Double clutching demystified

[Ken Chambers]

What exactly is double clutching some might ask? Does the driving technique
actually work? And are people
who attempt the procedure doing it correctly? In this age of modern automotive
advances, double clutching is
certainly an arcane and nearly obsolete technique passed down from yesteryear.
And sometimes the details get
lost in the translations.

Double clutching can be defined by engaging then disengaging the clutch
(releasing then depressing the clutch
pedal) while the gear shifter is in the neutral position when shifting between
two gears. But there's more to
it than that. Simply popping the clutch out in neutral doesn't in itself get
everything spinning at the right
speed to shift the gears. Furthermore, double clutching is usually only used
when downshifting - but there's
still more to it than that.

One of the objectives in shifting is to get from one gear to the other without
grinding them. The way to help
do that is to match the input shaft speed to the output shaft speed as closely
as possible. Now the input
shaft is connected to the engine via the clutch, while the output shaft is
connected to the wheels. However,
when the clutch is depressed the input shaft is disengaged from the engine and
is free to begin spinning
down. So when upshifting, the clutch is depressed at a high rpm and the input
shaft begins to slow down.
That's the ideal condition for shifting into the next higher gear. You want the
input shaft to slow down to
match the next selected gear.

Fortunately, our Amphicar transmissions have fully synchromeshed forward gears.
Synchromesh means there's an
internal mechanism used to get the gear speeds the same as they engage so they
don't grind. In normal
upshifting the synchros don't have to work very hard to mesh the gears since the
rpm's match pretty closely.

Downshifting is the problem. If you haven't heard it before, you haven't been
paying attention. Brakes are
much cheaper than transmissions. If you care about your transmission, downshift
only when your life depends
on it, like on steep grades. While it might seem cool to downshift every stop,
you're doing your beloved
Amphicar transmission a great disservice. Here's why.

Getting back to those synchros and speed matching. When you downshift you go
from low rpm in a high gear,
depress the clutch, rev up the engine a bit to match the speed of the next lower
gear, push the shifter into
the lower gear and release the clutch. Bet you thought you matched up all the
gear speeds about right by
revving up the engine during the shift. Right? Wrong!

Remember, when you depress the clutch you disengage the input shaft from the
engine. Your clutch was
disengaged when you revved up the engine. The input shaft was slowing down
precisely when you wanted it to
speed up. The synchros took up all the work of getting the rotational mass of
the already slow and
decelerating input shaft and clutch disk up to speed to mesh with the
corresponding gear on the output shaft.

So how can you save your synchros and get the input shaft turning faster during
a downshift in preparation of
the next lower gear? You guessed it. By double clutching. The ONLY way to
speed up the input shaft is to
double clutch. Let's go through it step by step.

You're in a high gear at low rpm, depress the clutch, shift into neutral,
release the clutch and rev up the
engine (and input shaft) to above where you think the rpm will be right for the
next lower gear, depress the
clutch again, shift into the next lower gear and release the clutch. You've
just matched the gear speeds
pretty close so your synchros didn't have to work so hard.

Sound easy, try it. Your synchros will thank you. If you've never double
clutched before you'll certainly
feel like a klutz the first few times.

Hope this helps.

Happy shifting,
Ken Chambers, CA
'64 Red

 

[nelson625@aol.com]

Ken-
Your dissertation of "double clutching"' was interesting and quite
comprehensive. I pesonally have never found any necessity to double clutch my
Amphicar, but of course, with the Synchromesh gears, it theoretically shouldn't
be necessary. However, you stated that "double clutching is usually only used
when downshifting." As I already said, I have never found it necessary to
double clutch my Amphicar. However, if you are speaking in general terms,
almost
anyone who drives a Model A Ford or similar pre-synchromesh vehicle or even an
early "crash box" transmission VW (i.e. without synchromesh), will tell you
that they routinely double clutch when upshifting to avoid some grinding of
gears unless they are in absolutely no hurry to get up to speed (up to speed
meaning 25 or 30 mph when you drop into high gear with e.g. a Model A Ford). If
one
doesn't double clutch these cars routinely when upshifting, it will
definitely shorten the time before some new gears will be needed in the
transmission
and nearby drivers will be treated to attention getting noises.
Vic "Splash" Nelson


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[nelson625@aol.com]

Re: Re: Double clutching demystified

Ken-
Again, thanks for the double clutching discussion. If you have a friend
who drives a Model A Ford (and it is probably the very commonest antique car
around), ask him if he doesn't frequently or routinely double clutch when
upshifting. With the Model A's, I do frequently "synchronize" the shaft speeds
when
downshifting into 2nd from high (3rd) exactly as you described with a quick
rev up with the acccellerator after the clutch is in, then smoothly go into 2nd
as it revs down. Practice a few times and it becomes natural as you would
agree.
You are a lucky guy to have had your DUKW ride. We had them in Daytona,
but that ended 2 or 3 years ago before I investigated them. Likewise, they
stopped this past April in Tampa Bay for insurance reasons, subsequent to the
Louisiana tragedy 3 or 4 years ago. I'm hoping they will overcome the Insurance
problem and resume. I know they are still going in Branson and Boston and the
Dells and a few other places, but none are super convenient to the Daytona area.
Your story of your mom driving the DUKW is great. It reminds me exactly of my
first Goodyear Blimp ride when I was joking with the pilot, prior to
boarding, as another Pilot was bringing her in. I said something like "we'll get
a
chance to pilot it too, right? - (totally joking,) and he said something like
"we'll see." By gosh, once aloft, sure enough each of us (that one held 6
passengers + the pilot) got a chance to pilot her. To me that was a thrill. As
proof that it is not necessarily what you know, but who you know, I've since
piloted 3 of their ships numerous times, inlcuding their now defunct ( due to a
mechanical failure) high tech Spirit of Akron, sometimes with only 2 of us on
board for 4 or 5 hours at a time. Talk about fun. I hold a Commercial Pilots
license and have several hours of "dual instruction" time in Blimps which is, in
a
sense, a bit like our Amphicars - something quite different that most people
will never experience. That's off the subject of Amphicars, but I'm still
plotting to get a DUKW ride before they all become extinct.
By the way, as others may have noticed, in the last Hemmings there is a
Schwimwagen listed for sale. (Th VW Amphibious version, as you all doubtless
know.) I called about it and spoke with the seller. He says it is in New Jersey
and is fully restored. The asking (maybe firm, I have no idea) price is $
58,000. From what I've seen on TV and read, they are pretty seaworthy. However,
since the prop gets its power directly from the air cooled rear engine's pully,
it is always turning in only one direction, which means they have no reverse.
In fact, the only neutral is by pulling on a rod to disengage the prop from
that pulley. The engine would be the original 30 H.P. I am fairly certain (= 34
P.S. European measure) which is quite adequate. I have no idea of what speed
they can go on land or in the water. On land, I would suspect 50 to 60, but I
wonder if any of our members own one and could say how fast they are in the
water. Very likely there is a Driver's Report somewhere which would answer that
and other questions relating to Schwimwagens.
Vic with the 67 Aqua "Split Personality"


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[Ken Chambers]

Vic,

I agree double clutching is not necessary in our Amphicars or other synchromesh
transmissions for that
matter. I was speaking in general terms in an attempt to explain exactly what
double clutching is, how it
works and why it might be used. I'll have to take your good word on what
technique some drivers use to shift
early non-synchromesh transmissions. You can't argue with success.

The topic of double clutching was on my mind after a recent family vacation to
the Wisconsin Dells. While on
a DUKW ride there, I was observing the relatively young driver double mashing
the clutch pedal to the floor
on each and every shift. He said the drivers are trained to shift the DUKWs
that way. While I had to wonder
if that was really necessary for upshifting, I did observe him doing the
"proper" downshift method. That is,
when releasing the clutch in neutral he raced the engine to get the input shaft
rpm's up before letting it
drop into the next lower gear. Ahh, the fine art of gracefully shifting a
10-ton DUKW.

A funny side note on that DUKW ride. I considered myself fortunate to have
ridden shotgun during that trip.
A few days later when visiting my family in the Chicago area, my 75 year old
mother said that was nice but she
got to drive one the year before while in Branson with her friends. "WHAT?!
You actually got to drive a DUKW
... from the left seat?" "Yes", she said. "When we were in the lake, the
driver asked me if I'd like to move
over and steer for a while while he entertained the passengers." Way to go Mom!

Getting back to smooth shifting our Amphicars. Having synchromesh transmissions
they should not grind when
shifting. If it does then something is wrong. It really gets down to mechanics
or driving technique.

As far as the latter, finger tip shifting really is the way to go. Remember
that input shaft and clutch disk
that starts spinning down when you step on the clutch? As soon as it reaches
just the right speed, the
selected gear will drop right in with just light finger pressure on the shift
lever. Ham fisted power
shifting is torture on the synchros, as well as downshifting.

Grinding due to mechanical problems could be worn synchros or it could be
something more easily fixed. The
fact that most Amphicars are really low mileage vehicles makes it more likely
its not an internal transmission
problem.

Make sure your clutch cable is properly adjusted. The Amphicar maintenance
manual states there should be
0.75" of free play at the pedal and 0.06" (1/16") at the release lever on the
transmission.

It's possible the clutch cable is stretching or has some broken strands. I have
found that free play at the
pedal increases after the vehicle is driven and warmed up. The symptom there
would be increasingly difficult
shifting if the clutch is not releasing enough.

I'd like to investigate someday if the clutch cable mounting points are stronger
enough and not flexing - like
our steering box mount.

Synchros rings operate with a film of oil between them and last a long time
unless abused or the oil is
abrasive. Good clean transmission lube is essential.

The talk of magnetic drain plugs is a good idea. Does anyone know the thread
size and pitch of those drain
plugs? It would be great if someone knew of a source or could have them made
available.

All the best,
Ken Chambers, CA
'64 Red




> Ken-
> Your dissertation of "double clutching"' was interesting and quite
> comprehensive. I pesonally have never found any necessity to double clutch my
> Amphicar, but of course, with the Synchromesh gears, it theoretically
shouldn't
> be necessary. However, you stated that "double clutching is usually only used
> when downshifting." As I already said, I have never found it necessary to
> double clutch my Amphicar. However, if you are speaking in general terms,
almost
> anyone who drives a Model A Ford or similar pre-synchromesh vehicle or even an
> early "crash box" transmission VW (i.e. without synchromesh), will tell you
> that they routinely double clutch when upshifting to avoid some grinding of
> gears unless they are in absolutely no hurry to get up to speed (up to speed
> meaning 25 or 30 mph when you drop into high gear with e.g. a Model A Ford).
If one
> doesn't double clutch these cars routinely when upshifting, it will
> definitely shorten the time before some new gears will be needed in the
transmission
> and nearby drivers will be treated to attention getting noises.
> Vic "Splash" Nelson


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