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What Gearbox Ratio?

Close ratio, wide ratio, what does it all mean? Can I get what I want from a British gearbox?

So you've got your British adventure bike (in this case a late sixties Triumph T100C) and it's geared to get along the road at about 5800rpm for the speed limit (100kph) in top gear. This is kind of OK, but when it comes time to get down and dirty and go off road, 1st gear is a little high and you start bouncing off trees, stalling in river crossings or really struggling up that steep pinch. What to do? Well, you could drop the overall gearing by putting a smaller front sprocket on, easy. But hang on, when you get back on the road, you're now only doing 80kph for the same engine revs and the poor little thing is haemorrhaging itself to get you home again. What you want is to retain the overall top gear ratio and drop the others down evenly (so you don't drop into a power free hole between 2nd and third, say) until 1st gear suits your purposes. Help is at hand. Triumph and BSA made a number of optional gear clusters available for their four speed gearboxes to suit differing uses. In the case of the unit construction 350/500 Triumph twin, it mostly came fitted with the standard road going cluster, but East coast US models of the T100C were often fitted with a wide ratio cluster to suit the off-roading the Yanks were throwing these things at. There was also a close ratio option for those who wanted to put these things on a race track. In theory the difference between standard and wide ratio is quite a lot with the 5.8:1 top gear ratio staying unchanged, but the 1st gear ratio changes from 14.1:1 down to 18.1:1. To illustrate the difference, an 18" tyre turning at 1000rpm is doing approximately 100kph. Multiply by the overall ratio and you get an engine speed of 5800rpm in top gear. To achieve that speed in 1st gear on the standard cluster you need to spin the motor to 14,100rpm but with the wide ratio cluster it's 18,100rpm...! This is just the math, it is not recommended that you try to prove it at home by this method as it will be extremely messy....More practically, at 5000rpm you are doing 86kph in top, 35kph in standard first gear, but only 27kph using the wide ratio clusters first gear. On paper this sounds like quite a big change so my next step is to suck it and see. Making the swap this week on the test bike, and will advise how it works out in practise.

Gearbox internals from 1966 Triumph 350/500 parts catalogue.

Optional listings for wide ratio and close ratio clusters


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