F1 cars use rather conventional transmissions, but adopt quite advanced control systems to operate them. In essence the transmissions use a clutch driving a manual gearbox of six or seven forward speeds, which transfers drive via a differential, weight reduction is critical with this component, as it is quite a lrge percentage of the total weight of the car and as it is placed so far back in the car, a recent (2002) mid-field gearbox is known to weigh 43Kg fully dressed.
The gear cluster sits between the engine and differential in a longitudinal axis, termed as a inboard longitudinal layout, previously the cluster has been placed behind the differential, for easier access at the cost of weight distribution and aerodynamic slimness. Also transverse layouts placing the bevel drive before the gear cluster made for a shorter casing and placed the weight inside the wheelbase, at the expense of width and casing rigidity as the cluster needed to removed from the side of the case.
Clutches are well known for using carbon-carbon plates, this allows the clutch to withstand heat created from gearshifts and standing starts. The detail development of the friction surfaces and basket, have reduced the size down to a minimum. Clutches used to dictate the crank shaft height of the engine, as there were of a larger radius than the crank stroke, now they are smaller than the crank stroke and further size reduction is irrelevant. The clutch is operated by a hydraulic release mechanism\bearing, operating via the gearbox control software.
The clutch drives the gearbox input shaft and is now often affixed to the shaft, rather than the tail of the crank, this simplifies the gearbox assembly as the whole clutch gearbox hydraulic system is removed as a whole. Gear clusters use a two shaft design (below in red and blue), with out drop gears where the output shaft drives a bevel geared crown wheel (CWP).
Gear selection is via an indexed selector drum operating the gears via selector forks (yellow) running in the key ways in the drum. The selector drum is rotated by a “cam and pawl” mechanism, which in turn is operated by a hydraulic piston run from the gearbox management software. The entire gear cluster and selector assembly sits is removable cartridge complete with bearing and oil-ways. This allows for easy assess and replacement of ratios. Ratios vary from six to seven depending on the engine characteristics, a peakier power delivery will require the full seven ratios (the maximum allowed) a reverse gear is mandatory and is also selected via the selector drum. Gears are often produced by specialist gear companies such as Xtrac or ‘Staffs silent gears’, as they have the dedicated machinery and heat treatment facilities. The width of the gear is dependant on its use, lower gears are placed under heavier loads and tend to be wider and have their gear detail and heat treatment adjusted to cope, where as higher gears can be made narrower with less intensive heat treatments. Gear engagement is via five ‘dog’ teeth.
The bevel crown wheel surrounds the differential which passes drive to the wheels via inboard tripode CV joints to titanium drive shafts towards the uprights. The drive shafts are often outsourced to specialist suppliers such as Pankl.
Gear cases need to provide the rear structure of the car as well and allocation for the gearbox. They need to provide mounting points for the rear suspension, impact structure as well and a rigid interface to the rear of the engine. Traditionally cast in aluminium alloys, the detail design of the casing has developed massively over the years. The current format is a large once piece casting from the rear of the engine to the centre line of the differential, with only apertures for the CWP bearing, differential and gear cluster cassette. Smaller castings are required for the differential cap, CWP bearing cover and of course the cassette.
Casting techniques have driven weight down, through the use of thinner wall thickness in certain areas, which are defined by careful Finite Element Analysis (FEA). Recent developments include the use of titanium castings created from a lost wax mould, adopted by Minardi and Ferrari, Renault also have part of their gearbox in cast titanium, but they use a carbon fibre upper section to create a split case, enclosing the gear cassette in the metallic part and the suspension mountings on the carbon composite part. Some teams have used cases moulded in carbon fibre, this provide a stiff structure good for suspension performance but the different rates of thermal expansion between the carbon fibre and metallic gearbox components caused problems initially. Arrows finally made a reliable carbon fibre casing in 2002 (see below) and the designer Mike Coughlan along with two other engineers have now joined McLaren to design the MP4-18 Carbon Fibre gearbox.
Since the Ferrari development of the semi automatic gearbox and subsequent Williams development of a sequential semi automatic box, control systems managing the timing of the hydraulic actuation of the gearbox have become just as important as the mechanics. The term Semi automatic should not be confused with Road car alternatives, in F1 terms it means the normally Manual gear selection is managed by electronics and hydraulics. Gear selection is initiated by the driver through paddles on the steering wheel (McLaren had an early version with buttons), the electronics decide the timing of the selector drum and clutch actuation, along with ignition cutting for upshifts and throttle blipping for downshifts. This ensures the engagement of the next gear is sequenced correctly to road speed and the wrong gear is not selected in error (i.e. first gear when at top speed). Escape routines are required for when a gearshift fails, such as when the gears connect dog-to-dog and not fully engaged. Gears now have to be selected in sequence, such is the design of the selector drum, the option of skipping gears is not possible (it was on the first Ferrari gearbox), but such is the speed of the shift that this is not an issue, pre selected change down buttons are allowed where the gearbox will automatically run a sequence of continuous down changes to drop the box to a selected gear (often second gear).
Also with the freeing up of electronic aids automated upshifts are allowed, where the electronics will make the shifts through the gears with out driver intervention, this is often use on straights where the precise gear change timing maximises acceleration.
Clutch actuation is also completed by paddles on the steering wheels, no longer is a clutch pedal required in the foot well, the paddles lack the feel of a real pedal and care is needed to predict the biting point when pulling away. With Launch Control (LC) allowed the driver no longer needs to control the clutch from the paddle to start the race as it is handled completely by the electronics all he needs to do is release the LC button on the steering wheel. Equally the clutch can be actuate by the anti stall software, this is designed to keep the engine running in a spin, by detecting the wheel speed has dropped to a point where the revs will drop to stall the engine, the system needs to be reset by the driver to re-engage the car in gear. This year (2003) McLaren have completely automated the clutch actuation as part of the of the combined anti-stall and LC functions, hence there is no clutch paddle on the steering wheel.