2nd gear Pre 74 vs post 74

My first post here…
I have a Vincent Comet powered collection of bits from the shed and I’m using a later Commando gearbox that has the 2nd gears that have a slightly taller ratio. Perhaps this is the proverbial dumb question, but is there a noticeable difference ( when riding) between the earlier gearbox and the later one? It sounds like some folks prefer the later setup, but for an anemic Comet engine the spread between 1st and 2nd seems a bit much. Made worse by the fact that the overall gearing is a bit too steep. I’m thinking of swapping out the gears for the earlier variety as well as dropping a couple of teeth on the crank pulley. I suppose I should change the crank pulley first and then see if I can live with 2nd gear. Any thoughts?

Get your overall gearing right first. You could as you say change the pulley. But spinning the box faster is generally a good idea. Maybe go down a tooth or two on the gearbox output sprocket?

A lot depends on how you intend to use the bike. For speed, a low first gear with the top 3 close and high can be good - but for normal road use, 4 gears evenly spaced might be better. If you want a Commando to be fast, higher overall gearing is better, and close ratios are better. But to get the bike moving from a standstill, you need the low first gear. Spinning the box faster reduces the loads on the gears.
If you have a peaky motor, more gears and close ratios are better. But close ratio gears will also help a torquey motor, when the overall gearing is high.

Thanks for the replies. The gearbox sprocket is currently 19 teeth. Normally I would just change the gearbox sprocket or rear sprocket, but there is a second reason for changing the crank pulley. It has a Bob Newby clutch/belt drive and the belt is too tight. Bob says you should be able to turn the belt 90 degrees and I can barely get it to 45. To be honest, I don’t know how much that matters, but I doubt if the roller bearings in the clutch appreciate the extra load. The gearbox is hard up against the crankcase, so no way to slack off the adjustment. Changing the crank pulley “should” kill 2 birds with one stone. Hopefully change the ratio enough that I can live with the 1-2 gear change. The bike will not see a lot of use and it will be mostly city riding. The motor is about as far from peaky as you can get. MK 1 Vincent cam and a piston that is below 8 to 1 fed with a 28mm Mikuni flat slide. It fires about every second lamppost. If the 1-2 gear change still bugs me, I’ll swap out the gears for the earlier variety.
The current setup is 36 teeth on the crank pulley, 68 on the clutch, 19 g/b sprocket and 45 at the rear wheel (110/90/18 tire).

Mongrel said:

Thanks for the replies. The gearbox sprocket is currently 19 teeth. Normally I would just change the gearbox sprocket or rear sprocket, but there is a second reason for changing the crank pulley. It has a Bob Newby clutch/belt drive and the belt is too tight. Bob says you should be able to turn the belt 90 degrees and I can barely get it to 45. To be honest, I don’t know how much that matters, but I doubt if the roller bearings in the clutch appreciate the extra load. The gearbox is hard up against the crankcase, so no way to slack off the adjustment. Changing the crank pulley “should” kill 2 birds with one stone. Hopefully change the ratio enough that I can live with the 1-2 gear change. The bike will not see a lot of use and it will be mostly city riding. The motor is about as far from peaky as you can get. MK 1 Vincent cam and a piston that is below 8 to 1 fed with a 28mm Mikuni flat slide. It fires about every second lamppost. If the 1-2 gear change still bugs me, I’ll swap out the gears for the earlier variety.
The current setup is 36 teeth on the crank pulley, 68 on the clutch, 19 g/b sprocket and 45 at the rear wheel (110/90/18 tire).

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I have a 35 tooth Bob Newby pulley, so I know he does those!

SteveA said:

I have a 35 tooth Bob Newby pulley, so I know he does those!

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I contacted Bob and he said he would make me a 35 or a 34. I found some larger Vincent rear sprockets under the bench, so I’ll graft one of them onto the existing rear sprocket (CBX550). That’ll be a cheap way to help figure out what overall ratio I’m looking for… plus I can find out if I can live with the 1-2 shift or need to swap out 2nd for the pre 74 variety.

From personal experience I would favour the earlier ratios every time.
It's most noticeable when downshifting, and I learned to allow the revs to drop right off before going back into first, otherwise the back wheel would step out.

B+Bogus said:

From personal experience I would favour the earlier ratios every time.
It's most noticeable when downshifting, and I learned to allow the revs to drop right off before going back into first, otherwise the back wheel would step out.

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Thanks… that’s the kind of feedback that I was looking for. I haven’t experienced what you are referring to, but that’s because I’ve only covered a few km’s so far. Basically shakedown runs to see #1 if I should just pull the motor/gearbox out and shove the rest off a cliff. #2 sort out any snags. It turns out that it is surprisingly well behaved, so it’s worth sorting out. The speedo needed a couple more magnets added to increase the pulses and smooth out the needle movement at slower speeds. One oil line return fitting leaks and the “ throw” on the shifter ( rearsets) is quite short which doesn’t make selecting neutral any easier. Just have to lengthen the arm on the gearbox. Once those are fixed and the engine beds in a bit, I’ll make it work a little harder and see what else it needs.
Back to those gears…. , as previously mentioned, it appears some folks prefer the later ones. Maybe I can swap my post 74 gears for the earlier ones. I might get used to it as it is, but my initial impression is that it kind of sucks. More noticeable on this bike I think.. an anemic 500 single instead of a 850 twin. My single would appreciate a more linear change in ratios.

My experience was that the 3-2 downshift was where the earlier 2nd gear ratio was really noticeable vs later. Up shifting doesn't really bring out the difference in gear ratios as much as down shifting. I went to a 5 speed and never wanted to go back. Either 4 or 5 gearset, the closer the gear ratios, the smoother the gear changes. In this case, it's only second gear, but I'd still go with later second gearset.

This thing would definitely be better with a 5spd box, but it won’t get ridden enough to justify topping up the fund. I do have a close ratio Norton 4 spd box. It’s an early laydown version. Too much farting around to fit it, plus then I would want to keep the taller gearing and not the best thing for city riding. I owned a few 750 Commandos, but don’t recall ever riding one of the post 74 bikes. I’m assuming/guessing the upshift on the later Commando is way less noticeable because it generates a lot more low end grunt than the Comet engine.

Am I correct in assuming that both 2nd gears be changed while the gearbox is still in the frame?

Mongrel said:

Am I correct in assuming that both 2nd gears be changed while the gearbox is still in the frame?

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Yes.

any mileage in filing odd bits off gearbox to get it a fraction further forward?

Mongrel said:

This thing would definitely be better with a 5spd box, but it won’t get ridden enough to justify topping up the fund. I do have a close ratio Norton 4 spd box. It’s an early laydown version. Too much farting around to fit it, plus then I would want to keep the taller gearing and not the best thing for city riding. I owned a few 750 Commandos, but don’t recall ever riding one of the post 74 bikes. I’m assuming/guessing the upshift on the later Commando is way less noticeable because it generates a lot more low end grunt than the Comet engine.

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The gear clusters in many Norton gear boxes are interchangeable. A low first gear and the other three high and close, might be better than standard ratios. On a race track when all gears are close and high, I never go below second gear. With a road bike, I would not go down to first gear when stopping. I would change into first gear when stationary. If second gear is a bit high, I would just rev the motor higher in first gear. Once the bike is rolling, only the top three gears matter.
If you go to a race track on a practice day, you will often see guys with their rear wheels hopping as they approach corners. It should never happen - they probably wonder why they crash.

I am an idiot. The last time I raced, I was using a 4 speed Manx gear cluster. First gear was too high to get good starts. So in the final race, I really revved the tits off the motor and the bike jumped away fast enough. I sold another bike I had and bought a 6 speed TTI box, knowing full well I would probably not race again. All I had needed to do was fit the standard Commando first gear into the Manx cluster, and live with the gap.
I was talking to one of our better riders about it - he told me he has three first gears for his Manx Norton. The Manx Norton gearbox was designed for push-starts. With the 850 Commando motor, the top three gears in the Manx cluster are superb. Many people do not realise the importance of that. I tried to race with a normal ratio Commando gearbox - the bike was far too slow. A lot of guys do not know how to use a gearbox.
My mate Russell got the shock of his life when I lowered the gearing on my short stroke 500cc Triton and led him and field of superbikes for almost a full lap. It was not something I would normally do. I was passed towards the end of a long straight by a couple of guys and entered the next corner behind them miles too hot. He still does not know why I was so much quicker than him on that day.

jan nelder said:

any mileage in filing odd bits off gearbox to get it a fraction further forward?

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Yes, but I’m trying to avoid that if possible. If that sounds like I’m being lazy, then guilty as charged. I’m fairly sure that in order to gain anything, I would also have to modify the engine plates slightly, plus it’s getting very close to real estate that is occupied by steel oil lines. If my guesstimate is close, the next largest belt would mean that I would have to pivot the gearbox back an inch .. give or take. The adjustment slots would allow me to move the gearbox back close to an inch, but unfortunately the clutch would come in contact with a frame gusset once it was moved about 0.050” rearward. The perils of assembling a motorcycle from the contents of a garbage dumpster without the aid of CAD.
I turned down a Honda sprocket ( because I’m using CBX550 wheels) and grafted on a Vincent 50 tooth sprocket. I haven’t test ridden it yet, but it should take care of my “too steep” overall gearing. Whether or not it makes the 1st-2nd shift bearable is yet to be seen. Upshift maybe better, but downshift worse?
Assuming I go ahead with swapping out the gears for the pre 74 variety, once the gearbox covers are off, I will have a better idea whether or not I can grind off enough to make a difference.
The belt doesn’t seem tight to me, but I can’t get the 90 degree twist Newby recommends. Where it sits now is only 45, but I can slide the belt sideways (a bit) on the clutch without any real force. It’s definitely not lose, but to me it doesn’t seem tight enough to cause any issues. Trouble is, I have zero experience with belt drives. I guess the question is how much does it change when things are hot. Only ridden a few miles in cool weather.

With close ratio gears, a bike becomes much faster - both in acceleration and top speed. Much higher overall gearing can be used. What slows a bike with normal gearing is the gap between the gears and the inertia of the crank - once you have the heavy crank spinning, you can use it to get a boost on every up-change, as long as you keep it spinning high, and make it pull.

acotrel said:

The gear clusters in many Norton gear boxes are interchangeable. A low first gear and the other three high and close, might be better than standard ratios. On a race track when all gears are close and high, I never go below second gear. With a road bike, I would not go down to first gear when stopping. I would change into first gear when stationary. If second gear is a bit high, I would just rev the motor higher in first gear. Once the bike is rolling, only the top three gears matter.
If you go to a race track on a practice day, you will often see guys with their rear wheels hopping as they approach corners. It should never happen - they probably wonder why they crash.

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I have another set of laydown box innards, but the 2nd gears are different 4 dogs vs 3 etc. I have piles of Dollshead stuff, but that doesn’t do me any good. I’m not positive, but a normal pre 74 box is probably what I need? Just slogging around town. I had a MK11 cam in the thing, but swapped it out for a MK1 thinking it might make it slightly easier to kick start. I’m biodegrading at an alarming rate so I need all the help I can get. In the end I doubt if the cam made a difference. Aside from the difficulty actually kicking the thing over, the programmable ignition system is a PITA which is kind of a long story. It counts the # of degrees rotation regardless of which direction it’s going. If the engine rolls backwards from compression, the next kick is advanced enough to really hurt. Plus… initially it would only fire if you turned the kill switch off and then back on again, but it would only run for 3 seconds and die. You could do that all day long if you wanted to. I have a paddock starter made from a treadmill and I’d be screwed without it. Never would have found the problem…. would have just shoved the whole thing off a cliff. Anyway I got started on that tangent because I was going to say the Comet engine is gutless in its current form, so a standard box may be the way to go?

acotrel said:

With close ratio gears, a bike becomes much faster - both in acceleration and top speed. Much higher overall gearing can be used. What slows a bike with normal gearing is the gap between the gears and the inertia of the crank - once you have the heavy crank spinning, you can use it to get a boost on every up-change, as long as you keep it spinning high, and make it pull.

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I definitely hear what you are saying. If the motor had more grunt and there was a likelihood I would use it, I could be easily talked into installing the (rebuilt) orphan close ratio box that is languishing under the bench. I also have a 48 Manx box, but not really a fan of the shift linkage and would require new engine plates.