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Discussion Starter #1 (Edited)
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.

:dontknow::dontknow:


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.'
 

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No you don't. You increase torque. Power is torque related to time
 

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Discussion Starter #3 (Edited)
No you don't. You increase torque. Power is torque related to time
on further research im not sure it effects torque either :dontknow:

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

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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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Torque is definitely increased. when you lower gearing
 

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Discussion Starter #6
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 :mrgreen:
 

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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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Cheers
 

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Discussion Starter #9
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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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...
 

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Thrill is correct , all the dyno can do is give you its best assumption based on math on what your torque is at the crank, engine torque never changes unless you alter the engine.

If you dont multiply the effort (gearing) you dont achieve anything.
 

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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...
:headshake:headshake

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
:+1:
The gearbox and the final reduction are torque multipliers. Torque is multiplied by the overall reduction ratio.
You have to take into account the primary reduction ratio, the gear ratio, and the final drive reduction ratio.
Lets put some examples:
ZX-10R 04/05
Primary R.R. : 1.611
First Gear R. : 2.533
Final drive R.R : 2.294

Overall reduction ratio for 1st gear: 1.611 x 2.533 x 2.294 = 9.361

6th gear R.R. : 1.304
Overall R.R. for 6th: 1.611 x 1.304 x 2.294 = 4.819

Torque at the crankshaft will be multiplied by this overall reduction factor and crankshaft RPMs will be divided by it. You can see that (obviously) you will have more torque in 1st than in 6th (at the wheel).

You also have to consider for exact torque calculation the overall wheel diameter. Rear wheel dynos consider drum speed vs. engine RPMs. Drum speed and rpm will be unique for each gear selected and for that given rear tire, and will correct (based on this) for exact calculation. If you are dynoing a bike, lets say in 4th, and you want to dyno it again in 5th, you have to correct the dyno calibration for the new 5th ratio, that will involve a diferent drum speed vs. engine rpm.
:thumbsup:
 

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Sextaafondo - i am correct. Have personally changed gearing on the dyno and seen the results. Ask Lee Shierts, Dave Owen, Carpenter, Wilburn Motorsports. Whoever you want. Even KWS. Changed the gearing on my car never changed the amount of TQ it had either - it changed the rate of acceleration based on the TQ it had due to the ratio of said gearing.

The question was not posed based on gearing in the trans (as yes it is a multiplier but to a much larger effect) - it was a simple question aimed (or so i thought) at sprocket changes. And if it was (as the first post suggests) - i am correct.
 

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You guys are talking at crossed purposes I think, by changing the gear ratios then the effort produced at the wheel will change at a given speed , but peak torque will always be at the same RPM,s , you cannot change the engine torque curve by changing the output ratio, but you change the effort at the contact patch.

So you multiply the torque through gear reduction but it will always have the same curve as the engine just a higher or lower amount dependant on how much you reduced or increased the gear ratios, the torque peaks and valleys will occur at exactly the same Revs, just at different speeds.
 

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Heres the physics.

If you have a nut stuck on a stud and requires a certain torque to release it, use a longer lever , the torque required will be the same but the effort is less although the movement at the lever is longer (the ratio).
 

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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...
All I can say is that torque is a measurement of force x lever arm. Dropping gearing increased the lever arm, while the force remains the same. The force cannot change unless it is altered at the motor... ie boring or stroking. When you you downchange gears, is it not easier to lock up the back wheel as the gears get lower.... This is why the clutch in the kawi is called a back torque limiting clutch. Because as the gearing gets lower, the (lever arm) increases in length so to speak. Since torque is measured by moment calculation when the lever arm (in metres) doubles, and is multiplied by the force (in Newtons) ie Nm the torque doubles.
 
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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...
Dude, this is not possible. The engine power curve won´t change no matter the gearing used, other than slightly modified by gears diff. friction losses.
If you have seen this happen (and I don´t doubt you) is because the dyno has not been recalibrated to the new gearing after the sprocket change.

Sextaafondo - i am correct. Have personally changed gearing on the dyno and seen the results. Ask Lee Shierts, Dave Owen, Carpenter, Wilburn Motorsports. Whoever you want. Even KWS. Changed the gearing on my car never changed the amount of TQ it had either - it changed the rate of acceleration based on the TQ it had due to the ratio of said gearing.

The question was not posed based on gearing in the trans (as yes it is a multiplier but to a much larger effect) - it was a simple question aimed (or so i thought) at sprocket changes. And if it was (as the first post suggests) - i am correct.
Sprocket change alters the so called Final Reduction Ratio, that is part of the Overall Reduction Ratio. So changing sprockets afects the ORR as so does the gearbox gear selection everytime you shift up or down.
 

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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...

Your gearing multiplies the torque from the engine to move the bike.

The lower the gearing the greater the multiplacation and the greater the 'thrust' hence easier wheelies etc.

A dyno takes gearing and engine speed into account.
 
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