S-Tronic Transmission. Design Peculiarities and Principle of Operation. Pros and Cons of this Transmission Solution.

Nowadays, the world’s leading car manufacturers actively install pre-selective gearboxes (also known as DSG or robotic gearboxes) in their new car models. This type of transmission is considered to be an ideal substitute for traditional automatic transmissions and CVTs. It is not surprising that the premium segment cars of Volkswagen Group (VAG) are equipped with dual-clutch transmissions. In this article, we will review a bright DSG option – S-tronic.

The S-tronic transmission is a preselective robotic gearbox which is installed on AWD and RWD Audi models. Taking into account that Audi is a part of the VAG concern, it becomes clear that S-tronic is an analog of the well-known DSG (Direct Shift Gearbox), but it also has some peculiarities. The S-tronic transmission has two clutch plates, which ensure rapid gear shifting without interruptions in torque delivery, high fuel efficiency, improved accelerating dynamics. Thus, S-tronic transmissions are very popular among aggressive driving enthusiasts.

Check out the 7-speed S-tronic transmission

Currently, there are 4 different versions of the S-tronic on the market. For longitudinal engine mounting, engineers have designed a special 7-speed S-tronic option. In this transmission, the 7-th gear is located at some distance from the first six gears (it helps to reduce engine RPM rate). The 3 S-tronic options for the east-west engine mounting are designed as in a single layout with 2 output shafts, ensuring a space-saving design. Thus, this gearbox can be installed on different cars (even on those with small space under the hood). In other words, the pre-selective S-tronic can be installed on compact Audi A1 and A3, as well as on Audi Q3 SUVs. There are 6- and 7-speed S-tronic options for east-west engines. The 7-speed version has a dry clutch, while the 6-speed option is equipped with a wet one.

6-speed S-tronic transmission

7-speed S-tronic for longitudinally mounted engines

7-speed S-tronic for east-west mounted engines and quattro drive

S-tronic design

The S-tronic design includes the following components:

• Two clutch plates;
• Two output shafts (for even and odd gears);
• Hydraulic control unit (mechatronics);
• Numerous special sensors that transmit signals to the TCU.

Technical characteristics of S-tronic:

• S-tronic is capable of transmitting up to 550 Nm of torque;
• The maximum RPM rate is 9,000 revs/min.

In terms of design, this transmission is quite complex, but it combines well with the AWD.

Principle of Operation

To understand how the S-tronic transmission works, you can imagine a classic manual transmission with the ability to shift gears automatically. To put it simply, the S-tronic transmission features two manual gearboxes combined in one housing. Even gears are located on the shaft of one gearbox, while the other shaft is responsible for odd gears. While the car moves at one gear (even or odd), the corresponding clutch plate is engaged (while the other clutch plate is switched off). However, the next gear is selected and almost engaged in advance.

Great video on S-tronic operation

At the moment, when the TCU completes the analysis of the car speed and identifies the perfect condition for shifting to the next gear, the hydraulic drive mechanism will automatically disengage one clutch plate and engage the second one. As a result, the gear shifting (downshifting and upshifting) is implemented in no time, the driver does not feel any jerks, delays, kicks, etc.

After the gear shifting reaches the 6-th or the 7-th gear (the highest, depending on the S-tronic version), the gear shifting process will be implemented in the reverse order. In this regard, the S-tronic transmission is similar to sequential gearboxes, in which gears are shifted in a strict sequence. The S-tronic transmission also has a manual shift mode which is performed with the help paddle shifters. Thanks to this mode, the driver can shift gears independently like in manual transmissions.

It is worth mentioning that the S-tronic transmission allows you to improve the turnability (cornering ability) of your car. It is possible thanks to the compression of clutch packs on the right turn, while on the left turn, the hydraulic actuator lock the differential, and as a result the right wheel gets more torque.

The 7-speed S-tronic transmission has a flexible system of control and self-diagnostics. If malfunctions and failures occur, the driver will see the error code. In this case, it is necessary to visit the nearest repair shop specialized in diagnostics and repair of this type of transmission. The mechatronics unit is the “brain” of this gearbox which ensures the maximum transmission efficiency simultaneously with convenience for the driver. There are also dual-clutch gearboxes equipped with electric drives, but such gearboxes are installed on budget-friendly car models.

There are similar gearboxes with the same principle of operation:

• PowerShift — Ford;
• MultiMode — Toyota;
• Speedshift DCT — Mercedes-Benz;
• 2-Tronic — Peugeot and many other options.

Audi models are actively equipped not only with the S-tronic transmission, but also with the R-tronic transmission (it has a hydraulic drive).

Pros and cons of S-tronic

After we reviewed the main peculiarities of S-tronic and its operation principles, it’s high time to consider advantages and disadvantages of this transmission. Let’s start with strong points of S-tronic :

• Improved car dynamics;
• Relatively small size and weight of S-tronic. It allows carmakers to install this pre-selective gearbox on compact cars that have a limited underhood space;
• Smooth and fast acceleration rates;
• Rapid gear shifting (it takes no more than 0.8 seconds to switch from one gear to another);
• Significant fuel economy (up to 10%) compared not only to standard automatic transmissions and CVTs, but also to manual transmissions;
• The process of gear shifting is unnoticeable for drivers and other passengers. In terms of comfort, this gearbox is similar to CVTs.

At first glance, it may seem that the S-Tronic transmission and its DSG analogs can be an ideal solution, since they combine the positive qualities of manual and automatic transmissions. However, in reality, this type of transmission has several drawbacks, which prevent dual clutch transmissions from pushing automatic transmissions out of the market.

The list of S-Tronic disadvantages includes the following:

• High cost of cars equipped with this transmission;
• Low reliability and short service life. As a rule, pre-selective gearboxes can operate without repairs no more than 150 thousand kilometers (under the condition that drivers comply with operation of operation).
• Complex and costly repair procedures. The design of this transmission includes additional shafts, two clutch packs, mechatronics, hydraulic actuators and servomechanisms, numerous sensors –thus, there are many potential damage points. Combination of hydraulics, mechanics, and electronics makes S-tronic quite complex in terms of diagnostics and repair;

Take a look at the S-tronic transmission repair process

• Taking into account that the S-tronic transmission is installed in Audi sports cars and operate under high loads, it is necessary to change transmission fluid timely, monitor the condition of the valve body (mechatronics) and its solenoids, use original spare parts or quality analogs;
• Not all transmission repair specialists can fix the S-tronic transmission. To fix this type of transmission properly, repair shops need highly-qualified specialists and special diagnostic equipment. For this reason, numerous repair shops frequently refuse to repair S-tronics, i.e. car owners have to go to official Audi /Volkswagen repair services and to pay for expensive repairs.

As the transmission components wear out, owners of cars equipped with S-tronic may have to deal with the following issues:

• If you drive at high speeds, you can experience kicks and jolts;
• When you shift from the 1-st to the 2-nd gear you may feel slight vibrations;
• Engine speed dips are also possible.

Transmission fluid change in S-tronic

Experienced owners of cars equipped with dual-clutch transmissions claim that a regular transmission fluid change is a ‘must’ for this type of transmission. The maximum mileage without fluid should not exceed 40-50,000 kilometers. Disregard of this recommendation can lead to the failure of friction plates or solenoids. For this transmission, it is necessary to use original transmission fluids recommended by the car manufacturer. Together with transmission fluid change, it is necessary to replace the oil filter, so that wear elements won’t find their way to the fresh transmission fluid.

Summary

Taking into account the above information, it becomes clear that S-tronic characteristics significantly contribute to driving convenience, enhanced car dynamics, and fuel saving. A new car equipped with this transmission can impress the owner with smooth gear shifting and great acceleration characteristics in comparison with cars equipped with classic automatic transmissions and CVTs. However, operation principles of this transmission are based on complex algorithms and its design includes the mechatronics as well as numerous servomechanisms. As a result, this transmission is not only expensive and complex, but frequently turns out to be less reliable than classic automatic transmissions and numerous CVTs.

All in all, if you can afford to buy a car equipped with the S-tronic transmission, you won’t be disappointed with its performance characteristics, but you have to be ready for strict requirements on maintenance services for this transmission.