WIDE RATIO DUAL PURPOSE GEARBOX
Triumph's wide ratio four speed gearbox as fitted to the 1975 TR5T.
The nature of dual purpose riding off road requires a wide range of gear ratios, from a stump pulling first gear that will let you climb a cliff, to a relaxed over-drive top gear to get you home on the road. Modern Adventure bikes cover this ground with 6 speed gearboxes, so how did the Triumph riders of old cope with only four relatively close spaced gears? Well the answer is that Triumph did in fact list optional wide-ratio transmissions for their T100C models all through the 60s and 70s. These ratios were based on those developed for the pre-unit TR5 Trophy production competition models. Triumph's final twin cylinder off road fling the 1973 TR5T Trophy Trail used a similar unit. Triumph works ISDT riders used a combination of gearboxes depending on the model and the particular event. In less demanding terrain on 500 and 650 cc models the standard ratios might be used, but if used in really steep terrain and on especially on 350 machines, the wide ratio might be preferred. Another option used by the factory riders was to fit only the wide ratio first gear.
Sourcing a full wide ratio gearbox proved a matter of luck after I discovered that the lads at British Spares in Nelson were also using their Triumphs off road and were looking into solutions. It really is great when you find that the people supplying classic bike bits really are enthusiasts just like yourself.
To my extreme joy after a couple of months British Spares came up with real gold - a new complete wide ratio Triumph gearboxes, still covered in grease and brown waxed paper. Fitting the gearbox was a simple exchange, no modifications were necessary. How it works work in real life remains to be seen.
Here's the technical side of the story taken from the British Spares website.
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.
Sourcing a full wide ratio gearbox proved a matter of luck after I discovered that the lads at British Spares in Nelson were also using their Triumphs off road and were looking into solutions. It really is great when you find that the people supplying classic bike bits really are enthusiasts just like yourself.
To my extreme joy after a couple of months British Spares came up with real gold - a new complete wide ratio Triumph gearboxes, still covered in grease and brown waxed paper. Fitting the gearbox was a simple exchange, no modifications were necessary. How it works work in real life remains to be seen.
Here's the technical side of the story taken from the British Spares website.
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.