Features - Technical
SEPTEMBER 15, 1998
BY PETER WRIGHT
In the 1960's, engines migrated to their current position behind the driver and ahead of the rear axle. Initially radiators stayed in the nose to gain the benefit of receiving maximum airstream energy to force air through their cores. Now the designer had to pipe hot fluids from the engine in the rear, past the driver's cockpit, to the radiators in the front. With the advent of monocoques, this meant holes or channels in the sheet metal structure, which was structurally undesirable, and the need for insulation if the pipes were located anywhere near the driver or fuel tanks. As the cars became smaller and more compact, the driver's feet nestled closer to the radiators and hot or burnt feet became a common complaint. Top-exit ducts - in the interest of reducing aerodynamic nose lift - ensured the driver received the hot air into the cockpit. As so often at that time, it was Colin Chapman who took the bold step of moving the radiators to the sides of the car, placing them in podded cooling ducts alongside the driver, on the Lotus T72. However, it was not concern for his drivers' comfort that drove him to make this change, there were sound engineering reasons for doing so.
Chapman's previous Formula1 car, the famous Lotus T49, had been his first to carry wings. Realising the importance of aerodynamic downforce, he put his mind to maximising it within the regulations. The wedge-shaped T72 was the result of efforts to persuade the car's bodywork to generate downforce, and the highly tapered nose shape would not accommodate radiators and their associated ducting. Placing them on the sides of the monocoque positioned them ideally for short cooling pipe runs from the engine. However, aerodynamically there was a problem: the energy of the airflow, back along the sides of the car was degraded by the wakes of the front wings and front tyres. Only around 50% of the free-stream energy was available for driving cooling air through the radiator cores. The radiators had to be twice the size of front mounted ones to do the same job. Only the core area needed to be increased, the header tanks remaining as before, so the weight penalty roughly cancelled out the weight saved by deleting long runs of water and oil-filled pipes to the nose of the car. It turned out that the drag difference between nose mounted radiators and side mounted ones was negligible (Lotus did not carry out wind tunnel testing in the mid-1970's). The final benefit Chapman sought, in mounting the radiators on the CG of the car rather than at one extremity, was to lower the polar moment of inertia - the difference was significant. The T72 was probably the most successful racing car ever, winning 20 GP's and 3 constructor's titles. It set the trend for Formula1 cars until ground effect changed the shape forever.
As I recounted in my previous article on the history of ground effect, I had been experimenting with airfoil-profiled sidepods on the BRM and March, attempting to generate more downforce from the body of the car. When Tony Rudd asked me to run the wind tunnel programme in support of the design of the Lotus T78. It was not too far into the programme before Ralph Bellamy (designer of the T78) and I found a way of incorporating the radiators into BRM/March-style sidepods. How this arrangement led to full ground effect, with the addition of skirts, is also covered in my previous article. One way or another, single-seater, open-wheel racing cars have had sidepods, incorporating the radiators, ever since.
The size, proportions and shape of sidepods have changed over the years, influenced by both the changes in the regulations governing bodywork and the progress of the teams' aerodynamic developments. Two requirements dominate: the ability to cool the engine and gearbox; and the required plan form of the car, providing the underbody pressure distribution desired by the designer. Secondary factors that also influence the design are: the provision of sufficient energy absorbing structure to meet the FIA side impact test; and provision of sufficient space to accommodate the boxes containing the car's control electronics and data systems and their associated wiring. Finally the sidepods are an important bearer of sponsors' logos!
Since 1983, when the new flat bottom regulations prevented designers tuning the distribution of underbody downforce by profiling the underneath of the car, the length of the sidepods has been one of the few options remaining for determining centre of pressure and the manner in which it varies as the car pitches and heaves. Long sidepods generate more downforce forward of the CG than the short ones currently in use. As the car's front axle ground clearance reduces under braking, they move the centre of pressure forward and tend to destabilise the car. Front wings have been developed to such a fine pitch that the underside does not need to generate so much forward-biased downforce, and sidepods have been steadily shortened. For a short time in the early 1990's, active suspension allowed the attitude of the car and it's all-important ride heights to be controlled, solving the problems of the longer sidepods. Banned for 1994, the move rearwards of the leading edge of the sidepods has been universal.
The width and height of the sidepods are determined by the need to accommodate the oil and water radiators for engine and gearbox cooling. The distribution of engine heat to water and oil will depend upon the detail philosophy of the engine designer, but the total will be similar for all the engines. Chassis designers press their engine opposite numbers to minimise heat rejection quantity and maximise coolant temperatures, in order to minimise the radiators sizes and airflow requirements for cooling. However, with engines producing around 800horsepower, it takes all the space available in the sidepods to package radiators and their ducting to cool them. If designers could lower the top of the sidepods they would do so, in the interest of improving airflow to the rear wing.
Shortening the sidepods by moving the leading edge rearwards, has not helped the task of providing enough cooling air to the radiators. The turbulent wake from the front wheel spreads as it moves downstream and so envelops the radiator intakes more, the further they are behind the wheels. A wide variety of barge-board arrangements has emerged since pods were shortened, to try and take control of both front wheel and front wing wakes and move them to regions where they can do some good, instead of hurting the cooling by entering the radiator intakes.
The aluminium radiator cores used in Formula1 cars are relatively thick and have high air pressure drop coefficients, and so do not require high velocity air to flow through them. Instead, the cooling air must be slowed efficiently, and the static pressure thereby raised, between the intake and the front face of the radiator. The internal ducting of the sidepods takes the form of a divergent duct to the one or more radiators in each sidepod. The pressure behind the radiators must be as low as possible to provide the greatest possible pressure difference across the core. Communicating the radiator exit ducts to the rear of the car ensures that the base pressure, lowered further by the rear wing underside, is used to "pull" air out of the radiators. The warm air behind the radiators is 60+°C, but cold relative to the 600+°C of the exhaust pipes, and so provides a useful secondary cooling function.
The outside shapes of the sidepods show little significant variation between cars, except around the intakes (styling) and the detail around the rear wheel. Each aerodynamicist has his own ideas how to influence the airflow onto the rear wheel and hence the pressure distribution around it. The high pressure ahead of the wheels "pushes" air under the car, filling and decreasing the desirable low pressure generated there. The high velocity air flow around the inside edge and along the side of the wheel and tyre creates low pressure, which is desirable as it inhibits the inflow under the floor area. Rear brake cooling air must be collected from the area between wheel and bodywork.
At some very hot races, radiator exit ducts appear on cars around the sides and rear of the sidepods, in an attempt to draw more air through the radiators. Only John Barnard, on the Arrows A19, has abandoned the coke-bottle shape that he himself pioneered on the McLaren MP4/2. Barnard has gone to great lengths to lower the rear deck of the Arrows in order to maximise clean air on-to the rear wing, and has probably squeezed the space beneath the bodywork to such an extent that the airflow through the radiators would have been compromised. Creating an alternative exit duct into the low pressure area around the rear wheels, restores the radiator flow without affecting the rear wing. Unfortunately, the lack of power of the TWR engine and problems with the car, have prevented a true assessment of this interesting and different approach.
The FIA introduced a side impact test, primarily as a result of Roland Ratzenburger's fatal accident at Imola in 1994, when the impact caused one front wheel to intrude into the driver's safety cell. The test calls for a 780kilogram impactor to hit the side of the car, 525 millimetres in front of the rear of the cockpit opening, at 7 metres per second, and to be arrested at no more than 10g. The sidepods provide the means of absorbing the energy, and the point of impact is chosen to encourage sufficiently long sidepods to provide adequate protection from the front wheel as well as the barrier. The space between the inner and outer ducts at the bottom of the sidepods provides a space that is filled with an energy absorbing structure. These follow the same principles as the nose cones, employing the controlled failure of a carefully developed CFRP structure to absorb the energy.
Unfortunately a couple of teams, Benetton and Minardi, chose to employ alternative structures to absorb the test impactor, freeing the design of the sidepods from needing to extend forward sufficiently to meet the test. The FIA is addressing this cunning way round the regulations.
The space between inner and outer surfaces, on the top and sides of the sidepods, is used to mount the car's electronic boxes. It is ideal space for this purpose as the mounting positions for these components are not dictated by the need to be directly mechanically connected to any other components, instead the flexible wiring looms can be kept reasonably short between the engine and dash panel - the prime areas of electronic activity. The disadvantage of this location is that when an actual side impact occurs, it is the black boxes that absorb some of the impact. It is not uncommon nowadays to find expensive printed circuit boards and electronic components littered around on the track at the scene of an accident, among all the shattered CFRP
How sidepods will develop in the future will be most influenced by changes in the regulations. First and foremost will be how the regulations determine the under-car airflow, which must surely be reduced in the future to limit speeds. If diffusers are abolished, it is unlikely the plan shape of sidepods will change much, as the loss of rear downforce will not encourage longer pods. It will be difficult to shorten them further while still meeting the side impact requirements. If the downforce of the front wing is controlled in some way, the pods could extend forward to regain lost front downforce.
Their height is unlikely to reduce, although the designers would like to do this to improve rear wing airflow, because the engines are not getting less powerful and their cooling needs are not reducing significantly.
The FIA is introducing a new regulation for 1999, which sets out to retain the wheel in the event of an accident. This is to be achieved by connecting the upright to the chassis with a Kevlar cable inside the lower wishbone. What the wheel does when the wishbone is destroyed will not become clear until a few Spa-type accidents have occurred. If the dynamics of the tethered wheel causes it to impact the driver's safety cell, it may require further revision of the side impact test and this may affect sidepod design.
Sidepods are here to stay, as they provide that valuable plan area for the under-car, low pressure conditions to act upon and create downforce; and they are the neatest method of accommodating the radiators and their associated ducting. We shall see them for some time, adorned with creative paint schemes and carrying the sponsors' logos.