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Thread: New sprockets?

  1. #1
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    New sprockets?

    Hi guys,

    I'm looking to change my worn out 09 F650GS sprockets. Should I stick with stock size or go for something different? Looking for better overall performance (mileage). My options are below.

    FRONT SPROCKET SIZE
    15T
    16T
    17T - Stock Size
    18T

    REAR SPROCKET SIZE
    41T - Stock Size
    42T
    43T
    47T

    Any suggestions would be appreciated. Thx.

  2. #2
    MOA #24991 Pauls1150's Avatar
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    With any chain- or belt-driven vehicle, a larger front sprocket will make the overall gearing "taller". This will increase the top speed and reduce engine RPM; but you lose low-end acceleration (necessary in the city, on hills, carrying a passenger and/or luggage, and when passing without downshifting) and you may increase clutch wear. The chain (or belt) and sprocket will also live longer with a larger front sprocket, since the "pulling" torque is now spread over a larger area and the change of angular velocity there is reduced.
    A smaller rear sprocket has the same effect on gearing.

    A smaller front, or larger rear, sprocket increases acceleration at the expense of top end.

    If better gas mileage is the only concern, one tooth larger up front would be the way to go.

    Remember that the chain (or belt) tension must also be readjusted for any change of sprocket.

  3. #3
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    Personally I wouldn't go any taller on an F650 than it already is because I'd be concerned about the clutch. I tried a 16T front which lowered the gearing almost as low as a F800GS. Overall I felt it was better everywhere except high speed. My choice overall would be the stock 17T front with a 42 rear (one tooth larger) which is what BMW uses on the new F700GS.
    14 R1200GSA, 93 R100R. No car is as fun to drive as any motorcycle is to ride.

  4. #4
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    Quote Originally Posted by rickatsf View Post
    Hi guys,

    I'm looking to change my worn out 09 F650GS sprockets. Should I stick with stock size or go for something different? Looking for better overall performance (mileage). My options are below.

    FRONT SPROCKET SIZE
    15T
    16T
    17T - Stock Size
    18T

    REAR SPROCKET SIZE
    41T - Stock Size
    42T
    43T
    47T

    Any suggestions would be appreciated. Thx.
    Stay with stock.

  5. #5
    BMW Pilot runnr548's Avatar
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    Cool

    Stock is ok, but I too had been considering one tooth taller in the rear.

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    Quote Originally Posted by runnr548 View Post
    Stock is ok, but I too had been considering one tooth taller in the rear.

    The nice thing is the stock chain has the adjustment range to change one tooth either way. Try it I think you'll like it.
    14 R1200GSA, 93 R100R. No car is as fun to drive as any motorcycle is to ride.

  7. #7
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    Quote Originally Posted by milo View Post
    Personally I wouldn't go any taller on an F650 than it already is because I'd be concerned about the clutch. I tried a 16T front which lowered the gearing almost as low as a F800GS. Overall I felt it was better everywhere except high speed. My choice overall would be the stock 17T front with a 42 rear (one tooth larger) which is what BMW uses on the new F700GS.
    42 in the rear? What added performance would that get you? Also anyone know what would result in going up one tooth in both front and rear - i.e. 18/42? Or is it a wash?

  8. #8
    Small road corner junkie pffog's Avatar
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    Quote Originally Posted by rickatsf View Post
    42 in the rear? What added performance would that get you? Also anyone know what would result in going up one tooth in both front and rear - i.e. 18/42? Or is it a wash?
    Here is a quick spreadsheet that shows the difference of any combination.. So basically, take the % and add that to the rpm at any given speed/gear combonation.

    Hypothetical example at 60mph in 6th, stock turns 4000 rpm, switch to the 18/42 as asked above, and at 60 your RPM would be 3% lower or 3880 RPM, conversely toss the 15/47 on and at 60, RPM would be 30% higher, or 5200. The 18/43 is the closest to stock, and in theory could give slightly better chain life as the chain does not have to rotate as sharply to wrap around the sprocket, saving wear

    front rear ratio % change
    15 41 2.733 13%
    15 42 2.800 16%
    15 43 2.867 19%
    15 47 3.133 30%

    16 41 2.563 6%
    16 42 2.625 9%
    16 43 2.688 11%
    16 47 2.938 22%

    17 41 2.412 Stock
    17 42 2.471 2%
    17 43 2.529 5%
    17 47 2.765 15%

    18 41 2.278 -6%
    18 42 2.333 -3%
    18 43 2.389 -1%
    18 47 2.611 8%
    2010 F800GS Full Ohlins package, '04 R1100S Replika
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  9. #9
    Small road corner junkie pffog's Avatar
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    As long as I had excel open, figured I would print the F800GS ratios too

    f800GS
    front rear ratio % change
    15 41 2.733 4%
    15 42 2.800 7%
    15 43 2.867 9%
    15 47 3.133 19%

    16 41 2.563 -2%
    16 42 2.625 Stock
    16 43 2.688 2%
    16 47 2.938 12%

    17 41 2.412 -8%
    17 42 2.471 -6%
    17 43 2.529 -4%
    17 47 2.765 5%

    18 41 2.278 -13%
    18 42 2.333 -11%
    18 43 2.389 -9%
    18 47 2.611 -1%
    2010 F800GS Full Ohlins package, '04 R1100S Replika
    '01 F650GS Wife's bike
    Maritime Alps and Vosges 2012
    Tuscany and Central Italy 2010

  10. #10
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    Thanks for posting that pffog!

    I have an early production F650GS twin, and the stock 17/41 is very nice for highway driving,
    but not so clever for rugged hilly terrain...

    I have almost 30K miles on the original sprockets/chain and they still are in good condition, but I do lube frequently. I think I will go with the 17/42 when I do swap out ... I would not mind a bit more low end torque...

  11. #11
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    Quote Originally Posted by pffog View Post
    Here is a quick spreadsheet that shows the difference of any combination.. So basically, take the % and add that to the rpm at any given speed/gear combonation.

    Hypothetical example at 60mph in 6th, stock turns 4000 rpm, switch to the 18/42 as asked above, and at 60 your RPM would be 3% lower or 3880 RPM, conversely toss the 15/47 on and at 60, RPM would be 30% higher, or 5200. The 18/43 is the closest to stock, and in theory could give slightly better chain life as the chain does not have to rotate as sharply to wrap around the sprocket,....

    This is very helpful info, thank you. So if I understand, the 18/43 will give me just a tab better mileage and chain life, but what about torque?

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    Quote Originally Posted by rickatsf View Post
    ....So if I understand, the 18/43 will give me just a tab better mileage and chain life, but what about torque?
    That combination is simply too tall IMO. Will an 18T even fit? You'll generate way less torque at the wheel and I would be surprised if mpg improves at all. Probably forget about even using 6th gear unless you're over 75 mph. It'd probably have to be in 5th, maybe even 4th to reach the bike's top speed? I couldn't imagine trying to start out on a steep hill with a passenger after adding 9% to (an already too-tall) first gear?
    14 R1200GSA, 93 R100R. No car is as fun to drive as any motorcycle is to ride.

  13. #13
    Small road corner junkie pffog's Avatar
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    Quote Originally Posted by milo View Post
    That combination is simply too tall IMO. Will an 18T even fit? You'll generate way less torque at the wheel and I would be surprised if mpg improves at all. Probably forget about even using 6th gear unless you're over 75 mph. It'd probably have to be in 5th, maybe even 4th to reach the bike's top speed? I couldn't imagine trying to start out on a steep hill with a passenger after adding 9% to (an already too-tall) first gear?
    On a 650 the 18/43 will only lower the rpms 1%, and is the closest to stock of ANY other combination. I don't know many people that could tell a 1% difference. Not sure about fit, but assume if it is offered it will fit.

    Just gearing up does not necessarily lower fuel mileage, there are a LOT of things that effect efficiency, and sometimes lowering the overall ratio can bring better mileage. In 2010 I was riding a F800GS 2 up, and I had one son on a F650GS, and the other on a F800R, both solo, riding around the Alps. There were times I would use less fuel than them, generally mileage was as close to identical as you can get, and I ride as fast 2 up as they do solo.
    2010 F800GS Full Ohlins package, '04 R1100S Replika
    '01 F650GS Wife's bike
    Maritime Alps and Vosges 2012
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  14. #14
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    Quote Originally Posted by pffog View Post
    On a 650 the 18/43 will only lower the rpms 1%, ...
    Thanks for the correction. I mistakenly referenced the F800 chart.
    14 R1200GSA, 93 R100R. No car is as fun to drive as any motorcycle is to ride.

  15. #15
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    Quote Originally Posted by milo View Post
    Thanks for the correction. I mistakenly referenced the F800 chart.
    I'm back to square one!

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