From: schaffer@cpsc.ucalgary.ca (Doug Schaffer)
Subject: Re: Double clutching & Toe-heel shifting
Organization: U. of Calgary Computer Science
Date: Thu, 2 Apr 92 17:59:15 GMT

In article <2248@isgtec.isgtec.com> karl@isgtec.com (Karl Sewlochan) writes:
[stuff deleted]
>
>A double-clutched heel-to-toe would actually take more time than a
>rev-matched heel-to-toe.  For example, going from third to second
>(eg. for a corner) the following steps would take place for a
>rev-matched heel-to-toe:
>    - brake
>    - clutch pedal down
>    - shifter to the gate of second (but not in yet)
>    - blip the gas to get the revs a bit above what you need it to
>      be if you were in second
>    - as the revs match the gate will "open" and the shifter will just
>      slide into second
>    - quickly let the clutch pedal up so as to engage the clutch with
>      the revs matched (if you let the pedal up too slowly then the revs
>      will fall and they won't be matched)
>    - still braking
>    - back on the gas (don't want to take a corner while braking)
>    - drive the corner
>
>Rev-matching allows you to quickly let the clutch up thus (among other
>things) reducing the time at the friction point and thus saving wear and
>tear on the clutch.
>
>Note: This is only one way to heel-and-toe.  You can also place the
>shifter into second and then raise the revs to the desired level and
>then let the clutch pedal up.  I prefer the first manner as it allows
>quicker shifting.  I am sure that there are other methods with their
>own pros and cons that someone else can explain.
>
>--
>Karl Sewlochan        ...uunet.ca!!isgtec!karl             karl@isgtec.com
>ISG Technologies Inc.  3030 Orlando Dr. Mississauga.   Ont.  Can.  L4V 1S8

I've been following this for a while, and I've got a couple of descriptions
of whats happening inside the clutch/transmission.  I don't purport to be an
expert on this subject, but the following excerpts give the best description
of what is actually going on during a double-clutch I've seen on the net.  
IMO, heel and toeing is simply driving technique to enable (people without 
three feet :-) to use all three pedals simultaneously for smoother shifts.

===========================================================================
I'm presuming the
"double-declutching" is to enable a shift fro  a high gear (eg. 4th)
to a lower gear (eg. 3rd).  When you blip the engine, with the clutch
in (pedal depressed) you only increase engine speed.  When you do it with
the gearbox in neutral and clutch engaged, you increase engine speed
AND the speed of the input shaft of the gearbox.

The input shaft has the gears on it which you are trying to "mesh" with
the gears that are directly connected to the output shaft (through
the layshaft).  So if you only increase engine speed and then try
to engage the lower gear, you will still "crash" gears due to the
speed of the input shaft never having been increased to match the
speed of the output shaft.

Now, I'm guessing this entire discussion must have something to
due with non-syncromesh gearboxes or a gearbox that has bad syncros,
is that correct?  The bottom line for any conventional gearbox found in
todays automobiles (manual full syncro gearboxes) is that double-clutching
(IMHO) is not ever required unless something is wrong.

Having said that, it is still a driving technique/ability that can
(if mastered) get a vehicle with a clutch that will not disengage (eg.
broken linkage or hudraulics) home by shifting without the clutch.
------------------------------------------------------------------------
Bill Sohl (K2UNK) BELLCORE (Bell Communications Research, Inc.)
Morristown, NJ             email via UUCP      bcr!dancer!whs70
201-829-2879 Weekdays      email via Internet  whs70@dancer.cc.bellcore.com
======== End of First Message ==============================================
The reason you have to let the clutch out while in neutral
*and then* blip the throttle comes from what it is we are trying to
acomplish in the first place (more below).  In neutral the gears *are
not* disengaged.  That's why they are called "constant mesh".  There
are *two* throttle blips in a double-clutch downshift as well as two
declutch-clutch operations.  Any less is not a double-clutch by simple
definition.

On most manual transmissions, reverse is not constant mesh.  Selecting
reverse involves actually meshing two gears' teeth together using
pressure applied to the shift lever.  All of the forward gears are in
constant mesh.  You may ask "then how can one shift?".  The gears
themselves mesh but the gears are not all splined to their carrying
shaft.  The transmission has two shafts, input and output.  Gear
selection is performed by locking one of the gearsets to the shafts.
In a syncromesh transmission, synchronizer rings "syncros" are used to
effect this process in a special way.  As pressure is applied to the
shift lever, two mating conical pieces of metal (pieces of a syncro)
are pressed together.  If the clutch is disengaged at this time, the
input shaft is physically disconnected from the engine and will begin
to change speed due to the friction in the syncro.  Once the two parts
of the syncro are spinning at the same speed, the syncro then slides
smoothly together engaging a number of small teeth and physically
connecting the gearset to both tranny shafts thus "selecting" that
gear.  In a non syncro but constant mesh tranny (like most
motorcycles) gear selection is performed by sliding a piece
("dog-ring") that is splined to the shaft into *lateral* engagement
with a gearset.  This operation is accompanied by a distinct "clunk".
On motorcycle transmissions, there is rarely a neutral between gears
and as such there is no such thing as a double-clutch downshift.
Engine rpms may still be matched so that the reengagement of the
clutch may be acomplished as smoothly as possible.

IMHO There is no point in double clutching on upshifts with constant
mesh transmissions.  In fact, with constant mesh but non syncro
trannies, use of the clutch during upshift is purely for reliablilty
reasons.  In a race, (car or motorcycle) you might use the clutch to
acomplish the 1-2 shift but from then on you just lift at the moment
you slam into the next gear.  If you use the clutch at all during
upshifts -much less double it- you will get blown away by those who do
not.  However, when downshifting it is *all important* in regard to
reliability (once again with non-syncro trannies).  If you need to
be smooth for example entering a corner under heavy braking with
wheels drifting, if you are sloppy with the clutch, you will upset the
balance of the car.  Of course, this upset only happens during the
final clutch which means in essense your approach is fine.

With a syncromesh transmission when braking that hard, you may have
difficulty selecting a gear rapidly enough.  This is where the first
clutch in a "double-clutch" comes into play.  Try this: drive down the
street in third gear at about 20 mph.  Now downshift into first as
quickly as possible.  It is not necessary to actually let out the
clutch once you have selected first gear.  Now try it again but go to
neutral, release the clutch pedal, blip the throttle, press the
clutch, and then attempt to select first gear.  Remember, you must
blip the engine up to or just above the rpm that it would be turning
if you were actually driving in first gear at the current speed.  I
think you will see what I mean.

Here is the complete procedure for a double-clutch downshift.  If you
use the brake simultaneously it is called "heel & toe" because you use
the toe of your right foot to brake and the heel or side of the right
foot to blip the throttle.

1. declutch (press the clutch pedal)
2. select neutral
3. clutch (release clutch pecal)
4. blip throttle to correct rpm to match *TRANSMISSION SHAFTS' SPEED
FOR DESIRED GEAR*
5. declutch
6. select gear
7. blip throttle to correct rpm to match CLUTCH speeds (flywheel to
transmission input).
8. clutch

Following are My Humble Opinions:
If this takes more than two seconds to acomplish you need more
practice.  An expert can do it in less than one.  If this takes longer
than to just lay into the syncro to get it into gear, it is a waste of
time.  Obviously, "laying-into" syncros will wear them out abnormally
quickly.  If you don't have syncros, this process *greatly* enhances
the life of numerous components in the drivetrain.  If you don't have
a neutral between gears (motorcycle) you simply can't do it ("it"
meaning steps 2-5).  If you aren't drifting (having to worry about
upsetting your car's balance) it ("it" meaning steps 2-5, 7) is
unnecessary.  Finally, (I don't know why I bother to address this but
here goes) regarding wear&tear.  clutch components only wear while
the clutch disc is slipping against the mating surfaces; the throwout
bearing only wears while the clutch pedal is depressed.  The syncros
wear only when pressure is being applied to select a gear.  Replacing
a syncro is undoubtedly more expensive than replacing the clutch disc
and throwout bearing.  The choice is up to you.  I personally believe
a person should drive the way they believe is proper.  When something
wears out, replace it.  Remember, 75% of people believe that they are
in among the "better" 50% of drivers.

{answer to specific question deleted}

Ok, that's my $0.02, now donning my nomex...
I welcome comment, criticism, contradiction, flames, and the like.
Erik
--
Erik van Bronkhorst KC6UUT erik@rftech.nwc.navy.mil DoD#4342585443 AMA#438054
======== End of Second Message =============================================

If anyone has a better description of what is going on inside the clutch
and transmission during a shift, I would appreciate a copy (email?).  Thanks.

Hope this helps.

Doug Schaffer, University of Calgary, schaffer@cpsc.ucalgary.ca