Soooooo i was watching Fast and Furious some days ago and i started to wonder what the hell does double clutching mean (Vin Diesel says “granny shifting, not double clutching like you should…”). I asked my all knowing brother and he offered a brief explanation. I have tried doing it and do feel a little extra oush but hey, I could be accelerating like hell instead of sticking the clutch back in, right? Any thoughts? Any street racing dopers out there?
BTW my car ain’t the fastest in the world but it’s decent (1.8 liter, 126 hp).
um that term is one I have Never heard applied to cars before, big ol 18 wheelers yeah but not cars. sometimes those big rigs need a little extra help between gears I guess.
Not much i guess. The theory i have so far is, by clutching again the car gets a little extra push but i’ve seen no significant difference in acceleration.
Double clutching, in my experience, was more useful in road racing as opposed to something like drag racing. The idea was that you wanted to bring the engine’s RPM’s up when you wanted to downshift to a lower gear, for a curve or whatever, while avoiding the bulk of the engine braking that would occur if you just straight downshifted.
Also, many trannies of old accepted the downshift more gracefully if if you did the double clutch RPM build.
Way back when automobile transmissions were not “synchro -meshed” double clutching was necessary so that the engine revs matched the transmission when changing gears…
Otherwise there were a could be a lot of loose teeth laying in the bottom of the box.
Double clutching or double de-clutching used to be essential to change down gears before synchromesh became available.
What is being done is matching the engine revs to the transmission revs before engaging the lower gear. This reduces wear on the transmission and avoids dangerously shifting weight to the rear of the car.
Tom and Ray mention that it helped you downshift with older, non-synchronized gearboxes. It was also useful to control engine braking - but yes, it is hard on the car.
Sounds like Tom and Ray were talking about up-shifting.
A Double clutch is when Aunt Ester hit Fred Sanford in the head with her hand bag and it shocked F.G.S. so much that he, 1, clutched his heart and fell in his chair, also, Aunt Ester forgot she had her silver plated Bible in her hand bag and that, 2, hurt his head which he clutched with his other hand. He then lost consciousness. When he woke up and was told what happened, he exclaimed, I double clutched!
Why can’t you just push in the clutch, rev the engine to the target RPM, move the stick into the gear you want to be in, then let out the clutch? Why do you have to put it into neutral and let out the clutch first?
First: Don’t watch the fast and the furious and think it has anything to do with cars. Anything they say about cars in that movie should be assumed faulty until proven correct.
Now, double clutching is done for downshifts only. You NEVER double clutch when you upshift. It makes no sense.
Proper double clutch procedure (downshifting!):
Push clutch in.
Move shifter to neutral.
Release clutch.
Rev engine to a slightly higher rpm than what the engine will be in the lower gear.
Push clutch back in
Move shifter to lower gear.
Release clutch.
So what does it do? It revs the tranny input shaft to the rpm where it will be in the lower gear. That means that when you attempt to move the shifter to the lower gear everything is already spinning at the proper rpm. Done right this pretty much eliminates wear on the synchronizers.
Yes, this was mandatory on old trannies that were not synchronized. It is still useful for downshifting multiple gears. For instance, I will often coast down in high gear to maybe 1200-1400 rpm. Then I need to accelerate and want 2nd gear. A quick double clutch will make 2nd just fall in place with no effort on the shifter lever.
I find it even more useful going into 1st gear while rolling. In many cars it is very difficult to get 1st gear while rolling. A double clutch will make the most stubborn tranny go into 1st.
It’s a historical question, actually. It’s wasted effort on cars made after the mid-1950’s. Before then, only the gearset you were using was spinning inside the transmission. To go from 1st to 2nd, you had to double-clutch (let up on the clutch pedal in nuetral and blip the throttle) to get the next gearset spinning before engaging 2nd. Otherwise, the gear teeth would get gnawed. Forget all that. Now, there are synchronizers to keep the gears spinning all the time. You no longer need to double-clutch unless you’re showing off.
Way back when, sports car race drivers did the “blip” thing routinely when up-shifting. Being able to accomplish “heel and toe” driving, where the heel of the right foot applied the brakes while leaving the toes of that foot just above the gas pedal was widely admired. The theory was that less time was required to get off the brakes and back on the gas. I don’t know if this is still in practice. I never heard of anyone routinely “double-clutching” when up-shifting UNLESS the synchronizers in the transmission were badly worn. When down shifting, “double clutching” simply allows a little extra time for the engine and transmission RPMs to match a little better.
Because you’re double clutching to reduce wear on the transmission synchro. When you rev the engine with the stick in neutral and the clutch out, you’re bringing the engine, both clutch plates, and the transmission input shaft up to the correct speed for the subsequent shift into lower gear. When you rev the engine with the clutch in, as in your example, you’re bringing only the engine and one clutch plate up to speed (since engaging the clutch opens a gap in the drivetrain); the transmission input shaft is still running at the lower speed. During your subsequent shift, the transmission synchro has to work to match speeds correctly between transmission input and output.
As I’m sure everyone is aware, my first car was a 1966 MGB roadster. It was not until 1967 that BMC provided synchromesh between second and first gears, so double-clutching to downshift into first was common and necessary.
Double-clutching is not used in high speed acceleration, but for downshifting and for when the engine is under load, like an 18-wheeler getting up to speed.
For acceleration, you don’t need to use the clutch. You must have a syncromesh trans. You get the revs up and even off the gas, so the drive train is not under load. You then shift to the next highest gear without clutching and while keeping the revs constant until the gears engage. Old VW Beetles were famous for snapping the clutch cable. You could drive them until you could replace the cable, by turning off the motor at a stop, starting it in gear and shifting without the clutch.
Used to shift without a clutch in an '87 Porche 944S all the time, just for the hell of it.
This was answered above, but here’s the brief version:
Why do you have to put it into neutral and let out the clutch first?
Because it not just a matter of raising the engine speed, the whole point is to raise the speed of a rotating shaft in the transmission. Gotta be in neutral and have the clutch pedal up to make that happen.
