Features - Technical
OCTOBER 8, 2000
"On your marks. Get set. GO!"
BY PETER WRIGHT
From the moment the cars complete their formation lap and are eased into their individual positions on the grid, they are monitored and controlled by Race Control via the TAG-Heuer timing and Seimens race information systems. The identification transponder fitted to each car, not only identifies the car to the TAG-Heuer timing system, but also precisely positions the car relative to numerous wire aerials embedded in the track and Pit Lane. By monitoring the signals from each car via an aerial embedded at each grid position, checks can be carried out to ensure that no car starts the race with a wheel over its individual "line". The system can also check the exact moment that a car crosses its starting line, and any jumped start is reported to Race Control, which decides on whether a Penalty is to be applied.
Once all the cars are stationary and in position on the grid, i.e. "On their marks." Race Control starts the automatic start sequence. The "Get set" command is the 5 second count down as the 5 red lights on the start gantry over the track are turned on at 1-second intervals. As with running races, there is then a pause of unpredictable duration before the "GO!" command, in this case randomly and automatically determined by the system and kept to a few seconds so engines, clutches and drivers' nerves do not become strained, then all 5 red lights are extinguished. From then on it is down to the combination of driver and car to make a better start than their competitors.
So important is the start that drivers and engineers begin to prepare for it well in advance of 2.00pm on the Sunday of a race. Unlike 100m sprint races, the cars do not all start in line and use straight tracks. The two-by-two, staggered starting grid means that grid position has more influence on starting results than anything else. It is not just a case of achieving the best possible Qualifying result, but notice has to be taken of track surface conditions on each side of the starting area and the line into the first corner. So slippery is the track on the 2-4-6Éside of the grid in Hungary, that it is often better to be third on the grid than second. David Coulthard - second in Qualifying - tried hard throughout morning warm-up at this year's GP, to drive down the slippery side to clean it up and yet was easily beaten away by Mika Hakkinen who started in third place and subsequently dominated the race.
There is little opportunity to anticipate the sixth light going out, and the 10-second penalty for a jumped start is so ruinous of a driver's chances that he would be foolish to take risks. Thus, starting a Formula 1 car in a race is almost 100 percent about controlling traction to get the maximum of the excess of available power to the road during the first 2-3 seconds before the aerodynamic downforce comes into effect and full throttle can be applied. Drivers and teams will practice for days to perfect the technique, try and analyze the track-tire combination to be used, and even try and practice on the actual track during practice sessions. However, it is impossible to reproduce the actual starting conditions, and so the driver must decide before the start exactly what combination of engine RPM and throttle will give him just the right amount of wheel spin to optimize the grip between the rear tires and the track. It takes too long for the signals to arrive that he has got it wrong, for him to immediately correct throttle and clutch and either recover a bogged down engine or to quell wreaths of tire smoke from excessive wheel spin. By the time a driver has recovered a bad start the competition is going past on either side.
There is little the car designer can do with the overall configuration of the car to help with starting, but the design of the driver's control systems has come in for close attention in recent years to assist the driver in carrying out the start process. At the start, the aerodynamics are right out of the equation due to zero air speed. The load on the rear tires is a function of the static weight distribution and the load transfer onto the rear axle. Accelerating an over-powered rear drive car is a sort of amplified process. The more acceleration there is, the more load transfer onto the rear tires occurs. The greater traction results in yet more acceleration and so on. Front wheel drive suffers from the opposite effect, as anyone trying to accelerate a powerful FWD car away from traffic lights knows. To maximize start line performance, the designer would opt for a rearward weight distribution and a short wheelbase with a high center of gravity height to achieve the maximum weight transfer. The problem is that in recent years, designers of Formula 1 cars have been putting more weight on the front axle, to better use the wide front tires, lengthening wheelbases to help stabilize the slightly twitchy cars and push weight forward. Big efforts have been made to lower the CG height, particularly through the use of ballast mounted in the undertray. All these trends go against starting performance. Combine these features with low-grip, grooved tires giving a low initial acceleration, and the gains to be had from load transfer are at a minimum. Getting a Formula 1 car away from the line is more difficult now than it has been for a long time.
The driver of a Formula 1 car has similar controls for starting to those used by the driver of a road car, but there are significant differences. The diagram illustrates, schematically, the systems on a Formula 1 car that are involved in starting. The actual throttle, clutch and differential actuator and sensor systems are compactly packaged and integrated - the clutch cylinder, for instance, being concentric with the gearbox input shaft - and so they are shown as simple linkages for clarity. Throttle and clutch signals are both processed by the central computer before they operate their associated systems. Most Formula 1 cars now have the clutch operated by the driver's fingers, via a paddle-lever mounted on the steering wheel. While this frees up his left foot for braking, it does deny the driver any real feel of how the clutch is taking up the drive. Both throttle pedal and clutch paddle simply operate position sensors, sending signals to the computer, and work against a spring return force. The spring characteristics will be designed to give the driver as much information as possible about the position of the controls, but will not tell him anything about what is happening at the other end, i.e. at the throttles or clutch itself, in the way mechanically linked systems do.
The other major difference between road cars and Formula 1 is in the engine characteristic. Even the highest performance road car engine is fitted with a flywheel to damp out the torque pulses generated by the firing of each cylinder, and to enable the engine to tick-over smoothly at a reasonably low RPM. Formula 1 engines have no flywheel, the tiny 4 inch diameter, carbon-carbon clutch being bolted directly to the end of the crankshaft. The rotating inertia is very low, as is evident when hearing an engine being blipped on the throttle and then the ignition cut: the RPM rises and falls at an incredible rate (more than 40,000rpm per second), and the engine stops almost dead when the ignition is cut. Engines tick-over at around 3,000rpm, and have an anti-stall system that opens the throttles if the RPM drops. Smoothness is not an issue. Low engine rotating inertia is essential as, in the lower gears especially, it adds significantly to the overall mass of the car that has to be accelerated. It also has a bearing on gear change times. The clutch diameter is kept as small as possible to keep the crankshaft mass low. As with other aspects of the car however, what is good for overall performance is bad for starting. The low inertia means that when the driver juggles throttle and clutch at the start, if he gets it wrong and engages the clutch too early without enough power, the engine won't just bog down, it will stall. There is no high-inertia flywheel to get him out of trouble.
With the car in gear, and as the 5 red start lights extinguish ready for the count down, the driver has just over 5 seconds to set up his engine RPM for the off. The sequential light "rev-counters" used in current Formula 1 cars only come on for the last 1000rpm or so of the RPM range, to indicate to the driver when to change up. They do not indicate at the lower RPM used for starting; judging RPM from the sound of the engine when there are 21 other drivers revving up there engines around you, is not an easy task. There is no reason, however, why the rev-counter cannot be programmed to indicate in the RPM range the driver wants to use for starting, re-setting to normal once the car is moving. Similarly, some other indication can be used to help the driver set his desired RPM for the start. The optimum RPM for starting depends on the grip available and the engine characteristics. There must be sufficient RPM to provide some inertia to make the clutch engagement not too sensitive, but not so much that it burns the clutch out slipping it. The RPM should be such that the throttle sensitivity is what the driver wants. With no load on the engine, it takes only a tiny throttle opening to rev it up onto the torque curve. Once the clutch is engaged, if the RPM drops off the fat bit of the torque curve, the engine is likely to bog down and take a lot of throttle to get it going again. As it comes on cam again, the torque increase is so sharp that wheel spin is inevitable in first gear. A slow take off from the line, followed by billows of tire smoke is a clear sign of just that situation having occurred.
Because piston engines, and racing piston engines in particular, do not like running at low RPM, something must slip when using them to move a car from zero speed. Either the clutch or the tires slip, allowing the engine to run at operating speeds until the car speed matches it in the starting gear ratio. For the tire to generate its maximum longitudinal force for accelerating the car, it must not slip at a rate more than the peak slip ratio for the vertical load it is experiencing. If the tire characteristics do not fall off too much at higher slip ratios than the optimum, it may initially be beneficial to spin the tires slightly to generate some heat in the tread surface and increase the coefficient of friction. Tread surface temperature, however, is not the key parameter for grip, it is the temperature within the tread layer. With 800PS and grooved, 15 inch wide tires it is all too easy to spin the wheels too much and burn the tread surface. Overheated rubber dust between the tire and the track are not good for grip. Thus the tires will only accept a certain proportion of the slip, and the clutch must accept the rest.
The driver's left hand works the clutch paddle on the steering wheel, and he must feel for the take-up point of the clutch just before the start. The actual take-up point on the clutch itself will vary as the clutch wears, but by measuring the slip ratio across the clutch, the control software can determine where the take-up point is and adjust the control law such that the driver is unaware that it is changing. This will allow him to learn where it is, even though he has no force feed-back through his control. Clutch characteristics also vary with temperature, though carbon-carbon has pretty consistent friction performance through a wide range of temperatures, and does not fade. The tiny clutch however, does not have much thermal inertia, and when it slips at high speeds it heats up very quickly indeed. It is not ventilated like a carbon brake disc, but is much more akin to a carbon, multi-disc brake pack on an aircraft. It is designed for single, high-energy absorption applications, with an extended cooling off period before the next application. If it gets too hot, the titanium and aluminum components loose their material properties and fail.
As the sixth and last light goes out, the driver engages the clutch to the correct extent to transmit as much torque as is needed to spin the rear wheels just over their optimum slip ratio. The electronically controllable differential will be set to ensure that there is no single wheel spin. If he gets it right, a faint blue smoke haze will come off the surface of the tires. At the same time he must adjust the throttle so that the engine maintains its RPM as increasing torque is transmitted to the wheels, and then increase RPM as the car gathers speed. If RPM is allowed to drop, the engine will bog down; if it rises too quickly, the clutch slip will overheat it.
For a few tenths of a second after engaging the clutch, the driver sits in his car in a sort of suspended animation, with the car not moving. The highly stressed gearbox shafts and half shafts are not infinitely stiff, and as torque is applied to them they start to twist. The tires too must distort torsionally to transmit load, and all this twisting must take place even before the wheels start to spin. Off the line, the initial acceleration is just under 1.0g. A reasonable estimate of this can easily be calculated from the weight (say 600kg + 70kg of fuel), weight distribution (say 57% on the rear), CG height (say 0.25m) and wheelbase (say 3.0m), and the coefficient of friction of the tires (say 1.5). These figures yield an acceleration of 0.98g. As the car gathers speed, the aerodynamic downforce starts to add load to the rear tires, and the acceleration will rise to around 1.1g at 80kph, after just over 2 seconds. It is about now that the driver can apply full throttle and start to concentrate on positioning his car to pass those that have made worse starts, or discourage those that have made better ones from passing him. Acceleration will continue to increase to a maximum of around 1.4g at 140kph, around 3.75 seconds after the car starts to move. From then on the car is power limited, and as it goes up through the gears acceleration will fall off to around 0.5g at 250 kph, which comes up after 7.5 seconds, finally falling to zero at the car's maximum speed, if achieved before the first corner.
In those 2.5 seconds after the red lights go out, the drivers' skills are tested in a unique way that all too often determine the outcome of the race. It is inevitable that the design and engineering departments of the teams will try to do everything they can think of to help their drivers, within the wording of the Technical Regulations. The temptation to help him in ways that are outside the rules is obviously very strong too! There are two main areas where car systems can assist: control of engine power and control of the clutch. What they would like to do is to control the engine through the ECU (ignition and fuel injection) and drive-by-wire throttle, and the clutch to keep the rear tires turning at their optimum slip ratio. As with traction control on road cars, this is easily achieved by measuring the front and rear wheel speeds and closing a control loop around calculated slip ratio. That is definitely not permitted. Nor is deducing the car speed and rear wheel speeds by other, less direct means. Nor is it allowed for the rate of change of rear wheel speed (or rate of change of engine speed) to be controlled by comparing it with car acceleration. Controlling the engine by subtle "tuning" of the ignition timing and fuel injection duration was outlawed in the last series of rule clarifications, introduced earlier in the season. Systems that control the rate of take-up of the clutch, similar to the passive systems used on dragsters, are not permitted either.
All that is left is for the control law writer to try and give the driver the best set of control characteristics possible for handling the engine and clutch during the start. Smooth, progressive characteristics that are always the same are what he needs.
However, rumors still abound that there are some teams ("Not me!" the chorus chants) that are cheating. To do so requires either control software that "disappears" after use and before the FIA up-loads it for checking, or a control law that is so subtle that it is not recognized as being traction control. For teams to risk being thrown out of the World Championship if caught, they must be pretty confident that they cannot be detected, or else it is just rumor. Whichever, the FIA is currently sticking to its ban of driver aids, particularly the one that would come into play at the start. It would be a pity if an algorithm decided the outcome of races.