How do you double-clutch, and why?
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Double-clutching, or double-declutching (the British term), is a driving technique that went out of style in the 1960's. It was an important skill for racing car drivers in the decades before the advent of really efficient synchromesh mechanisms, and an absolutely necessary skill for truck drivers and race drivers alike in the early decades of the 20th century. Hardly anyone uses it anymore, even on a racetrack.
When a driver shifts gears, there is a mass of spinning metal inside the gearbox whose speed must change radically. Nowadays that mass is not so large as it once was, and modern synchromesh mechanism can bring the rotational inertia of what mass there is up to speed fairly easily, but there was a time when the driver had to:
1: disengage the clutch and shift the gearbox into neutral
2: engage the clutch again and simultaneously give the throttle a shot with the right foot to bring the gears up to speed
3: disengage the clutch and shift the gearbox into a lower gear
4: re-engage the clutch while bringing the revs up to match the higher RPM necessitated by the lower gear.
All this took much less time to accomplish than it takes to describe. An hour's practice is enough to learn the basic skill. Race drivers had a harder time of it because they had to blip the throttle with the same foot that was being used to stand hard on the brakes to set the car up to enter the corner for which the downshift was necessary. This was a technique called "heel and toeing". For a virtuoso demonstration of the art, look for a 1966 movie starring James Garner entitled Grand Prix.
So is double-clutching a skill worth learning nowadays? Hardly, but I still use it occasionally. I learned it driving a John Deere tractor that had a square gearbox, no sychromesh whatsoever. Coming fast into a corner these days, I just shift from 4th to 3rd in the normal way, letting the sychromesh do the work, but for a fast, immediate downshift from 3rd to 2nd, which would otherwise provoke a mild, protesting "grunk" from the gearbox, I double-clutch. Besides relieving stress on the gearbox it reminds me of my youth and a certain 1953 MG TD. Ahhhh well . . .Incidentally, you can double-clutch going UP through the gears too. In this technique you do not blip the throttle when the clutch is in and the gears are in neutral, for the object is to slow the rotating mass of gears down. There is only one time when the ability to do this is necessary, and that is when your clutch, for whatever reason, won't disengage. If you're good, you can start the engine with the car in 1st gear and then shift up through the gears without stressing the synchros and without using the clutch. In the 1920's most racing car drivers had to drive like that all the time, clutches being so delicate then that they were used only once, at the start of the race.