Question about double clutching

Do double clutch, you
first put in the clutch,
then put the gear in neutral.
Then let out the clutch.
Then rev match, ie raise the revs for the downshift.
Then put in the clutch and
then let out the clutch.

Is that right?

What good does letting the clutch out after you put the car in neutral? Can’t you raise the revs with the clutch still depressed?

You have got the sequence right. What you are doing when you double clutch is that you are spinning up the clutch disc; input shaft;counter shaft and gears; and main shaft gears to match the speed of the output shaft to complete the gear engagement. If you are really good you don’t need to declutch after the first steps – just match the speed; slip the shift sleeve over the gear dogs and gas it. Most drivers are not that good so they rely on the syncronizer/blocker to match any difference in the spin speeds before the engagement occurs and the clutch is reengaged.

Some drivers don’t even bother to declutch once. They release the gas until the torque is relieved on the exit gear; shift to neutral; rev the engine to the next gear speed; use the syncro/blocker as an indicator for the engagement; shift fully into the next gear; and back on the gas. If yiour technique is sloppy, you will be wearing the syncro ring which is expensive to replace.

Hope this is enlightening.

Double clutching is used more with bad or non-existent synchronizers (like used to be for 1st gear in some manual transmissions). The better technique to use for smooth downshifts is called ‘heel and toe’ downshifts, where your right foot heel’s on the brake while your toe give the gas a nudge to match revs.

But I never used it in 20 years of driving a manual…

Until the early 60s 1st gear was not synchronized on most transmissions and a great many cars and most pickups had manual transmissions. Before the mid 30s many transmissions were non synchronized. I don’t believe that farm and utility tractors have ever had synchronized transmissions. Double clutching was almost a necessity driving all the Luddite vehicles.

But in normal driving why force a downshift? When slowing, shift to neutral and coast down until you are stopped, or if you wish to resume driving while still moving shift into the appropriate gear and continue on. There should be very little stress on the synchronizer if the proper gear range is selected.

I agree, RK, I never had the need to do it. But you can buy some cars now that rev match on downshifts electronically. 370Z and 911 are two of them, I think.

I believe all the high powered supercars do.
I’ve always tapped the accelerator when downshifting. But, than, I’ve never owned a high powered supercar…

Unless you drive a 1940’s or older antique car, a big semi truck with a manual transmission, or a car with a damaged transmission you probably don’t need to double clutch. I do double clutch pretty frequently but it mostly just a habit I got into and a timing thing now. I did learn to drive in an old 1950 Chevy Convertible with a straight 6 and 3 on the tree manual transmission. Double clutching was very helpful on that car because the tranny wasn’t in the best of shape. There was also a Model A Ford somewhere in my teens that you had to double clutch because there were no synchro’s in that transmission at all.

The reason to double clutch is to bring the 2 sets of gears that you shifting into close to the same speed inside the transmission. Then you can get the teeth of the gears to mesh together with minimal grinding. Doing a double clutch on a modern manual transmission would save some wear and tear on the sychro’s IF you are doing it correctly.

@kenberthiaume may we ask why you need to learn double clutching?

I had to do that on my 89 Mustang GT when the throw out bearing went without warning, and had to drive 15 miles home…At a stop with engine off put it in 1st gear then used the starter to get moving, at 20 mph left off on the gas slowly and you could feel when you could move it to neutral…now rev the engine a bit keeping slight pressure on the shifter and when the rpms matched it would slide into 2nd with no grinding. Never had to go beyond 3rd to get home…When coming to a stop you could feel when you could get it into neutral and at about 5 mph get it into first, then just stall the engine with the brakes and repeat the process with the starter to get rolling again.

Not too good for the starter but it works. I have been driving manuals all my life and still have this car with only 111k on it and had it professionally repainted… i know just about what speed when slowing down where you can slide it into neutral without using the clutch to disengage no matter what gear you are in but do not do this as a habit.

The first cars I drove were from the late 1940s through the mid 1960s. These cars were three speed manual transmission cars with the shift on the column… First gear was not synchronized, so to engage first gear on a steep hill while the car was moving, you had to double clutch.
I rented a U-Haul truck when I moved 120 miles away to do my second round of graduate school. Money was tight, so I rented the truck on a return basis. Stenciled on the front bumper was “Local Use Only”. The transmission would clash between shifts unless I double clutched. Apparently the synchronizers were shot. When I gassed up the truck for the return trip, the oil was 2 quarts low. I poured in 2 quarts and when I got the truck back to the U-Haul dealer, it was again 2 quarts low. We did the move in ten hours, so I was only out one day’s truck rental.

Another reason why a double-clutch is helpful going down into second or first (while still rolling) is that the way most transmissions were designed (and often still are) places the synchros on the output shaft side of the gear ratios. During a normal downshift first and second synchros have to speed up the clutch back through the gear ratio, which in magnitude is seen as the clutch inertia times the square of the ratio. That’s why going down into second in just about any vehicle is a little harder than going down into 3rd or 4th, and why going into first is much harder again until you are nearly stopped. The wider gear ratio spacing typically found in the lower gears doesn’t help either.

Re: What good does letting the clutch out after you put the car in neutral? Can’t you raise the revs with the clutch still depressed?

The crankshaft is bolted to the flywheel. The clutch disc is pressed against the flywheel when the clutch pedal is out. Letting the clutch out allows the crankshaft to spin the clutch disc too, which in turn spins the shaft that drives the transmission. The idea is to get that transmission input shaft turning at the right speed so the gears mesh like a hot knife through butter. If you didn’t let the clutch out, you wouldn’t increase the rotational speed of that xmission input shaft, so the gears would have to sync up by spinning the sychronizers in the xmission instead, which wouldn’t be quite as smooth.

Search for “Importance of rev matching on downshift to reduce clutch wear” August 14, 2012. There is a great video showing a race car driver double clutching on down shift(s). Watch his/her left foot when doing a down shift. The foot goes to the floor; neutral is engaged; foot comes up half way as the throttle is blipped; left foot back down; the shift is completed; clutch pedal comes up; and throttle is mashed. Towards the end he/she does a three gear downshift before getting back on the gas.

Of course you are watching a demonstration of “heel and toe” work. I use “heel and toe” even though he/she is using sides of the right foot for brake and accelerator. Takes a lot of practice to get that good and a pedal arrangement that matches the foot width and pedal height.

Thought you might enjoy this while you are working on your double clutch technique.