Double De-clutching
| Field | Driving |
| Went Obsolete | Somewhere around the 50s |
| Made Obsolete By | Newer gear-boxes (with synchro mesh systems) |
| Knowledge Assumed | Basic driving skills |
| When useful | Changing gear smoothly |
In very basic terms: Gear-boxes have two sides - an engine side and a final-drive side. To get a smooth gear change (and not damage the gear box) the speed of both sides need to be matched.
The idea of double de-clutching (also called double clutching) is that you clutch, shift to neutral, de-clutch, rev the engine to the right speed (which will affect the speed of the gears on the engine-side of the gear box), clutch, engage gear, de-clutch.
I think Leyland DAF invented the manual box with synchro mesh that made this obsolete, but my memory isn't so good.
If you want to know more, I'd recommend the Hillyers workbook designed for trainee mechanics. It's a big book, but covers all of this in great detail.
This is not completely obsolete, if you shift when there is a large difference between the gear speeds, you can actually grind the synchro-mesh.
This is still a necessary skill for driving a lot of race/rally cars which have non-synchro straight-cut gearboxes.
Actually, still necessary to drive many heavy trucks (semis, dump trucks, etc.). Commercial license manuals in all US states I'm aware of even detail the procedure and many require it to be demonstrated to earn a CDL (Commercial Driver's License).
Also, if the synchro gets damaged, this can be a useful skill.
Drivers of big trucks only use the clutch when making a complete stop. The rest of the time we shift by slipping the transmission into neutral, then matching the engine speed with the drive line speed, then slipping into the next gear.
Some drivers of big trucks use the above method (including myself) but it takes skill and experience. Double-clutching is still the accepted method of shifting. Regardless, the term "double-clutch" is unfamiliar to most drivers.
This is extremely useful when downshifting, particularly into first gear. You can downshift sooner, without grinding the gears.
Three speed manual transmissions without synchro rings on first gear were used - although not common - as late as the early 1970s. These "crash boxes" had to be double-clutched to shift down to first gear at anything other than a standstill.