Double Clutching
 

I read an article on the Motor Trend news ticker list about how manual transmissions are becoming even more rare etc etc.... Yet again though, the function of 'double-clutching' was brought up. Could someone explain to me what this double-clutching is? I drive a stick Cavalier and find that disengaging the clutch once works sufficient enough for me to change gears. Do you need a certain type of clutch for double-clutching? Race application? What does it do? Advantages/disadvantages?

It was mainly an an idea used in pre-synchromesh transmissions, to minimize time between disengaging from one gear, and engaging another. The engine and transmission RPM would be changing, so the plan was to clutch into neutral, adjust engine RPM using the throttle, clutch again into the desired gear, therefore (hopefully) better matching the RPM the engine and transmission were at, or soon to be at.

Synchromesh pretty much spelled an end to double clutching, but some people still do it anyway, for effect. If you'd like to hear what it sounded like, watch the movie "Bullitt", as Steve McQueen goes through the gears in that Mustang, which legend has it he drove himself in the chase scenes. It's a shame about that Charger, though.

Essentially you would only need to double-clutch on a worn out / unsynchronized transmission, although double clutching can extend the life of the synchros (although modern transmissions are pretty robust and can usually go beyond 300,000mi)

Here is an example:

I am slowing down off the highway, I'm going about 60mph at 2500rpm in fifth gear. To slow down, I want to downshift to fourth gear. Normally I would declutch, move the stick to fourth and re-engage the clutch, which would force the clutch disc and input shaft to speed up from ~2500rpm to let's say 3400rpm instantly. To double clutch, I would declutch, shift to neutral, engage the clutch, rev ("blip") the motor to 3300rpm , declutch, move to fourth, engage clutch and smoothly let off the throttle to slow down.

Sounds terribly difficult, right? Well it isn't with practice.

mariner> Did you notice that the Charger somehow loses five hubcaps during the chase??

You noticed that too? I thought I was the only one who picked apart car movies for inconsistencies. Is it sacrilege to say that the Dodge sounded really sweet? I wonder if they dubbed in the exhaust note for dramatic purposes? And why does McQueen double clutch almost every shift? Poor synchro in the 'Stang? :D

I should stay on topic. As a toy, I once bought a 1956 GMC truck, with a 3 speed on the column. As there was NO synchro, you either learned to double clutch, or the transmission took a lot of abuse in a big hurry. I have no doubt that had I not learned to double clutch, I would have had it for a matter of days only. And as Smart Aas says, it's really easy to do, once you try it for a while. Even I learned how to do it.

Thanks for clearing that up gyus.

Semi-truckers still use this technique.

Same result as if you were doing a heel-toe shift. As you come into a corner or are going to be down shifting you push in the clutch and shift gear and while applying the brake with the toes on your right foot, you press the gas with your right heel to get the revs up to get the engine and tranny speeds the same when you engage the lower gear and let up on the clutch. This is a vital technique for autocrossers. I've read about people doing both, but haven't felt adventurous enough to try. Getting the heel-toe shift was one thing, trying to get them both seems like way to much work and unnecessary movement, but I guess it all depends on the situation and what kind of car you're driving.

double clutching = bullshift

its really moronic. i've tried it many times. doesnt really make a difference, might as well jsut power shift. besides double clutching isnt for rookies, if you can successfully do it, then you're pretty good. but its not that big of a change. if your lucky you might get .01 less on your 1/4 mile time slip.

just know when to shift (rpm) and do it fast. that's always worked for me. i shift so fast i get rugburn lol. i know people that shift faster than me though. my friend has an rsx-s with a cold air intake. runs 14.8 thats almost stock and he runs 14.8

that's skillful driving. he shifts like a madman on speed with crack all sprinkled in a jar of espresso.

Double clutching is mainly for down shifting. You don't have to double clutch to upshift because you want the engine to drop in RPMs.

The point is this. When you downshift the input shaft is going slow, but needs to be sped up to ease the engagement with the lower gear. Sometimes even on a synchronized transmission this needs to be done.

-T