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Thread: Effects of changing sprockets

  1. #1

    Default Effects of changing sprockets

    NOTE: The following and much of the resulting discussion in this thread relates to how gearing changes affect full throttle acceleration when shifting gears as appropriate such that you are getting the best possible acceleration out of the bike at any given speed (like drag racing). There is some useful discussion later in the thread about how gearing changes will effect "normal" daily-driving type riding here: http://www.600cc.org/forum/f89/effec...tml#post413514

    I originally posted this on a Yamaha FJR1300 forum. Since quite a few people seem to change sprockets on the FZ6R, I thought it would be useful to post this here too. There are a few references to the FJR, but it is mostly generic info. When you see "taller gearing", think "larger front sprocket" and/or "smaller rear sprocket". The opposite for "shorter gearing".

    ****** BEGIN COPY FROM FJR FORUM ******

    This isn't quite relevant to the FJR, since we can't simply change sprockets to adjust the overall gear reductions, but it's an interesting subject anyway...

    What are the effects of changing the overall gearing of a bike (or any vehicle, for that matter)? The common misconception is that taller gearing gives you higher top speed, but less acceleration, while shorter gearing gives you more acceleration, but a lower top speed. Those generalizations have partial truths to them, but people seem to misinterpret the idea to mean "shorter gearing means that I can accelerate faster across the board!"

    Time for some visual aid:


    Exact values are intentionally left off this chart to avoid focusing on unimportant details. Before anyone asks: NO! this is not Gen I gearing vs Gen II gearing. This is just two different arbitrarily different gear ratios for illustrating the trends in taller vs shorter gearing.

    Explanation of the parts of this chart:
    The two jagged mountain lines represent the amount of force applied by the rear wheel at full throttle when in the best possible gear for max acceleration at any given speed. One line represents tall gearing, and the other is short gearing. The gear shifts are pretty easy to spot - they're the creases in the lines.

    The "Max Power" dashed line shows how much force the rear wheel would apply if max power was available at all speeds. This is essentially what a CVT (continuously variable transmission) could do.

    The "Redline Power" dashed line shows how much force the rear wheel would apply if the engine was running at maximum RPMs at all speeds.

    The "Drag Force" line shows how much rearward force is applied by aerodynamic drag at all speeds.

    The "Max Force" dashed line represents the most most rear wheel force that can be practically applied to the road. This could be the point at which the bike starts to wheelie, or the back tire loses traction and starts making smoke. Either way, if you see the rear wheel force line go above this "Max Force", you would need to use less than 100% throttle at that point to keep the wheel force at/below the max.

    Analysis of the chart:
    Acceleration is directly proportional to rear wheel force minus the drag force. Top speed is determined by where the two lines intersect.

    I hereby declare that the "Power Corridor" is the space between the "Max Power" line and the "Redline Power" line.

    Ideally, once you reach peak power in 1st gear, the rear wheel force should never fall outside of this "Power Corridor". On the FJR, 1st and 2nd gear are too far apart from each other, so they don't actually intersect, and the RPMs fall back too far when shifting into 2nd to stay in the "Power Corridor".

    No matter what you do to the overall gearing, the rear wheel force will always follow the "Power Corridor" (except for the beginning of 1st gear, which will always be outside of the corridor). Shorter gearing "squishes" the entire tire force line left and up within the corridor, and taller gearing "stretches" it right and down within the corridor.

    Adding more gears to the transmission would allow you to "tighten up" the rear wheel force line so that it stays closer to the "Max Power" line, rather than bouncing back and forth so much.

    Shorter gearing can improve acceleration in first gear to a certain point. Anything that goes above the "Max Force" line is useless and just makes the bike harder to ride.

    Shorter gearing does NOT improve acceleration everywhere. After 1st gear, it just causes the peaks and valleys of the wheel force line to move around within the power corridor. In some areas you'll have more acceleration with shorter gearing, but in other areas you will have less acceleration.

    Taller gearing will only give you more top speed if the original gearing had peak power in top gear occurring before top speed. Go too tall, and you'll start decreasing your top speed. Best possible top speed is achieved when the bike is geared to have peak power at the speed where "Drag Force" and "Max Power" intersect.

    Gearing changes alone cannot give significant top speed increases, unless the bike was geared incredibly so low that it reaches max RPM before "running into" the drag force line. If a bike is already limited in top speed by drag, then the only way to significantly increase top speed is to reduce drag (improve aerodynamics) and/or increase power. The amount of power required to overcome drag is proportional to the cube of speed. This means that doubling the top speed would require 8 times the power!

    ****** END COPY FROM FJR FORUM ******


    Here's the original thread with some additional discussion: Effects of changing gear ratios - FJRForum
    Last edited by UselessPickles; 08-05-2013 at 11:07 AM.

  2. #2

    Default

    Wow... nothing? I hope I didn't fry all your brains


    BTW - to put the effects on top speed in perspective, the difference between the best possible and worst possible drag-limited top speeds on the FJR is only 3.5mph.

  3. #3

    Default

    This is bound to get some more reactions...

    I simulated a comparison between stock gearing, the common -1/+2 gearing that people use for "more acceleration", and the common +1/+0 gearing that people use for lower cruising RPMs (and accurate speedo readings), but sometimes fear loss of acceleration. Simulated with a 170lb rider, at SAE standard air conditions (77F, 990mbar pressure, 0% humidity), shifting at optimal points for max acceleration, and 0.32s spent shifting (that's how fast my clutchless shifts are). 1/4 mile runs assume 11 inches of "roll-out" (amount of travel at the start line before the timer actually starts). Aerodynamic drag was based on a reported actual (not speedo-indicated) top speed of 125mph from some motorcycle website.

    The "soft launch" is at 3000rpm, which is probably comparable to a casual quick launch (taking off from a light quickly, and going to full throttle right away, but not revving the crap out the engine to get a drag-racing launch).

    The "hard launch" is at 8000rpm. This is the fastest possible launch, and also assumes that you launch perfectly so that you are using full throttle during the whole launch and working the clutch to keep the engine at 8000rpm.

    (more sprocket combinations have been added than what I listed above)

    Fastest results in each grouping are bold.
    Slowest results in each grouping are red.

    *NOTE*
    I don't know at what point the FZ6R will begin to wheelie, so the times for the shorter gearings are possibly unrealistic. The point at which it will begin to wheelie depends much on how high/rearward is the center of mass of the the entire bike+rider system. That means that if you are a heavier rider and/or have a passenger and/or are sitting upright rather than laying on the tank, you are less likely to be able to actually make use of the extra early (1st gear) acceleration of the shorter gearings.

    0-60mph (soft launch)
    -1/+2: 3.94s
    -1/+1: 4.00s
    -1/+0: 4.06s
    Stock: 4.26s
    +1/+0: 4.46s

    0-60mph (hard launch)
    -1/+2: 3.54s
    -1/+1: 3.58s
    -1/+0: 3.62s
    Stock: 3.75s
    +1/+0: 3.89s

    0-100mph (soft launch)
    -1/+2: 11.47s
    -1/+1: 10.94s
    -1/+0: 11.04s
    Stock: 11.39s
    +1/+0: 11.06s

    0-100mph (hard launch - 8000rpm)
    -1/+2: 11.07s
    -1/+1: 10.53s
    -1/+0: 10.60s
    Stock: 10.89s
    +1/+0: 10.49s

    1/4 mile (soft launch)
    -1/+2: 12.56s @ 105.3mph
    -1/+1: 12.57s @ 104.9mph
    -1/+0: 12.59s @ 104.5mph
    Stock: 12.66s @ 105.9mph
    +1/+0: 12.75s @ 104.8mph

    1/4 mile (hard launch)
    -1/+2: 12.23s @ 105.4mph
    -1/+1: 12.24s @ 105.1mph
    -1/+0: 12.24s @ 104.6mph
    Stock: 12.25s @ 106.1mph
    +1/+0: 12.28s @ 105.0mph

    Top Speed
    -1/+2: 122.9mph
    -1/+1: 123.3mph
    -1/+0: 123.6mph
    Stock: 125.0mph
    +1/+0: 126.2mph


    There you go. Lots of numbers to compare different sprocket sizes. This clearly shows that shorter gearing does not always mean more acceleration. The top speeds do actually somewhat follow "conventional wisdom" this time, in that the taller gearing gives higher top speeds, but not for the reasons that people usually think.
    Last edited by UselessPickles; 02-25-2012 at 08:31 AM.

  4. #4
    Junior Member zaclaw's Avatar
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    Is this for the FJR1300? What year?

  5. #5
    Senior Member djf18374's Avatar
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    Quote Originally Posted by zaclaw View Post
    Is this for the FJR1300? What year?
    This appears to be for the FZ6R, since there are sprocket changes mentioned, and the numbers are exactly those for the 6R. 126 MPH corrected seems about right for the +1 +0 gearing.
    2009 Cadmium Yellow FZ6R / T-Rex Frame Sliders / Pro Grip CF Tank Pad / GYTR Clear DB Windscreen / Mod-stock Exhaust / Driven 17T Sprocket / SW Motech Center Stand / ZX6-R Mirrors / Motodynamics Flushies / Motodynamics Integrated Tail / GYTR Yellow Seat Cowl

  6. #6
    FZ6R_Blue
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    These numbers cant be right. i have had mine up to about 145 on a flat stretch of road

    Top Speed
    -1/+2: 122.9mph
    Stock: 125.0mph
    +1/+0: 126.2mph

  7. #7

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    Quote Originally Posted by zaclaw View Post
    Is this for the FJR1300? What year?
    The first post is just generic info. The sprocket size comparison is for the FZ6R. The FJR has a shaft drive, the overall gear ratio cannot be changed.

    Quote Originally Posted by FZ6R_Blue View Post
    These numbers cant be right. i have had mine up to about 145 on a flat stretch of road
    Indicated speedo speed, or actual GPS speed? Any wind? Do you know the road was level? A slight downhill slope and a tailwind can add a lot of top speed. If you have -1/+2 sprockets, then your speedo will be quite optimistic. Like I said in the post, the aerodynamic drag I used is based on a reported top speed of 125mph from a motorcycle website (can't remember which one). Without knowing you at all, I'd have to put my faith in the motorcycle website for having a better testing procedure to minimize error (probably multiple runs in opposite directions, average results, using GPS or radar gun).

    Also, the exact numbers don't matter anyway. Just look at the relative comparison between the sprocket sizes.
    Last edited by UselessPickles; 12-20-2011 at 07:59 AM.

  8. #8

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    Quote Originally Posted by FZ6R_Blue View Post
    These numbers cant be right. i have had mine up to about 145 on a flat stretch of road
    So the most likely way you got to 145mph was by using -1/+2 gearing and looking at your speedometer.

    Redline is at 11500rpm. You can't go faster than that. With stock gearing, that would be 138mph (but you can't get there due to drag). With -1/+2 gearing, you would be at 124mph (actual speed). An actual speed of 145mph is just impossible.

    A stock bike has already been shown to have about 6% speedo error (+1/+0 sprockets are 6.25% taller than stock and seem to make the speedo accurate).

    Stock gearing is 11.3% taller than -1/+2.

    That gives a grand total of 18.3% speedo error with -1/+2 gearing, so your 145mph speedo reading would be an actual speed of 145mph / 1.183 = 122.6mph, which is very close to my simulated top speed of 122.9mph.

  9. #9
    Senior Member djf18374's Avatar
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    Quote Originally Posted by UselessPickles View Post
    So the most likely way you got to 145mph was by using -1/+2 gearing and looking at your speedometer.

    Redline is at 11500rpm. You can't go faster than that. With stock gearing, that would be 138mph (but you can't get there due to drag). With -1/+2 gearing, you would be at 124mph (actual speed). An actual speed of 145mph is just impossible.

    A stock bike has already been shown to have about 6% speedo error (+1/+0 sprockets are 6.25% taller than stock and seem to make the speedo accurate).

    Stock gearing is 11.3% taller than -1/+2.

    That gives a grand total of 18.3% speedo error with -1/+2 gearing, so your 145mph speedo reading would be an actual speed of 145mph / 1.183 = 122.6mph, which is very close to my simulated top speed of 122.9mph.
    Thank you. I was going to go through the math and explain this exact thing to Mr. Blue...but you did all the work for me I find a lot of people trusting their speedo to be accurate from the factory. Even worse, they think the speedometer should still be accurate after changing the sprockets to a different ratio
    It's math, plain and simple math, and the physics of drag have not changed. our HP of the FZ6R just can't overcome the drag force at those speeds. ~125 MPH actual speed is all she wrote, on flat road.
    Downhill may be a different story...
    2009 Cadmium Yellow FZ6R / T-Rex Frame Sliders / Pro Grip CF Tank Pad / GYTR Clear DB Windscreen / Mod-stock Exhaust / Driven 17T Sprocket / SW Motech Center Stand / ZX6-R Mirrors / Motodynamics Flushies / Motodynamics Integrated Tail / GYTR Yellow Seat Cowl

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    Super Moderator dart1963's Avatar
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    Quote Originally Posted by UselessPickles View Post
    Wow... nothing? I hope I didn't fry all your brains


    BTW - to put the effects on top speed in perspective, the difference between the best possible and worst possible drag-limited top speeds on the FJR is only 3.5mph.
    I've thought about that before, just never put it, or seen it, in numbers... figured you could only effect it slightly, as shorter gears meant more shifts to reach a given speed, and longer gears would have a big dip in the powerband between gears... think the best reason for a swap is the rev/cruising speed, if you're not comfortable cruising at 7k rpm...
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