taylor
1000+ Head-Fier
- Joined
- Feb 6, 2004
- Posts
- 1,089
- Likes
- 10
Quote:
Exactly.
However, in some old (and I mean really old, not like '10 years ago' old) transmissions, or heavy duty truck or race car transmissions, it is necessary.
The way your transmission works is that there different gears for each speed are always spinning. Moving the lever engages each set of gears. If you do that while it's connected to the spinning engine, it will destroy the gears. So you use the clutch. However, the clutch is still spinning at whatever speed the engine was at. Normally, when you engage the gears, they slow down the clutch to the same speed as the new gear it should be in. However, without a synchro, you need to do that yourself.
Let's say you're at 3,000 rpm in 1st gear, which is a 3:1 gear. So your flywheel is 3k rpm, your clutch is 3k rpm, and the driveshaft is at 1k rpm. You want to shift to 2nd gear, which is a 2:1 gear. So you disconnect the clutch and slow the engine down to 2k rpm. The driveshaft is still spinning at 1k rpm. The clutch, however, is still at 3k rpm, because of inertia. In a regular car, when you move the shifter from 1st gear to 2nd gear, it connects the clutch to the 2:1 gear in the transmission, and the clutch slows down to 2k rpm. That's because there are little clutch-esque things (synchro-something they're called) in the transmission to make it do that. They aren't as big as the regular clutch, but because the freely spinning clutch doesn't have nearly the momentum of the engine, they don't have to be.
In a car without them, however, you might grind some gears. So what you would do is go in 1st at 3k rpm. Then you would push the clutch down, and put the car in neutral. Then you would lower to 2k rpm. Then you would let the clutch, so that the engine, not the synchros, slows down the clutch to the right speed. Then, you put the clutch out again, and put the shift lever into second, and it will catch right away. Then you can let off the clutch and drive.
It's more complicated and takes longer, but some transmissions require it.
Double clutching is often confused with toe-and-heel clutching, which is completely different. Toe and heel is when you want to downshift while braking. Like if you're taking a turn in 4th gear, but you need to slow down, but you want to be in 3rd gear at the end of the turn, so you can accelerate again. So you put the clutch in, hit the brake with your toe, and hit the gas with your heel, to get it to the correct RPM for the lower gear you're going into.
Edit - heh got beaten to the punch!
| Originally Posted by pne /img/forum/go_quote.gif what I don't understand is, you are trying to drop the engine rpm to match the transmission speed correct? Once you press the clutch the engine is released from the transmission and already begins to drop rpm, what does letting it out in neutral, disengaging it, shifting up, and letting it out again achieve? Seems to me that all you need to do is clutch in, wait a fraction of a second for the rev's to drop, and shift up. |
Exactly.
However, in some old (and I mean really old, not like '10 years ago' old) transmissions, or heavy duty truck or race car transmissions, it is necessary.
The way your transmission works is that there different gears for each speed are always spinning. Moving the lever engages each set of gears. If you do that while it's connected to the spinning engine, it will destroy the gears. So you use the clutch. However, the clutch is still spinning at whatever speed the engine was at. Normally, when you engage the gears, they slow down the clutch to the same speed as the new gear it should be in. However, without a synchro, you need to do that yourself.
Let's say you're at 3,000 rpm in 1st gear, which is a 3:1 gear. So your flywheel is 3k rpm, your clutch is 3k rpm, and the driveshaft is at 1k rpm. You want to shift to 2nd gear, which is a 2:1 gear. So you disconnect the clutch and slow the engine down to 2k rpm. The driveshaft is still spinning at 1k rpm. The clutch, however, is still at 3k rpm, because of inertia. In a regular car, when you move the shifter from 1st gear to 2nd gear, it connects the clutch to the 2:1 gear in the transmission, and the clutch slows down to 2k rpm. That's because there are little clutch-esque things (synchro-something they're called) in the transmission to make it do that. They aren't as big as the regular clutch, but because the freely spinning clutch doesn't have nearly the momentum of the engine, they don't have to be.
In a car without them, however, you might grind some gears. So what you would do is go in 1st at 3k rpm. Then you would push the clutch down, and put the car in neutral. Then you would lower to 2k rpm. Then you would let the clutch, so that the engine, not the synchros, slows down the clutch to the right speed. Then, you put the clutch out again, and put the shift lever into second, and it will catch right away. Then you can let off the clutch and drive.
It's more complicated and takes longer, but some transmissions require it.
Double clutching is often confused with toe-and-heel clutching, which is completely different. Toe and heel is when you want to downshift while braking. Like if you're taking a turn in 4th gear, but you need to slow down, but you want to be in 3rd gear at the end of the turn, so you can accelerate again. So you put the clutch in, hit the brake with your toe, and hit the gas with your heel, to get it to the correct RPM for the lower gear you're going into.
Edit - heh got beaten to the punch!
