Double-clutch downshifts seem to be the butt of all car related jokes since F&F made the line famous. I have seen a lot of contradictory information across the automotive forums about how double-clutching a synchro-equipped transmission is useless, it's a technique only for big-rigs, it's slower then a traditional heel-toe downshift, etc, etc.
I double-clutch all my downshifts on synchro-equipped transmissions for two reasons. The first is it is easier on the gearbox. Double-clutching spins up the layshaft to match engine-speed and road-speed, virtually eliminating synchro wear on downshifts. The second is that downshifting is faster because there is no waiting on the sychro while applying pressure to the shift lever while the layshaft comes up to speed. Granted it only takes a fraction of a second for the synchro to do this, but there is zero delay with a properly executed double-clutch downshift.
Now, before you start arguing that double-clutching CAN'T be as fast as single clutching because it is so much more complex, let me explain how it isn't. When most people discuss the steps of double-clutch heel-toe downshifts, it goes something like this:
1. Right foot steps on the brake.
2. Left foot pushes the clutch in.
3. Right hand move's gearshift lever to neutral.
4. Left foot lets the clutch out.
5. Right foot rolls over and blips the throttle.
6. Left foot pushes the clutch back in.
7. Right hand moves gearshift lever to next lower gear.
8. Left foot lets the clutch back out.
9. Right foot lets off the brakes.
This serial processing is accurate when you are thinking about an action consciously. But when you process subconciously, your body can parallel process and do quite complicated things. Take walking for instance. Try and figure out all the steps required to walk down a hallway. There are so many it would be overwhelming and we'd decide to just stay on the couch.
So, when you consider how double-clutching works when your body processes in parallel, it isn't any slower. The reason is, you are not consciously going through the 9 steps listed above (at least once you've properly learned how). Things are going in that order, but the actions are occurring in parallel.
There are three ballistic actions that occur during a double-clutch heel-toe downshift. A ballistic action is an action that your brain starts consciously, but doesn't have to finish consciously because your body is programmed to complete the series of actions. Here are the three actions my body performs when I double-clutch heel-toe downshift. These are not in order, they are parallel processes that are synchronized appropriately.
1. Right foot depresses brake, throws in a throttle blip at some point and comes off the brakes later.
2. Right hand moves shift lever through neutral to next lower gear.
3. Left foot pumps the clutch twice quickly.
So, when you think about the body's ability to parallel process, it becomes a much simpler task of performing three synchronized actions simultaneously rather than performing nine separate actions serially.
To show how a double-clutch is no slower than a single-clutch, let's look at what happens during a single-clutch:
1. Right foot depresses brake, throws in a throttle blip at some point and comes off the brake later.
2. Right hand moves shift lever through neutral to next lower gear.
3. Left foot pushes clutch down for the duration of action 2.
So the only difference between the two is the left foot pumps the clutch twice quickly instead of holding the clutch down for a longer single cycle. The KEY here is that there is no pause while the transmission is in neutral. As the shift lever is passing through neutral, the clutch is out and the throttle has been blipped. No extra time was used.
Consider these three parallel processes (rather than nine individual steps) and watch the video again. It's clear that there are three processes taking place that are just tightly synchronized.
As for only using double-clutching for non-synchro transmission, my argument is that I left-foot-brake and don't use the clutch at all since there are no pesky synchros to get in the way...
