What exactly is a dual-clutch transmission, and how does it work?


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What exactly is a dual-clutch transmission, and how does it work?

Post by WingAdmin » Mon Oct 30, 2017 12:14 am



With the arrival of the new 2018 Goldwing and its inclusion of Honda's Dual Clutch Transmission (DCT), some people may wonder what a dual-clutch transmission actually is, and how it benefits the rider.

To understand a DCT, you first need to understand how a transmission functions. The job of a transmission is to engage different sized gears, to alter how fast the output shaft (connected to the wheel) spins in relation to the input shaft (connected to the engine). At slow speeds, the engine is spinning much faster than the wheel, so the transmission needs to reduce the ratio. At highway speed, the engine may be spinning the same speed as the output drivewhaft, or even slower (called overdrive), in which case the transmission needs to increase the ratio.

How the transmission does this is with a series of gears that are different diameters. These gears are constantly meshed with one another - that is to say, the teeth of the gears do not engage and disengage, they are always meshed and turning - for all the gears - at the same time.

So how then does the transmission select which of these gears is actually connected and turning the wheel? This is the job of the engagement dog, shown in pink in the diagram below. In this diagram, we show a two-speed transmission. The input shaft (in red) spins the two mainshaft gears, which in turn spin the output shaft gears (in blue). The output shaft (also in yellow) is not actually connected to the output shaft gears - the blue output shaft gears can (and do) spin freely on the output shaft. When the shifter pushes the engagement dog (pink) into one gear or the other, notches (or dogs) engage into holes in the side of the output shaft gears. This spins the engagement dog, which IS connected to the output shaft (in yellow), which in turn spins the output shaft , and the rear wheel.

Animated two-speed transmission
Animated two-speed transmission

When we expand that from two gears to six, and package it in a small area suitable for a motorcycle, we see something like this:

6 speed motorcycle transmission
6 speed motorcycle transmission

The engagement dogs don't move all by themselves - they are dragged along by shift forks. A shift fork is a piece of metal in a "C" shape on one end, which slides into a slot in the engagement dog. As the fork moves from side to side, it drags the engagement dog along with it, engaging one gear or the other, or if it is in between the two, neither of the gears.

How does the transmission know which forks to move where? It uses a shift drum, which has a series of curvy slots in it. As the drum rotates, it moves the shift forks in sequence, so that only one gear is ever engaged at the same time. As the shift drum rotates (bit by bit, as you push the shifter up or down), it moves the shift forks back and forth to engage and disengage each gear.

Shift forks and shift drum
Shift forks and shift drum

Now you can't shift gears when you're riding the motorcycle, you first have to pull the clutch in. The clutch disconnects the engine from the transmission. This stops a load from being put on the transmission, so that you can slide the engagement dogs out of the gear. When the transmission is under load, it is harmful to slide the dogs in and out of gear, as it wears the edges of the dogs round, and if they get too worn, the transmission will begin jumping out of gear unexpectedly. Therefore we need to use the clutch to disconnect the engine from the transmission.

Unlike cars, most motorcycle clutches are bathed in oil, so they are called "wet" clutches. And unlike a car, where there is one big disc sliding against another, a motorcycle as a multiplate clutch, so essentially there are a series of small clutches, all stacked up against one another. The plates alternate: one is a friction plate, the next is a metal plate, the next is friction, the next is metal, and so on. All of the friction plates will be connected to the engine and all of the metal plates will be connected to the transmission (or the other way around, depending on the manufacturer).

The friction plates and metal plates are pressed together tightly with very strong springs, so that power gets from the engine to the transmission. When you squeeze the clutch lever, the plates are pulled apart, so they can spin against one another, and power is no longer transmitted from one set to the other:

Clutch
Clutch

Now - dual clutch transmissions: As you would expect from the name, DCT's have two clutches. In fact, a DCT is actually two small transmissions in one, crammed into the same space. Each clutch is responsible for one half of the transmission's gears, and they alternate: so the first clutch will be responsible for 1st, 3rd and 5th gears, and the second clutch will be responsible for 2nd, 4th and 6th. The second set of gears, 2nd 4th and 6th, are on a tubular shaft that rotates around the shaft controlled by the first clutch, so that both sets of gears can be packed into a small area. Many high-end sports cars with dual-clutch transmissions have separate shafts for each set of gears, as they don't have the space restrictions found in motorcycles.

Dual Clutch Transmission
Dual Clutch Transmission

When both clutches are disengaged, the transmission is in neutral. To engage first gear, the shift dog for first gear engages, and its clutch (the red clutch) then engages. The motorcycle is now being driven in first gear. The motorcycles knows that the next gear will be second gear, so it gets it ready, by sliding the shift dog for second gear into place, engaging second gear. This is OK to do while first gear is already engaged, because the clutch for second gear (the blue clutch) is not engaged. When it comes time to shift from first to second gear, the red clutch is disengaged, and at the same time the blue clutch is engaged. Now second gear is being driven, and first gear is not - the shift is instantaneous. The motorcycle knows that the next gear will be third, so it shifts the red gear out of first and into third. When it comes time to shift into third, it simultaneously disengages the blue clutch and re-engages the red clutch.

Obviously working two clutches and two shifters like this is beyond the capability of most riders, and besides - holding one of your clutches in all of the time would get tiring - not to mention how catastrophic it would be to your transmission if you accidentally engaged both clutches at the same time! So instead, a DCT is controlled by a computer. The computer controls the shift forks as well as the engaging and disengaging of the clutches. The rider shifts by pressing buttons on the handlebars, or by setting the transmission to "automatic" and letting the computer do it all.

How does this benefit the rider? Instant shifts mean less time spent shifting gears when accelerating. More efficient use of gear selection for acceleration and engine braking as selected by the computer offer benefits in terms of performance as well as fuel-efficiency. And of course, no more tedious clutching in stop-and-go traffic!

Because DCT clutches are disengaged until the computer energizes the clutch to engage it (either with oil pressure or electrical actuators), it means that when the engine is off, neither clutch is engaged. That means that when the engine is off, the engine is not connected to the wheel - so the motorcycle could potentially roll away! Therefore, DCT motorcycles typically come with a parking brake.



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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by jim34481 » Mon Oct 30, 2017 5:34 am

WingAdmin
Great explanation.
Thanks
Jim

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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by landisr » Mon Oct 30, 2017 12:54 pm

Thanks a lot for this. I never understood the Dual Clutch concept. ie it told me Two Clutches, but why, how, and for what purpose. Also, I honestly had a misconception about "automatics" on a motorcycle. My experience tells or told me that they were 'lazy' and inefficient, like the old(?) centrifical clutches either mechanically or perceptionally. I have to admit that DCT has a number of advantages and that a number of riders would like to have it.

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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by WingAdmin » Mon Oct 30, 2017 7:32 pm

landisr wrote:
Mon Oct 30, 2017 12:54 pm
Thanks a lot for this. I never understood the Dual Clutch concept. ie it told me Two Clutches, but why, how, and for what purpose. Also, I honestly had a misconception about "automatics" on a motorcycle. My experience tells or told me that they were 'lazy' and inefficient, like the old(?) centrifical clutches either mechanically or perceptionally. I have to admit that DCT has a number of advantages and that a number of riders would like to have it.

Ron
The centrifugal clutch used with a CVT (constant velocity transmission) like you will find on scooters, ATVs and snowmobiles is a different thing. It consists of a set of weights around a ring. When the engine spins the ring fast enough, the weights pull out against the resistance of springs pulling them in (centrifugal force) and rub against the lining of a cylinder around them, turning the cylinder which then powers the transmission.

There was also a car-style automatic transmission used by Honda as well. Honda adapted its "Hondamatic" car transmission to motorcycles, and it was used on:

CB750A (1976-1978)
CB400A Hawk Hondamatic (1978)
CM400A Hondamatic (1979-1981)
CM450A Hondamatic (1982-1983)

It had a torque converter (otherwise known as a slushbox) that used viscous coupling in place of a clutch - basically if you have a blade that stirs a pot of oil fast enough, and there is a set of blades in the oil, the oil will turn the second set of blades, even though there is no physical connection between the two sets of blades. It was lazy and inefficient (and heavy) as you state.

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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by 4given » Wed Nov 01, 2017 5:56 am

How does it act between shifts? Is there a bump since there doesn't appear to be a torque converter?

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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by WingAdmin » Wed Nov 01, 2017 8:05 am

4given wrote:
Wed Nov 01, 2017 5:56 am
How does it act between shifts? Is there a bump since there doesn't appear to be a torque converter?
The computer also has control over the ignition - which is why you won't find a DCT on a bike without throttle-by-wire. When upshifting, the computer cuts the ignition for just a moment as one clutch engages and the other disengages. This, along with the fact that the rotating engine mass itself is relatively light (at least compared to a car), with little or no flywheel weight means that the instant ignition is cut, the engine RPM drops almost immediately. It's all timed very precisely by the computer, so that by the time the next clutch engages, the engine is at, or very near the speed that it will be when the next clutch is engaged.

Downshifting is similar: When a downshift is executed, the first clutch is disengaged, the computer waits an instant for the unloaded engine to spin its RPM up to what it will need to be in order to match the next lower gear, and when it's there, it engages the second clutch.

The end result is absolutely seamless upshifts and downshifts. The only indication you really have that a shift has actually occurred is that the sound of the engine changes. Basically, all that skill and practice you have done in order to be able to shift smoothly, matching revs as you do so, can be replaced by a computer doing the same thing, 100% perfectly every time, in milliseconds - far faster than any human can shift.

Here is a Honda Africa Twin with DCT, and you can hear the instantaneous shifting on this video:




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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by fullarmorcycle » Wed Nov 01, 2017 10:42 am

Sorry, but just not interested in a DCT or automatic on a motorcycle. Riding a bike is one of the last places anyone can be or anything one can do to enjoy the machine and all the things that go with it. Automatic shifting is just one more thing that sucks the fun and life out of riding a bike. My two cents is that if someone wants automatic shifting then stick to a car.

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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by flanzajr » Wed Nov 01, 2017 12:10 pm

Since there is no torque converter,how does the transmission operate from a standing start? Does it begin to engage the first clutch as you raise engine rpm? Also how does reverse work in this manner or is there no reverse?
Thanks!
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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by WheezyRider » Wed Nov 01, 2017 12:51 pm

Scott that was a brilliant explanation, thank you! I never understood before. The Africa Twin video is a perfect addition to the explanation too.

Its hard for me as a 71-yr-old iron butt on my 3rd Wing I love to death, to even imagine going to the Dark Side of an auto tranny on a motorcycle. On the other hand, as you imply in your fine article on the new bike -- better is better.

Always a wincing annoyance since new is my Round-The-World 2006 GL1800's "clunk" when going into gear with cold oil. Ditto finding neutral sometimes is a trial-error pisser. The DCT is probably the future like it or not. Initially I am a hater, but I bet after one ride I'll be sold.

Put another way, I swore I'd never get an auto tranny -- not ever -- when I was a teen and twenties. Three speed on the column was just fine. Now, I drive a pseudo-growling Porsche Cayenne GTS with paddle shifters, and almost never use the paddle shifters because the computer is so much better than I am! Guessing 'twill be thus with motorcycles now that Honda is perfecting it.

The reduction in cargo capacity is a serious error for serious road warriors: BIG-BIG MISTAKE HONDA!!! Speaking here as one who actually toured on his 3 Wings, two up with months of luggage at a time. The fuel capacity reduction is just foolish weight-cutting nonsense: A slightly larger plastic gas tank adds maybe an ounce, so what? There is no law that says one has to fill up with gas if about to do a drag race! (How many Goldwings do drag races?) It's nonsense, they NEED the fuel range and they NEED the luggage capacity! I am deeply disappointed with Honda's engineers on these items, they are nearing perfection, then backing away from it.

I love most details of this bike as you describe them Scott, especially the front end. (It always was a lousy front end I rebuilt totally with excellent Traxxion + fork brace.) But the luggage capacity and fuel capacity are almost deal killers for me.
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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by WingAdmin » Wed Nov 01, 2017 5:23 pm

flanzajr wrote:
Wed Nov 01, 2017 12:10 pm
Since there is no torque converter,how does the transmission operate from a standing start? Does it begin to engage the first clutch as you raise engine rpm? Also how does reverse work in this manner or is there no reverse?
Thanks!
Frank Lanza Jr.
'83 Interstate w/ alternator conversion
When the bike is in "drive" mode, and the bike is stopped, the transmission computer holds the clutches open, while staying in first gear. As you raise the RPMs with the throttle, it gradually engages the clutch, much as you would do with your clutch lever, maintaining the correct RPM and torque to keep the engine from stalling and keep the bike accelerating.

The DCT-powered Goldwing does have reverse, and it does this by shifting into its reverse gear and modulating the clutch just enough to let the bike creep backwards. It also takes care of the throttle for you while you do this, to keep the bike from going too fast.

For the same reason, the DCT Goldwing has a "walking mode" which is essentially the exact same thing, only in first gear instead of reverse - it creeps forward very slowly, with the transmission computer doing all the work required to keep the bike at an exact, slow speed.

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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by 4given » Thu Nov 02, 2017 5:55 am

Totally amazing technology and explanation of the technology. Thanks Scott. I can't see myself getting any farther than marveling at this machine and the work and technology that went into it for various reasons. 1 reason being that I couldn't afford one and another being that I enjoy my GL1500 too much too look for a better one for me. I will turn 66 in Feb. so I probably won't be outlasting my Aspencade as it only has 67,000 miles on it. I've been riding since I was 15 and I enjoy it as much/more now as/than I ever have . Praying that God gives me the wisdom to know when to take my last ride. This forum has been in valuable to me since I try to do my own work and I'd like to thank everyone who lends their knowledge to me.

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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by Rednaxs60 » Thu Nov 02, 2017 9:49 am

The one aspect of the new GW that I like is the hill assist. Be easier on the transmission and will benefit all who ride where there are hilly/slope conditions. There are a lot of other options as well, but not in the market.
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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by WingAdmin » Thu Nov 02, 2017 7:42 pm

Like most new car/bike technology, DCT was originally developed on the track, specifically for Formula 1, and gradually trickled down. You can hear the instant upshifts and downshifts occurring in this video - move ahead to the 7:30 point to see when the car actually heads out on to the track, and you can hear the shifts:




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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by Mzomaxrph » Sun Nov 12, 2017 8:06 am

Thanks for the detailed explanation on DCT! I also enjoyed watching the video of the dct in action. At one point the rider gets off the bike and twists the throttle while the bike is on its side stand. How does that happen without the bike taking off? How does the computer know you don’t just want to accelerate, is it because the side stand is down? Do you have any videos showing slow speed maneuvering? I really would like to get an idea of how to control the bike without feathering the clutch.

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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by IBArider » Sun Nov 12, 2017 8:25 am

Great Thank You to WingAdmin!! Explanations and Illustrations are great informing about the DCT. Whether you want/like an Automatic motorcycle
or not knowledge is good to be able to make a better decision on a purchase or just to have a good conversation. :)

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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by WingAdmin » Sun Nov 12, 2017 7:45 pm

Mzomaxrph wrote:
Sun Nov 12, 2017 8:06 am
Thanks for the detailed explanation on DCT! I also enjoyed watching the video of the dct in action. At one point the rider gets off the bike and twists the throttle while the bike is on its side stand. How does that happen without the bike taking off? How does the computer know you don’t just want to accelerate, is it because the side stand is down? Do you have any videos showing slow speed maneuvering? I really would like to get an idea of how to control the bike without feathering the clutch.
Because it has an automatic mode doesn't mean it's like a scooter with a CVT, which would in fact leap off the sidestand if you twisted the throttle like this. Just like other Goldwings, the DCT Goldwing has a neutral:

Right side switchpod
Right side switchpod

On the right side, are the transmission control switches. You can press the "N" button which switches it to neutral (and is what the bike starts in when you first start it up), "D" to activate the DCT, and A/M to switch between automatic (where the transmission decides when to shift) and manual, where the shifter switches on the right handgrip are used to shift up and down:

Shifter switches
Shifter switches

It's not specified in the manual; but I would expect that if the sidestand is down, the bike will either refuse to shift out of neutral, or will kill the engine like any other bike (I expect the former would be true).

If this bike runs like the other Honda DCT's (and I expect it will), you use a little bit of rear brake for trail braking, and gently open the throttle. The bike will feather the clutch for you, and pull you along at a constant speed. Open the throttle, and the clutch will lock up and the bike accelerates hard away.

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Re: What exactly is a dual-clutch transmission, and how does it work?

Post by Mzomaxrph » Sun Nov 12, 2017 8:49 pm

Wow, thanks WingAdmin! That answers a lot of my questions. Sounds like I would have to learn a new style of riding but that could be a fun experience as well. Of course, I will be test riding both the 6 speed manual as well as the DCT. I’ve put 104K plus miles on my 2009 and have been holding out for the expected big changes as we are seeing on this upcoming model.



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