920 gearing...

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You just lost yer nerve when you thought about stripping the primary. :)
I think one tooth will put you around 3700 where I am. I doubt you would notice power loss with the big engine and a few less revs is always nice for general riding. Sprocket is around 100 quid for the TTI. It doesn't work out no great loss. Try it and see.
 
Onder said:
You just lost yer nerve when you thought about stripping the primary. :)
I think one tooth will put you around 3700 where I am. I doubt you would notice power loss with the big engine and a few less revs is always nice for general riding. Sprocket is around 100 quid for the TTI. It doesn't work out no great loss. Try it and see. Hey you have the five speed, use it.
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I may be wrong, but it is my recollection that the spread of gears in the TTI box is the same as the standard Norton box. That is, you cannot space the gears out from 1st through 5th. So, apart from racing, what's the advantage of a five speed box? Before my change of cam to a Webcam 312b, I had a 23 tooth sprocket, but at highway speeds, (70 mph at about 3500 revs) the bike was a bit breathless at around this speed, ie acceleration was sluggish. At rebuild time, I went to a 22 tooth sprocket. With the cam, it was much stronger, and I think it would pull a 23 tooth much better now. However, I am now happy with the 22 which suits me as the best all round gearing for my city and wide open space riding.
 
Fullauto said:
I may be wrong, but it is my recollection that the spread of gears in the TTI box is the same as the standard Norton box. That is, you cannot space the gears out from 1st through 5th. So, apart from racing, what's the advantage of a five speed box? Before my change of cam to a Webcam 312b, I had a 23 tooth sprocket, but at highway speeds, (70 mph at about 3500 revs) the bike was a bit breathless at around this speed, ie acceleration was sluggish. At rebuild time, I went to a 22 tooth sprocket. With the cam, it was much stronger, and I think it would pull a 23 tooth much better now. However, I am now happy with the 22 which suits me as the best all round gearing for my city and wide open space riding.
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There are various options with the TTI, I think post 8 shows a few. I don't thing there is a disadvantage with a 5, or 6 speed box apart from not making much use of the lower gears. I am happy with the close 4 speed in my 920 road bike, doesn't need anymore, but I wouldn't say no to a cheap TTI EXHD 5 for it :)
 
Fast Eddie said:
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,
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If your rev counter is reading right :D
 
Fullauto said:
I may be wrong, but it is my recollection that the spread of gears in the TTI box is the same as the standard Norton box. That is, you cannot space the gears out from 1st through 5th. So, apart from racing, what's the advantage of a five speed box? Before my change of cam to a Webcam 312b, I had a 23 tooth sprocket, but at highway speeds, (70 mph at about 3500 revs) the bike was a bit breathless at around this speed, ie acceleration was sluggish. At rebuild time, I went to a 22 tooth sprocket. With the cam, it was much stronger, and I think it would pull a 23 tooth much better now. However, I am now happy with the 22 which suits me as the best all round gearing for my city and wide open space riding.
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I‘m inclined to agree that on the road, the extra gear probably does not offer any real advantage. But personally, I do prefer the ratio spacing.

There are two main advantages to a TTI box over the stock one. First is the smooth clean shifting with less lever travel. Second, and most important, is the strength. This isn‘t a necessity to many, but it is a huge advantage with big tuned motor and / or a bike used on the track.
 
Hi Eddie
I would go 23. The engine will pull it. I think it's more a matter of sweet spot & where it really takes off to overtake. Can't remember what the revs were but It only needed a few hundred revs to hit 70 from cruising along.
 
Essentially, if you have a 20/40-ish ratio between front and back sprockets, and your gearbox and primary ratios don't change, then all you are doing by adding or removing each tooth is changing the RPM range by about 5% per tooth. Meaning that if you are in any gear at 5000 RPM going "X" speed, then if you add a tooth to the final drive sprocket you'll be doing about 4750 RPM at that same "X" speed.

So in the big picture you have to decide if your bike would still deliver good power if you lowered the RPM range by 5%? (one tooth added) How about 10% lower RPM?? (2 teeth added) As many have said, the bigger/biggest final drive sprockets are most advantageous for cruising at high speed on the highway. If you have mixed highway/twisties use, I wouldn't go for the 23 sprocket.

I run a 21 on my strong running 750. It's just right for my intended use. I can hop in the highway and not be too high in the RPM range to go a few exits, but I'm by no means geared to run the highways regularly with that 21 sprocket. In the case of my engine, I wouldn't go with any larger of a final drive sprocket because I don't want to "gut" the bike's low range power. And, while I realize that running around with a parallel twin at high RPM's is not a good thing, I don't think lugging the engine in the lower RPM range is good either...

I vote 22... but I don't have any experience with the power of your 920cc machine, so maybe you could go 23 with no issues and it's more a case of what you like and what roads you ride.

You can calculate your numbers here... https://www.gearingcommander.com/
 
Even on my 750 I run a 4 to one overall ratio. That gives me 1/2 the RPMs as MPH. So when I'm cruising at 70 MPH the motor is turning 3500 RPM etc. This way I don't have to run a speedometer. My MPH is always double the RPM and easy to calculate. I live in the country and like the comfortable easy RPM to and from the city. My bike is only 340 lbs. A 920 could certainly pull that or more.
 
Nigel, is the engine smooth at 2500-3000 revs?
I don't intend to ride at that rpm but sometimes ride for hours on twisty, hilly 50 mph speed limit country roads that are police patrolled. At 55-60 mph on these roads one can have lots of fun and probably sneak thru police radar unscathed.
My bike with 21 tooth fitted is smooth as glass in top gear at 55 mph and anything above.
Below 55 some vibration shows up and at 50 in top it's quite annoying, so I shift down into third just to get rid of the vibration. Running for miles and miles in third is also annoying when there is plenty of torque to run in top, but the vibes say no.
Your configuration is so different that I expect the smooth running threshold is also different. Does your bike start to vibe below 2900 revs as standard Commandos do?

The Commando is a bit of an oddity as gearing it lower actually makes it smoother at low speeds. All of the other parallel twins shake more with a smaller gearbox sprocket, hence you find overgeared 650BSAs and Dommies that can't pull skin off rice pudding.

Glen
 
Brooking 850 said:
Hi Nigel, how about a six speed TTI gerabox, then you have a gear for every event!!!
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I think Nigel likes my 6 speed box. I haven't yet used it in anger. It should be good, but it might actually be too many gears. One thing which often happens with close ratios, is all the ratios are either all together too high, or all together too low. Close ratios give you much better acceleration, but sometimes that does not happen until you get the bike moving above a certain speed. So you can be either too slow coming out of corners, or too slow at the ends of the straights. I simply relied on Bruce's judgement about the spread of ratios, but I asked him to put first gear between standard Commando first and a Manx first. I might find that first gear is still too high. The major advantage and the reason I bought 6 speeds, is to get the bike off the start line during clutch starts. The first corner I approach during a race should be taken at the same speed on the first lap as it is on the last. Four gears close ratio are perfect everywhere else other than the start and revving the tits off the motor to get a good start is a career limiting strategy with a Norton box. Five speeds should be enough.
If you think back about all the races in which you have participated - what happens at the first corner ? Most guys don't approach it with their arse on fire. What you lose at the start is usually very difficult to regain.
My Seeley 850 will accelerate hard in any gear, once it is moving. Every time I have raised the overall gearing my bike has accelerated faster. In races the first 40 MPH is always a problem.
 
worntorn said:
Nigel, is the engine smooth at 2500-3000 revs?
I don't intend to ride at that rpm but sometimes ride for hours on twisty, hilly 50 mph speed limit country roads that are police patrolled. At 55-60 mph on these roads one can have lots of fun and probably sneak thru police radar unscathed.
My bike with 21 tooth fitted is smooth as glass in top gear at 55 mph and anything above.
Below 55 some vibration shows up and at 50 in top it's quite annoying, so I shift down into third just to get rid of the vibration. Running for miles and miles in third is also annoying when there is plenty of torque to run in top, but the vibes say no.
Your configuration is so different that I expect the smooth running threshold is also different. Does your bike start to vibe below 2900 revs as standard Commandos do?

The Commando is a bit of an oddity as gearing it lower actually makes it smoother at low speeds. All of the other parallel twins shake more with a smaller gearbox sprocket, hence you find overgeared 650BSAs and Dommies that can't pull skin off rice pudding.

Glen
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A 54% balance factor probably produces smooth running at about 3000 RPM. The Commando was designed to compete against CB750 Hondas in the commuter market. If you rebalance your crank to 72 %, your motor will be smooth at 7000 RPM, but the motor will rock the bike backwards and forwards at 3000 RPM. You balance the crank to suit the way you intend to use the bike. If I had a road-going Commando, I would balance the crank high, but I'm an idiot. - I don't ride on public roads because I don't like getting booked.
 
worntorn said:
Nigel, is the engine smooth at 2500-3000 revs?
I don't intend to ride at that rpm but sometimes ride for hours on twisty, hilly 50 mph speed limit country roads that are police patrolled. At 55-60 mph on these roads one can have lots of fun and probably sneak thru police radar unscathed.
My bike with 21 tooth fitted is smooth as glass in top gear at 55 mph and anything above.
Below 55 some vibration shows up and at 50 in top it's quite annoying, so I shift down into third just to get rid of the vibration. Running for miles and miles in third is also annoying when there is plenty of torque to run in top, but the vibes say no.
Your configuration is so different that I expect the smooth running threshold is also different. Does your bike start to vibe below 2900 revs as standard Commandos do?

The Commando is a bit of an oddity as gearing it lower actually makes it smoother at low speeds. All of the other parallel twins shake more with a smaller gearbox sprocket, hence you find overgeared 650BSAs and Dommies that can't pull skin off rice pudding.

Glen
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Glen, I’ll try and be purposefully mindful of vibration next time I take it out (currently have the rearsets off again for some minor fiddling).

But I think the answer to your question is that it is really quite smooth at 50mph. Before I had my 850 balanced and fitted with JS kit, it vibrated badly at around 3,000 rpm, most noticeably when decelerating through that range, I don’t know why. It was so bad I initially thought something horrible was happening like the gearbox coming loose or something! But after the refurb, that was all gone.

The 920 runs a lightweight Maney crank. Statically balanced by Steve to the specs given by Jim Schmidt. Steve was adamant that dynamic balancing was unnecessary. All I can say is that Jim’s specs and Steve’s workmanship speak volumes!

I run the iso’s quite tight, I nip them up and then back them of so I can just prize the engine cradle with a bar. I do that in the belief it helps tighten up the feel of the handling (right or wrong, I don’t know) But even set up like that, the bike shakes around quite a lot at tickover, plenty enough to amuse modern bike onlookers, but as soon as I’m riding, vibes just don’t come into the equation. So much so in fact, I am convinced the engine would be fine solidly mounted.

It had new vernier iso‘s from AN when I first rebuilt it, and it runs a comnoz / cNw link type head steady.
 
acotrel said:
I think Nigel likes my 6 speed box. I haven't yet used it in anger. It should be good, but it might actually be too many gears. One thing which often happens with close ratios, is all the ratios are either all together too high, or all together too low. Close ratios give you much better acceleration, but sometimes that does not happen until you get the bike moving above a certain speed. So you can be either too slow coming out of corners, or too slow at the ends of the straights. I simply relied on Bruce's judgement about the spread of ratios, but I asked him to put first gear between standard Commando first and a Manx first. I might find that first gear is still too high. The major advantage and the reason I bought 6 speeds, is to get the bike off the start line during clutch starts. The first corner I approach during a race should be taken at the same speed on the first lap as it is on the last. Four gears close ratio are perfect everywhere else other than the start and revving the tits off the motor to get a good start is a career limiting strategy with a Norton box. Five speeds should be enough.
If you think back about all the races in which you have participated - what happens at the first corner ? Most guys don't approach it with their arse on fire. What you lose at the start is usually very difficult to regain.
My Seeley 850 will accelerate hard in any gear, once it is moving. Every time I have raised the overall gearing my bike has accelerated faster. In races the first 40 MPH is always a problem.
Click to expand...
Are you using tyre warmers for classic bike racing over there? First corner as fast as the last, maybe Kenny Robert's but not anyone I know this side of the pond.
 
Fast Eddie said:
I run the iso’s quite tight, I nip them up and then back them of so I can just prize the engine cradle with a bar. I do that in the belief it helps tighten up the feel of the handling (right or wrong, I don’t know) But even set up like that, the bike shakes around quite a lot at tickover, plenty enough to amuse modern bike onlookers, but as soon as I’m riding, vibes just don’t come into the equation. So much so in fact, I am convinced the engine would be fine solidly mounted.

It had new vernier iso‘s from AN when I first rebuilt it, and it runs a comnoz / cNw link type head steady.
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Have you measured that clearance?

When I first bought my Commando, it came with a broken frame since the DPO tightened the rear ISOs too much. I wouldn't go much tighter than 0.005".
 
My theory is that so long as it can move freely, it won’t crack.

‘Too tight’ would mean it can’t move freely. That will surely crack.

My thinking is that the link type head steady reduces the side to side movement that might cause stress on the frame, so with little or no side to side movement, there is no need for any clearances above that needed to allow free up and down movement.

Reducing overall vibration at source, by balancing, is obviously a factor too. Less vibration needs less isolation.

There is no way I’d have dreamt of setting it up like this before the rebuild, the way it used to shake around at 3000rpm felt very mechanically wrong to me, I can easily envisage that would have led to damage.

I sometimes think the isolastics were too successful, meaning that some people simply ignore the whole concept of balancing, taking the line that ‘it doesn’t matter, the isos will handle it’. My thinking is that they’ll handle a balanced motor better / easier.

Caveat: I’m in NO WAY putting this out there as some kind of suggested method for all, jus’ sayin’ what seems to work for me.
 
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FastFred said:
Are you using tyre warmers for classic bike racing over there? First corner as fast as the last, maybe Kenny Robert's but not anyone I know this side of the pond.
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I am a very old rider and I am not so tyre-dependent. The geometry of my bike means I don't lean the bike so much going into corners and halfway round I can gas it very hard. In any case the poor old bike probably hasn't got more than 60 BHP. If you don't have extreme angles of lean and silly power, you don't need so much grip. Also the first corner is not usually so far away that your speed is extreme. If you know what it looks like on the last lap, you can take it almost that fast on the first. Most guys like to get settled-down a bit before they get stuck into it.

Nigel, gearing on a Commando is very deceptive. When I first installed the 6 speed box, I did not realise it had the wrong selector drum. I tried to ride the bike around the pits and it would not go. Being stupid I revved it to about 5 and slipped the clutch. It must have been in 4th or 5th gear - it took off like a rocket and I almost stuck it through the fence. You might be able to pull a much higher gear than you imagine. If you raise the gearing - as you accelerate down a straight, you might find that you reach exactly the same maximum revs in each gear, but you are going faster. It pays to know how long it normally takes you to get down at least one straight on a circuit. Then as you raise the overall gearing, you know if you have an improvement. If you have a normal Commando crank, it is extremely heavy. When it is spinning at 6,500 RPM it has a lot of stored up energy. If you have close ratios, the crank almost does not slow down as you change up.

On my 850, I am using a 24 tooth engine sprocket with the Manx clutch. So the primary drive is pretty standard. On the back, I use 21 tooth on the gearbox and 38 on the back. One thing I've noticed in most of Ken's photos, it looks as though most guys are using about 45 teeth on the rear sprocket. If you do that the bike will accelerate at a certain rate, dropping it to 40 tooth will probably make the bike go faster with no drop in revs.
 
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