Features - Technical
OCTOBER 6, 1997
Wet Settings
BY PETER WRIGHT
The scene is familiar: 22 Formula1 cars are assembled on the grid and the Pit Lane is closed. The start of the race is 15 minutes away and after two days of practice the cars are set up as well as they are ever likely to be.
The level of uncertainty this situation brings, undoes all the careful preparation and throws the biggest possible wild card onto the table. Suddenly all bets are off, the Grid is probably in the wrong order, and Team Managers' and strategists' minds click into high gear, ready to take advantage of any opportunity that presents itself. Nobody knows what will happen next.
Thanks to advances in visible and infra-red satellite images, and supercomputer modelling of the atmosphere, weather forecasting has become quite good on an area basis. However, there is no computer that can accurately predict the local weather down to a few square kilometres. To predict when a storm-cell is going to arrive over the circuit, how much rain it is going to drop and when; feeling the seaweed is as likely to give the right answer as a bank of satellite receivers. The process of deciding which tyres and settings to put on the car at the start of the race is a bit of a lottery when there is rain about, yet some drivers and teams (Michael Schumacher for instance) seem to get it more right, more often than others (Williams for instance).
When it rains the car is subjected to three main differences, compared to being on a dry track:
1. It is fitted with wet tyres.
2. The acceleration, braking and cornering forces are reduced by up to 65% or more.
3. The balance between aerodynamic downforce and drag is tipped heavily in favour of downforce.
Because wet tyres have treads, more or less open depending upon whether they are optimised for very wet, wet or drying conditions, they run at greater slip angles than slicks. Also, because the forces generated by them are so much less, they can be constructed with lower lateral and vertical stiffness. It is inevitable that as soon as wet tyres are fitted to a car optimised for dry conditions, the suspension settings will be wrong - probably too stiff and over-damped.
In addition, the wet tyres will have a different rolling radius to the slicks. Rolling radius variation is a combination of the radius reduction due to aerodynamic downforce, plus the increase due to wheel speed i.e. the tyre radius reduces as speed increases due to vertical load, and increases due to centripetal tread forces. Because the changes in rolling radius affect ride height and hence downforce, the characteristics are crucial to aerodynamics balance settings. Wet tyres will probably squash down more due to being softer, but grow more due to heavier treads. What the net result is, only the teams and competing tyre companies know, but the ride height will most likely need changing when wets are fitted.
If it is very wet, or there are streams of water running across the track in the braking area, the front ride height may need raising further to prevent the "plank" from surfing or aquaplaning when the nose dips under braking and hits the water. The sudden loss of front grip can be disastrous.
Current Formula 1 cars can generate around 3-5g cornering and well over 4g braking, using the products of the tyre war between Goodyear and Bridgestone. In wet conditions these figures can reduce to little over 1g. The first and greatest effect of this radical reduction in grip is on load transfer - front to rear under acceleration and braking, and across the car under cornering. Many of the suspension settings are concerned with accommodating these load transfers. Springs, anti-roll bars, anti-dive, anti-squat, and roll centres are all set to control the attitude of the monocoque and its ground effect undertray under high loads. Suspension kinematics and static settings attempt to optimise the presentation of the tyre tread to the road under these conditions. Reduce the loads and they will all be wrong.
Brake balance is set to optimise the braking loads according to the vertical loads on the front and rear axles. The differential settings assume load distributions across the rear axle under acceleration and trailing throttle. The drive-by-wire throttle characteristics will be as aggressive as the driver can stand for a given rear axle grip level. All will need changing when the load transfer is reduced. Clutchless up changes in semi-automatic gearboxes take a few hundredths of a second, often faster than the engine can slow down to match the RPM. The sudden engagement between the wheels and the inertia of the engine sends a torque pulse through the powertrain that shows up as a ±1g oscillation in longitudinal acceleration. That is usually not a problem on slicks, in the dry, but may well exceed the available tyre adhesion if it is wet. The sudden loss of grip at the rear has sent more than one driver off the road. Slowing down the gear change and using the clutch to introduce some slip during up-changes are all options that will smooth out the gear change and make it less likely to cause the car to spin.
Wing settings in the dry are a compromise between grip and top speed. The rear wing configuration and setting, and the balancing front wing setting, will depend on the circuit and the relative importance of - i.e. time spent in - corners and on the straight. Monza, Spa and Hockenheim still demand the least possible rear wing, but more and more modern circuits need almost all the downforce possible. Monaco and Hungary usually see all sorts of appendages appearing on the cars, in attempts to maximise downforce.
One problem that can occur during a full length wet race is that, because the engine spends so much time in low gears and at high RPM, and the because the duration of the race is longer than anticipated, the oil consumption of the engine ends up much greater than calculated. Teams that have had bad experiences due to this problem increase the capacity of their oil tanks in anticipation when going to the circuits where rain is most likely.
Let it rain, and the driver is only interested in grip. If he cannot get through the corners quickly and cannot put down the power effectively, it does not matter what the top speed is as he will never get there before he has to brake, hundreds of metres early.
An optimised wet track Formula1 car is very different from one optimised for dry conditions. In testing or practice, if it rains, it is possible to explore all the requisite changes in order to arrive at the ideal set-up for these conditions. Some of them - springs, anti-roll bars, camber - take a significant amount of time for the mechanics to make a change. Others, such as wing settings, damper settings and toe-in are quite quick. Some teams have followed the lead of CART practice and fitted quick front wing flap adjustments for speedy changes. Throttle characteristics and gear changes are controlled by computer and so it is easy to set them up from a plug-in terminal, or give them to the driver to adjust with switches in the cockpit or on the steering wheel. Differentials and brake balance are steadily falling into the clutches of the central computer and the driver can fiddle with these too.
During the race, when it really matters to have the right settings on the car for whatever conditions prevail at the time, many of these adjustments take too long to make. This is when the teams that know their cars through and through, can score. It also helps to be a prophet.
The only settings that can be adjusted during a race are those to which the driver has access, and small adjustments to wing settings during a Pit Stop to change tyres. The latter are limited to front wing flap incidence, if the car is fitted with the quick adjuster, and the removal of a taped on Gurney flap on the rear wing. Thus the choice of settings for the start of the race is crucial and results in feverish activity and discussion on the Grid if the conditions for the next one to two hours are changeable and uncertain.
Well organised teams that have conducted plenty of wet weather testing and bother to use a wet or damp Practice and Qualifying fully, will know quantitatively what the compromises are between wet settings in the dry, and vice versa. Armed with this knowledge they will be able to make value judgements about spring, damper, geometry, and ride height settings, based on the best prediction of the track conditions available to them. A sensitive and intelligent driver such Schumacher will be able to tell his Race Engineer whether he wants to drive with a too stiff car, with lower down force and good top speed, for 20 laps in the wet, in order to have a really good car when it dries, or to go with wet settings. If the latter is decided on, because it is likely to be wet on and off throughout the race, he may decide go for a high downforce, soft set-up and take the risk of being overtaken if the track dries. The equation is so complex and the unknowns so numerous that there is no right answer - though there are always plenty of experts after the event, ready to criticise decisions with the benefit of 20:20 hind-sight.
As the drivers sit on the grid, with the rain pouring down, waiting for the five red lights to extinguish, how do they prepare to drive in these conditions? The next few seconds may well determine the outcome of the race, as one of the few opportunities for overtaking is about to happen, during the rush to the first corner. The Parade Lap will have been used to fine tune brake balance for the prevailing conditions, ready for the latest possible application of the brakes into the first corner. If possible, the traction available will have been calibrated with a couple of bursts of throttle from a low speed. Standing starts during the Parade Lap are not permitted. The front two rows of the Grid think only of being in the lead after the first corner, for he who leads will be able to see ahead, while those who follow must peer through walls of spray thrown up by the front runners. Reining-in 750 horse power for conditions that will only accept 100 or so, takes phenomenal self-control. Those who have it and who maximise their performance for the given conditions, gain seconds over those who take several laps to adapt to the wet track - that is if they have managed to remain on it at all!. Senna became a legend in the rain and Schumacher is his heir, both winning races in the first few laps, before their rivals have realised what was happening to them.
Both these drivers are renowned for using completely different lines from the usual dry ones. Over a Grand Prix weekend a layer of rubber is laid on the driving line, and "marbles" are scattered over the rest of the track effectively denying use of alternative lines. When it rains however, the whole track is washed of dust and "marbles" and it is the rubbered line that becomes slippery in the wet. Alternative lines that avoid this part of the track often yield greater grip, and the cunning drivers explore these to find where it is best.
Driving in the wet is about smoothness and the avoidance of inputs that excite the car so that it exceeds its limits. The inertia of a Formula1 car is so low that it spins quickly and, with so little grip available to control it, it quickly exceeds the slip angle from which recovery is possible. The grip in the wet is not much more than that of a Super Touring Car in the dry, so driving it is not a big problem for a Formula1 driver, providing the car is reasonably stable. The problem is that, under the varying conditions the driver must know where the limit is without exceeding it. That is the skill of the great "rain-masters".
Once the race settles down, the Team Managers and each driver's Race Engineer watch the TAG-Heuer monitors to check on their immediate competitors. As conditions change, and possible tyre changes become likely, lap times will be monitored very closely. If anyone has changed tyres (either from wet to slicks or vica versa) their times will be of the greatest interest, providing potential indication of the optimum moment to make the change.
Drivers will often try and stretch the use of a tyre if they believe there is likely to be a sudden reversal of the conditions - either because the rain was due to a short, sharp shower and the sun will dry the track again quickly, or there are a series of showers coming through and the dry spell is only temporary. To make wet tyres last on a drying track the driver must go to enormous lengths to avoid over-heating them and wearing the edges of the tread blocks. Any tendency to spin the rear wheels, slide the car or lock up the brakes has to be avoided. Drivers will go off the dry line on to wet sections to cool the tyres and, somewhat surprisingly, head for any puddles left on the track. Driving fast on slicks, on a wet track, requires tremendous skill and bravery.
Rain is the great leveller. So much of the performance of the cars is controlled and limited by the adhesion between car and track. Maximum power is much less important than the ability to use it. Braking performance is no longer controlled by the detail optimisation of the braking system and its cooling. Likewise, it does not really matter how optimised the gear ratios are. Aerodynamics are still important, but corner speeds are so reduced that mechanical grip becomes much more important. Potentially good drivers can show their car control skills in an under-powered car that has little chance of being competitive in the dry. Thinking drivers can use their strategic skills to make Pit Stops at the best time, while real racers take advantage of changing conditions to gain seconds as others hold back.
Jackie Stewart once said that the secret of a successful racing driver is to drive as fast as is possible all the time, whether it be a test, Practice, Qualifying or the race, because you can never know what will happen next. It is possible to spot the really great drivers by their ability to be in a position to take advantage of an unexpected situation. This is never truer than when it rains.