650 Dakar Gearing..

[Scott Nettleton]

I regualarly cruise on the motor way at around 6000 RPM. On the clock it shows 90 MPH (probably around 85). It seems fairly happy but after long periods (2 hours) I figured it was probably excessive. I had an idea to bring this down to 5000. I would plan to do this by putting larger front sprocket on , any idea how many teeth I would need to go up by and any idea if this should be of concern ?

Thanks in advance for any help..
Scott

 

[Mouse]

You might find there's no room for a larger front sprocket. Also bear in mind if you fit a larger sprocket you'll need to fit a longer chain.

You could also gear it up by fitting a smaller rear sprocket, if one is available.

You can work out what gearing you'd need for what you want ... to bring the revs down from 6k to 5k at a certain speed you'd need to increase the gearing ratio by about 17%. So 17% more teeth on the front spocket or 17% fewer teeth on the rear. Say the existing gearing is 15 front 40 rear (it probably isn't I'm just using example figures). This gives a ratio of 15/40 = 0.375.

Add 17% to that gives approx 0.44. So your new front / rear has to equal 0.44. 17 front 40 rear would give 0.425, which is close. 15 front and 34 rear would give 0.44.

You'll be limited by what sprockets are actually available.

And bear in mind that gearing the bike up like this might make slow control in traffic etc harder, as you'll have to slip the clutch more. But I suppose you can only try it and see

 

[Der Lustenauer]

This might help.

The first link is an Excel sheet, the second a program which can be used for different bikes, the third the data for the F650 GS.

Excel:
http://www.anpeki.de/F650/F650Gang.xls


Geardata:
http://www.schmidt-sven.de/texte/geardata.htm


Data for F 650 GS (open with Geardata):
http://www.abenteuer-land.de/rock/htm/geardata_datenbank.htm

Wunderlich has 15, 17 and 45 tooth sprockets.