What is Double Clutching?

@anon166320: An engine is also used for slowing a vehicle. That's why automatics have low gears like first and second, and most new cars can be up and down shifted like a manual, usually in a sport mode. If your car doesn't have the option to be downshifted manually into a low gear (some don't), you can't drive up Mount Washington because they want you using the engine to help slow your car on the descent, because if you use brakes alone they'll overheat and the car may not stop.

I drove a tractor trailer for years. It was supposed to be double clutched. I would just match the road with the gear. I never used the clutch while shifting, up shifting or down shifting. I didn't use it while stopping; I would just put the rig in neutral. The only time that I would use the clutch would be to start moving. I have 40 years of tractor trailer experience and never hurt a tranny or a clutch.

In Ontario now, there's a few highways that allow dual 53-foot trailers and if I didn't double clutch with those, it would pretty much be a runaway train with 160,000 pounds pushing me with the only thing slowing me down being some ceramic pads on metal.

Also with 18 gears, if you're in bad traffic, if you can't power shift, your leg will go numb from pushing in the clutch. So anybody who says you're wearing out your clutch double clutching or clutching (same thing then), if you know anything about your vehicle then don't use a clutch on your up shift.

Nascar guys will never clutch on road courses when upshifting or they would blow their clutch out because those cars are built for all out throttle so yes, for Watkins Glen and the other one or two road courses. they use a different car with a different setup. but a lot of the specs are similar to the oval tracks. It's a lot of stress on brakes and clutches and trannys for those Nascar vehicles.

This is an interesting discussion of shifting, and several references were made about teaching or training drivers in the art of shifting dump trucks.

I am a propane tanker driver in the northern panhandle of West Virginia and Northern Virginia area. Where can I find a person, school or company to teach me effective shifting and downshifting of a dump truck? I am already CDL licensed with the proper endorsements.

The bad part is that I can do it, but just not smooth enough to convince dump truck companies to hire me. Does anyone have suggestions as to how I can get this guidance?

To the people who "change to neutral and use brakes," they need to understand that engine deceleration is part of the engine's function. Unless you are using a Lenco drag box, every time you take your foot off the accelerator your engine is decelerating. It is just not getting any fuel, but it is still operating in the exact same way as accelerating.

The brakes, however, are only designed to be used for very short periods of time. They have a serious overheating problem, hence slotted or vented rotors and if you can afford it, ceramic discs. It only takes one hill to warp your rotors and boil your fluid, whereas your car could roll for hours.

Those of you who are saying that downshifting to slow a vehicle down is unnecessary, are missing the point. First off, as someone else said, downshifting is a way to slow a vehicle down without hitting the brakes. This is particularly valuable in two situations. The first is to maintain control of the vehicle in slippery/icy conditions where hitting the brakes, no matter how carefully, may cause you to lose control. The other is also about maintaining control, and that's when you're going down hill. Semis and cars/trucks with trailers can be particularly susceptible to brake fade on long and/or steep grades. This is when the brakes heat up to extreme levels and when this happens, the brakes won't hold.

I've never driven a "big rig", but I have driven a small SUV with and without a small trailer going between NY, and the Appalachia area -- VA, WV, KY -- and have on occasion felt the beginning of brake fade, but fortunately knew what it was. A group I was with had a driver who had no experience with mountain driving and when we had to make a stop, you could smell the brakes which had overheated. We had to tell him that he needed to force it into second (automatic) on the downgrades to keep the brakes from failing.

By the way, truckers have another reason why downshifting and staying in gear is valuable, If they are in neutral, they can't use the "jake brake" to slow the truck down. If you've ever been driving on major highways where there are serious grades (either steep or long) you may see what are called runaway ramps. These are in strategic places and are usually meant for big rigs or big trucks as a way to stop their vehicle if the brakes do fail (usually from brake fade/over use).

@150461: You are wrong. That's what the brakes are for. You are causing wear on an expensive motor instead of cheap brake pads, and your passive-aggressive tone is

something to look into.

Thank you. I've known how; and have driven trucks where I knew it was needed - but never understood exactly why. And the correct answer, although 'logical', I dismissed thinking that brief moment would not be enough time for the two to sync. Now I know - it is; and that is the reason for the action. Thanks!

I've driven a lot, and all but one car (cadillac, can you blame me?) was a manual tranny. The only one I have ever had the sync go out in was my first car, an Eagle Talon TSi. (which were damaged when I bought it and they are notorious for going out in my gen.)

Anyway, to me, double clutching is a waste. If you have a stage 2/3/+++ clutch then sure, if your accelerating rapidly (or decelerating, be it the case). Just match the rpm to what they should be. It becomes second nature if you have the ear for it.

I speed shift quite a bit (shifting without clutch), but I don't recommend it unless you can get it exact every time.

But it is true, it will make your syncs and in turn, transmission, last longer- IF you do not have an ear for the engine (or feel, really. I drive with my music drowning my flowmasters). My coppers anyway.

@anon12381: This has been answered already without anyone asking When you press the clutch, you disconnect the engine from the transmission, so the active gear is still revolving relatively to the wheels.

But, if you shift into neutral (and then release the clutch), the engine is again connected with the transmission and you can get the transmission (now disconnected from the wheels) to revolve faster or slower, using the engine to regulate. You then disconnect the engine again and shift into the correct gear, hopefully having roughly matched the RPM of the wheels.

To the person who said downshifting is pointless and you know because you have been driving for 23 years, i don't want to be on the road with people like you.

I've only been driving 5 years and even i know it's stupid to slow down in neutral. it puts all the stress on the brakes, causes more wear and tear, and a higher possibility of lockup or failure. you want the engine to help slow you down. it's safer and more efficient. it's actually against the law to ride in neutral for this reason.

When you're driving a 2 ton speeding machine, you endanger everyone around just because you won't use common sense. a pity so many people die each year due to lack of thought.

When shifting a big truck (tractor trailer), double clutching is preferred by transmission manufacturers. The reason to do this is because there is only one synchronizer in the transmission.

First, you must know how to read and understand the tachometer. You must be able to find out the rise and fall of the tach as you shift the gears. As an instructor, I teach the 500 RPM method on most trucks and 300 on others, depending on the way the truck handles the shift and fuel efficiency.

Most people have no idea why, when or how to use the process. On up shifting, simply bring the tach up to the desired RPM, press the clutch in half way (thereby releasing the transmission from the engine), bring the gear selector out of gear and release the clutch. Allow the tach to fall to the RPM desired (thereby matching the speed of the engine to the speed of the gear you wish to enter) press the clutch in half way again and put the gear selector into the next desired gear (this is usually 300 to 500 RPMs lower than the the RPM you came out of), then ease off the clutch.

Down shifting is just the opposite. You must first slow the truck down to the desired RPM (which is below 1400 RPMs because most teachers are governed around 1900), press the clutch in half way, bring the gear selector out of gear and then release the clutch. Press the accelerator and bring the tach up to the desired RPM (which is usually 300 to 500 RPMS above wherever you pressed the clutch in the first place), press the clutch in half way, put the gear selector into the next lower gear and ease off the clutch.

Easing off the clutch when you put the gear selector into gear is very important so as not to dump or put shock load onto the drive train components thereby twisting the drive line or severing the bolts off the U-Joints. This process takes practice and is not common to the "normal" driver.

The reason to press the clutch in half way is to not engage the clutch brake. The clutch brake's job is to stop the transmission parts from moving. The only time a driver should engage the clutch brake is when putting the truck into the initial (or take off) gear, be it first or reverse. This is also something that takes practice.

Most drivers do this process wrong, by allowing the gears to "roll through". This causes damage to the gears by breaking the teeth or shaving the teeth off, thus causing the transmission's life time to shorten.

Let me raise one point. In order to understand the essence of double-clutching, you need to understand that there are three separate moving components: engine, gear box, and wheels. The connection between the engine and the gear box is the clutch, and the connection between the gear box and the wheels is the collar (which is connected to the drive shaft). The collar has teeth (called dog teeth) that engage different gears rotating at different speeds for a given clutch rotation speed.

Most modern cars have a synchro that allows the collar to engage different gears having a different rotation speed than itself. The synchro forces the engaging gear to rotate at the same speed as the collar (much like the clutch forces the engine to rotate at the same speed as the gear box). Simple rev-matching (without double-clutching) saves the clutch from excessive wear while rev-matching with double-clutching saves both the clutch and the synchro from excessive wear.

The point is, the only benefit (albeit an important one) of double-clutching is to save wear on the synchro. As mentioned by somebody else, even a synchro equipped gear box would have problems trying to engage a gear that is rotating at a substantially different speed than the collar. Double-clutching will be an invaluable tool in this case. Of course, a gear box without a synchro needs double-clutching all the time unless you are very proficient at rev-matching.

I've been driving 23 years. when you need to slow down, just knock it into neutral and slow down. When ready to accelerate, just select the appropriate gear and accelerate.

I understand double clutching a lot better now just from looking at this article and more from the comments.

Here is what I was wondering. Double clutching is required on most tractor trailers, but with all the weight being carried is double clutching the best thing to do when traveling up a long hill and then down?

Shifting from 5th to 4th to 3rd to 5th with gas and braking to 3rd gear out from 2rd into the 1st to stop.

have learned to shift my baby girl (my GMC pickup) without the clutch and am getting a lot better gas mileage.

Downshifting is 100 percent pointless the way you all are going about it. if you know you need to stop, throw it in neutral and then brake as needed. when starting again, just decide which gear you need to be in and go. it makes things 10 times easier.

I've always done this out of pure natural instinct. I think I'm made to race cars.

you don't need to give gas, just let off the clutch halfway and then release fully.

"There is no need for even touching the clutch other than first or reverse gear. "

I agree. on my manual trans lease cars. But on the cars I own, I will use the clutch and live with a little synchro wear with non-perfectly matched rev/wheel speed.

Shifting without the clutch is very entertaining when done well.

When you have a loud car, you don't need to watch the tach when downshifting -- just listen to the roar!

Basically, you are matching the engine speed to the actual rpm speed of that gear. For instance, if I am going 40 mph in fifth gear I may only be turning 2,000 rpms. However, in second gear I would be turning more like 5,000 rpms. So, to effectively downshift the car from fifth to second, without locking up the gears I will throw in the clutch and blip the throttle up to 5,000, perhaps slightly past to allow for the lag time of shifting the gears. The shift will be very smooth and avoid the wheels (if two wheel drive) from locking up during if I just downshifted straight into second.

thanks for such a wonderful article! helped me a lot.

Learn to heel toe down shift. Practice. Read "Going Faster" -- best driving book period!

There is no need for even touching the clutch other than first or reverse gear.

When you are shifting up, you actually want the engine to slow down, so the order would be more like (1st), (neutral-short pause to let RPMs drop), (2nd - gas as necessary now)

L plater speaking:

when I shift up from 1st to 2nd, do I do 1st, netural, gas, then second?

There's one thing I don't understand about double clutching - if I'm in fourth gear and want to shift down to second, should I go about it like:-

4th - Neutral (and gas) - 3rd - Neutral (More gas) - 2nd or can I work around it more like

4th - Neutral (gas) - 2nd?

The amount of gas to match speeds is proportional to the number of steps you down shift. For instance, you add more gas when you down shift from fifth gear to third than from fifth to fourth.

Actually its proportionate to your gear ratio...

You want to rev the engine to the same RPM that it would be at X speed in Y gear. Really not that complicated

anon12381: Good question. My guess is that the engine revs alone isn't what's important. When you press the clutch, you disconnect the engine from the transmission, so the active gear is still revolving relatively to the wheels.

But, if you shift into neutral (and then release the clutch), the engine is again connected with the transmission and you can get the transmission (now disconnected from the wheels) to revolve faster or slower, using the engine to regulate. You then disconnect the engine again and shift into the correct gear, hopefully having roughly matched the RPM of the wheels...

For shifting up, you should be able achieve the same thing with regular single shifting, except it would take longer time for the gears to reduce their RPM on their own without the assistance of the engine.

What I struggle with is downshifting. On a modern car you don't really hear the engine. Tapping the gas pedal becomes a hit or miss thing. Maybe it is just a matter of practice.

Why would you have to pause in neutral?

Why can't you just shift to the gear wanted (while holding in clutch)rev the engine to get the rpm's to where you want them then let the clutch out?

The amount of gas to match speeds is proportional to the number of steps you down shift. For instance, you add more gas when you down shift from fifth gear to third than from fifth to fourth.