• Welcome, Guest! We hope you enjoy the excellent technical knowledge, event information and discussions that the BMW MOA forum provides. Some forum content will be hidden from you if you remain logged out. If you want to view all content, please click the 'Log in' button above and enter your BMW MOA username and password.

    If you are not an MOA member, why not take the time to join the club, so you can enjoy posting on the forum, the BMW Owners News magazine, and all of the discounts and benefits the BMW MOA offers?

New sprockets?

rickatsf

New member
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.
 
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.
 
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.
 
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.
 
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?
 
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%
 
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%
 
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...
 
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?
 
....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?
 
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.
 
So just to close the loop on this matter, decided 17-42. Results, higher RPM and very noticeable increase in acceleration, but not at the expense of top end.
 
Back
Top