I always thought it was double de-clutching.
I also thought you used it in older cars to spin the flywheel to match the rpm of the engine, which you dont need to do in modern cars?
I might be wrong and im sure ill be hung if i am
1. Double (de)clutching as people did back in the 1930s serves three purposes:
- allow the driver to blip the throttle to match the rpm between engine and gearbox shafts;
- allow the gear change without grinding, as there is no synchromesh to spin the gearbox shaft to required speed;
- protect the clutch (as 1920s-1930s fabric-lined clutches were much weaker).
2. Modern (post-WWII) synchromesh gearboxes use synchro rings to spin the gearbox shafts to speed before engagement. So the normal driving technique is to shift the gears with clutch down: just separate engine and gearbox, and the synchro does the rest.
During open road run at moderate rpm, any car can shift without clutch disengagement at all. But this is not good - it wears synchros down. Better wear down the clutch, which is cheaper and easier to replace.
3. By doing a double-clutch in a synchromesh gearbox (where it's not absolutely needed...), we gain three benefits:
- better rpm match -> less wear on the gearbox's synchromesh;
- better rpm match -> less jerking on downshifting. Try it in a Leon, after some practice to do it quickly. The shift is very, very smooth. Like there is no gear in the gearbox at all.
- (why not simply keep the clutch down and blip the throttle as many people do?) less wear on the clutch linings. This is the first reason for
double clutch, i.e. release the clutch during throttling up: if one revs up with clutch disengaged, the clutch disk will slip like mad, heat up and burn the linings. By leaving the clutch engaged during rpm match, there is no load on the lining and no wear.
Obviously, the rpm match is not precise (and there is no grinding sound to warn you when you mismatched). But the synchromesh will compensate for slight mismatches, 100-200 rpms upwards or downwards. This is what synchros are designed to do.
4. The point where the synchromesh is overloaded and performs poorly is the quick, violent downshifting from 4th gear to 2nd gear, jumping the rpm, diving nose-down and jerking the car. Jumping from 2000 rpm to 4000 rpm into an instant puts tremendous wear on clutch, gears and dual-mass flywheel. By training yourself to double-clutch most of the time, you avoid this.
5. Double-clutch is usually slower than clutch-down-shifting and it's not performed during street racing, drag racing, autocross or other forms of racing, if the gearbox is a road-model, with synchromesh.
Only WRC cars with dog-clutch gearboxes require double-clutch, while WRC drivers train themselves to do the shifting almost by instinct, faster than a road driver anyway.
