Rain Setups

First, sincere apologies to faithful readers for the long, long dry spell in new blog posts. I’ve been busy with two racing programs. That’s the reason, but it’s no excuse. Oh, well.

If you followed the 2010 ALMS race weekend at Lime Rock, then the topic of this blog will come as no surprise at all. Let’s end the dry spell in blog posts by talking about rain.

I’ll be the first to tell you that I am never totally comfortable with rain setups, despite having run in the rain quite a bit and even having engineered a major series race win in the rain. Rain setups are always a conundrum. First, you don’t get a lot of opportunities to develop your rain setup. Then, there’s the really big question of how far do you take the rain setup. Then, there’s the other really big question of how skilled and comfortable your driver is on a wet track. Compared to those, the technical issues of the rain setup are almost “cut and dry”, to make a weak pun. Contrary to what the advice columns say, I’ll tackle the easy part first.

For starters, let’s assume we have a properly wet track (at least light rooster tails), with rain still falling at a steady enough rate to keep the track from drying. It’s been raining for a while, and there isn’t much heat left in the pavement. We don’t anticipate that it will quit raining during the session.

Let’s start with a small handful of basic principals. We need grip. The brake bias needs to be shifted to the rear. We absolutely cannot tolerate understeer. Let me say that one more time. We absolutely cannot tolerate understeer.

Chasing grip first…

First, we need all the downforce we can produce. Wing(s) to the maximum, more rake, add on all the gadgets, do it all. Drag be damned. That much, I know for sure. It’s essential that we remain respectful of the car’s fundamental aero characteristics. Pitch and ride height sensitivity will affect decisions to adjust rake, as well as spring or bump rubber rate decisions. I believe that as you add total downforce, you need to try to keep the aero balance as far forward as possible. All in the interest of avoiding understeer.

In pursuit of grip, it’s tempting to dramatically open the bump and rebound bleeds in the shocks. On the other hand, we need to remain respectful of the car’s needs for aero platform control, which may or may not change in the rain. We also have to remember that the bumps, curbs, and other surface irregularities in the track will not go away just because they get wet. So, how much bleed is enough? It depends.

In race series where the rules allow driver-adjustable anti-roll bars, it’s easy to soften the bars for some grip. Perhaps equally important, it’s easy to reverse the adjustment. In other series, it becomes more of a judgment call whether to soften the bars, and if so, how much.

So that’s it for the easy changes. Now, if it’s really clear that the rain will continue, and at a track-soaking pace, we can also consider softening springs and reducing negative camber. Again, we need to be cognizant of platform control, if we choose to soften springs. It’s much safer to soften dramatically on a low-dowforce car than on a high-downforce one. Meanwhile, the camber change can be quite helpful, presuming the weather has the courtesy to stay properly wet. If not, you’re in trouble, aren’t you?

Finally, let’s have a look at the rain tires themselves. How fresh are they? If they’ve been previously run on a drying track, they will have worn away the chiseled edges that are so essential to cutting through the water to the track. If it’s really wet, fresh rains always help.

Now, a brief word about brake bias…

Braking Gs in the rain won’t be as high as in the dry, so we won’t have as much forward weight transfer. Lacking that, we’ll need less front brake bias. It’s pretty easy to make a rough calculation of the needed change. The whole process is almost painless if you have driver-adjustable bias and brake pressure sensors wired into onboard math in the data logger and displayed on the dash. A word to the wise – brake bias adjusters can be cranky little pieces of crap, and the time to debug them is not when the rain starts falling.

OK, so let’s talk about understeer…

Understeer in the rain is more than just an inconvenience. It’s an opportunity to arrive at a corner and plow straight off into the muddy grass, gravel, wall, or whatever other dramatic and damaging fate awaits the car. Not good, and it won’t endear you to either the driver or the mechanics.

Let’s remember that our dry-track handling balance assumes a lot of weight transfer to the front in braking, helping corner entry. Then, on exit, the rear lateral grip is reduced with an aggressive application of power. Neither condition is there for us in the wet. It will be necessary to shift the handling balance in the oversteer direction. It’s not as simple as aggressively softening the front springs and bar, because that might lose aero or roll platform control in a way we won’t like. You should work within the tuning options you’ve proven for your car in the dry.

The oft-mentioned option to disconnect both the front and rear anti-roll bars can, in the right circumstances, be a good move for preventing understeer. The wheel rate of the front bar is almost always higher than the rear. Nuff said.

Here are two serious “gotchas” related to cooling…

First, be careful of your engine temperatures. Some ECU calibrations may default into “cold-start” mode if temperatures drop too much.

And, some brake materials don’t take kindly at all to getting wet or getting too cool.

Everyone wants to just lay a strip or two of duct tape over the radiator opening or the brake ducts. Step back for a minute and think about how well duct tape will stick to a wet, dirty, oily surface, and you will see why I strongly prefer sheet aluminum blanking plates held in place by Camlocs.

Now, let’s talk drivers for a minute…

Some are really good in the rain, some are OK, and some are, well, not OK. I hope yours is in the first group. If so, you’ll look pretty good, no matter how well the car works. I once had a special case of a driver who was good in the rain, but didn’t care for losing any of his cherished platform support to softer springs, bars, or shocks. We just turned up the wing, reset the brake bias, bolted on the rains, and went off to win the race. As always, tune to the driver’s comments.

So, here’s the ugly question…

How far do you go with “messing up” your nice dry setup for the rain?

Here are some of the questions we’ll need to consider:
Will the rain let up soon, or get worse?
Is it real rain, or just a shower?
How much of the track is wet?
What can we learn from radar and the sky?
When will the track be wet enough to need rains, or dry enough to take them off?
How much heat remains in the track surface?
If this is a practice session, do we expect to qualify or race in the rain?
Do we even go out on the track?
All questions are for another day. Hopefully soon. See you later.