Five speed gearbox on 750

pommie john said:

No.

Standard Commando first gear is 2.56
Quaife offer three different first gear options: 2.36, 2.11 and 1.98. obviously these are all higher than standard so your advice of gearing it up one tooth on the rear ( which is impossible on a Commando rear hub) would make it even higher.

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If you are fitting a 5 speed box, why would you use the bike in situations where you were continually stopping at traffic lights? The first gear problem only presents itself when you are getting the bike mobile from a stand-still. The beneficial affect of the 4 closer gears above first is better acceleration. But a commando engine tends to always spin up at the same rate, regardless of gearing. When you use the top 4 gears closer ratio, the engine revs drop less on up-changes, so the engine has fewer revs to regain before it pulls to the max. But you don't get seem to get full benefit unless you raise the overall gearing.
So in effect there are two considerations - first gear - and the top 4 gears. If you raise the overall gearing, first becomes worse, but the top 4 become better. When I was using the 4 speed CR box, first gear was higher than a Commando standard second gear, and it was still not that difficult - just a bit slow at first until the bike is rolling.
What amazed me is that when I first got my 6 speed box, the gear-change was inverted. The 850 motor pulled 5th gear from a standstill and still accelerated away.

Personally, I always believed that the lower the gearing, the faster the bike should accelerate. But with my Commando 850 engine, that is not the case. With the 4 speed close box, every time I've raised the overall gearing the bike has accelerated faster. I would not have thought an 850 engine would bog down, but that is what seems to happen, if you let the revs drop when changing up. The standard gearbox is absolutely hopeless, on an up-change you wait forever to get the revs back.
I don't know what the torque curve looks like, I only know what happens.

acotrel said:

If you are fitting a 5 speed box, why would you use the bike in situations where you were continually stopping at traffic lights?

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To get across town.

For road use I never felt a need for a fifth gear. I have have been riding Commandos since 1973 on freeways, twisty backroads and in city. “Whole Lotta Torque about Norton”.

My first experience of the 'standard' gearbox was an 850 MkIIa with the raised 2nd gear. Once in second the ratios were pretty well spaced going up the box, but the gap from first to second was occasionally a problem. Downshifting into first from 'normal' revs would have the back wheel stepping out, so I had to modify my riding style to allow the revs to drop right off in second before downshifting, with a consequential loss of engine braking effect.
The earlier box feels like the gap between second and third is slightly too big, but still better than the later box, so in my humble opinion, neither is 'right'.
Having been brought up on Triumphs with 5 speed boxes I always felt that when making a comparison, the only negative to the Commando was the gearbox.
In the end I bought a Quaife. Just need to finish the bike it's going in!

I only ever tried to race with the standard box, once. I almost could not ride the bike - it was appalling. With 4 speeds close ratio, once the bike is rolling, you never go back to first and five speeds should be much better - as long as the five speeds are not so close that when you have a reasonable top gear, first gear is not as high as 2nd gear in a standard Commando box.
I think that in most five speed boxes, the higher four gears would be fairly close together and first would be almost as low as first in the standard box.
It is possible that some five speed boxes are designed to be used with very short stroke motors and have the five gears very close together with first at the level of second gear in the standard box.
I have not yet had the opportunity to race with the TTI 6 speed box. If I find the ratios are awkward, I will have to play raising or lowering the overall gearing. All it means is that somewhere on any race circuit, the gearing will be unsuitable and the bike slightly slower. With the 4 speed close box, the clutch start in races has always been a problem - the bike does not jump fast enough and what you lose at the start is very difficult to get back.

If you have a near standard Commando, try riding away from a stand-still in second gear. It is not difficult. And that is the worst case scenario. When I was using the 4 speed close box, I never used to slip the clutch - just dump it and the bike will lumber off. Commandos have loads of torque.

When I replaced the standard gearbox cluster with the 4 speed close ratio set, I had a different bike. It was so much better, it was incredible. The 5 speed box is definitely a good way to go, regardless of what happens with first gear. With a 2 into 1 exhaust, you have good power from about 3000 RPM, and the close gears really get the bike going. My bike is on methanol fuel, but apart from that, the 850 motor is still near standard. The bike is quick enough to win against some pretty formidable opposition.

illf8ed said:

For road use I never felt a need for a fifth gear. I have have been riding Commandos since 1973 on freeways, twisty backroads and in city. “Whole Lotta Torque about Norton”.

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Actually, me too. I've been using Quaife 5-speeds in the race bikes since 1972, but not in the street bikes. I'm using a 5-speed in a MK3 restomod I'm building now, but more because I thought it might be interesting, and to use up some of my old road race parts in the process, than for any real need. In general, unless you build the Commando to be a high rpm, narrow powerband, monster for the street, the 4-speed is more than adequate. If you can pick up a TTI with wider ratios, that would let you gear really tall for long distance highway flying, but still have a decent first gear for town travel, but unless you do that sort of riding regularly (like comnoz), you might not find the benefit worth the cost.

Still, almost every modern bike has either a 5-speed or 6-speed gearbox, even the ones with large air-cooled pushrod engines, and that makes me wonder if I'm missing something here.

Ken

Yes my 2007 XL1200R Sportster has five gears. It doesn’t wind up as far as the Commando so maybe needs an extra gear like a diesel truck.

illf8ed said:

Yes my 2007 XL1200R Sportster has five gears. It doesn’t wind up as far as the Commando so maybe needs an extra gear like a diesel truck.

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I think that top gear in the 5 speed sportster box is an overdrive. It is higher than 1 to 1. With 5 speeds close ratio, a Sporster on the road, would be a weapon. That motor has a lot of potential. I looked at them when I was building my 850 - but back then they still had problems. The KR750TT Don Emde raced at Calder in the early 1970s ran a timing side bearing. And anything Harley is expensive. When you build a bike, if you have not got the gearbox, you have not got the bike.

Even if the motor is slow, if it delivers loads of torque, a close ratio gear box acts as a torque converter. If you keep the motor spinning high as you race-change up through the gears, the bike will accelerate at least as fast as any other with a lighter crank. When I built my Seeley 850, I thought the heavy crank would be a problem - if you use the standard gearbox, it is. Throttle response is not good with the heavy crank, but in many other ways, it is much better than a light crank. If you ride on bumpy circuits or on windy days, the heavy crank is excellent. Once the heavy crank is spinning, nothing stops it.
I think that if you were using a close box on public roads in traffic, you would be changing gears a lot more than with the wide box. With my bike, I only really use the top 1500 RPM - between 5,500 and 7000.

I have only raced my Seeley 850 on two circuits - Mount Gambier and Winton - both small circuits. If I wanted to race at Phillip Island, I would have difficulty in getting the overall gearing high enough. The 850's power delivery is mainly about torque rather than top end. If the overall gearing is too low, the bike does not pull, it just coasts and throttle response takes over. To get the most speed, it takes high gearing and race-changes up through the box

acotrel said:

I think that top gear in the 5 speed sportster box is an overdrive. It is higher than 1 to 1. With 5 speeds close ratio, a Sporster on the road, would be a weapon. That motor has a lot of potential. I looked at them when I was building my 850 - but back then they still had problems. The KR750TT Don Emde raced at Calder in the early 1970s ran a timing side bearing. And anything Harley is expensive. When you build a bike, if you have not got the gearbox, you have not got the bike.

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I wasn’t sure about top gear ratio on my XL1200R so below is from HD Forum:
But the transmissions themselves are 1:1 in their top gears, whether the top gear is 4th or 5th. In top gear, the power never goes through the countershaft, it comes straight in the mainshaft and goes straight out the final drive gear. Both the 4-speed and the 5-speed transmissions are designed this way.

In most gearboxes, the drive in top gear does not go through the layshaft. When I looked at the ratios which are specified for the Sporster, top gear was higher than 1:1. At that point I lost interest, because converting it to close ratio would probably have involved buying an entire gear cluster, instead of having a couple of pairs of gears made. It is pretty obvious that the Sporster is designed for road use - one with a good close ratio gearbox would be very interesting. You would not have to do much to the motor, if your bike was nice and light, and the frame handled.