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article details
Author Ryan Leecock
Categories All Honda/Acura
Create Date January 15, 2002 17:50
Last Update February 08, 2005 18:24
Double Clutching

By Phil Ethier

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 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 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. 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.
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.

For the other bit of confusion:

Left-foot Braking = Using your left foot on the brake instead of your right. Sometimes this is done in conjunction with clutch-less shifting. It is not the same as trail braking. I used to left-foot brake (in a straight line) autocrossing the Midget, as long as I did not have to shift. In the Sonett, I often found that I could adjust the attitude in sweepers by stabbing the brake with my left foot while on the power with the right. Such strategies are probably not in the cards for the Europa due to the steering column running between the clutch and brake pedals.

Trail Braking = Braking while turning. Done to unload the rears and transfer weight to the fronts to minimize understeer or cause oversteer. It can be done with either foot. Generally done in FWD cars while entering a turn. Also can be done to delay braking as long as possible at the end of an important straight, in conjunction with an early apex. Be sure you have runoff room if you do this. If you screw up, you will buy some cones, just make sure there is nothing harder to hit.

Left-foot braking and trail braking are not synonymous, but are often done concurrently.

Copyright 2002, Temple of VTEC

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04-21-2008 10:06
Why are you copyrighting something you pirated?   (Score: 1, Normal)
Okymek
04-12-2006 10:18
Would this mess up your engine?   (Score: 1, Normal)
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