Re: Double clutching (long)

[Josh Pinkert <flush@radix.net>]

At 04:36 AM 7/14/97 +0300, Steve Adams wrote:
>Double clutching came about because of non-syncromesh gearboxes.You
>simply can't engage a gear properly on a non-synchro box without
>matching engine revs to road speed for the particular gear you are
>engaging. The point of releasing the clutch and engaging it again while
>revving the engine is to allow the clutch plates to be spun up as well
>to match the gear and crank speeds.
>
>At least thats how it was explained to me! Probably someone out there
>who knows better!!
>

Correct, but here are some of the details as to what is going on.

Mechanically speaking, double clutching speeds up the input shaft of the
transmission enough to make the gear change.  On one side of your clutch,
there is the crankshaft coming from the engine.  On the other side is the
input shaft to the tranny with a small flywheel.  Then, of course, there is
the output shaft from the transmission.  All three of these have to be in
*sync* to execute a smooth shift.  The crankshaft is easy to control...just
blip your throttle.  The output shaft from the tranny is coupled directly
to your wheels (via driveshaft, half shafts, etc).  So that speed is pretty
much fixed as well.  So with the double clutch, you are controlling the
speed of the transmission's input shaft.

Figuratively, your driving along in 3rd gear at 3000 rpm traveling at 40
mph.  The clutch is engaged.  When you push the clutch in, the input shaft
to the transmission starts free-wheeling.  The more time you wait with the
clutch in, the more time it has to slow down.  A downshift is nearly
impossible without syncromesh.  To pull off a downshift to 2nd, you need to
speed the transmission input shaft up to 4000 rpm.  So you shift from 3rd
to neutral, and let the clutch out.  Now by blipping the throttle, you can
effectively control the speed of the transmission input shaft.  So you rev
it to 4500 rpm, push the clutch in, and shift into 2nd.  In a
non-syncromesh car, without that double clutching you'd have to grind the
gears long enough to speed the input shaft up to the 4000 rpm required to
*engage* the gears in the tranny.  Note that you can't even put the car
into gear...unless you perform a double clutch.

For upshifts, double clutching should not be necessary even on
non-syncromesh transmissions (the input shaft from the tranny is already
spinning fast because of the rpms from the previous gear).  But for
downshifting, double clutching is the ONLY way to get the gears to engage.
Without a double clutch downshift, you'll just grind away at your
non-syncromesh tranny.  The act of double clutching is not only used in
racing...trucks with slow and balky transmissions will downshift smoother
with a double clutch downshift.

So what does a DC buy you?  Maybe less wear and tear on your syncromesh
mechanism.  But then again, its there for a reason.  I DC out of habit,
even though my car doesn't require it.


- Josh Pinkert
- flush@radix.net