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A lot of racers in the United States, especially those of you blessed enough to live in the dry southwestern part of the country, regard racing in the rain as something that should be avoided. I've seen some very reputable SoCal racing teams opt to park their car rather than risking a big off-track excursion in the rain. But in the rest of the country and throughout Canada, the UK and Europe, racing in the rain is not only seen as a required part of a racer's skill set, it's often relished by drivers and seen as a way of separating the men from the boys. Racing in the rain is also an excellent way to develop smooth and precise inputs with the steering wheel, brakes and throttle, since the repercussions of upsetting the car's tenuous grip are greatly magnified on a wet surface. And the thing is, the smooth and precise driving technique required in the rain just so happens to translate to faster laps in the dry too, since the same principles of conservation of momentum and minimizing weight transfer apply just as much in the dry as they do in the rain.
A good place to start when thinking about how to go fast in the rain is to consider the one part of the car that's actually in contact with the wet tarmac: the tires. Before you even consider whether you're on slicks, semi-slicks, a rain-specific race tire or a street tire and its relative ability to evacuate water from the contact patch, it's vital to realize that even the best rain tire reacts differently during use on a wet surface than it does on a dry surface. As illustrated by the graph below, a tire in the dry is able to build considerably more traction and as it reaches its peak the available traction levels off for some time before gradually falling off as you overdrive the tire and create excessive slip angle (experienced as understeer, oversteer, or a four-wheel drift depending on which tires lose traction). Since traction loss is fairly gradual in the dry (depending on the compound), it means as a driver you'll be able to feel it happening soon enough to counteract the slip by reducing velocity and/or changing trajectory – in other words, you can catch the skid by braking a bit, easing off the gas, and/or countersteering, depending on what the particular scenario calls for.
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| It doesn't take a rocket scientist to understand that a tire has less grip on a wet surface, but what a lot of novices fail to understand is that traction falls off its peak more dramatically and with less warning in wet conditions (source: speedsecrets.com). |
In the rain, however, a tire will reach its traction peak much earlier than in the dry and it will also level off more quickly and drop off more suddenly as slip angle increases. This means less time to react to a loss of traction and if you don't react quickly enough things can go from bad to worse in a hurry. Whether you're conscious of it or not you're probably using a slip angle of 6 to 10 degrees in the dry (which is generally considered ideal in terms of generating maximum grip from the tires), but in the wet the optimal slip angle is more in the range of 3 to 6 degrees. Understanding how your tires operate in the rain is the first step to maximizing speed while averting an unscheduled trip across the infield, but there are lots of other factors that will also help you find the fastest way around a wet circuit.
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| Gary Sheehan is a master in the rain. He found traction (and thus speed) at this sopping wet Global Time Attack event at Infineon Raceway in the LIC Motorsports STI when a lot of other teams were afraid to put their car on the track. |
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| Notice how Gary is well off the dry line and apex curbing? Instead he's out on the more porous portion of the track where there's more grip to be found in the wet. Clever monkey! |
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| Mike Kojima is a good rain driver too. He says to drive fast in the rain, drive slower and if you have to drive off line to miss standing water in areas where you need grip, do it. |
