Double Clutching. why? (1 Viewer)

[Ryan Tsang]

My understanding is that you double clutch when you downshift to match revs so as to not jerk the car and reduce wear and tear. Can someone outline the procedure because I'm thinking it is not necessary. I mean, why can't you just press the clutch down ONCE, shift gear while stabbing the throttle, then let go of the clutch (release) when the revs are right. I don't see the "double."

 

[Yee-Ming]

I've heard of "double de-clutch", which only relates to big lorries and the like: they have a heavy-duty clutch which requires operators to "pump" the clutch pedal twice in order to disengage the clutch and free the gears, unlike ordinary cars where a single press frees the clutch already. apart from that, I can't think of any reason to double anything when downshifting (and I do drive a manual).

 

[Evelio Figueroa]

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 18 wheelers are still set up with a non-synchromesh gearbox. With trucks 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. Unless you know how to "float." Thats shifting gears without pushing down on the clutch. Something I'm very good at.

 

[John Miles]

Ryan, what you describe is exactly the right thing to do on all modern passenger cars with synchromesh-type gearboxes. On older gearboxes without synchros, you needed to release the clutch temporarily while in neutral to bring the transmission input shaft up to speed before selecting your gear. But not anymore.

Some people still double-clutch, though, on the grounds that it theoretically makes life easier for the synchros. But in reality, as long as you make it a habit to match revs, you don't need to worry about it.

 

[Shane Bos]

So is that still what is implied in the Fast and the Furious about "double clutching" even in a Japanese sports car?

 

[Greg Kolinski]

ignore anything from "the F&F" it was just funny entetainment,not a whole lot of techical correct automotive /racing stuff.Ive been a dealer tech for almost 20 years,and watch that movie just for the humor ."example" the Charger pulling the front wheels waist high.......while blistering the tires to huge clouds of smoke:frowning: after around 15 years of playing around at the local 1/4 mile track,i have NEVER seen that happen about the only thing technically correct is that people WILL put a huge useless wing on the back of cars

 

[Todd Hochard]

and stickers. Don't forget the stickers.

 

[Charles J P]

Double clutching while drag racing would be about the surest way to lose the race (it would be way to slow, even if you were good at it). People that hardcore about racing would likely shift without the clutch, damn the transmission.

 

[Dave Falasco]

Shane, I've been wondering about that for a while myself--what the heck is "granny clutching" versus "double clutching"? I'm relieved to hear it is a bunch of hooey; not that it matters in my '93 Toyota Corolla.

 

[Scott L]

I may be wrong but I think granny shifitng is not pressing the clutch in at all. Jsut matching the gears to the higher one so the clutch doens't need to be engaged.

Here are some cool videos for download on upper level shifting tecchniques:

How to drive an M5

Check out the Porsche uphill video

 

[Carl Johnson]

i'm no auto expert but if that's the way that granny shifts i would think a single trip to the store is all it would take to burn out both her clutch and transmission. am i wrong here?

 

[Michael Varacin]

"People that are hard core about racing would likely shift without the clutch, damn the transmission."

Uh, not quite. (I am a race car driver.) We do shift without the clutch, but baby the gear box. That mentality ensures you will not finish. We do what is called in generic terms "heel and toeing." Since our gearboxes have no synchros, each gear must match the output shaft speed. So here's how it works:

On the up shift, it's a no brainer. Say I'm at 6700 RPM in 4th gear, but for the same road speed in 5th gear the engine would need to be at 6300 RPM. I simply lift of the throttle for an instant, pull back on the lever, and it falls right into place. Then I slam the throttle back down. The whole event takes place in under .1 seconds. (It can be seen with the data acquisition system on the car.) For the brief moment the throttle is lifted, the engine RPM drops close to what it needs to be for the next gear, in this case a 400 RPM drop. Since everything is matched speed wise, the lever drops right in with a flick of the wrist. No clutch ever needed. It's a very sweet feel. You could almost blow on it to get it into the next gear.

On the down shift, it gets a bit trickier since we need the engine RPM to be higher to match the next gear. So I simply pull the lever to take it out of gear, then while it's in neutral, I blip the throttle with the "heel" of my foot that has it's "toes" on the brake pedal. (Or the right foot if I'm left foot braking.) This brings the engine RPM up to the right speed for the next gear. Then I simply flick my wrist again to drop it into the lower gear while the engine RPM is still hi. No clutch ever needed. This becomes second nature, and happens almost as quick as the up shift.

Of course, street cars never have the pedals set up good enough to do this.

 

[Charles J P]

Uh, not quite. (I am a race car driver.)

Well, I assume you would agree that race car driver and street racer are not the same thing. What you are describing is exactly what I am talking about (shifting without the clutch) and I even do it sometimes in my regular car (Mazda Protege 5). Done right it is not bad for the clutch or gearbox. I can do it best from 2nd to 3rd and 3rd to 4th. 4th to 5th wants to grind, and 1st to 2nd is hard because my first gear is so low that the RPM spread is higher and sometimes hard to get (ie going from 2nd to 3rd when at 3000rpm in 2nd = 2000rmp in 3rd which is easy to hit, but 4000+ rpms in 1st might only be 2000 rmps in second so its harder to not try to shift too soon or to let it drop past your mark and then you have to try to blip the throttle again.