Graham - imagine a 4 speed box with a range change - effectively same as a Landrover 4 speed with high/low.
So you can go up through the Low box, then change range on-the-move by hitting a button / lever as you change from 4 back to 1 (which is now 5) and go through the second set of 4 (4 over 4 - except that on the Landrover 4L is about 2 1/2H so more like 4
trying to climb up on to 4
).
Then imagine on top of this the ability to automatically 'split' each gear using a simple button/lever - in other words get two speeds out of each gear stick setting with the ratio of the split gear 1/2 way between the two gears. If the ratio of the Fairy overdrive was different it might have been able to act as a splitter for each gear (ratio 1/2 way between two gears- as it is, from memory the ratios are all over the shop.
Here's a typical shift pattern with a single splitter button/lever - 4 speed box with range change (4 over 4) which with the addition of a split gives 16 forward gears total (18 includes the 2 x reverse gears):
So when, say, accelerating in 7L (which is gear stick in 3 high range position and split in Low), while the power is on you flip the split lever from L to H - nothing will happen until you momentarily ease off the power and the drive goes 'slack', at which point it automatically splits or changes up/ into 7H.
Power back on - then at some stage when slowing down (with truck pushing the engine) you may flip splitter lever back from H to L - again nothing will happen until you blip throttle when it will change from 7H to 7L.
At some point flip split lever from L to H - then when you use clutch and change gearstick from 7 to 6 (2 high range) it will automatically revert to H and you will have gone from 7L to 6H.
And so on -
6H split 6L split & gear stick shift 5H split 5L split + range change + gear stick change 4H split 4L split gear stick shift 3H split 3L.
Not surprising they're mostly automatic now.......but all the fun and skill has gone.
The
Twin Splitter is usually a 4 speed box without high/low ranges, but with 2 splits in each gear lever position, giving 3 gears in each gear lever position, giving 12 gears.
As Col says, once on the roll you don't need to use the clutch even when using gear lever to change gear if you get the engine revs right, but if you don't know or forget about the clutch brake, it gets
extremely embarrassing until you work out that there must be a clutch brake (once had a demo Foden 8 wheel tipper for a day - it was fine after the first 10 minutes
).
Then there are those people who just like to show off in old American trucks with a 4 speed main box and 3 speed auxiliary or 2 x auxiliary boxes - have a look on Youtube for some unbelievable cogging tunes, but it will seriously interfere with progress on Scrufts Junction.
T