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Le Mans transmission development

The Le Mans 24 Hours is one of the toughest tests of racecar endurance in the world. Nowhere else are components and personnel presented with the same combination of relentless racing, an unforgiving circuit and ever-changing track conditions. In the past, transmission reliability has proved particularly troublesome and, while the current generation of Prototype and GT gearboxes are generally reliable, constant r&d by manufacturers is required to ensure they stay that way.

The internals of a Prototype or GT gearbox are very similar to those found in most other high-end racecars, be they single-seaters or saloons. However, they are subject to many small modifications to enable them to withstand the pounding meted out at circuits like Le Mans or Sebring. Some of the biggest challenges are presented by circuit topography, notably the bumps and kerbs.

This pummelling effects the transmission in two ways. First, it can create issues with lubricant distribution, and second it can cause shock loading of the driveline.

Extreme loadings are experienced under these conditions because shifts take place when the car’s wheels are not on the floor. This means the wheels are overspeeding and, as the car hits the ground and traction is regained, the driveline is shock-loaded. Clearly the way to ensure reliability is to engineer the transmissions to be tough enough to accept such abuse, but it is also imperative that the performance compromise this entails is as small as possible.

One manufacturer has invested heavily in test equipment to simulate such occurrences, allowing it to accurately model the demands placed on a transmission at any circuit. The system consists of a gimbal-mounted test rig that can reproduce the g-loadings a transmission encounters in use, based on actual track data. This rig, combined with an active transmission dynamometer to simulate the loadings on internals, has allowed the manufacturer to produce transmissions with a level or reliability that would have been unheard of even a decade ago.

The company also has another test rig that aids race reliability by allowing engineers to gain a greater understanding of transient differential operation. This knowledge then allows them to better adapt their differential settings during a race, for example moving from wet to dry running, something often required to account for the ever-changing weather conditions at Le Mans.

Previously, the only way to measure the operating characteristics of a differential was to use an entire gearbox ‘three-motor’ rig test facility, a sophisticated piece of equipment that characterises the whole driveline. Not only is this an expensive procedure, however, it’s also very difficult to isolate and characterise just the differential. Until now, the only other alternative was to measure the characteristics in the vehicle and on the track, which is even more costly

The differential rig features a single 45 kW motor that drives both sides of the differential under evaluation, thereby eliminating the complexity and cost of synchronising two motors. Despite its simplicity, the computer-controlled rig can operate in the ‘quasi-transient’ test mode necessary to characterise the dynamic behaviour of ramp- and plate-type differentials. In this test mode the rig can perform a combination of closed- and open-loop tests in order to analyse differential response to quasi-transient inputs. 

Other modes of operation permit torque values to be ‘dialled in’ to the rig to evaluate steady-state characteristics of the differential with respect to torque, speed and temperature sensing. The control system allows measurements to be sequenced manually or automatically for both 2D and 3D mapping, and can be fed with simulated or actual cornering data. It was also written to provide measurements with high rates of data logging.

To simulate a corner the relative motion across the differential can be varied from 0 to 500 rpm, and the rig has an input torque and locking capability of 1000 Nm (737 lb-ft). An oil system has also been incorporated to simulate the differential oil system, while the test environment can be pre-heated and is fully temperature controlled.

Overall, the development of test systems such as these go to show that, although transmission reliability is considered a given in modern endurance racing, it is only kept this way through relentless r&d.

Written by Lawrence Butcher

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