Wednesday, 13 February 2013

Dual Clutch Transmission

The drivetrain is shown below. The gearbox is mounted longitudinally, in line with the engine crankshaft and connected via a driveshaft to the rear differential which directs torque to the wheels, tires, and road.

The gearbox on the M5 is the Getrag GS7D36BG M Double-Clutch Transmission (DCT) with Drivelogic. The "Drivelogic" is  marketing speak for the computer that controls the gearbox: BorgWarner's DualTronic clutch module (also called the "mechatronics" as a hybrid of mechanical and electronic in one package).

It is classified as an "Automatic Manual" because it uses clutch plates and a clutch mechanism to connect and disconnect the engine from the geartrain as for a manual transmission, but is has no clutch pedal and the gears can be made to shift automatically as for an automatic. It does not use a fluid-coupled torque converter as would a traditional automatic transmission, instead using hydraulically controlled wet clutches. 

It is a "Dual-Clutch" because it has two set of clutch plates, one for the even gears and one for the odd ones. This creates two sub-transmissions. As one is driving along in a certain gear using sub-transmission A, the electronics know if you are accelerating or decelerating, and will automatically engage the next gear in advance on sub-transmission B. This sub-transmission B free-wheels until the one clutch is disengaged and the other engaged. This allows extremely rapid gear changes with no interruption of power to the wheels.

This type of transmission was first used on a BMW in the M3. The Getrag BG used in the M5 is a beefed-up version of the SG used in the M3, and shown below in a partial cut-away view.

The principle of operation is as shown below.

The engine (A) inputs torque to two clutch assemblies (1,3) within the transmission (B). Depending upon which clutch is engaged, torque flows to either the top transmission sub-assembly (2) or the bottom one (4). These gears engage the propeller shaft which drives the rear wheels through a differential (C).

The dual-clutch assembly pulled away from the gear trains is shown above. It is a wet clutch, meaning that it is bathed in oil for smoother operation and longer life, and uses multiple clutch plates to compensate for any resulting slip. The two clutches are concentric, and the two transmission sub-assembly shafts nest one inside the other (observe the nested toothed gears towards the middle right, each drives its own sub-assembly).

The illustration below represents the inside of the M3 gearbox, but it is similar in principle to the M5's. The gear train on the bottom is called the countershaft. It is permanently rotating and meshed with the constant gear on the output shaft. There is an additional small gear train sticking out the side for reverse gear. The various gears are always meshed with one another. Some are permanently rotating with their shaft, others are free-wheeling on their shaft until a dog clutch pushed in place by the shift mechanism meshes them to their shaft. The dog clutch uses a synchomesh mechanism to match RPMs before locking the gear to the shaft. The main shaft is actually two entirely separately rotating shafts, one nested inside the other. Some of the dog clutches mesh the gear to the inner shaft, others to the outer shaft, and some of the gears are permanently rotating with either the inner or outer shaft.

Below we see the gear diagram showing sensors. The blue rectangles are the sliding dog clutches. They slide right or left to "mate" a gear to its shaft. Each has its own shift travel sensor.

For example, 2nd gear is always turning with the main outer shaft, and it is meshed with its counterpart on the countershaft. That gear, however, is free-wheeling on the countershaft until the dog clutch mates with it.

The system predicts what gear it will likely go to next depending on if the car is accelerating or decelerating. It will then pre-engage the appropriate dog clutch. The next gear is always on the other shaft which is not yet clutched to the drive shaft, so its shaft can free-wheel. The engaged clutch can then start decreasing and the other clutch can start increasing in pressure to match RPMs. 

The following animated diagram shows all the various power channels through the transmission. 

Note how the clutches alternate as we go up and down the gears.

The diagram below shows the entire system.

The transmission (2) is lubricated and cooled by oil via the air/transmission oil cooler (1). The transmission has its own integral oil pump driven from the center input shaft. Therefore the engine must be running for the oil pressure to build up. It is connected by wires to the gear selector level.

Within the transmission there is a pipe running down the side with nozzles to lubricate the gears.

On the drive end of the casing there is the parking brake that prevents the shaft from moving, with the mechanism shown below.

Park is engaged when the engine is turned off and the gearbox is not in neutral. If the parking brake ever needs to be released manually, this is accomplished inside the front cup holder by removing the cover and sliding the parking lock lever as shown.

The "mechatronics" (electronics + hydraulics) module plugs into the side of the transmission, receiving input from the various position, rotation, temperature, and pressure sensors within, and effecting via hydraulics the shifting and clutching functions.

The mechatronics communicate with DME-1 via the PT-CAN computer bus to get pertinent information and to "blip" the throttle on shifts. Blipping means bringing the engine revs up to match the wheel speed at the next lower gear down during down-shifts. The gearbox blips in all shifting modes.

Sequential manual gear shift control can be effected by the '+' and '-' paddle shifters on the wheel.

The main control is by means of the gear shift selector.

Moving the selector to the right toggles between "Drive" (automatic shift) and "Sequential" (manual shift). Pushing the lever forward and back will shift. Pushing the lever to the left will engage neutral. Left and up: reverse.

Each of D and S modes have three "Drivelogic" settings accessed from (5). In D these are Efficient, Comfortable, and Sporty, which will move the shift map higher in the RPM range and shift faster. In S mode, the three are Comfortable, Sporty, and Maximum which refer to the speed of the gear changes. The shifting speed is also affected by the accelerator position and how quickly it is changed. In my experience, D1 mode is dreadful in that it robs the car of all its torque by shifting too early and keeping the revs very low. This is the most fuel efficient setting, however.

There is a mode called "Launch Control" for maximal acceleration off the line allowing the optimal amount of wheel slip (17%) and using the fastest shift speeds at the highest RPMs. This is engaged by deactivating DSC, selecting the third "S" mode, pressing the brake pedal gently, holding the gear selector forward, waiting for the flag symbol to appear, flooring the accelerator, releasing the brake, and then releasing the gear selector switch. It's not meant to be easy!

In order to simulate the bahviour of an automatic gearbox, a light tap on the accelerator when stopped will cause the car to move forward (or backwards if in R) very slowly without holding it. On hills, the brakes hold the car steady for 2 seconds after releasing them until the accelerator is depressed. After that, it starts rolling.

The gear ratios are as follows. 1st=4.8x, 2nd=2.6x, 3rd=1.7x, 4th=1.3x, 5th=1.0x, 6th=0.84x, 7th=0.67x. The final drive ratio at the rear differential is 3.15x. Driving at less than 145 km/h it's never necessary to get out of 3rd gear, though the RPMs are at 6000. 4th gear will get you to well above 200 km/h, and 5th will get you to its maximum speed of somewhere around 300 km/h (though the electronic speed limiter keeps it to under 240 km/h or so). 6th and 7th gears are for fuel economy, for cruising quickly on the highway at relatively low RPMs to conserve fuel.

The DCT gearbox on the F10 M5 is an absolute delight to drive. First it adds to the performance of the car by keeping power applied through the shifts. Second, the shifts are lighting quick, like gunshots, which is just a whole lot of fun to drive, and finally the blips on the downshifts are just right, and make it feasible to get yourself easily into the right gear when tackling a corner.


  1. Replacing the clutch is not an easy job and you should not try to do it if you don't know how.

  2. I think when you change another gear,therefore not to mess up the tranmission.Double clutch is the act of keeping your rpm as high as possible to stay in your power.

  3. I agree that it is not that easy to replace the clutch and you will take a lot of time ,when the first time i used to do it i failed about it and the fact that i called a friend to help me fix.Well good things to see some information about it.

  4. Bob's Transmission27 May 2014 at 11:10

    Replacing the clutch is definitely a job for a professional. It is an essential part of the car, and doing it wrong can be very costly.

  5. My dad and I have been working on a car that he rescued from getting broken down into scrap, and it's been a pain the butt. We've come a long ways though, and are starting to work on the transmission of it. I didn't know to much about the transmission at first, but articles like this have really helped me out a ton, and I can't wait start working on it.

  6. Great post and very helpful. While I love Mercedes-Benz AMG vehicles, I'm really hot to get an M car and the one thing that holds me back is this DCT transmission. I want to be able to leave it in automatic mode and it's hard to get an good information on how well the car drives and behaves if JUST left in automatic mode 95% of it's life. Thanks for the article.

  7. I'm thinking about getting a current M5 but I only drive sticks, I thought I read somewhere the dual clutch is more geared to the paddle thingie. Is that correct???? Any extra info about it????