The modern racing car gearbox
It can be sobering to sit back and consider what we ask of a racing car gearbox when we require it to perform a structural role as well as functioning as a change speed box...
Generally the loads that we will ask it to absorb are a significant proportion of, or even a multiple of all up vehicle weight, and they will act in a variety of directions and combinations.
First, if it is to do its job of providing the rearmost section of chassis, we ask it to take the same torsional loading as the chassis itself. We also ask it to accept lateral bending loads from cornering forces, and vertical bending loads from car weight, load transfer and aero forces and the magnitude of these moments will increase with the distance from the rear axle. This in part explains why most casings are nearly always much larger in cross sectional area towards the front than they need to be to house the gearbox itself.
Second, we almost always hang the majority of our rear suspension either directly from the casing, or from subframes mounted to it. We have to provide reinforcement locally where we choose to feed the loads in, and we will generally try to tie opposing sides together internally with webs and diaphragms to stiffen and distribute these high loadings through the case. These, in turn, often have to double as bearing supports for the gear shafts themselves.
Thirdly, we invariably cantilever the rear wing structure off the back, and sometimes bolt up the underbody tunnels or diffuser, once again feeding in high point loadings.
Finally, we often mount a pneumatic airjack to the casing, and so, when stationary in the pits, we ask it to support 60% or more of the car’s weight.
All of this in addition to the internal loads set up by the gears themselves, in the crown wheel and pinion, and by the clutch…
Amazingly, when one considers that the average wall thickness of a case can often be only four or five millimetres of magnesium, we almost always get away with all of this and still retain a transmission system inside that works for us – most of the time. We usually achieve this despite being obliged to make provision for sensibly sized access holes in order that we can open the box up and change the ratios in a hurry. And although we would like to make the complete unit from diff housing to engine in one unit, practicalities invariably force us to split the whole into front and rear sections.
As a final challenge, we almost always shroud the box in tight fitting bodywork, and pack the exhaust system in as close as we possibly can for good measure, so that the case itself runs at elevated temperature, to the detriment of the life of the gears within, and the stiffness of the case itself.
This stiffness, both in torsion and in bending, is every bit as vital in the transmission as in the chassis, or in a stressed engine. A structure is only ever as stiff as its most flexible component, and if the gearbox has a lower unit stiffness than the chassis – which it often has – then this will be the limiting factor in the key axle to axle stiffness figure. The relatively large cross sections that we can usually afford to design into our gearcasing at its forward end is a bonus when it comes to increasing stiffness, but by the time we reach the rear axle we invariably seek to reduce it to the practical minimum to house the final drive and differential assembly, because we are almost always looking to maximise the diffuser or tunnel area that activates the aerodynamic underfloor. This works against us and it is not uncommon to see a graph of torsional deflection, plotted against the wheelbase, that suddenly starts to climb in the vicinity of the diff housing. Sometimes this comes down to detail design, and it is always standard practice to dowel individual sections of the casing to one another, and to provide generous flanges and internal ribs.
Lack of Stiffness is not just the enemy of wheel loadings though, a flexible case can cause havoc with the gears themselves, which in extreme cases can slip out of mesh or engage more than one or a time.
All in all, when it works, it is a major engineering achievement!
Written by Peter Elleray.