Transmissions have been constant mesh since the 1930's, except for reverse
(exclude Aston-Martin and similar). That is, there is an input shaft,
primary shaft and a secondary shaft, with the pairs of gears for 1,2,3, and
5. The one piece primary shaft has the gears cut on it. The secondary shaft
has separate gears with the synchro cones/rings. In neutral, the input (from
the engine, via the clutch) spins and there's no connection to the output.
Select a gear and one of the sets of secondary gears is connected to the
primary shaft.

Now, the idea of double declutching is to perform what synchro rings do when
not worn out. The input gear speed, governed by the motor, has to match the
output gear speed, driven by the road wheels. When the speeds are matched
the desired gear can be selected without crunching.

So ... changing say 2nd to 3rd: release the accelerator, depress clutch,
select neutral, release clutch. The engine revs will slow down. When the
engine revs match the road speed for 3rd (this happens in a couple of
seconds), depress clutch and get it into 3rd. If you let the revs drop to
idle, you won't select 3rd without a crunch, and you'll have to give it a
blip in neutral.

In fact, you can change gears without a clutch, with care. I had to do this
once during a forest rally when the clutch cable broke. Getting started was
entertaining: select 1st gear and start engine...  drove the car 200km back
home clutchless too.

If you hold some de-select pressure on the stick in 2nd, when you release
the accelerator, the load comes off the transmission, and the stick will
glide perfectly safely into neutral. Keep some 3rd position select pressure,
and when the revs drop to the right speed, it will glide into 3rd, at which
point you get on the gas again.

So ... changing 3rd to 2nd ... always harder, but the same technique. As you
pass through neutral, with the clutch out, give the engine a rev. This
synchronises 2nd road speed with engine speed for that speed.

All this does is speed up the engine: when the clutch is in, it does not
cause the input side of the transmission to speed up. In fact it slows down
due to oil viscosity.

-
regards
Stewart DIBBS
www.pixcl.com/lancerproject

BTW, what exactly does releasing the clutch while in neutral do? Why
can't I just rev up the RPMs w/o releasing the clutch and not pausing
in neutral?

On May 15, 7:48 pm, runcyclexc...@gmail.com wrote:

You can try double-clutching, as was done in the days before synchros.
Basically, when you shift into third (either up into it or down into
it), you would pause while in the neutral position, release the clutch
pedal and rev the engine simultaneously, then push the clutch pedal
out again and complete the shift into third. I have to do this
occasionally on my MGB as the second-gear synchro is a little weak.

Dan D
'99 Impreza 2.5 RS (son's)
Central NJ USA

After several days of desert dirt road driving the 3d gear of my
manual impreza transmission strated having problems shifting: I always
get a grinding noise when I stick the 3d in. Two separate mechanics
say it's the 3d gear synchronizer and that the whole transmission
needs to be rebuilt, with quotes above $3K? I did not expect that
much, is it what it costs for Subarus? Can I just keep driving until
the car dies (it got 120K). I found that if I bring the RPMs as low as
possible I can shift the 3d fine, but this creates a delay in the
shifting.