re: double clutching v. Fred shifting (Long)

["John & Deb Dohrmann" <jdohrmann@domain.elided>]

Lets clarify that double clutching involves releasing (engaging) the clutch
while in neutral as part of shifting up or down between gears.

Picture the clutch-trany system as having three sections:  The
flywheel/pressure plate which always turns at engine RPM, a center section
including the clutch friction plate and the front half of the transmission
that is turned by either the clutch or the rear section but will slow down
in neutral, and the output end of the transmission that always turns with
the wheels (through the differential).  The flywheel and center sections
connect through the clutch and, since the clutch slips while engaging, can
be connect with a significant speed difference.  The center and output
sections connect through the five gears and reverse.  The center and output
sections will not fully mesh unless they are turning at the proper relative
speeds for the selected gear.  The synchros use friction to bring the center
and output sections to the correct  speeds.  Gear clash comes from forcing
the center and output sections to engage before they are at the right
relative speeds.

Upshift
From a dead stop, the center and output sections connected through 1st and
zero rpm.  Assume engine is at 1200 rpm.  Slip clutch until car is going 6
mph (in my spider) when the flywheel and the center section will both be at
1200 rpm, and release clutch completely.  All three sections are connected,
give it the gas and accelerate to 4000 rpm (20.7 mph).
Fred shift by fully depressing the clutch and lifting foot from gas AND
putting the center section of the transmission into neutral so it will slow
down.  Engine speed drops to 2400, which is the engine rpm for 20 mph in
second gear.  But before second gear will engage, the center section of the
transmission has to slow down from 4000 rpm to 2400 rpm.  Fred shifting
relies on the friction and lube drag to slow the center section while it is
in neutral and disconnected on both ends.  With practice, you shift the
center section into second gear just when the relative speeds of the center
and output sections are close enough for the synchros to handle (and the
synchros only need to change the speed of the center section).  You release
the clutch and go on.
Doing the same upshift with double-clutching, you  would depress the clutch,
close the throttle, shift into neutral, and then release the clutch.  This
connects the center section to the engine.  When the engine speed drops to
2400, you know the center section is at the right relative speed and (with
or without the clutch) will engage with the output section with little need
for the synchros.  In fact, the synchros are little use because the output
section is turning with the wheels and the center section is turning with
the engine and it takes real work to change the speed of either.

No really much difference in upshifting, in both cases hearing the next
engine speed is important.

On to Downshifting.

Say your in third gear at 2400 rpm and 30mph.  You want to downshift to
second.  30 mph in second is 3500 rpm for the flywheel.
In a Fred downshift, you hold the gas steady and depress the clutch and
shift into neutral.  Engine and flywheel rev up quickly to 3500 rpm.  But
the center section is turning at less than 2400 rpm, due to friction and
drag.  You then shift into second, with the synchros applying the necessary
force to accelerate the center section of the transmission from under 2400
rpm to 3500 rpm.  Once the gears engage, you release the clutch and are in
second.
Double clutching the same downshift, you would depress the clutch while
keeping steady on the gas, shift the center section into neutral, and engage
the clutch.  You give enough gas to rev the engine to 3500.  Now the
flywheel and center section are at the right rpm to engage second gear.  You
depress the clutch (or not) and shift into second, release the clutch and
you are there.
In the downshift case, double clutching reduces the work done by the
synchros, because you use your control of the engine rpm to bring the center
section of the transmission to the correct speed to engage the gears.

Bottom line, I don't care how Fred treats his cars but I THINK I'll keep
double clutching downshifts on my car.  I don't believe the Italian tune-up
applies to synchros.
--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided