It’s raining, it’s pouring, and you have to be on grid in 15 minutes. What do you do?
Fortunately you won’t be the first person to face wet tarmac, so these aren’t uncharted waters. The basic wet setup hasn’t changed in years: deeper tire tread, softer suspension.
But what exactly does that mean? Should you mount rain tires at the first hint of moisture in the air? And should you soften the entire suspension or just parts of it? Thanks to BimmerWorld and the BMW Performance Center, we recently tested some popular wet racing options under varying conditions. Racing in the rain may be an art, but we hoped to make setting up for it more of a science.
HOW DO YOU TRACK THE WEATHER?
The first thing I do when it looks like rain is the same thing every racer does: I turn into an amateur meteorologist–although Dark Sky, RadarScope and a collection of weather apps have helped us all raise the bar with predictions more recently. Dark Sky will tell you with excellent precision what precipitation is coming.
The most important question is “How wet is it going to be?” which will then inform other critical factors, including whether the track will get wetter or drier, what level of grip the car will have and, ultimately, how far I will need to chase the grip with a wet setup.
WHAT DO YOU DO WHEN IT LOOKS LIKE RAIN?
In general, I have a full-dry setup that optimizes speed on the perfect track and a full-wet setup that maximizes grip–and ideally minimizes the risk of losing the rear of the car and spinning. The in-between conditions are covered by a compromise of these settings. I evaluate the weather and the likelihood that I will be changing my settings to chase the forecast.
I like to work smarter and drier, not harder or wetter. I usually start with the fast and easy things, like shock settings, if I have good adjustable dampers, and wing adjustments–things I can do without crawling under the car on wet pavement. I try to save adjustments to the sway bar and especially the springs for days when I know there is no chance of anything but pouring rain.
“I’d rather start my morning changing rain tires to drys in the sun than getting soaked while switching to rains.”
And if I think it’s going to rain before I get to the track–whether I’m just arriving or planning for the next day–I put on the rain tires. I’d rather start my morning changing rain tires to drys in the sun than getting soaked while switching to rains.
WHAT IS YOUR LINE IN THE SAND FOR GOING FROM DRYS TO WETS?
It may be because we don’t get the opportunity to pull the rains out often enough so we feel the need to use them, but I think way too many people slap on rains at the hint of a sprinkle. Ultimately, it is important to be prepared–car in the air, on stands, or even rains on one side and drys on the other to make the swap take half as long–but don’t stress over it. Make a decision and be done.
Keep in mind that on a warmer or less humid day, cars will dry the track, so the rain needs to be coming down at some more-than-minimal rate. If it’s 40 degrees and humid, the track might not dry all day, even with no rain.
And dry tires are only minimally worse on a wet track if there is no water to channel, and much better if a dry line starts to form. Rather than jumping to use those stupid rain tires that I’ve carried around for four years while they dry-rot, I typically give a strong nod to the drys–unless it will clearly stay wet.
WHAT IS YOUR TYPICAL WET-WEATHER SETUP?
I put my tires at an optimum pressure. If I get 8 psi of dry pressure rise, I’ll plan on about 5 for a wet tire due to the lower grip and heat, and adjust my starting pressures accordingly.
I’ll drop my shock control–always more clicks in the rear than the front to prevent a spinning car. And if I have separate compression and rebound controls, I almost fully drop compression but keep some rebound in the car. You still need to control the release of spring energy.
If I have a car with a wing, I add some. This means more rear aero grip, and it also helps in braking and keeping the car from spinning, which means in the greatly reduced grip I am comfortable going faster.
If it’s really raining and I have to get wet under the car–or if I thought ahead and set up an awning or E-Z Up canopy and something to roll around on besides my shirt and running water–I go full soft on the front bar and disconnect the rear to maximize roll and weight transfer from the reduced grip.
It takes either having a crew or an obvious rain weekend to make me want to change springs because of the work required–and the long time it takes to reverse if the weather changes. With a good set of shocks, I can make clicks in a few minutes versus an hour or two to change springs and achieve similar results.
WHY NOT SIMPLY LOWER TIRE PRESSURES?
Lowering pressure to add grip is an autocross trick. It worked for this test because our small track most closely resembled that type of course. But add higher speeds and more load like you find on a road course, and you’ll need more support via air pressure. Depending on the car’s weight, a DOT tire–wet or dry–has an optimum pressure somewhere in the range of 34 to 42.
Just like we don’t pump up a tire for rain conditions to “decrease contact patch and increase pressure on the existing patch,” which is not a real thing, we also don’t drop a lot of pressure in search of grip. At road course speeds, that becomes an underinflated tire and induces its own problems.
HOW DO YOU ALTER YOUR DRIVING STYLE IN THE RAIN?
“Point and shoot” usually describes a driving style that is either poorly developed or best suited for a car that is poorly set up or just won’t handle. But it’s the perfect technique for rain driving: Slow the car down, turn it, then get on the gas out of the corner.
Rain minimizes grip, so staying in turns longer bears a larger penalty. If I can minimize the agonizing period of time spent in that mid-turn, no-grip situation and maximize my time traveling in a straight line, I go faster.
Certainly there is more to it, including knowing where the grip is on a certain track; staying off oiled or polished areas of the surface; and avoiding slick paint, curbs and puddles. In general, though, you need to maximize your straights more than ever and minimize risk. Don’t spin the car.
WHY DIDN’T THE SOFTER ANTI-ROLL-BAR SETTINGS MAKE YOU FASTER?
It was the right move and a step forward, but our track didn’t show it. When chasing wet weather, the name of the game is compliance, and less roll stiffness helps weight transfer in lower-grip situations.
The adjustment we did–soft front, no rear bar–softens roll, but more so on the rear. This is my standard approach.
Poor handling in the rain can cost you a half-second or a little more, but a spin can cost 10 to 20 seconds or even cause a crash.
I make larger changes to the rear than the front and go for more of an understeering car that won’t spin. This can lead to more of a point-and-shoot driving style, which is what often wins true rain races.
IF YOU HAVEN’T TESTED WET SETUPS BEFORE AND IT STARTS TO RAIN, WHAT IS YOUR HAIL MARY PLAN OF ATTACK?
Zero front compression; take out the same clicks plus one or two more in the rear; take out a few clicks of front rebound and one or two more in the rear. Add some wing and let it rip. And don’t spin the car!
WHAT ARE YOUR FINAL THOUGHTS ABOUT OUR DATA FROM THIS TEST SESSION?
Now take these gains and extrapolate them over VIR.