Hard, but rewarding.
Double clutching is without a doubt a very popular driving technique, one that is still being used nowadays in particular situations or circumstances, like truck driving or when a cars’ synchros are completely or very badly worn out.
Nevertheless, what is it exactly about? Well, basically, it allows you to shift (downshift mostly) very smoothly in case of absence of synchromeshes in the gearbox. To go a little bit more into detail, let us go with a practical example: let’s say that (with a manual transmission obviously) you want to go down from third into second. Double clutching means that the first thing you want to do is pressing the clutch, which will allow the engine to rotate at its own speed while the clutch and the transmission will have matching speeds, faster than the engines’. You now move your gear selector from third to neutral. This will let the clutch and the transmission to now also rotate at different speeds. You then release the clutch, meaning that the engine and the clutch will now be coupled back together and rotate at a same speed. What is happening at this point is that you have synchronised the clutch and the engine and disengaged your previous gear, but the transmission (and so the gear you need to move in) will still be rotating at a faster speed than those two. Therefore, you blip the throttle, while you may or may not be holding the brake depending on the circumstances of why you are downshifting, and so you will get the engine and the clutch to rotate at a higher speed, very much near the one of the transmission. At this point, you can then quickly engage back your clutch and move your selector from third to second, providing this way a smooth and synchronised downshift, and thus completing the process.
Why double clutching is not very frequently used while upshifting is because when you are actually going into a higher gear, it means that your engine is already going at a speed that virtually matches the next gear. Therefore, they are already pretty much in sync.
With synchromeshes though, this process is basically unnecessary. You just have to heel and toe to provide a smooth downshift, given that the car doesn’t feature an auto-blip system in which case blipping is useless too, and the synchros will match the speed of the gear you have selected with your engine.
So why should we bother with double clutching in sim-racing? Well actually, just for more realism and immersion. While the heel and toe technique can suffice with road cars, unless we are talking about cars from the Veteran or Vintage era, it is definitely not accurate for sportscars and single seaters up to the early nineties. These cars featured dog-rings, rather than synchromeshes, and while this particular kind of setup, called a “Dog Box”, could allow you to shift without even working the clutch pedal, it was only in case of necessity that it was operated this way. Mark Donohue recalls an episode of his racing career when he had to work up a sweat to learn how to drive not double clutching a Ferrari 250 LM, which he was sharing with Walt Hansgen, only to find out, truth be told, that rev-matching was easier than he feared.
Here is an excerpt:
Walt drove the first two-hour stretch. When it was my turn, he came in and told me that the clutch linkage was out. It wasn’t possible to disengage the clutch at any time. I thought, “Oh ****! Now what?” He must have seen the panicked look on my face, because he got right to work explaining how to drive without a clutch. I could get going by using the starter with the car in gear. From then on I had to shift by matching engine rpms with the throttle. To upshift, I put pressure on the lever while still at full throttle, then let up on the throttle for just an instant, and it would slip into the next gear. The three-four shift was a little harder, because I had to move the lever laterally in the gate and match engine rpms better. Downshifting was somewhat like double-clutching without the clutch, with a little blip in neutral.
Racing drivers used to use this technique while competing and, in fact, those who still teach nowadays the racing technique to amateur drivers suggest learning and mastering the double clutching technique. While the use of it is, in truth, mostly useless today thanks as we said to synchromeshes, except in very particular cases as noted in the opening of the article, it is a testament to the importance it has had in the history of the motorsport. Even in a movie like “Grand Prix”, it can be clearly seen that all drivers (remember that while the film featured actors as main characters it was actually professional drivers who worked as stunts) are practising it.
And since sim-racing is the safest way to learn new driving techniques, because errors and mistakes won’t result in a fatal breakdown of your daily car, why not take the chance to try it just to learn something new, which as always in life may or may not also come useful down the line. As a plus, it would let you get closer to experiencing what racing drivers of the golden era and recent past had to endure inside a racecar for hours, sometimes even days!
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 Mark Donohue with Paul van Valkenburgh, The Unfair Advantage, Massachussets-USA, Bentley Publishers, 2000, p. 35.