>>Yes, now I'm thinking I've definatly heard of the term "double
>>declutching" used from those days :o) No idea what it is, but still...
>Simply as it sounds - declutching once to move from gear to neutral,
>then depressing the clutch again to slip into the lower gear. It
>was/is primarily for non-syncromesh 'boxes, ...
That sounds correct, but this next bit is quite wrong...
It's precisely --because-- most cars lack syncromesh on reverse that
it is counter-productive to double declutch when shifting into
reverse.
With the vehicle stopped, engine running at idle, transmission in any
forward gear (let's say first gear), and clutch pedal depressed, the
engine is disconnected (by the clutch) from the transmission's input
shaft. At the other end, the transmission's output shaft, linked to
the drivewheels, is not revolving because the vehicle is stopped. The
output gear (first gear) is locked to the output shaft via the
syncromesh gear dogs. The heavy lower countershaft, permanently
meshed with each of the output gears, is also stationary as it is
linked to the stationary first gear and output shaft. The
transmission input shaft is linked to the lower countershaft,
therefore it too is stationary. With nothing rotating in the gearbox,
and clutch depressed, it should be possible to shift from any gear
into any other gear including reverse with no risk whatsoever of
grinding gears.
Now try momentarily letting the clutch up in neutral before again
trying to select reverse.
With the car stopped, engine running at idle, transmission in NEUTRAL,
and clutch pedal LIFTED, the engine is driving the transmission's
input shaft which in turn is spinning the heavy lower countershaft
splashing oil up onto all the gears. The countershaft is constantly
meshed with (and thus spinning) each of the output gears. The output
gears are each spinning freely (disconnected in neutral) on the output
shaft. Only the output shaft is stationary as the car is stopped.
When you next depress the clutch, the heavy countershaft will continue
to spin for some time depending on the viscosity of the gearbox oil.
If the oil is cold and thick, the gears will come to a rapid stop.
But if the oil is hot and thin, the gears will continue to spin for
quite a while even though the engine is no longer driving them once
you depress the clutch.
If you attempt to immediately select reverse at this point, the gears
will definitely crunch. Reverse typically has no syncromesh. Trying
to select reverse in this manner introduces the reverse gear between a
stationary output shaft and the still rotating heavy countershaft.
Gear teeth don't enjoy that sort of abuse.
There are two easy ways to prevent crunching gears when selecting
reverse. You could depress the clutch and simply wait until you are
certain the gears have spun to a halt before sliding the gear selector
into reverse. Or you can depress the clutch, momentarily select any
forward gear, and then select reverse.
The forward gears all typically have synchromesh cups which act as
brakes to match the rotational speeds of the output gear to the output
shaft before mechanically meshing the gear dogs. Momentarily
selecting a forward gear will brake the rotating gear (and the
constantly meshed heavy countershaft and input shaft) to match the
same rotational speed of the stationary output shaft, thus bringing
everything to a smooth halt. Now, keeping the clutch depressd, you
can select reverse with no risk of grinding gears.
-Ferdinand-
