Now that I've actually had a valid reason to double clutch, and have had some time to get good at it, I feel that I am now in a better position to explain this better than I have in the past. Hopefully this will be the double clutching thread to end all double clutching threads :) Hopefully the layout of this post makes it easy to follow the train of thought and the thought process used to understand this technique.
Some back story... The 1g I just bought for a daily driver has a completely banjaxed 3rd gear synchro. It grinds badly with every attempt to upshift to 3rd, and downshifting to 3rd is not possible. The previous owner found that by using a super heavy weight oil, its not so bad. I believe it's 120 weight. This will be important later. I figured its time to learn to double clutch, in an effort to put off the 800 dollar tranny repair expense ;)
Lets recap what I have posted in the past about double clutching. The point is to manually synchronize the speed of the input and output shaft in the tranny so the synchros don't have to synchronize anything. This is especially useful in trannys that either dont have synchros, or don't have synchros good enough for downshifting (Some racing gear boxes and truck gear boxes have trouble with this especially, by design). This is not to be confused with rev matching, which is to rev the motor to where it would be in that next lower gear before the clutch is let out, to reduce the shock and keep the balance of the car. But double clutching on the down shift MUST include rev matching, so they are sometimes confused. ;)
The technique, on the upshift, consists of clutching, shifting to nuetral (between gears), reclutching (let up, push back down) while between gears, and shifting into the next gear. When downshifting the technique is the same but while the clutch is UP in the neutral/between gears aprt of the shift you blip the throttle to rev match (bring RPMs up to where they would be in the next lower gear at this particular vehicle speed). So you can see that there is nothing fast about this process, with all of the added steps. It takes much longer than a normal shift, so forget everything you learned in the F&F movie ;)
The theory will have to come after some explaination of how the motor, clutch, tranny, and car are all related.
The clutch will engange/disengage the motor from the input shaft of the tranny. The input shaft is connected to the output shaft only when the car is in gear, and the output shaft always matches the wheel speed since its a direct mechanical link to the tires (via rear axles, rear end, driveshaft, tcase, or front axles, front diff, etc). So when the clutch is up, the input shaft rpm will match the engine rpm. When you are shifting but in between gears (nuetral), with the clutch in, the input shaft is not connected to either the wheels or the motor, and is allowed to decelerate due to friction and the drag from the fluid (and stop if you wait long enough). The output shaft its mated to however is still rotating, connected to the wheels. So the job of the synchros is to to match the speed of the two shafts so the gear in question can be engaged. The gears themselves are constant mesh anyway, but the synchro/slider/etc is what locks that gear to its shaft making it a part of the shaft. Check out www.howstuffworks.com for an explaination of this with pictures.
Now, the problem comes in when the clutch is down and you are between gears. The input shaft is not connected to anything and is free to slow down and maybe come to a stop. If there are no synchros, or they are not capable of matching the speed of the shaft to the respective gear (as in my case), you get that sweet ass crunch when you try to engage the gear.
The solution is double clutching. You are in second gear. You put the clutch down, shift half way to 3rd (in between gears, or nuetral), let the clutch back UP, which accelerates the input shaft back to engine rpm (which has now fallen a few hundred rpm), you put the clutch back down and push the shifter into third. If you timed all this right, the engine speed was where it should be for 3rd gear at your current road speed (rev matched), and the input shaft was right there with it thanks to the double clutch action, and the synchro/etc engaged perfectly with no sweet ass crunch.
For downshifting: You are in 4th gear at some rpm. Put the clutch down, shift halfway to third, let the clutch UP, blip the throttle to raise RPM (say 1000 rpm higher, whatever the rpm would be in 3rd gear at the current road speed, again "rev matching"). Clutch back down, push the shifter into 3rd. If you did all this right, the engine speed was where it would be in 3rd gear at the current road speed (if you are good with rev matching this will come naturally to you), the input shaft was at the same rpm since it was connected to the motor while the clutch was up when you reved it, and the shaft speeds match and it slides right in with no sweet ass crunch.
Now on to my experiences. I found it difficult to get the hang of this. At first it just took time to get my hand and legs to cooperate with this bizarre driving technique. Then its all about timing. I would get it right most of the time, but then it would crunch again occasionally. Turns out I was just getting lucky some of the time. I finally found that while upshifting I need to pause slighty before reclutching and shifting into 3rd. This lets the RPM drop a little further so it matches the road speed in that gear. I wait until 4000 rpm, clutch in, shift halfway, clutch up, pause, clutch in and shift. I say those last two steps at the same time because this is important! If I wait too long after pushing the clutch in to shift, it crunches. Lets think about why. Remember back to that 120 weight gear oil. Thats some thick shit, and provides a lot of drag on the input shaft when it is left to spin down. By taking too long after the clutch is in, the input shaft slows down too much and the speeds dont match any more, and it crunches. So I put the clutch in and push it into gear at the same time. Problem solved. With thinner fluid this may not be so critical.
Also note that the length of that pause will depend on the RPM I shift at, which is why I mentioned it above. The higher the RPM, the bigger the drop in RPM in the next gear. This may seem like stating the obvious, but its very important. Shifting at say 6000 rpm, that pause has to be a little longer, and this is where practice and experience comes in handy.
Downshifting was much harder at first, but I can now nail that too every time. Again, like with rev matching, the trick is in how much you rev it up. Rev too little, things dont match up. Rev too high, things do match up. But, if you rev too high, you can increase the pause ;)
Thats the basics. I'm surprised that I was able to learn how to do this in just a couple weeks, and I'm more surprised at how much I learned about tranny operation and how all these elements relate to each other. Things I always knew, but this validates and reinforces things I've known for years but had no actual experience with. Good times....
