"From David Lane:
Double clutching is really only useful for truck transmissions, which don't have synchromesh gearboxes, and is generally only used when downshifting.
I am not an expert on this, and it is complicated to explain without drawings or a working model, but I can probably give you a mostly accurate description of how it works. In theory, the transmission transmits power from the engine to the car's wheels and tires--we will call them the road wheels for short. In the simplest terms, the road wheels are connected to a single gear in the transmission. There are five gears of different sizes attached to the engine. Each time you upshift, a smaller diameter gear from the engine is meshed with the road wheel gear.
Now, since the road wheel gear can only turn at one rate of speed (depending on how fast you are going) the engine speed must change to accommodate the different gear ratios in the transmission. This is not unlike riding a multi-speed bike. If you go at 10 mph and switch gears, your legs will change speed while your road speed stays the same.
It would be damn difficult to change gears if the engine were always connected to the transmission. Fortunately, when you depress the clutch, the engine is disconnected from its gears. When you push the shifter to the next gear, there are "synchromesh rings" in front of the gear which matches the speed of the engine-driven gears to the speed of the road wheel gear--speeding the engine-driven gears up, or slowing them down as necessary. Once the synchromesh gets the two gears going at the same speed, the teeth will mesh and the shift is accomplished. Without synchromesh, if the two gears are not going at the same speed, the gear teeth will grind against each other, which is not good for the transmission. You have no synchromesh in reverse, which is why you have to be stopped to shift there. If you are rolling forward, the road wheel gear will be moving, and you will hear a grind.
Anyway, double clutching is a way to match the engine speed in a given gear to the road wheel speed, in a transmission without synchromesh. In a downshift, you depress the clutch and come into neutral. The engine needs to be going at higher rpms to accommodate the speed of the road wheel gear, so you let the clutch out (still in neutral)--which re-attaches the engine driven gears to the engine. Then you blip the throttle to increase the engine speed, which gets the engine-driven gears spinning faster, until they are going at the right speed to mesh with the road wheel gear. You push the clutch back in, and if you have done it right, the shifter will move easily into the lower gear. If you miss, you will hear a lot of grinding and probably a lot of cursing will come from your lips. Oh, well. That's why they have truck driver's school. Let the clutch out, and you are on your way.
As you can see, synchromesh makes the whole process much easier. When downshifting, you can hold the clutch in just once and blip the throttle, which will do some of the work for the synchromesh and make the mechanical parts last longer. If you are not exactly on target, the synchros will do the rest of the work for you.
Now you can see why upshifting does not require any fancy clutch work. When you upshift, the engine slows down between gears. It will do this anyway as soon as you take your foot off the accelerator and depress the clutch. If you pause for a moment in neutral, it will give the engine a chance to slow down enough to align the gears, and the stick will easily slip into the next higher gear.
If you are in a race, push harder on the stick and go directly from gear to gear. The synchros will match speeds for you.
In the broadest view, all we are talking about is smooth driving and saving some wear and tear on the synchromesh. There are a lot of old ladies out there who have never heard of any of this, and who's transmissions last a good long time. Performance drivers are much more likely to stress their cars, so they tend to find ways to make less work for the transmission if possible.
Here is an experiment for you. Go out on a road where you have some room. Put the car in third and go at about 3000 rpm. Maintain speed and see how fast the engine is going when you shift to forth. let's say it is about 2200 rpm. Now go back into third at 3000 and put a little pressure (one or two fingers) on the shifter toward neutral. Don't depress the clutch. Nudge the accelerator a little to remove the tension between gears, and the the stick will move out of gear, into neutral. Take your foot off the accelerator, and put a little pressure on the stick toward forth gear. When the engine rpm drops to 2200 rpm, the stick will probably slide into forth without you ever pressing on the clutch. If you miss, and the engine rpm drops below 2200, just give it a very little gas to get it back to that speed. When the engine speed matches the road wheel speed, it should drop into gear. That should illustrate how it works. Be real gentle with the shift lever, and you won't hurt anything. Push too hard, and you will hear a grind.
It is more difficult doing this trick going from forth to third, but all you have to do is use the accelerator to raise the engine speed while in neutral. Very few drivers know how to shift without the clutch, and I offer this description to you for your amusement and experimentation. Obviously I won't be responsible if you break something--although if you stay gentle with the shifter, the worst you may experience is a little, soft grinding sound. If you hear that, go back to neutral and either try again, or give up.
I had to learn this trick when I blew my clutch at an autocross, thirty miles away from home. It was my own fault, I lost track of how fast I was going and tried to downshift into first, which over-reved the engine and disintegrated the clutch friction surface. I actually drove the car home, matching gears, without ever using the clutch. I had to go real slow near stop lights, and once or twice I had to stop the car and start it in first gear to get going again. That was in the days before cars had to have the clutch pedal pressed in order to start them.
David Lane email@example.com "
-sorry so long...
-all new cars have synchros, so its really not necessary to double clutch --> still looks cool though!
-BUT downshifting with revmatching and heel-toe is damn good enuf. [ 04-27-2003, 02:51 AM: Message edited by: Jd ]