920 gearing...

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Fast Eddie

Fast Eddie

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I’m running a standard primary ratio, and a 21 tooth gearbox sprocket on my bike currently (Maney stage 3 head, Maney cam, 920cc).

To me, it feels very under-geared, meaning, I’m in third before I can blink and when using in top it so obviously feels like it wants another gear (cos it had the torque to pull when acceleration is required).

I’ve got a 22 and a 23 tooth in stock... can’t decide which to try...

So, I’d like to ask other 920 riders, what gearing are you using...?
 
When I had my MK3 street bike with a built 920 engine, I ran a 22T countershaft with the stock 42T rear and an 18" tire about the same diameter as the original 19. Worked well for me and had no problems, with plenty of torque for starting from a stop. If I were riding it today at freeway speeds, keeping up with the 80 mph + speeds common around here, I'd go with the 23T, but for the kind of riding I normally do, which mostly avoids freeways, I'd stick with the 22T.
 
Brooking 850 said:
Hi Nigel, how about a six speed TTI gerabox, then you have a gear for every event!!!
Click to expand...

Ha. Yeah right!

Maybe Bruce will,just sell me the 6th gear cogs to drop into my 5 speed ?

Interestingly, the 6 speed box lists 5th as 1:1 and 6th as direct...

Which is my cue for another dumb question: what is the difference between 1:1 and direct? I thought they were different ways of saying the same thing ?!
 
lcrken said:
When I had my MK3 street bike with a built 920 engine, I ran a 22T countershaft with the stock 42T rear and an 18" tire about the same diameter as the original 19. Worked well for me and had no problems, with plenty of torque for starting from a stop. If I were riding it today at freeway speeds, keeping up with the 80 mph + speeds common around here, I'd go with the 23T, but for the kind of riding I normally do, which mostly avoids freeways, I'd stick with the 22T.
Click to expand...

Thanks Ken, yeah it’s the 22 and 23 I’m trying to weigh up. I’m currently thinking 23...
 
My bad (that’s what happens when you post too early BEFORE coffee)...

5th gear in the 6 speed box is 1.1 NOT 1:1 ...

920 gearing...
 
I am running a 22, but I cannot remember the primary reduction for the Norvil belt drive on the bike. Plus I have a 19 rear.
I would say I get 70mph at just under 4000rpm, probably about 3850/3900 and it would pull a 23 no problem. I was 2 up the other day and even that didn't seem to make a difference to how it pulls.

It's the age old problem though, the bigger the gearbox sprocket, the higher the loading on the gears that are less able to cope, ie any of them under top. I cannot remember if you are running a BruceBox, if you are It shouldn't be a problem.

I was in my box yesterday because the kickstart pawl had been eaten, 1st and second gear sets are starting to wear. The standard box just isn't up to the job with a 920 if you like to stretch the throttle cable. Come to think of it, it was never up to the job as an 850. :)
 
You live in the flat zone FE so more likely you can get away with higher gearing. But if you have built a scratcher then it is all about
power where you want to use it. Top end never will be used and on the motorway how far do you intend to go? Not very far
probably.
 
Adding another gear will do nothing on top, 1:1 is as low as they all go.

Go to a 22 if you must.
 
Im at 3750 @ 70mph with my relatively stock 850. I don't beat it much and it is doable. But it isnt enough engine really. And dont forget Im 9 stone full kit.
 
All good points guys, thanks.

Ralph, yes I’m running a Bruce box.

Maybe a 22 is the best compromise. I’ll ponder a while longer (good excuse to delay the job of stripping the entire primary side, ics cNw starter, etc).

Onder... 9st with kit ?! Doubt I was that light when I was 12 ! At that weight I’m afraid your opinions just don’t count. Go and find some skinny people to talk to... ;)
 
Yeah you're right. I'll just mosey on over to the Ducati stand and chat up those models they favour with the painted on leathers....my fellow Skinny Folk.
 
What revs are you at at 70 Nigel?

I find that I gear for the smooth spot. Round here our road allow what we used to call bend swinging, as soon as you got on the throttle coming out of a corner it wouldn't be long before you would be back on the brakes for the next one, there is a rhythm to it all and a natural speed where it all become a lot of fun, that is what I gear for. I am lucky because I find that fairly high gearing suits just fine.
 
Hi Nigel, I ran a 23t gearbox sprocket on my 920 for at least 3 years and was quite happy with it, usually 2 up with my 9 stone wife and found it was very nice to ride. Maybe a bit of clutch slip required setting off on a steep hill but apart from that no problems, no layshaft issues (although I fit the FAG 6203 P63 as recommended by Mick Hemmings) but with you having the TTI I assume that this isn't going to be a problem. Also I've had no gear wear issues. I'd go 23t but it's up to you of course.
I have gone back to a 22t since my engine has morphed back into an 828cc low compression engine.
 
Ralph, I was chasing my tail for a while thinking it was very under geared. Then I discovered the speedo is reading a good 10% SLOW.

According to my sat nav, it’s more or less 70mph at 4000rpm, so it’s not ‘wrong’ by any means. It just feels like it would do that at less revs without any detriment. I think it might also be something that’s exaggerated in my mind due to the noise emitted by the 2:1 pipe !

Well, that and this bloody lockdown giving me too much time for unnecessary thoughts about unnecessary things ...!
 
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