Alright, I'm back. Quick double clutching info (if any of your are familiar with my posts in the hybrid forum you know this is not going to be quick). There are 3 parts to your drivetrain. The engine/flywheel, clutch/halfshaft (which is probably the wrong name, but I'm using it for this example), and other halfshaft/diff/wheels. There are two shafts (I'm calling them halfshafts but I know the wrong name, I just can't remeber the correct one) that run parallel to each other in the tranny.

The flywheel is obviously bolted to the engine and turns the same speed as the engine no matter what. The halfshaft/diff is attached to the wheels and can only turn how fast the wheels are turning. The clutch/halfshaft is the one your concerned about when you double clutch. When the car is in gear and the clutch is not pushed in, everything is turning the same speed. When you push the clutch in, the engine disconnects and only the clutch/halfshaft and diff/halfshaft are still turning the same speeds. Slide the tranny out of gear and they all turn seperate speeds (they'd all start to slow down at different rates, in that example). If you put the car in neutral and leave the clutch pushed in, the engine is turning whatever speed it is (idle or if your holding the gas) and the halfshaft/diff is turning whatever speed the wheels are. The clutch/halfshaft is just freewheeling.

Lets say for a second that you don't have any syncro's in your tranny (you do, but some much older and some racing transmitions do not). When you push the clutch in and slide the tranny into gear, a shift fork engages one of the gears on one of the halfshafts (usually the clutch/halfshaft, but not on all trannies). If the clutch/halfshaft and diff/halfshaft aren't turning the same speed, your going to grind your gears. With a syncromesh tranny, when you start to engage the gear, the syncro's go to work and help to spin that clutch/halfshaft up to speed so it doesn't grind.

Lets say your moving 40 mph and you want to go into 3rd gear from 4th (you would need to up the revs for the new gear). With a syncromesh tranny, you would only need to push the clutch in, slide the shifter from 4th to 3rd (the syncro's will equalize the speed of the clutch/halfshaft diff/halfshaft, they work very fast), tap the gas to bring the engine rev's to around 3000 rpm (what most Honda's are at about 40mph) and let the clutch out and you'd have a smooth shift.

If you don't have syncro's, You would have to push the clutch in and slide the tranny into neutral and then release the clutch and use the engine to spin the clutch/halfshalf up to speed. You'd then have to push in the clutch again and put it in gear before the clutch/halfshaft slows down. If you did it correctly, when you disengage the engine, the two halfshaft are at equal speeds and the shift will be smooth. If not, you'll get grinding as it equalizes anyway. You then need to keep the engine rev's at the correct level and let the clutch out to re-engage the engine. That's double clutching, fun huh.

Most (probably all) modern tranmsitions use snycro's. They make rev matching a lot easier and they are also very tough and will last the length of the car. The downside is they do take up space in a tranny, specifically part of the gears themselves. This means you have to make the actual gears smaller to accomodate them (smaller = weaker). Syncro's also weigh something, it's not much, but on serious racecars, they're a luxury that can be skipped for weight savings. If you've ever heard of Honda's 3rd gear syncro's going bad, I think there's a reason for that. When your drag racing (or any racing) you sometimes leave your hand on the shifter, in 3rd gear specifically. What kills those snycro's is overengaging the gear (pushing and holding the shifter up rather than letting it rest in the gear) that doesn't allow them to disengage and rest. I don't know if that's true or not, but it's been my experience and I still need to rebuild my tranny. Seeya.