Five speed gearbox on 750

[pierodn]

Hi.
How many sprocket teeth for a five speed gearbox?.
Thanks.
Piero

 

[kommando]

Depends on what its being used for.

 

[L.A.B.]

4-speed or 5-speed 'top gear' will be 1:1 ratio.

 

[acotrel]

With 5 speeds, the ratios are usually closer together, you should be able to pull one tooth less on the rear sprocket, and still accelerate faster.
There is a very marked difference in acceleration between a standard box and 4 speeds close ratio. Five speeds are similar for the top four gears, however with a 4 speed close box, you tend to get a bit of a lurch when moving off from a stand-still.. With 5 speeds you should not get that.

 

[lcrken]

5-speed gearsets for Nortons have been available in a variety of close-ratio clusters, with similar gear ratio spreads from 2nd to 5th gear, but with first gear ratios ranging from 1.91 (Quaife close ratio set) to 2.36 (Quaife Road set with kick start provision). All are 1:1 in 5th gear. So, if you have the tall first gear (1.91), you might want a tooth less on the countershaft than if you had the low first gear (2.36), just to avoid having to slip the clutch as much on starts. But like Kommando said, it really depends on the use, i.e. drag racing, road racing, putting around town, or freeway flying.

Ken

 

[acotrel]

The 4 speed close ratio cluster which I have was sourced by a friend from America. From 1st to top gear, the ratios are perfect. However 1st is where 2nd would be in a standard box. For racing of most kinds, you need to get the jump off the start. Those 4 gears, with another one evenly spaced below, would be excellent. I once tried racing with a standard gearbox - it was absolutely hopeless. The acceleration rate as I came up through the gears was far too slow, and coming down through the gears - I had to use a heap of revs on every gear change - so never smooth. If you are not smooth when racing, you crash. If you are fitting a 5 speed box, you must intend to ride the bike fairly fast in whatever your intended application might be. For normal road use, the standard box is probably adequate.

 

[pierodn]

Hi.
I would have a freeway flying use.
The gearbox is from a PR.
Thanks.
Piero

 

[lcrken]

The Quaife 5-speed that was optional in the PR for customer delivery had (I think) a 2.23 ratio first gear. The page below shows the top speed at 7,000 rpm for a variety of countershaft sprockets. Those values are for the original 19" tires and stock triplex primary drive. That might help you decide what gearing to use.

Ken

 

[MichaelB]

Cool brochure...

 

[acotrel]

If you have been using the bike on public roads and top gear has been OK, I would go one tooth down on the rear sprocket, so the overall gearing is slightly higher. When you ride with close gears, the bike accelerates faster so you will reach top speed earlier between bends in the road. In the end top gear is probably top gear and if you ride long enough wind resistance determines your top speed. But if you start getting to top speed quicker, you might over-rev. Peak revs on most Commandos is 7000 RPM.

 

[SteveA]

If you have been using the bike on public roads and top gear has been OK, I would go one tooth down on the rear sprocket, so the overall gearing is slightly higher. ........

That will be a neat trick with a standard Commando 750 rear wheel!

 

[B+Bogus]

Going down a tooth on the back sprocket is easier said than done with a standard Commando drum rear wheel!
EDIT: LOL! beat me to it

 

[acotrel]

Going up one tooth on the countershaft sprocket is usually like going down three on the rear sprocket, so you would need the lowest first gear in the box. Once you are rolling, there is no problem. But off the mark when first is too high, can be a pain. I noticed that when I changed from the standard box to 4 gears close - once the bike was rolling, it did not matter much how high the overall gearing was, the bike was faster. For highway use, where top gear is might be important when you are riding long distances. I think that when you have 5 gears close, you might tend to ride faster. When you change up you don't lose so many revs, so the revs come up faster - you stay further up the torque range.

 

[acotrel]

When I was using the 4 speed close box, first gear was too high and the bike used to just lumber away from the start. Only once did I take the bull by the horns and rev the tits off the motor, at the start of a race. The bike jumped away well without destroying the box, so I got my only good start. With 5 speeds, you should not have that problem unless you are very inexperienced.

 

[pommie john]

Just use standard gearing.

As others have said, both gearboxes use the same top gear so your cruising speed rpm will be the same.

Don’t gear it up because most close ratio boxes have a higher first gear and you don’t want to make it even higher.

 

[acotrel]

I would have thought that in most 5 speed boxes, first gear would be slightly lower than in a standard box. But the top four gears are closer, so as you accelerate changing up through the gears, the bike is faster and can tolerate slightly higher gearing. Overall close ratio gears give you a better ride, but more gear changes. If there is a gap between first and 2nd, it usually does not matter. I have only used one 5 speed box in a race bike, it was the Triumph 5 speeder. It was better than the Triumph 4 speed close box. Personally, If I had a road going Commando, I would use my 4 speed close box and live with the high first gear. With 5 speeds, you should not have a problem with first gear, even when you raise the overall gearing a bit.

 

[acotrel]

With a Commando, if you don't make it pull, it is slower. With close ratio gears, you need to make it pull as hard as it does with wide ratio gears. The close gears keep you higher up the rev range and the drop in revs at each up-change is less.

 

[pommie john]

I would have thought that in most 5 speed boxes, first gear would be slightly lower than in a standard box.

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.

 

[FastFred]

Doesn't the engine power output figure into this?

 

[lcrken]

Doesn't the engine power output figure into this?

Not unless you are trying to gear it to pull the maximum top speed. Then you need to know what rpm the horsepower peak is at, and gear it so that it will just pull that rpm in top gear, or maybe a little past it, in case you get a tail wind. But that gearing with a close ratio first gear might be a real pain for pulling away from stops.

And if you are planning to win drag races, you would choose a lower gear, but that might be a real pain on the highway.

With a stock Commando, you might be happy with anything from a 19T to a 22T countershaft sprocket, depending on if you are riding twisties and never get much past 100 mph, or cruising cross country on freeways at 95 mph (might even like a 23T for that). There's just no single answer to "what's the best countershaft sprocket for a Commando". AFAIK, standard fitment from the factory was mostly 19T for the 750s, with 20T and 21T for the 850s, but you could get them on special order with other choices, and 22T, 23T and 24T sprockets were available as parts.

Ken