorter gears.'

Last edited by Fluke; 04-12-2008 at 03:41 AM.
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post #2 of 26 Old 04-12-2008, 03:38 AM
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No you don't. You increase torque. Power is torque related to time


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post #3 of 26 Old 04-12-2008, 03:44 AM Thread Starter
 
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Quote:
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No you don't. You increase torque. Power is torque related to time
on further research im not sure it effects torque either

if it does increase like you say I dont think dynos show the increase as they calculate torque at the crank not wheel afaik

Last edited by Fluke; 04-12-2008 at 03:46 AM.
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post #4 of 26 Old 04-12-2008, 03:47 AM
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your quote "at that point you wouldn't be traveling very fast at all. Think of it this way. The torque increases on the output shaft of any gearbox when the gearing is lower. But the machine takes more time to perform said task. power is torque divided by time and in this case the torque increased with the down gearing, at a directly proportional rate to the time it takes to do the job. Therefore power remains mainly the same, yo may gain a little, or in some cases lose, due to things thrown into the meld like friction etc


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Last edited by thrill; 04-13-2008 at 03:33 PM. Reason: error... inversely should read directly
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post #5 of 26 Old 04-12-2008, 03:48 AM
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Torque is definitely increased. when you lower gearing


Bill - GI Pro, Hindle slip on, Smoked screen, ZX14 brakes.... Well, you feel lucky, punk? You shouldn't, this is a 10R
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post #6 of 26 Old 04-12-2008, 03:49 AM Thread Starter
 
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Quote:
Originally Posted by thrill View Post
your quote "at that point you wouldn't be traveling very fast at all. Think of it this way. The torque increases on the output shaft of any gearbox when the gearing is lower. But the machine takes more time to perform said task. power is torque divided by time and in this case the torque increased with the down gearing, at an inversely proportional rate to the time it takes to do the job. Therefore power remains mainly the same, yo may gain a little, or in some cases lose, due to things thrown into the meld like friction etc
cheers man, great explanation
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post #7 of 26 Old 04-12-2008, 03:52 AM
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Torque at the output shaft is increased, but torque at the crank remains the same. The only way a rear wheel dyno can measure crank torque is to be calibrated with the final drive ratio. If your torque at the crank is say 50Nm and you hook up to a gearbox that is a 2:1 ratio you torque will be 100Nm at the output 3:1 ratio 150Nm at the output


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post #8 of 26 Old 04-12-2008, 03:53 AM
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Cheers


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post #9 of 26 Old 04-12-2008, 03:55 AM Thread Starter
 
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it seems google can be your worst enemy as so much duff information about.. actually finding the facts is hard for a squid lol
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post #10 of 26 Old 04-12-2008, 07:03 AM
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Quote:
Originally Posted by thrill View Post
Torque is definitely increased. when you lower gearing
Not according to the dyno.

All gearing does is move the tq/hp lower down (if you go - on front or + on rear) in the rpm range by about 300rpm for every "1" change on the rear...

09 R6S Slow and steady...

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