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I found this on the net. Article starts after the dotted line.
---------------------------------------------Everything you ever wanted to know about Double-clutching. (and why you don't have to know it) By Phil Ethier ([email protected])

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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 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 La
 
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