Features - Technical
MAY 23, 1997
Formula 1 Transmission Trends
BY PETER WRIGHT
With Williams, Ferrari and Prost retaining transverse layouts, while all the other contenders have returned to a longitudinal arrangement, as described in V6N3, there is no consensus on which is the optimum for the current cars. Prost still uses the ex-Benetton gearbox, so it maybe expediency that dictates their choice. Ferrari's CFRP/Titanium/CFRP sandwich favours the transverse layout as this concentrates the hot, highly stressed parts of the casing, that cannot yet be in CFRP, into the smallest volume. It is questionable whether the stiffness benefits of carbon fibre and titanium compared to magnesium are realised in a design that necessitates extra casing joints. Ferrari have overcome their initial problems with cracking around the suspension pickups and the unit appears very reliable now. How the specific stiffness of the whole (stiffness per unit weight) compares with Williams' box is an unanswered question. Certainly no other team has chosen to follow in Ferrari's footsteps down this exotic and costly path.
Just why Williams have retained their transverse layout is their secret. Maybe it is because their is no real advantage in a longitudinal's weight or efficiency, and maybe they know something about weight distribution that the others do not know, not wishing to take advantage of pushing the engine and gear masses forward. Ratio changing on a longitudinal requires the box to be split from the engine and inevitably takes longer than on a transverse layout. It could be simply that, with the gearbox still the Achilles' heel of the car (e.g. Villeneuve at Imola), they would rather stay with the devil they know.
Gearbox R&D efforts are focused on reliability, design for lower weight, and transmission efficiency. All three come from very careful detail design, specification and quality control. Material and heat treatment specifications have to be more closely controlled to exploit further material properties, reducing weight without compromising durability. Bearing, shaft and gear tolerances, allied with precise control of lubrication, can yield efficiency gains and lower running temperatures.
Nearly all the teams use Xtrac for the manufacture of key gear and shaft components. The exception is Ferrari, who source all their components in Italy. Jordan splits their suppliers between Xtrac and Hewland. Williams works closely with their technical partner Komatsu, for the manufacture of final drive gears, tapping into the specialist steel manufacturing skills for which Japan is renowned.
The choice of fitting 6 or 7 forward ratios is provided for in some of the gearboxes, notably Benetton and Jordan. The decision on how many to actually fit for a given event, is mainly a function of matching engine characteristics to the circuit and the driver's preferred style. With gear-change times down to around 2 hudredths of a second, any development to reduce them yields a diminishing return in exchange for potential greater risks of a miss-shift.
The ability of the carbon clutch to continue to shrink in diameter and weight, whilst transmitting ever greater power, still amazes me. The current 4.0 inch diameter AP Formula 1 unit must be getting near the limit, and it is unlikely that the engine designers will be able to lower the crankshaft centreline much more. The tiny clutch would not look out of place on a 250cc motorcycle engine. Clutch duty cycles have been made slightly easier with the advent of computer controlled gear changing, where the clutch is not operated at all during up-changes, and is controlled precisely by the computer on down-changes. It is the start that takes clutches to their limits of temperature, and they are still expected to accommodate the bulk of the slipping needed to maintain engine RPM when leaving the start line. Tyres lose grip if slipped too much, while the carbon plates do not, unless their surface temperature becomes too high. Ultimately it is the thermal capacity of the clutch that will determine where the size limit lies.
For a long time in Formula 1, the Salisbury type, limited slip differential has survived as the preferred unit, in spite of attempts to dislodge it by ZF's cam and pawl, the Torsen and similar concepts. In the last couple of years however, the viscous differential has proved to be the most versatile and easy to set up for a given track. However, it looks as if it will be a short reign as the computer controlled friction-plate differential, borrowed from rallying and road cars, gains in popularity.
The viscous differential has proved tolerant to changing track conditions, which is essential in the race environment when the half to one hour needed to change a differential is not always available. Characteristics can be changed by varying the number of plates, and the quantity of the fluid as well as it's viscosity. However, it is the desire to alter characteristics quickly in the garage, and even on the track that has led to the development of computer controlled electro-hydraulic units.
The regulations permit such devices, provided they emulate a purely mechanical system whose characteristics can only be altered by instructions from a computer, plugged into the car while in the pits, or by the driver selecting a setting on a cockpit control. No dynamic parameter from the car can be fed-back to close the loop around a car response. In particular the slip ratio of either rear wheel cannot be used, as to do so would be construed as traction control. Mechanical differentials apportion torque to the two rear wheels, either according to the overall torque, or the differential speed of the wheels, the different systems doing so in different ways depending on how the internal friction between the two outputs is varied. The computer controlled differentials vary the friction by changing the hydraulic pressure in the actuator that clamps the friction plates. Control laws that regulate the pressure according to various permitted parameters are constructed within the software. Software inspection is necessary to ensure that a sense of reality is retained in defining what is considered to be a possible mechanical system for emulation.
Varying degrees of success with these devices, is evident on the cars that are using them so far; all the top teams are reputed to be at least testing them, if not using them in races. A differential can steer the car, and one that possesses a mind of it's own, which computers sometimes do, will upset the driver a great deal by taking control at critical times and altering understeer/ oversteer characteristics when least desirable. Once fully sorted however, they offer a versatility and adjustability with which no mechanical system can ever compete.