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Thread: Need more power

  1. #21
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    Quote Originally Posted by MZ5 View Post
    The _power_ output of an engine is the only thing that matters to performance (acceleration as well as top speed).
    I will begin by assuming by "power output" you are referring to the entire power curve and not just the reported peak power figure so as to not get in to the finer details of engine performance. We will also ignore the effects of wind resistance, friction, weight, etc. for simplicity's sake, even though these have a huge impact on performance (acceleration as well as top speed).

    Your statement above is not accurate. Take two identical motorcycles and change the final drive gear ratios on one of them and one will undoubtedly accelerate faster than the other. This comes as a trade-off as you will lose out on top speed. Identical engines with different gear ratios WILL have different acceleration and top speeds. This is a fact. Many other things can have an impact on performance as we are talking not about the performance of the engine as a separate unit, but of the entire system of a motorcycle.

    Providing his determination that he "needs more power" was made based on lack of acceleration and he is willing to lower the top speed of the machine proportionally, changing the final-drive gearing could provide the answer he is looking without increasing the performance characteristics of the engine. From a strictly technical point of view, this changes the relationship between the motorcycle's speed and the power output of the engine, as you are moving the desired speed range up into the higher power band of the engine. With this in mind, you could argue (semantically) that this is an increase in 'power' but that's more a language-based argument than physics-based.

  2. #22
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    If you shorten transmission you get higher torque (to the wheel), lower rev, and same power. Power = torque x rpm and thus juste follows the torque (engine) curve as rpm increases.

    The NC 750 has decent torque, it's power is limited by the low max rpm. In road use, it is a far better choice than small 4 inline that may produce more than 100 bhp but at very high rpm due to limited torque.

    To me torque is the real thing. I own two other more powerful bikes but love the engine response of the NC. Of course for track use I use the triple.

  3. #23
    Super Moderator Need more power 670cc's Avatar
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    Quote Originally Posted by TheIronWarrior View Post
    Your statement above is not accurate. Take two identical motorcycles and change the final drive gear ratios on one of them and one will undoubtedly accelerate faster than the other. This comes as a trade-off as you will lose out on top speed. Identical engines with different gear ratios WILL have different acceleration and top speeds. This is a fact. Many other things can have an impact on performance as we are talking not about the performance of the engine as a separate unit, but of the entire system of a motorcycle.
    Oh, I love these power debates!

    So in the statements above, changing final drive ratios resulted in a trade off between acceleration and top speed. But, what if the manufacturer widened the ratios, or lowered the final drive ratio and added another gear on top, such that the acceleration was there at the low gear ratio, but the top speed was still there at the high gear ratio? Do you get the best of both worlds with no trade off?
    Last edited by 670cc; 6th December 2017 at 09:35.
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  4. #24
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    You can't have your cake and eat it. If you shorten you will hit the rev limiter before you get to top speed. If you lengthen you will not get to the top speed as you will not have enough torque (at the wheel) to overcome air resistance.

    If you have short first gear(s) only you will have a large drop of rpm when switching that will put you out of the torque curve and make the bike sluggish.

    Factory ratios usually are best for road use. I played with this on one bike, and could not decide if it was better or worse, just different. If really you need more torque at the wheel then you may shorten secondary transmission, top speed not being an issue IMHO.
    Last edited by Jos; 6th December 2017 at 10:03.

  5. #25
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    Quote Originally Posted by 670cc View Post
    So in the statements above, changing final drive ratios resulted in a trade off between acceleration and top speed. But, what if the manufacturer widened the ratios, or lowered the final drive ratio and added another gear on top, such that the acceleration was there at the low gear ratio, but the top speed was still there at the high gear ratio? Do you get the best of both worlds with no trade off, or something for nothing?
    Consider this a 'thought-experiment':
    Let's take Bike A to be a completely stock machine and Bike B will have a modified transmission.
    For simplicity's sake, let's assume final drive ratios, as well as top and bottom gear ratios remain the same. This way, the top speed of the bike will remain unchanged, as will the speed/torque in 1st gear.
    Let's then add a few more gears to the trans (arbitrarily say 3 more gears) and adjust the ratios accordingly so they're relatively evenly distributed. We now have a completely stock Bike A and a Bike B that has a 9-speed transmission.
    Bike B will now have less of an RPM drop between gear changes, keeping the engine operating higher in the power band. Theoretically this will increase acceleration. I say theoretically because now you have more gear changes, increasing the time that there is no power to the wheel at all (clutch in). I would suspect that someone somewhere determined 5 or 6 gears in the transmission of a typical motorcycle hits the balance well, as these seem to be the most common numbers of gears.
    So assuming a shift of the gears takes exactly zero time (impossible) you could consider that getting "something for nothing" in a perfectly theoretical world however this is not possible in real life.

  6. #26
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    Quote Originally Posted by Jos View Post
    You can't have your cake and eat it. If you shorten you will hit the rev limiter before you get to top speed. If you lengthen you will not get to the top speed as you will not have enough torque (at the wheel) to overcome air resistance.
    As an addition to the above, I had read somewhere, and experimented with, top speed on my NC700 (closed course in Mexico, of course...) and have found that you can actually hit a higher speed in 5th than in 6th for this exact reason.

  7. #27
    Super Moderator Need more power 670cc's Avatar
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    Quote Originally Posted by TheIronWarrior View Post
    Consider this a 'thought-experiment':
    Let's take Bike A to be a completely stock machine and Bike B will have a modified transmission.
    For simplicity's sake, let's assume final drive ratios, as well as top and bottom gear ratios remain the same. This way, the top speed of the bike will remain unchanged, as will the speed/torque in 1st gear.
    Let's then add a few more gears to the trans (arbitrarily say 3 more gears) and adjust the ratios accordingly so they're relatively evenly distributed. We now have a completely stock Bike A and a Bike B that has a 9-speed transmission.
    Bike B will now have less of an RPM drop between gear changes, keeping the engine operating higher in the power band. Theoretically this will increase acceleration. I say theoretically because now you have more gear changes, increasing the time that there is no power to the wheel at all (clutch in). I would suspect that someone somewhere determined 5 or 6 gears in the transmission of a typical motorcycle hits the balance well, as these seem to be the most common numbers of gears.
    So assuming a shift of the gears takes exactly zero time (impossible) you could consider that getting "something for nothing" in a perfectly theoretical world however this is not possible in real life.
    Or, we could just use a Continuously Variable Transmission to eliminate the gearing steps, and have the engine constantly operating at it's peak horsepower RPM throughout the entire road speed range. The benefit of this is, of course, ignoring any additional losses in the CVT drivetrain vs traditional gears.
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  8. #28
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    Quote Originally Posted by TheIronWarrior View Post
    I will begin by assuming by "power output" you are referring to the entire power curve and not just the reported peak power figure so as to not get in to the finer details of engine performance. We will also ignore the effects of wind resistance, friction, weight, etc. for simplicity's sake, even though these have a huge impact on performance (acceleration as well as top speed).

    Your statement above is not accurate.
    Yes, it is. Your assumptions and entire comparative setup are fallacious. The correct and useful comparison is two engines (or powerplants; don't assume an engine vs. a motor, nor anything else, because it's irrelevant), one with 30% higher power output than the other, and both power 'curves' completely flat. The engine with greater power output will result in greater acceleration at any given vehicle speed. The engine speed is irrelevant since the power curves are totally flat.

    Your argument is fallacious because you've assumed many things, including that more power is made at higher rpm. That is normally correct, and that is why the lower-geared vehicle accelerates more quickly; your assumptions have caused the powerplant to make more power, which allows the tranny to exchange speed for more wheel torque.

    He needs more power.
    Last edited by MZ5; 6th December 2017 at 15:49.

  9. #29
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    Quote Originally Posted by TheIronWarrior View Post
    Consider this a 'thought-experiment':..
    We don't need though experiments; work, power, forces, HP, torque, etc., are settled defined things. Anyone who took high school physics was taught that F=MA, therefore A=F/M. There's no power in the formula, because power is a scalar quantity, and force is a vector and they are different things. There are formulas to calculate HP from Torque and vice-versa and can be hound here.

  10. #30
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    Quote Originally Posted by MZ5 View Post
    Yes, it is. Your assumptions and entire comparative setup are fallacious. The correct and useful comparison is two engines (or powerplants; don't assume an engine vs. a motor, nor anything else, because it's irrelevant), one with 30% higher power output than the other, and both power 'curves' completely flat. The engine with greater power output will result in greater acceleration at any given vehicle speed. The engine speed is irrelevant since the power curves are totally flat.

    Your argument is fallacious because you've assumed many things, including that more power is made at higher rpm. That is normally correct, and that is why the lower-geared vehicle accelerates more quickly; your assumptions have caused the powerplant to make more power, which allows the tranny to exchange speed for more wheel torque.

    He needs more power.
    I never said a more powerful engine would NOT accelerate faster.
    I also think it's comical that my assumptions were fallacious, yet your "correct and useful" comparison uses two powerplants with completely flat power curves. This is not even theoretically possible without some severe throttling in the higher RPM ranges, which would kill any performance anyway. Your "correct and useful" comparison has zero real world applications, where as my comparison which is clearly "fallacious" has been shown in many cases to produce the results noted. (try a Google search for 'gearing and acceleration' and see what you can find, there's plenty out there).
    Please tell me more about how my comparison of two identical machines with different gear ratios having different performances is wrong, I'd love to hear about it.

    I will certainly concede that I may have misread your initial post. I understood you meant he needed an engine with a larger power output. If your intent was to say that he needed more power delivery to the wheels at a given time, that would be correct and this is achievable in many ways, not just with a more powerful engine.

    Bottom line is that changing performance of the entire system of a motorcycle is not limited to only changing the engine.

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