Why F1 clutches have very little in common with roadgoing counterparts

Components that are hidden deep in the car are often overlooked and yet play a vital part in providing function with performance. One such item is the clutch.

The clutch in a Formula 1 car performs a similar function to that of the clutch in a road car in that it provides a coupling between the engine and, ultimately, the road wheels. An automotive clutch is a friction device connecting the engine to the transmission: it’s capable of slipping and hence can provide an infinitely variable ratio between the two, ranging from the engine turning and the gearbox input shaft stationary when the clutch is disengaged to a one-to-one ratio when the clutch is fully engaged, and the gearbox input is rotating at the same speed as the engine.

This ability to slip allows the driver to pull away from rest by gradually engaging the clutch and thereby altering the amount of slip, effectively altering the ratio of engine speed to gearbox speed until the clutch is fully engaged. The difference between clutch use in a racing and a road car occurs not at the start but during gear shifts. In a road car the clutch is fully disengaged to make a shift, thereby allowing the gearbox shafts – aided by synchromesh – to synchronise their speed when engaging another gear.

On an F1 car this isn’t necessary since the complex control electronics will ensure that when a gear change is called for, the next gear is engaged when the drive dogs on the gears are in exactly the right place, negating the need for the clutch to disengage and providing what are termed ‘seamless shifts’. Such a shift ensures there is effectively no loss of drive during an upshift. In effect this control is an extremely fast-acting electronic analogy of the mechanical synchromesh used in road cars.

The architecture of a road clutch and a Formula 1 clutch are similar, but the differences are significant.

A road car clutch consists of a friction plate which is pushed against the engine flywheel by a diaphragm spring. The friction plate has splines at its centre hub which are connected to the gearbox input shaft. This gives a path for the engine torque from the crankshaft to the flywheel and then via the clutch-driven plate to the gearbox, before finally being transmitted to the wheels.

To disengage the clutch on a road car the driver presses the clutch pedal, which moves a piston in a hydraulic cylinder or operates a cable which is connected to a release bearing mechanism that bears on the clutch diaphragm spring. Pressing the pedal pushes the centre of the diaphragm spring which then pivots around a fulcrum, releasing the pressure on the clutch plate.

F1 clutches, used for starts and reversing only, are operated by paddles on the back of the steering wheel

Photo by: Mark Sutton

The road car clutch will have a housing made of pressed steel and the clutch plate will have an organic friction material similar to that of a brake pad. For a mid-sized family car, the clutch may be 240mm diameter and would weigh over 5kg. Its torque capacity would be around 450Nm.

The Formula 1 clutch is a highly developed version of this but the release mechanism pulls on the diaphragm spring fingers rather than pushes on them, which allows a smaller diameter and lighter weight design as well as enhancing the cooling. It is much smaller in diameter, between 100 and 110mm; and, rather than a single clutch plate, it would have four or five driven plates internally splined to the gearbox input shaft. These act against similar alternately spaced plates externally splined to the clutch housing. The torque capacity of a clutch is a function of the plate type and area and the number of plates as well as the diaphragm spring load rating.

Despite its tiny size, a Formula 1 clutch will handle 2,000Nm of torque. The small diameter allows the engine to be mounted low in the car and the clutch would be much lighter at around 1.5kg because the housing is made from titanium and the friction plates from carbon. This all adds up to lower rotational inertia which benefits acceleration. It’s often not appreciated that rotational inertia kills acceleration just as weight does.

The engineering behind the massive reduction in size of clutches is very much down to the development of carbon friction surfaces which were developed alongside carbon brakes, the materials being very similar

Furthermore, the inertia of the engine or clutch is seen at the wheels as the square of the total gear ratio. An F1 clutch has an inertia value of around .0066kgm2 which is around one tenth of that of a road car. Without wishing to put equations in this column it can be shown that as our Formula 1 car accelerates, the inertia of the clutch is resisting the acceleration in a similar manner to adding 8.5kg of mass to the car. Even a small to mid-size road car has a clutch inertia which would equate to adding 85kg to the car so it’s easily seen why the reduction in inertia of the clutch is even more important than the reduction in weight.

The engineering behind the massive reduction in size of clutches is very much down to the development of carbon friction surfaces which were developed alongside carbon brakes, the materials being very similar. Prior to this the friction materials for racing clutches were a mixture of copper, iron, bronze and silicon dioxide sintered onto a backing plate. While superior to the organic facings, they were still temperature limited with friction fading at less than 1,000C. Carbon materials can easily handle this temperature which can often occur during a launch. Even the hubs and clutch cover can see 500C on a bad day.

A carbon clutch was first run on a Williams car in 1983 and, by the mid-1980s, was accepted as the norm. Tilton, ZF and AP Racing were all in the early development race but now AP Racing supplies eight out of the 10 teams and celebrated its 865th win with a carbon clutch in Abu Dhabi last year.