Primary gears 1200 Sport vs. Griso

[mdagbro]

Hi

Been studying the specifications for my 1200 Sport and the Griso. They have different primary gear ratio but it's compensated in 5th and 6th gear on the Sport, so the total engine/rear wheel ratio is almost identical. The Griso has 26/35 ratio on the primary, while Sport has 24/35.
My idea is to swap primary gear(s) on my Sport to reduce engine revs. Tables below show the gear ratio for both bikes and the last one is a theoretical calculation my Sport with Griso primary ratio. My question is, since the 35 teeth wheel is identical on both bikes, how can the clutch shaft wheel have 24 and 26 teeths, respectively? Can I really achieve this with the Griso (26) clutch shaft, without any other modification?

Griso 8V
Prim   1,346:1 (26/35)
I   2,235:1
II   1,700:1
III   1,348:1
IV   1,115:1
V   0,968:1
VI   0,862:1
Final   3,667:1
Total@6th gear 4,255:1

1200 Sport 4V (per cyl, why ..? )
Prim   1,458:1 (24/35)
I   2,235:1
II   1,700:1
III   1,348:1
IV   1,115:1
V   0,935:1
VI   0,8:1
Final   3,667:1
Total@6th gear 4,278:1

Sport w. Griso primary
Prim   1,346:1 (26/35)
I   2,235:1
II   1,700:1
III   1,348:1
IV   1,115:1
V   0,935:1
VI   0,8:1
Final   3,667:1
Total@6th gear 3,949:1

Exploded views:
Griso
Clutch shaft Z=26 (pos 1):
https://us.fowlersparts.co.uk/parts/5719249/griso-griso-griso-s-e-1200-8v-chassis-number-prefix-zgulsg01-2005-2013-griso-s-e-1200-8v-chassis-number-prefix-zgulsg01-all/clutch-shaft

Drive gear Z=35 (pos 2):
https://us.fowlersparts.co.uk/parts/5719285/griso-griso-griso-s-e-1200-8v-chassis-number-prefix-zgulsg01-2005-2013-griso-s-e-1200-8v-chassis-number-prefix-zgulsg01-all/primary-gear-shaft

1200 Sport
Clutch shaft Z=24 (pos 1):
https://us.fowlersparts.co.uk/parts/5716816/1200-sport-1200-sport-1200-sport-8v-chassis-number-prefix-zgulpj20-zgulpr00-zgulpr01-zgulpj01-2006-2013-1200-sport-8v-chassis-nu/clutch-shaft

Drive gear Z=35 (pos 2):
https://us.fowlersparts.co.uk/parts/5716854/1200-sport-1200-sport-1200-sport-8v-chassis-number-prefix-zgulpj20-zgulpr00-zgulpr01-zgulpj01-2006-2013-1200-sport-8v-chassis-nu/primary-gear-shaft



Mikael

[derekwiltshire]

Looks to me like you might lose some acceleration in the sport by making that change, though yes it would lower engine revs for any given speed


Sent from my iPhone using Tapatalk

[beetle]

Why not just buy a Griso gearbox?

[mdagbro]

Why not just buy a Griso gearbox?

The Griso has almost identical TOTAL engine/wheel ratio. Using Griso primary gears in Sport gearbox would improve the situation, lowering engine revs ... but naturally it's a trade off when it comes to acceleration.
But I spend more time running at constant speed than accelerating ... and never ever used the bike's acceleration to the limit, why I think it would suit me.

[pete roper]

Sports certainly seem to be geared higher than Grisos. I thought the Breva/Sport primaries were higher but I'll happily defer to your research, it's been a long time since I worked it out.

Pete

PS. If you want a Griso gearbox I have a few.

[mdagbro]

The Griso has almost identical TOTAL engine/wheel ratio. Using Griso primary gears in Sport gearbox would improve the situation, lowering engine revs ... but naturally it's a trade off when it comes to acceleration.
But I spend more time running at constant speed than accelerating ... and never ever used the bike's acceleration to the limit, why I think it would suit me.

BTW, driving in gears 1-4 it would be like an ordinary Griso.
Only 5 & 6 would be "overdrive".

[mdagbro]

My concern is the fact that a 26 teeth wheel would mean a larger diameter than 24. Guess they must have solved it with less engagement of the 24 wheel teeths ..?

[Idontwantapickle]

My concern is the fact that a 26 teeth wheel would mean a larger diameter than 24. Guess they must have solved it with less engagement of the 24 wheel teeths ..?

I got to wondering about that. It looks like the 35 tooth gears in both transmissions are the same. They have the same part number 05215631.
I looked at the housing numbers as well thinking that maybe they subtly changed the position of the primary shaft. Although the main case number is different the clutch housing is the same so that's not it.
That must mean your supposition is right, the fewer teeth of the clutch shaft gear have been designed to engage the same primary shaft gear.
That's some frugal engineering right there!
I'm still a doubter though. I would have to check that fit carefully before trusting it in service. A careful comparison of the clutch shafts would probably reveal the answer.


http://www.harpermoto.com/parts-by-motorcycle/2000-up-moto-guzzi-motorcycles/1200-sport-8v-1200-2008/clutch-shaft-1200-sport-8v-2008.html

http://www.harpermoto.com/parts-by-motorcycle/2000-up-moto-guzzi-motorcycles/1200-sport-8v-1200-2008/primary-gear-shaft-en-1200-sport-8v-2008.html

http://www.harpermoto.com/parts-by-motorcycle/2000-up-moto-guzzi-motorcycles/griso-v-8v-1200-2007-2009/clutch-shaft-en-griso-8v-1200.html

http://www.harpermoto.com/parts-by-motorcycle/2000-up-moto-guzzi-motorcycles/griso-v-8v-1200-2007-2009/primary-gear-shaft-en-griso-8v-1200.html

Hunter