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Transmission Gear Swap

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Contributed by:

Dennis Murfin, UK


Changing Gears the CB77 Way

Doing the X swap on the CB72 /77 series bikes is very straight forward and well within the ability of anyone who can build the gearbox correctly in the first place.

The results may not be quite so clear cut;

Taking a standard (pre 1966) 305cc with a 15 teeth gearbox sprocket and a 30 teeth rear wheel sprocket. The OVERALL reduction ratios from the engine to the rear wheel are as follows:

First gear = 17.52, Second = 10.46, Third =7.32 and Top = 6.26.

Looking further, the DROP in engine revolutions at each gearchange are as follows:

First to second = 40%, Second to Third 30% and Third to top 15%

Note that the steps are not equal, nor should they be, logically you need a smaller drop in engine RPM when shifting into the higher gears, the higher road speed needing more power, hence the need to make sure the engine stays in the 'power band' when shifting up.

With the second and third gear pairs swapped over the figures become:

First gear = 17.52, Second = 11.70, Third = 8.26 and Top = 6.26.

Note that First and Top have not changed as the positions of first and top gears is unaltered.

The DROP in engine RPM at each gearchange now looks like this:

First to second = 33%, Second to Third = 30%, Third to Top = 24%

What does all this mean:

With the Modified set up the change from First to (the now lower) second gear will probably be made sooner, I.E. at a lower road speed, which may help initial acceleration.

The RPM drop between second and third remains at 30% but again the actual gearchange will take place at a lower ROAD SPEED,

A successful change from Third into Top cannot be made until the bike is travelling fast enough to pull TOP gear comfortably which will be the same road speed regardless of whether 'standard' or 'crossed' gears are being used.

To reach this critical speed with the crossed ratios will mean holding onto (the lower) third gear for longer and thus reving the engine harder to get a good shift into TOP.

In conclusion these figures suggest that if you have a standard spec. 250cc bike, regularly ride with a passenger or live in a hilly area it may be best to leave things the way they are.

If you have a 305 or 350 conversion and /or an engine with a with a bit of tuning thrown in, you might like the bike better with the gears crossed over, particularly if you don't carry a pillion or don't frequently ride in hilly regions.

Best bet is to try it and see, it isn't too big a job and if you aren't happy with the results itís just as easy to change it back again and it hasnít actually cost you anything has it?

There is yet another option of fitting TWO pairs of second gears or even TWO sets of third gears which give different figures again but you will have to work these numbers out for yourself.

What all this does prove is what we all know, I.E. the CB72/77 would have been an even better bike with a five speed gearbox as standard !


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