Speed Secrets: Tuning Your Car for the Rain

By Ross Bentley

April 25, 2017

We all love driving in the rain, right? Well, some of us. But even those of you who are like me (I love it!), we'd prefer to drive a car in the rain that was well set up for the rain! In other words, driving a poorly- handling car in the rain is not so much fun.

There are a lot of theories about what setup changes to make to your car for the rain - some based on science, some on experience, and some on what seems to be old wivesʻ tales. That's why I went to Jeff Braun when asked by a reader about what to do with the car's setup for the rain. There are not many race engineers in the world with more knowledge than Jeff, and very few who are able to take that knowledge and put it into words in a way that we (not-so-bright?!) drivers can understand.

Enjoy! Then go find some rain to drive in. - Ross

Racing in the rain... yeah, well, I just did the Rolex 24 at Daytona in January as race engineer on the CORE Porsche GTD car. We started at the back of the pack, and were two laps down in the first three hours (complex driver ranking rules and strategic implications played into this). Then it started raining and by ten hours we were leading, and lead for quite a while. So I am going to disclose the BIG Speed Secret of how to be great in the rain.

Hire really good pro drivers and sit back and watch it all unfold. That’s exactly what I did and it was easy.

Hope you enjoyed this installment of Speed Secrets Weekly, and how to race well in the rain.

Thanks.

Oh, you want more than that? Before we get into specifics, there are different kinds of rain. Bubba, the shrimp guy on Forrest Gump describes about fifteen kinds of rain, but I don’t worry about “sideways” or “stinging” rain. For us and our race cars there are three types of rain.

  1. Track getting wetter
  2. Falling out of the sky steady, saturated
  3. Track drying out

If you're in a long race with pit stops and it rains youʻre likely to deal with all of these. If you’re at a track day or SCCA race you may only deal with one type during your session.

The tricky part of setting up a car for the rain is the changes to the car you would do are often directly opposite, depending which of the three rain types you have. This is why the great rain driver is still the best asset for going quick in the rain. A guy that can feel the grip and feel how it’s changing and be right at the limit as the track changes is what you need. At Daytona this year, we started a stint in 11th place running 2:10 laps; in the center of the stint, we took the lead and the top three were running 1:56 laps; and by the end of the hour, we were still pulling out our lead, but running 2:10 laps again. No car setup will make that happen. That’s the driver feeling the grip and adapting corner by corner. So again, I will say itʻs driver and skill way more than setup that will make you fast.

Hang in there. We will get to the “secrets” soon, but here are a few more observations from my experience on the approach to racing in the rain. It amazes me the number of non-pro drivers who say, “Oh no! It’s raining this session. I am not going out.” WHAT??? Really??? Are you kidding me???? That’s a situation you should dream of and wish for and be ready for! Living in West Texas made me appreciate rain and when your kid is a kart racer in West Texas, youʻre not going to get many chances to hone the rain skills. We used to cancel home school for the day when it started to rain; we went out and ran the karts as long as it would rain. There is nothing better for feeling the grip, understanding how to adapt to the rapidly-changing conditions, find extra grip, and learning what you can do with the controls to change the grip on each tire as needed, than actually doing it – no substitute at all. I would like to think that all that silliness may have had something to do with how we went from 11th to 1st in the super-changing conditions at Daytona… same kid.

Why does everyone think the British drivers are so good in the rain? Is there a top secret UK rain driving school they all go to that Americans are banned from? Don’t think so. They actually RACE in the rain in the UK, a lot!

So NEVER give up an opportunity to go drive in the rain. I feel bad when, as an engineer, I have to skip the session before some races because I know it’s going to be dry for the race and we can’t take the risk. Unless you’re in a pro race and the risk is too high, go drive in the rain and learn and try things and experiment with the grip. Heck, if you crash, at least youʻre probably going slower when you hit something hard than you would have been in the dry. So it’s not a stretch to say itʻs safer to practice in the rain…..ok….it is a stretch, but do it anyway. There is nothing better.

Right – the “secrets” part of how to set up the car for the rain. Some thoughts below in no particular order:

  1. Go softer – When the grip is lower due to the rain, we can’t load the tires as hard and expect them to grip; there simply is nothing for them to grip onto, so run softer springs, bars, and shocks. Think of it as being on a skating rink. Be easy on the tires, gently load them and do it over a long time. A softer car does that.
  2. Wind some brake bias to the rear if your car allows. Because of the reduced grip there is less pitch and weight transfer to the front when you brake in the rain, so the vertical load on the front tires is reduced and they can’t take as much of the total brake load as in the dry. On average shoot for 1% from the front.
  3. Put rain tires on. Seems obvious, but not nearly that straight forward. Depending on which of the three types of rain you have, you will likely find yourself with the wrong tires on the car at the wrong time, and as soon as you correct your mistake, the rain type will change and you will be wrong again. I can promise you this will happen. That’s when you unleash that driver stuff you practiced so hard on every time it rained in practice.
  4. Add tire pressure. Even if youʻre running your normal DOT tire in the wet, add 4 psi and see if that helps. The harder the rain and more standing water the more you should add. Ten psi extra is not crazy. First the rain cools the tire and you will not reach the hot pressure you may need. You may have seen the tire failures this year at Daytona. That was caused by too low pressures and the tire failing - not because it was bad or defective, but because it was run too low on air, had no structure, and failed the sidewall. The other reason to add air is it “crowns” the tire and helps it cut through the water better.
  5. Lower the rear wing angle. I know youʻre thinking Iʻm nuts. Well, you’re right but that’s beside the point. The best thing is that no one listens to me on this and always puts wing IN to plant the car and make more grip in the super-slippery rain. That’s great for me because I take out wing and pass them on the straights and dare them to pass me back around the outside of the corner while it’s raining. Picture it, you blast by in the driving rain on the straight and have a three-car-length lead into the brake zone. Your competitor can go a little deeper with the extra drag stopping power, but its sketchy, right? Rain, hard to see, car right alongside, going fast, not sure how much grip there is, just got passed. Unlikely they fire it off into the unknown. Now they are behind you and faster in the corners by three mph. What...are they just going to drive around you? Nope. They wait and you gap them another five car lengths on the next straight. But don’t believe me and keep putting in wing. It makes my job easier!
  6. Don’t assume more ABS and TC is always good in the rain. Most of the time it is and you should try and play with it, but youʻre going to have to chase it hard as conditions change and the rain type changes. Be aware of the TC “holding you back” as it starts to dry out. You have to stay ahead of it. The ABS can start to be too intrusive as it dries and reduce the braking power. Of course you have to stay ahead the other way, as well, when it starts to rain harder.

I have never seen all these tricks, combined in perfect coordination as they might be, overcome a well-driven car with no changes. The rain IS the great equalizer. It takes the car performance and makes it less important, and brings out driver skill for all to see. That’s cool from a fan standpoint and a driver’s standpoint. We all get to “wheel” the cars or see the drivers “wheel” the cars, sideways and on the edge, chasing that elusive grip. Modern racecars have taken much of that enjoyment away from the drivers and fans. That’s why racing in the rain is so great. Please don’t pass up a chance to go do it. Itʻs special.

Have fun… itʻs really fun in the rain.

- Jeff Braun

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