Bertie
Registered user
The F650GS is thought to be slightly overgeared. If the rear sprocket is increased by one or two teeth, what impact will that have on the speedo/odometer? Presumably, it would under-read?
Gearing question
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None, the speedo reading isn't dependent upon gearing.
The 650 has a higher geared 17:41 (front:rear) sprocket combination whereas
the 800 has a lower geared 16:42 sprocket combo.
For a given speed you can calculate the impact on revs with the formula
oldrevs * newrear / oldrear / newfront * oldfront = newrevs
Moving to a 43-tooth sprocket on the rear gives
4000 * 43 / 41 = 4195
As it has fewer teeth you get more of an impact changing the front sprocket. If you switched to F800GS sprockets the result would be
4000 * 42 / 41 / 16 * 17 = 4353
It's such a pity BMW used the close ratio gearbox out of the F800 sports bikes; you'd have thought with six gears available they would have managed to create the best of both worlds with a low first gear AND the 'overdrive' top gear.
Tim
The 650 has a higher geared 17:41 (front:rear) sprocket combination whereas
the 800 has a lower geared 16:42 sprocket combo.
ignore this load of bollocks said:
For a given speed you can calculate the impact on revs with the formula
oldrevs * newrear / oldrear / newfront * oldfront = newrevs
Moving to a 43-tooth sprocket on the rear gives
4000 * 43 / 41 = 4195
As it has fewer teeth you get more of an impact changing the front sprocket. If you switched to F800GS sprockets the result would be
4000 * 42 / 41 / 16 * 17 = 4353
It's such a pity BMW used the close ratio gearbox out of the F800 sports bikes; you'd have thought with six gears available they would have managed to create the best of both worlds with a low first gear AND the 'overdrive' top gear.
Tim
Bertie
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Thanks Tim - another for you!
The standard F650GS (twin) has a geared top speed of 140 mph (unattainable with standard engine output).
What sprocket combination can I use to lower the top speed by 15% and thus increase the torque by 15% ?
The standard F650GS (twin) has a geared top speed of 140 mph (unattainable with standard engine output).
What sprocket combination can I use to lower the top speed by 15% and thus increase the torque by 15% ?
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Torque is a measure of engine output so is unaffected by gearing.
If you want to gear the bike so that it's at maximum revs at approx 119 mph (15% less) then you would need to use a 15 front, 43 rear combo (if that's possible).
140 / 43 * 41 * 15 / 17 = 117
Someone will probably be along shortly to point out any possible disadvantages.
Tim
If you want to gear the bike so that it's at maximum revs at approx 119 mph (15% less) then you would need to use a 15 front, 43 rear combo (if that's possible).
140 / 43 * 41 * 15 / 17 = 117
Someone will probably be along shortly to point out any possible disadvantages.
Tim
Bertie
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Thank you again Tim. I recall that 15 teeth is the minimum number that should be used on a gearbox sprocket due to the loading on the smaller no of teeth. And a longer chain........ Of course, could go for the 15% more power option (fit Remus headers, K & N filter, Akropovic etc) and leave the gearing alone!
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It sounds like the F650GS isn't the bike for you.
Bertie
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Very perceptive! I have a 1200GS, but my brother have recently bought an F650GS but isn't on the forum yet. He's just considering altering the gearing because of what the journos have said, but I reckon he's better off just keeping it as is and stirring the excellent gearbox for more go when he needs it. He is getting a ridiculous amount of miles to the gallon.....
Cole
Registered user
Yes i recall they skimped on the 1200 in the same way and that remains (along with the fuel pump controller thingy) my biggest gripe about the bigger bike.
At least you can change the general gearing (final drive ratio) on the chain drive bikes
I see what you're saying there Tim.
However, surely if you went to a 43 tooth from a 41 at the rear you'd increase revs for a given speed.
Therefore for 4000Rpm with the 41 you'd be looking at the following:
4000x(43/41) = 4195 (Or thereabouts)
Apologies if I've read the thread incorrectly but I've now read it twice just to make sure.
However, surely if you went to a 43 tooth from a 41 at the rear you'd increase revs for a given speed.
Therefore for 4000Rpm with the 41 you'd be looking at the following:
4000x(43/41) = 4195 (Or thereabouts)
Apologies if I've read the thread incorrectly but I've now read it twice just to make sure.
HR0
Registered user
F800GS xl gearing spread sheet
i don't really know if this is of any use
the data should be specific for the F800GS as its ot of my hand book
and with luck you can play around with the parametres eg. front rear sprocket tooth numbers regardless of whether they are available off the shelf ????
hope the attach works for you
its not mine and if i could remember where its from i'd reference them !!!
H
i don't really know if this is of any use
the data should be specific for the F800GS as its ot of my hand book
and with luck you can play around with the parametres eg. front rear sprocket tooth numbers regardless of whether they are available off the shelf ????
hope the attach works for you
its not mine and if i could remember where its from i'd reference them !!!
H
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Oops. Where's the Dunce's Cap when you need it. I've edited the post to reflect the correct figures. Suprised nobody else picked it up, or were they all being nice to me today.
Tim
Tim
Neiljohnuk
Registered user
Bumping this slightly, my 650 twin now runs a 15/47 pairing, the 47 rear is a twin ringer so doesn't flex and wear oddly, unlike the original single ring 42, it's an aftermarket version of the F800R one IIRC.
A second benefit beyond better low speed control is that an Abus Granit shackle lock can pass through both the sprocket and the brake disc improving security!
Dean
Registered user
Personally I have found the f650 GS twin gearing just fine, so I have always replaced standard sprockets. Maybe I'm just used to it
I have wondered about Touratech's quoted facts for the 15/47 - as follows:
"Chain pinion for BMW F650GS (Twin)/F700GS/ F800GS 15 teeth
chain pinion steel
Compared to the original 16 teeth front and 42 teeth rear (F650(Twin)GS: 17/41), combined 15T front plus 47T rear results in approx. 10% reduction in speed and 10% increase in torque"
10% is a fair bit, do you notice the 10% increase and decrease ?
ahutcheon
Registered user
From a quick hunt on the web, the step between 5th and 6th is about 8.5% and between 4th and 5th about 12.5%. So you're looking at the equivalent, roughly, of dropping a gear. Steps between the lower gears are bigger: if you're trying to make 1st a bit lower it might be worthwhile, but if you're happy with the lower gears then unless 6th is so tall as to be useless it might be easier to ride around in 5th some of the time.
Disclaimer: I've never ridden a 650GS twin, but I have agonised about choice of bicycle gearing enough to have a reasonable grasp on the maths.
shaunj
Registered user
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