What does "double clutching" do?

(2010-11-03 02:10:07)

1. Rhys Jones Says: What i mean is when your driving and press the clutch down, then release it and you get a little burst of speed, why is this, and does it damage the car in anyway?

2. MooCowMan Says: Holy cow, that was a long answer and I still have no idea what his point is.

It uses the intertia of the flywheel to give that little burse of speed. But you're not really gaining any speed. When you press on the clutch to disengage and let the engine reve higher, you've lost speed. When you let go of the clutch pedal and get that little burst, you'll just be catching up to the speed you were at before, but not quite.

It causes wear and strain on the clutch. Don't do it.

3. Divya Chutani Says: First, in most cars, the actual gears are always in constant mesh, and the synchronizers only decide which pair of gears to connect to their shaft. But many trucks and some racing cars are still set up with a non-synchromesh gearbox. With trucks, because they have so many gears, it's noticeably more efficient not to have all of the gears in mesh all of the time. So with the "crash-box," you HAVE TO double-clutch, or you will not be able to shift. The same holds true for racing cars—to gain the last couple percent of efficiency, only one set of gears is in mesh at any time, and you have to actually synchronize their speeds or you can't get it in gear. Despite the obvious drawbacks of having to double-clutch, the gearbox is stronger and more efficient than a comparable synchromesh one, and has less tendency to overheat.

Other reasons for double-clutching: Because it is the right way to operate the clutch. Because it saves wear and tear on your synchronizers in the long run, if you're planning to run your car over 200,000 miles, as I do. Because it is fun to do. Because in very cold weather, (-10° F, for example) you may have to double-clutch to shift gears at all, at least for the first few miles.

One very important reason is that, if your clutch linkage ever fails, you can still shift and get home by double-clutching, getting the engine and gears' speeds synchronized and then just EASING the shifter into the right gear. In the last 1,050,000 miles of driving VWs, I have lost my clutch about 3 times, and each time, with careful planning, I've been able to drive home safely. One time I pulled into the Customs House at Calais, Maine, and discovered my clutch was out. I eased along carefully and managed to get all the way home, 350 miles, to Boston, where it was convenient to put the car in the shop to have the clutch repaired—much more convenient than in the middle of a vacation, or the middle of Maine.

Another reason is that on some old cars, first gear isn't synchromesh, so if you need to shift into first without coming to a full stop, you have to double-clutch. Also, a lot of cars these days are made with weak, chintzy synchros, so they soon wear out, and to drive them gracefully, you need to double-clutch.

Note, when down-shifting, you have to shift into neutral and then blip the throttle momentarily before you shift into the lower gear. It requires practice and a good feel, a good touch, to do it right, especially considering the embarrassing noises you make if you miss your shift into a low gear on a crash-box. For example, you should aim to have the revs just a little high, so if you miss, the engine will soon slow down, and then gears will be at the right speed to mesh and the cogs will slip in....

OKAY, Pease, I'll try this double-clutching some day; but why do you bring up all this stuff in an electronics magazine? Ah, there's an excellent analogy: In most conventional switching regulators, the power transistor turns on while there's lots of voltage across it, and after it turns off, the voltage usually increases to a large voltage. When the transistors turn on, the diodes are already carrying significant current, and the transistors have to turn the diodes off. This is all somewhat stressful, and causes the transistors and diodes to have large turn-on and turn-off surges—pulses of power on every cycle. Of course, diodes and transistors have been designed to withstand these stresses and surges with excellent reliability; we see them all the time.

4. Larry E Says: When you press in the clutch and rev the engine and then let it out your car jumps a little bit because the engine is going faster than the car is moving. It's hard on the clutch because there is a little slippage when you reengage it. My old truck needs double clutched when you are down shifting because the syncros are worn out.