Double Clutching

Double Clutching

Understanding the transmission(understand this first…):


The green shaft is connected to the clutch disc, which contacts the flywheel(spinning w/engine) when the clutch is out(engaged) The layshaft (red) always spins with the green shaft, so if the clutch is in (disengaged), it will eventually stop spinning. When the tranny is in neutral with the clutch engaged, the green and red shafts spin at the same rate as the engine. The blue (actual gears you hear people talking about) and red gears always mesh, but the blue gears are not attached to the yellow (output) shaft. The blue gears just spin on bearings around the output shaft. The yellow shaft is connected (through other stuff) to your wheels and spins at a rate proportional to that of the wheels. The purple collars are attached to the output (yellow) shaft and spin with it. When you move the gear shift lever, it will cause the dog teeth in the sides of the collar (you’ve selected) to lock into the sides of the gear and cause the wheels to spin at the same rate as the engine(if the clutch doesn’t slip and nothing breaks:) Generalized flow: Wheels->yellow output shaft->purple collar->blue gear->red gear/shaft->green gear/shaft->clutch disc->flywheel->engine crankshaft.

The above is without synchros. So let’s say the green shaft (attached to flywheel) is spinning at 1800RPM when you’re in 4th at 35mph. So the leftmost purple collar is pushed to the right and locks 4th gear to the output shaft. Now you want to shift to 1st (where the engine would need to be at 5500RPM). So you push the clutch in and bring it out of 4th, and the green/red shafts slow down because the clutch is disengaged and the blue gears are spinning freely. Now, if you were to push the rightmost collar leftward (into 1st), the red/green shaft would have to be spinning at
5500RPM, and it’s probably at about 1500RPM by now. So the purple collar and the blue gear (1st) are spinning at very different rates. Consequently, the dog teeth will not mesh and you’ll hear this nasty grating noise. The solution to this(without double-clutching) is synchros. They’re cone-shaped protrusions from the gear near the output(yellow) shaft that gradually allow the purple collar to bring the red/green shaft up(or down) to the appropriate speed before the dog teeth lock into the gear. Just FYI, if you try the above scenario with a 3/S, you probably wouldn’t even be able to force the shift lever into 1st (probably because the synchro would have to do way too much work, and would probably start to slip). Without double-clutching, I can’t really shift into 1st until I’m below 15-20mph, and even then, it fights back.

Double-clutching(what you really wanted to know)….

Here’s the steps: (re-using the above scenario, 4th->1st @35mph)

1. In 4th, going 35mph
2. Clutch in, gearshift into neutral
3. Red/green shaft slow down since not connected to flywheel or wheels
4. Clutch out (engaged)
5. Red/green shaft spin at engine speed
6. Rev engine to 5500RPM (expected speed in 1st)
7. Assuming you’re still going about 35mph
8. Blue gears(connected to red shaft) spinning at rate close to that of the purple collar for 1st gear.
9. Clutch In
10. Gear shift to 1st
9. 1st gear synchro barely has to do work, so tranny slides smoothly into 1st.
10. Clutch out
11. Going 35mph in 1st with very little bucking, just louder:)

Try the above with your 3/S if you haven’t double-clutched before, you’ll
be surprised at how easily it shifts into 1st. It took me about 2 weeks or normal driving to be able to double-clutch almost as quickly as regular downshifting without thinking about it. Now it’s just habit- I don’t think about it at all.

Next try heel-toe braking while double-clutching/rev matching 🙂