Double-clutching and Heel-and-toe
Dec 15 '00 (Updated Apr 16 '06)
The Bottom Line It's good for driving trucks, but also good for saving the synchromesh in a transmission.
Most readers are already asking why do we need double-clutching at all? The benefits of double-clutching is multifolded. First of all, it improves the longevity of your transmission. Secondly, it put the sport into sport driving.
Most of you already know about bliping the throttle when the clutch is disengaged during a downshift. This technique works for tranmissions with synchromesh. As the clutch is reengaged, the synchromesh matches the speed of the layshift with the drive shaft. www.howstuffworks.com has a good explanation of the manual transmission design (use keyword transmission to search for it).
The previous technique avoids excessive clutch wear by matching the speed of the drive shaft to engine speed. HOWEVER, the layshaft is still spinning at a slow speed when the lower gear engages. The sequence of speed matching with gas bliping while the clutch is disengaged is as follows:
1 engine turning at a slow speed, higher gear is engaged, layshaft turns at a slow speed (to match the higher gear)
2 clutch is disengaged, engine is now independent of the transmission
3 gear shifted to neutral, the layshaft is now independent of the driveshaft
4 blip gas, engine speed is higher (to match the need of the lower gear), layshaft is still turning at a slower speed to match the previously engaged higher gear
5 gear shifted to the lower gear, the layshaft needs to turn faster to match the lower gear, the synchromesh uses dynamic friction to match the speed of the layshaft to the speed of the driveshaft
6 clutch engaged, the engine is now connected to the layshaft, which is already turning at a higher speed
Although clutch wear is minimized, synchromesh wear still exists. This may not be an issue because the synchromesh only needs to match the speed of the drive shaft (lots of momentum) to the speed of the layshaft without the engine engaged. The layshaft has a relatively low mass, therefore it is easy to match its speed to that of the driveshaft.
In order to minimize even synchromesh wear, double clutching works as follows:
1 higher gear is engaged, engine and layshaft are turning slow to match the higher gear
2 clutch is disengaged, layshaft turns independent to the engine
3 neutral is selected, driveshaft and layshaft are now independent
4 clutch is engaged, engine and layshaft are connected
5 blip gas, BOTH engine and layshaft speed up
6 disengage clutch, layshaft is independent to engine
7 shift to lower gear, layshaft is already turning at a higher speed to match the lower gear, NO SYNCHROMESH ACTION!
8 reengage clutch, engine is already at a higher speed, no clutch action
Step 7 of double clutching is the key to minimize synchromesh wear. How much trouble is this worth? It depends on the durability of the synchromesh. Of course, the other factor is how often you need to down-shift.
The previous steps work when you want to downshift and accelerate. What if you are braking and want to downshift at the same time? Downshifting during braking has several advantages. First of all, when you are done braking and ready to accelerate, you are at the right gear already. Secondly, downshifting also allows you to use engine braking. Not only does this help decrease the wear on your brake pads, but it is also safer when you are on a *long* downgrade.
The steps are exactly like the previous 8-step sequence. However, this time you need to press the brake while performing double-clutching. How do we do this? You need to use your right foot for both the brake pedal AND the gas pedal. How you do it depends on your car and your right foot. I put my toe and ball of the right foot on the edge of the brake pedal, and use the outer edge of the same foot for the gas pedal. My knee is bends to the left when I only need to press on the brake, and swings to the right when I need to blip the throttle.
The full sequence of toe-and-heel is then:
1 brake with the right foot (toe and ball of the foot) and hold
2 disengage clutch with the left foot
3 shift to neutral
4 reengage clutch with the left foot
5 blip throttle with the outer edge of the right foot, keep pressure on the brake pedal!
6 finish blipping (lift outer edge of right foot off throttle)
7 disengage clutch with the left foot
8 shift to lower gear
9 reengage clutch with the left foot
It really isn't all that difficult, just needs practice. My tiny size 7 1/2 feet work just fine with my Subaru Legacy. You can probably get after market pedals and/or tuning to make it easier on your car.
Being able to toe-and-heel doesn't mean being able to do it quick! For SPORT driving, you need to do this quick as well.
To practice, I STRONGLY suggest an empty parking lot and a relatively used car. It also helps a lot to dry practice. Sit in your chair and practice the steps. I did that for two hours before going for a road practice!
If done right, both double-clutching and toe-and-heel are methods to improve the longevity of the transmission and safety.
Update (2006-04-16)
I continued to use this technique for four years before an automatic (minivan) replaced the manual Subaru in 2004. Two years after this change, I am now test driving used Mazda MX-5s with manual transmission. And, guess what? I have no trouble with the double clutch technique!
This is just amazing to me. At first, I thought that I would grind the gears because in my mind, I could not even recount the sequence of double clutch downshift (shame on me!). However, when I was driving the vehicle, I just did it without thinking. In fact, when the (potential) seller said that I should try single clutch downshift, I could not undo the double clutch instinct.
Note that I am not particularly skilled as a driver, let alone being an enthusiast.
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