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Double Clutching Explained
The basis of misunderstanding about double-clutching, rev-matching and
heel-and-toe is a lack of understanding of the basic way the
engine/clutch/transmission combination works. So take a deep breath and
follow along.
What do synchros do? They are used to make the the layshaft match the gear
that you are going to. To do the job in a car without synchromesh, you
have to make the layshaft go the speed of the new gear yourself.
(Note, I have always just called the gears on the layshaft "the layshaft"
so it is easier to visualize. This is a schematic description, not an
analysis of which gear slides where on which shaft, or how synchros work,
or what oil to use for them. So if you don't agree with the schematic
concept I outline here, tell me where I went wrong, and we will look at it.
Technical statements about gear movements on shafts or the like will be
sent to the bit bucket. I know that modern transmissions are constant-mesh
but it really does not change the concept.)
You have three rotating parts in series (1,2,3) with two ways to connect
and disconnect them from each other (A B):
1 A 2 B 3
engine -> clutch -> layshaft -> gear selector -> various transmission gears
1) The engine speed is controlled by the throttle alone when the clutch
is disengaged ("in"). It is equal to the layshaft speed when the clutch is
engaged.
A) The clutch disconnects the engine from the layshaft when you step on
the pedal.
2) The layshaft speed is equal to the engine speed when the clutch is
engaged. The layshaft speed is related to the road speed when a gear is
selected. Therefore, when the clutch is engaged AND a gear is selected,
the engine speed is related rigidly to the road speed. (Nobody spins the
wheels in this schematic outline!) When the clutch is disengaged AND the
selector is in Neutral, the layshaft coasts down freely, regardless of
either engine or road speed.
B) The gear selector disconnects the layshaft from the transmission gears
when you select "Neutral", and connects the layshaft to a specific
transmission gear when you select one.
3) The various transmission gears are being pushed around by the road
speeding under your car via the differential and driveshafts. All of them
are spinning at different speeds in rigidly defined relationship to each
other. If your tires are not slipping, the speed of the system is rigidly
proportional to your road speed.
If you have working synchromesh: You are loafing along in Third gear and
want to change to Second gear. When you put in the clutch and change the
gear selector from Third to Neutral, the layshaft is going a particular
speed that matched Third gear at the present road speed. If you do not
change your road speed (your brakes are broke, say), the layshaft is going
too slowly to match Second gear. As you approach the Second gear selection,
the synchros will speed the layshaft up to the same speed as Second gear is
now rotating. How do they do that? Ask a mechanic. They just DO it. I
have a rough intuitive grasp of how they work, but it does not matter here.
If you have no synchromesh (Crash Box City): You are loafing along in
Third gear and want to change to Second gear. When you put in the clutch
and change the gear selector from Third to Neutral, the layshaft is going a
particular speed that matched Third gear at the present road speed. If you
do not change your road speed (your brakes are broke, say), the layshaft is
going too slowly to match Second gear. (Sound familiar so far?) As you
approach the Second gear selection, the gears go GRAUNCH.
What should you have done? This:
Push in clutch Layshaft equals Third gear speed
Select Neutral Layshaft coasts
Let out clutch Layshaft equals engine speed
Blip throttle Engine and layshaft speed up to Second gear speed and a
little
Push in clutch Layshaft coasts
Select Second No GRAUNCH if you have the speeds right. Ideally,
layshaft has coasted down to exactly Second gear speed
Let out clutch System is all locked up in Second.
This is double-clutching. It INCORPORATES rev-matching, or there is no
point in doing it. Therefore, in my opinion, there is no reason to have to
say "rev-matching" to describe this process. The rev-matching is the
tricky part. How high do you rev it? Experience is the only teacher.
Does double-clutching do any good if you have a synchromesh box? Yes, it
makes the synchros work less hard than they were designed to, so they will
last longer. The synchros will make up for any errors.
Can you rev-match the layshaft without double-clutching? Yes. Don't use
the clutch at all:
Ease the throttle Takes the load out of the system
Select Neutral Layshaft equals engine speed
Blip throttle Engine and layshaft speed up to Second gear speed
Select Second No GRAUNCH if you have the speeds right
This works exactly the same in a synchromesh box as a crash box. Since the
layshaft is always connected to the engine, synchros will NOT help you. In
fact, I think errors in this method can wreak your synchros. Normally,
synchros only have to deal with the rotating mass of the layshaft. Now you
are asking them to deal with your whole engine. They ain't gonna like it.
You have to be perfect. This is what Rick Mears and many other
disgustingly able drivers do. I couldn't do it on a bet.
Can you rev-match the layshaft to the transmission gears by throttle -
blipping with regular single clutching? No. The layshaft is not connected
to the engine while the clutch is in. Those that recommend
throttle-blipping rev-matching are right that it gives a smooth transition
when they let the clutch out, but it does absolutely nothing to match the
layshaft and does not save your synchros any work at all. (See the way to
get around this in autocrossing below.)
OK, here is the hard part. Racing. You are NEVER loafing along in Third
gear. The scenario for double-clutching changes: You are blasting along
in Third gear and want to change to Second gear for the exit of the corner
you are approaching. When you put in the clutch and change the gear
selector from Third to Neutral, the layshaft is going a particular speed
that matched Third gear at the present road speed. When you brake hard for
the corner, the layshaft is going some speed that may or not be right for
the speed Second gear will be turning when you are ready to use it.
Classic driving style is to double-clutch into Second while braking. This
requires heel-and-toe to blip the throttle to rev-match the layshaft. The
only reason that this is the "classic" method is because they used the
engine braking in Second to help out their lousy brakes.
So here we come to the in my opinion part, where I tell you why I think all
the above work is not necessary. In autocrossing with modern brakes,
engine braking is counterproductive to slow down for a corner. It upsets
your brake balance and does not really help very much in slowing the car
down. So instead of revving the engine way up to make Second while you are
still going fast, wait to select Second until you are going the entry speed
for the corner. The synchros iron out the small difference between the
layshaft and Second gear.
Push in clutch and brake together Layshaft equals Third gear speed,
coasts down a bit
Select Second near end of braking The car has slowed, layshaft
coasts while you are still braking, gears
are slowing, no need to hang about in
Neutral. No GRAUNCH, the speeds are
close and the synchros work.
Off brakes and on throttle Here is where you do the rev matching so
that there is no big jerk when you let
out the clutch
Let out clutch It is still well before the (late) apex.
Add power Well, this what you came here for. Go!
Mind the attitude and unwind the wheel.
You will note that the actual downshift is one move, and each foot has only
one job to do. I am convinced that this method is the second-fastest way
to do this. Rick Mears will be able to do the left-foot-brake, no-clutch
method faster, due to the lack of transfer time of the right foot from
brake to throttle. But since most of us cannot pull that off at all, I
think my method is the fastest for non-gifted mortals. I always say to
make the synchros work for a living, but if you don't shift too soon, they
don't work very hard.
This works very well with a streetable car having a normal flywheel and
synchros in an autocross situation. The reason it will not work as well in
road racing is that the braking time is longer. A light flywheel that
allows the engine to rev down faster is not going to help either.
__________________
96 Civic Hatch
***12.9 @ 103 drag radials WOOT!!**
211 WHP @ 9500
JDM B18C (only 1000 miles on it 
Bored .20 over w/JDM Type R pistons
11.5:1 Compression
Portflow ported head
3 Angle Valve Job
Skunk2 titanium valvesrings/retainers
skunk2 stage 2 cams
skunk2 intake mani
00 civic SI tranny
Hyperclutch stage 2
light flywheel / crank pulley
JDM 4-1 header 2.5 inch collector
2.5 inch test pipe
5Zigen Fireball cat back
B&M FPR with gauge
AEM CAI
MSD ignition
P28 ECM OBD-1
Bubba Built and Dyno Tuned
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yellow dude is tunerjunction haha.
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