Just to fire this up - here's some footage of Walther Rohl in the Group B Audi.  That left foot braking is incredible.  The crowds are just insane also.

Both.

Heel/Toe is primarily to ensure you have no engine compression lock up on the drivien wheels by matching road speed to engine revs.
In a car with syncromesh (in good nick) you don't need to double, the syncros do the gear speed matching for you, you are just matching revs to road speed.

In the Dulux i double clutch back to 2nd to avoid grinding, sometimes to third - just trying to make the gearbox last.  If your syncros are good you don't need to double which is slower.  Doubling does match engine to road much better but it is slower.

I agree with Colin - in theory it shouldn't take any longer during your braking.  Maybe when I learn to brake a bit later I'll find it harder...

I also tend to skip gears on the big braking corners - e.g. at Honda I go fourth to second gear directly, no way my gearbox would handle that without double declutching.

I've been double-declutching for 18 years, so i definitely find it faster than just heel-and-toe.  I can DD without thinking, I'd need to think to h&t.  I find when I'm right up it, while DDing I can literally change gear as quickly as my hand can move the gear lever if there was no resistance at all (which of course is the result when you get it right anyway).

DD is much smoother than h&t also i find, but that maybe cause I'm more experienced at DD.  I think the braking is a good point also, and one I bought up when we were having our slightly-drunk-and slightly-heated-discussion-about-it.  Under hard braking you have ample time to DD through every gear.  But as said, with a fast car, that may be a different scenario.

Either way, either technique is an absolute necessity on the track, or even fast road.  I can change down smoothly in a half-second, and still be pulling 5 grand in the lower gear, right in the meat of the power.  try letting the clutch out without blipping the throttle while you do that as you're turning in.

I was taught to DD by my Dad, who'd learnt drive in cars that required it.  And he probably assumed that one day I would have a car that also required it.

Once he saw that I could do it, he taught me how to change gear without the clutch, which is something relatively easy to do, but only if you're pretty good at double declutching.  He also said "don't EVER do this in a Japanese car".  Sorry Mum.

You're probably thinking "why would I ever need to change gear without the clutch".    It's come in handy for me once when I had to nurse a car home with a buggered clutch.  Also, I have a mate back home in NZ who has done Formula Ford, now doing Formula Toyota (or whatever it's called), he said that in FF, he was always left-foot-braking and changing without the clutch, and if you weren't doing that you'd be back of the field. 

It's not something I do at all nowadays, but as far as I'm aware it doesn't harm the gearbox at all if you get it right.  Happy to be corrected on this.  (Dad if you're reading this, feel free to use the clutch on my car at Phillip Island in December!)

It's not Formula Toyota, it's the Toyota Racing Series.   Looks like a good class, wish they had it here.  http://www.toyotaracing.co.nz/

Sheldon always told me he was a big fan of DDs.  Didn't know it had anything to do with gearboxes though.

I tripped across this by mistake, thought it might be of interest.

Taken from http://www.hiperformancestore.com/guiboDSclutch.htm



Double Clutching, or how to not wear out your syncros.

     This was originally written and posted on gtv6.org which is now alfagtv6.com. Since then I still have not managed to wear out a single syncro. They seem to last nearly forever if you do the work for them by double clutching whenever possible. Save the syncros for when you really need them. For example during a critical drag race you DON'T want to double clutch on an upshift despite what Dominic says in The Fast and The Furious. This is when it's acceptable to put extra wear on the syncros.

     I seem to have run into quite a few people who are not too clear on the method of double clutching, nor the reasons.
Well the reasons are simple, double clutching will greatly reduce your syncro wear, in fact if you could do it perfectly all the time it would totaly eliminate all syncro wear. In many transaxle cars with worn syncros, double clutching is the only way you can downshift at speed without grinding the gears, and in some really worn out gear boxes it may be the only way you can shift period.
Before we get started lets have a basic review of what goes on in your gearbox. Inside there are two main shafts, lets call them the INPUT shaft and the OUTPUT shaft. The Input shaft spins at the same speed as the engine when the clutch is engaged (that means pedal up). The output shaft is connected solidly to the wheels via some other gears, half shafts, etc. so it spins at what we can call road speed. Get it so far? Input shaft spins at engine speed, output shaft at road speed.

     Now when your car is in nuetral these two shafts are not connected which is why you can rev the engine up in nuetral with the clutch engaged and the car does not go anywhere. In order to make the car move we need to connect these two shafts together, we do that with one of the gears. Lets do that with first gear in ohh, say an 85 GTV6. With the car at a stop, first gear selected and the clutch DISengaged (pedal down) the shafts are connected but not turning. Remember they can't be turning because the output shaft spins at road speed and the car is stopped. The input shaft is not connected to the engine because the clutch is disengaged.
Now, we will let the clutch out and the car will start moving because the clutch by definition engages and CONNECTS the engine to the Input shaft of the transaxle (via the driveshaft of course. Now the Input shaft is turning at engine RPM and the Output shaft is also turning but a whole lot slower due to the 3.5:1 first gear. That means that when in first the input shaft spins 3.5 times for every one time the output shaft spins.
Now the fun part. We rev our already warmed up car up to 6300 RPM in first and shift into second (just a normal shift, we are not double clutching yet). Now what happens? As we all know this will result in a big drop in engine RPM. IN fact it will drop to about 3600 in this particular year GTV6. However the output shaft continues to spin at the about the same speed because road speed has not changed much during the shift. However what is the input shaft doing? Well, it's spinning a whole lot slower, it just dropped from 6300 rpm to 3600 rpm. So, what caused it to slow down while the clutch was in during the shift? If you guessed SYNCROs your right! The syncro is simply a metal ring that uses friction, like a break to slow down the input shaft to match the speed of the output shaft so the two shafts can be connected by the gear. Every time you shift up a gear the syncro slows the input shaft enabling it to be reconnected to the output shaft. If the syncro is worn and it can't get the shafts to the same speed you will hear that horrible crunch and grinding sound.

     Now the more the syncro has to slow down the input shaft the harder its job is. A shift at max RPM causes a much bigger RPM change then a shift into second at 3000 rpm. That means that the syncro will have a much easier time dealing with a 3000 RPM shift then a redline shift. So your car is much less likely to grind with a low RPM shift and will have much less wear on the syncros. Of course it's not fun to drive around at low RPM all the time so we need another way to reduce the load on the syncros, and I am getting to that.
Now lets consider a downshift. A downshift is just the opposite of an upshift meaning engine RPM and Input shaft speed will increase. That means the syncro will need to speed up the input shaft which is a whole lot harder then slowing it down on an upshift. On an up shift the input shaft slows down on its own and the syncro just helps it along but on a down shift the syncro has to speed that shaft up a couple thousand RPM and that's very hard on it. Again a key to minimizing the wear is to shift at the lowest speed possible and only go down one gear at a time. A shift from 5th into second at 40 mph will require the syncro to speed that shaft up a whole lot more then a shift from fourth to third at twenty for example. In fact even if your syncros are new they may give trouble in that 5th to second scenario and won't last long if you do that.

     Understand that that was just a very basic non tech description of how the transaxle works but should give enough info to understand double clutching.
Double clutching is a method of reducing the load on the syncros by matching the speed of the input shaft to the output shaft manually during shifts. During an upshift it will be neccesary to manually slow down the input shaft and during a down shift you will need to speed it up.
How do we do this? It's easy, at least in theory although it will take a little practice. First of all lets sit in the car, start the engine and rev the motor with the clutch engaged (pedal up) and the car in nuetral. Right now you are speeding up and slowing down the input shaft. Whatever speed the engine is spinning, thats the speed of the input shaft. As soon as you depress the clutch pedal the input shaft will start to slow to a stop since it is no longer connected to the engine. After you depress the pedal wait a moment for the shaft to slow down to make things easier on the first gear syncro which will need to stop the input shaft. Now lets get the car rolling by letting the clutch out and run the car up to max RPM in first. Now we are going to DOUBLE CLUTCH into second gear. First let off of the accelerator press the clutch pedal in and move the shifter to nuetral. Let the clutch pedal out and wait a moment. Now engine RPM and consequently Input shaft RPM will begin to drop. Thats exactly what we need. In an 85 GTV6 we know that it will need to drop to about 3500 RPM to be equal to the output shaft, in a Milano 2.5 it's closer to 4000rpm due to its closer gear spacing. Once the engine is approaching to desired rpm (it's better to do this too early then too late) press in the clutch pedal and select second gear. If you time it right the syncro will only have to slow down the input shaft a couple hundred RPM instead of the 3500 it needs to slow during a normal shift. Even the worst syncro can handle this job so you can understand why this method reduces wear and enables shifting with really worn syncros.
Practice upshifting this way and soon it will be second nature and you will be able to do it really fast with no wear or damage to your transaxle. You will soon learn just how long you have to pause with the clutch out in nuetral for the RPM to drop the right amount. It's a little different in each gear and for each year but basically it's about about a two second pause for the first to second shift and about one second in all the others except fifth. Shifting into fifth the RPM drop is so minor that double clutching is not needed but if you want too, make the pause as short as you can and it will be about right.

     OK, now the hard part the double clutching down shift! This is what separates the amatures from the experts.
Really it's not that bad Lets start with a fifth to third shift at around 60 mph. This is the typical passing scenario. First of all the car at 60 will be at about 3000 rpm in fifth. In third gear it will be turning around 4000 rpm at 60, that means we will need a 1000 rpm INCREASE.
We start out by pushing in the clutch pedal, putting the car in nuetral and letting the pedal out. Now we can speed up the input shaft to 4000 RPM with the accelerator pedal. Once it is at the required RPM or close enough push the clutch pedal in and select third gear, then let the pedal back out. This really takes a lot of practice but once you get it you can downshift to second or third from and other gear smoothly with no gear crunching or grinding. When practicing this don't get too hung up trying to look at the the tach and match rpm, you just need to blip the throttle to an RPM higher tnen needed and let it settle into the desired range and then select the gear before the input shaft slows down too much.
Good luck and lets preserve those syncros.