Your shopping cart is empty.
Product Qty Amount

[email protected]
/ Categories: Archive, transmission

Dual-clutch transmissions

For many years, sequential transmissions using dog engagement have been the standard fitment for most serious racing machinery, thanks largely to their durability and short shifting time. However, Seat’s latest Leon Cup touring car features a dual-clutch, semi-automatic transmission as an option, similar to those found in many new roadcars. Beyond this latest manufacturer effort, there are also a number of racers at club level successfully running cars equipped with Porsche’s PDK transmission.

The dual-clutch transmission is by no means a new idea, having been conceived by engineer Adolphe Kegresse in the late 1930s (incidentally, Kegresse also developed the half-track drive system for off-road vehicles). He hoped to use the dual-clutch system in Citroen’s Traction Avant, but World War II intervened. There then followed a 40-year hiatus until Porsche picked up on the idea.

Porsche used the PDK gearbox in its 956 and 962 Le Mans cars. Its first success with the transmission was a win at the Monza 1000 km race in 1986, although the system’s reception among the team’s drivers was mixed. Derek Bell, who won the 1986 World Sportscar Championship, liked the ease of use of the system but was less than impressed with the extra weight it added over a standard five-speed sequential, going so far as to describe the cars as feeling like they had “a trailer on the back”.

The biggest benefit the system delivered from an endurance perspective was reliability – not so much in terms of the physical durability of the transmission components but in terms of saving the driver from having to operate the clutch. In a 1000 km race, one mis-shift on a manual transmission of the period could destroy it; the PDK removed this possibility. With improvements in sequential transmissions, however, and the introduction of various pneumatic and hydraulic shift systems, the need for the PDK disappeared, but Porsche did not forget the technology and continued to develop the system for roadcar use. Other companies, including transmission manufacturer Borg Warner, also developed dual-clutch systems, and since the turn of the century they have become common in many roadcars.

At the heart of a dual-clutch system is a two-piece main shaft, with one shaft section running inside the other. Each shaft carries three gears, with odd-numbered gears on one and even numbers on the other. Attached to each shaft is a multi-plate wet clutch, one running inside the other, with the engagement of each clutch being controlled by a hydraulic circuit. Gear selection is also controlled by hydraulic servo motors, and the vehicle’s control electronics govern overall operation, the theory being that the ECU determines which gear is likely to be needed next and pre-engages it. As soon as the driver initiates a shift, the clutches engage and disengage, selecting the next gear with minimal lag.

Porsche claims a shift speed of less than 100 ms for its PDK system, and given that Seat falls under the same VAG Group banner as the Stuttgart manufacturer, there is good reason to believe that the Seat system operates with similar rapidity.

But how effective is the transmission in racing use? Because of its extra weight and complexity over traditional sequential transmissions (and often regulations outlawing the system), the upper echelons of racing have not seen the adoption of dual-clutch gearboxes. However, anecdotal evidence from those who have used them in club motorsport, particularly in the US, have found they provide very short shift times without the expense of a dedicated motorsport sequential transmission. The main issues experienced have related to keeping the clutch oil temperatures under control, but the addition of extra cooling capacity has minimised such problems in all but the most punishing endurance races.

Whether the systems will see more widespread adoption in racing remains to be seen. It is telling though that one motorsport outfit, which prepares cars for SCCA competition, has found the control software on PDK transmissions so effective that it encourage its customers to let the system operate in fully automatic mode – under competition conditions!

This video provides a very good illustration of a dual-clutch transmission in operation.

Written by Lawrence Butcher

Previous Article Preventing roller lifters from rotating
Next Article Data logging