Tuesday, September 7, 2010

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.


  1. Thanks Buddy, hopefully we won't need this advice at Atlanta in a few weeks.

  2. Hi Buddy,
    Check out paragraph 5, where you say you need more front brake bias. I think this is wrong as you also state later in the post...
    Regards, Ian

  3. Great post Buddy, was wondering if you had finished blogging :).

  4. Buddy, thanks so much for returning to your patiently-waiting audience! Great post.

  5. hey Buddy, good to see some new stuff.

    for me a lot depends on if there's a balance or handling difference from the wet to dry tires on how much you change.

    and then the all important tires pressure discussion; rains-vs-wets. that might be a good follow up for this article.

    keep'em coming. see you at Petit.

  6. Less downforce in the rain I say, trim it out. If anyone wants to know why I think this let me know. Or better yet don't ask, just keep putting downforce in when it rains and lets race.

  7. Thanks, Rick.

    Ah yes, tire pressures. That one slipped right by. The perils of blogging impromptu, without working up an outline.

  8. OK Jeff, let's hear about running less downforce in the rain. If it's too long for a comment on this post, shoot me an EMail and I'll post it as a guest blog.

  9. Hi Buddy,how are you doing?

    Once again I come to you with some (in this case maybe be too much) questions.

    The following questions are about your last post,rain tuning/setup.

    Paragraph7-You prevent understeer by putting more downforce in the front.Is it because : more vertical load=more grip on the front=less understeer?

    P7-By adding rake you mean increasing the relation between Rear/Front height ,i.e., less height in the front causes less load transfer between front tires = more grip in the front?

    P10-Why reducing negative camber will help in rain?Is it: lower speeds(rain)=lower camber increase during corners so we need to straighten up the tire relative to the dry setup?)
    P13-You say,more rear braking bias with rain.Is it because Rain=Lower speeds=less forward weight transfer=less traction in the front comparing to dry setup (higher speeds,more load transfer overall?)

    Is it a good idea to reduce the overall braking in the car(rain),to help prevent locking the tires very soon?Or should it be driver's job to be gentle?

    P16-Why do a lot forward weight transfer in braking helps in corner entry?Heard something about pivoting around the front tires??…how can we achieve it?

    P16- Should I assume we always want oversteer on corner exit?Should the car be tuned to oversteer with power, i.e., if there isn't enough engine power to do it,should I set up the car so it will be easy for it to oversteer on corner exit?

    P16-Softening front springs and bars will reduce understeer because there's less weight transfer between the pair of tires in the front?what would be the first one to tune?springs or bars?why?

    P23-The good wet track driver would soften everything in the car,this way you achieve less weight transfer overall=more grip overall?

    Hope to read from you again soon!

    Best regards,

  10. Buddy - Why less or the same downforce in the rain? Lets say you add downforce and gain 10 % grip over the competition. Now where are you going to use that? You going to drive around the outside of them in the turns? Can you really pass in the rain into a corner or on the exit because you had more downforce? You pass in the rain on the straights. If you pass a guy due to the less drag and get to a turn first, what is he going to do, blast around you on the outside because he has more downforce...nope. Take it to an extreme we are racing on glare ice, how much will the added downforce help? It will help some but enough to make a difference in lap time over having less drag. Thats my theory and I just love it when guys crank in downforce when it rains. All the drivers I get are better at passing on the straights than in the turns. I could be completely wrong.....happens all the time to me.

  11. Good points, Jeff, and thanks for sharing them. I'll confess to never having tried less downforce in the rain. Hope I get a good opportunity to A-B these two approaches. There are two schools of thought at work here, lap time (from grip and driver confidence)and racecraft. It will be interesting to discover how they compare. I usually tilt a little in favor of racecraft, and it's clear that you do the same. Thanks.

  12. Thinking about this a little more...

    Maybe it works out that both lap time and racecraft benefit from less downforce. Lacking a sim, I'll have to wait for the next trackside opportunity for some insight.

  13. What about the issue of putting the power down out of the corner? If I'm low downforce and have to delay putting power down then I'll be easy to pass on the straight.

  14. I bought a 25yo FFord from someone unknown. First time I used it was on a wet test day. I knew nothing of the car and did the usual FF wet setup tricks... less rear damping, no RARB, less front FARB, high tyre pressures, rear bias on the brakes. It was OK not fast though. Later that day it seemed to dry up so I popped the rear bar back on, stiffened the rear dampers back to what I considered was a reasonable rebound by feel/pushing up and down (I had no info on this car at all). Tyre pressures slightly high for te dry because it was cold... It pissed down the moment we got out really heavily. However I was fastest of everyone even in those in the brand new cars.
    The next day in the dry the car was awful - constant oversteer. It turned out the guy had put really soft springs on the front which were really unmatched compared with the rear. It was obvious from the first moment I drove it in the dry. Running whatever the opposite of rake is made it handle a bit better. What really struck me though was in the wet it was fast even with what I thought was a dry setup. The soft front springs made all the difference to this car.

  15. Firmtec, I've had some interesting rain runs since writing this blog. When it's been clearly we and we've been able to go all the way wet with the setup, we've found gains by not only softening the springs but by also changing the spring balance - softening more in the front than in the rear. This, of course, helps by combating the dreaded understeer. Your experience is much the same.