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Discussion Starter #1 (Edited)
clutch in, out of 3rd, clutch out, quick stab of the gas(this is where the input and output shaft get their respective groove on and match up nicely in speed - you're making things waay easier for the synchros), clutch in, into 2nd(silky smooth as you've gotten the input/output shafts to a similar speed), clutch out (and your foot should be steady on the gas while the clutch is coming out - a little more to accelerate, a little less to slow down)

Ok, so I recently told this to someone while trying to describe how to double clutch a downshift from 3rd to 2nd.

Any advice, comments, questions?

I'm pretty sure that's right, I've only double clutched once in a blue moon, maybe 50 times in my life.
 

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BTW this has been discussed a bazillion times, but you are correct. I also have refined my explanations over time, the more you understand the easier it is to explain.

In order to double clutch properly (and have it make sense) you first need to understand how the transmission and synchros work.

The input shaft and clutch disc spin inside hollow gears. The gears are "mated" to the shaft by the synchros only when a gear is chosen.

This input shaft spins 1:1 with the engine when the clutch is engaged, it slows when the clutch is disengaged due to oil friction. In neutral, the shaft simply isn't engaged to any gear. The gears floating on the input shaft are locked to the gears on the output shaft. Since the output shaft speed is directly related to wheel speed, all gears on the output shaft are always spinning the same rpm as if it were a one-piece shaft. The input gears are floating on their shaft, and because of the different ratios, the floating gears are always spinning at different speeds when the car is moving (i.e at 120mph 1st gear is spinning INSANELY fast around the input shaft!!! ...which would tremendously over-speed the engine if you tried to engage it).

so...
Simple (single clutch) rev-match down shift:
clutch in
rev engine and select lower gear
clutch out

When selecting a lower gear while the clutch is in, the shaft and clutch disc are being sped up by the synchro of the lower gear. (Remember the hollow gears around the input shaft are being forcefully turned by the gears on the output shaft). By revving the engine, you're trying to match the higher rpm of the gear you've chosen. Upon engagement, it feels smooth.

Double clutch rev-match down shifting:
clutch in
neutral
clutch out
blip
clutch in
gear
clutch out

When you push in the clutch and go into neutral, the input shaft is separated from the gears. By engaging the clutch in neutral and revving the engine, you're using the engine to speed up the shaft and clutch disc instead of the synchro. Now, when the clutch goes back in and you select the lower gear, everything is already at the same speed and the synchros do nothing. By the time you let the clutch out it feels smooth again.

I only ever single rev and heel/toe. Modern transmissions would only benefit from a very slight heat reduction at best by not using the synchros.
 

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Double clutch rev-match down shifting:
clutch in
neutral
clutch out
blip
clutch in
gear
clutch out
This is the correct sequence for a down shift. I haven't seen anybody mention how you double clutch on an up shift. It's pretty close, except replace the "blip" step with a "release throttle and wait for engine RPM to drop to appropriate speed".
 

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double clutching in my opinion is sort of useless, the only real advantage it has is it helps the next gear spin up(using the synchro and brass blocker ring) to match RPMs, and mainly reduces heat in the trans and wear, but unless you quite literally drag race EVERYWHERE you go, its problably gonna slow you down more than speed you up
 

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Discussion Starter #5
$0.02 - In a perfect world I'd be able to revmatch/heel-toe/double clutch in my sleep

double clutching in my opinion is sort of useless, the only real advantage it has is it helps the next gear spin up(using the synchro and brass blocker ring) to match RPMs, and mainly reduces heat in the trans and wear, but unless you quite literally drag race EVERYWHERE you go, its problably gonna slow you down more than speed you up
+1

Yes and no. If we had the time, training, and resources of a rally driver then I'm sure that with time, we'd have sufficeint skill and confidence to seize certain rare moments where we could use double clutching in situations that it would be more appropriately needed. However, as everyone is quick to point out, synchros do their job just fine and the chances of human error seem to increase exponentially when trying to perform a perfect double clutch downshift. Revmatching and heel/toe have always just seemed the more logical choice(2 out of 3).
 

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The critical thing, to my mind is that when driving near the limit around a corner, sudden big changes to power at the wheels (more OR less) can upset the balance of the car.
Result: at the least, you don't hit the line you want and end up with a slower
speed through the corner.
Worst case: spin. Which will really kill your time. ;)

The closer you are to the limit, the more critical being smooth is.

Since very little driving is really right at the limit, and driving at the ragged edge on public roads is simply not smart, whichever way you go about it is fine. Do what works for you.

Less time in neutral is good.
Smooth power delivery is good.

Rev. matching with a single clutch shift will get you less time in neutral.
Double clutching is smoother, when mastered.

The difference, as far as I can determine, really depends on the driver, driving style and condition of track/race surface.

I am still getting used to my WRX, but so far I seem to get smoother shifts double clutching.

Still gotta get the miles up a bit before I start shifting up at redline though, so that may change. ;)
 

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double clutching in my opinion is sort of useless, the only real advantage it has is it helps the next gear spin up(using the synchro and brass blocker ring) to match RPMs, and mainly reduces heat in the trans and wear, but unless you quite literally drag race EVERYWHERE you go, its problably gonna slow you down more than speed you up
Double clutching on upshifts can be very slow, and really has very little to do with drag racing. Big trucks sometimes use it because there's so much mass in the engine and transmission. Anyways, its not really a performance trick for "speed". When people refer to DC'ing with a performance car they are 99% referring to downshifts. If done correctly it will not slow you down at all because downshifting is done while the car is slowing down (typically while braking). It has more to do with being smooth during "off throttle" moments.

Whether DC'ing is "useless" or not due to how well synchros work, a lot of people want to learn because its fun and can help you become a better driver by being more connected to your car.

"Ragged edge" is relative, i.e. driving on snowy and icy conditions at 15mph may require just as much grace as an 80mph sweeper on a roadcourse.
 
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