Dry and rain lines both seek to minimize the time a car it takes to travel the course. Unlike the dry line which is largely rooted in geometry and car power level, rain lines are often a function of surface peculiarities at a particular track. As such, while one can often reason about dry lines without ever driving a course with good results, rain lines often can only really be figured out on track.
The following discussion assumes that the car is largely traction limited in corners regardless of the line chosen. What I mean by this is that in the dry in, say, a low power car a driver would often be on full throttle at the apex of most turns. In the wet, on the other hand, a driver would not be able to go full throttle while still having significant steering angle, regardless of whether the car is taking a normal dry line or a rimshot.
Given full race rain tires it may be possible to go full throttle in the corners; in such cases, the rain line discussion becomes a moot point.
I postulate that performing even very gentle braking, cornering or acceleration is better than correcting a car that has lost grip. A car that exceeded the grip limit may be sliding on its tires, understeering or oversteering - generally speaking, the car is not facing the desired direction and/or is not applying desired acceleration or deceleration.
The first step in finding a good rain line is identifying any abnormally slick spots on the track where the car completely loses grip. A good example of this is NJMP Lightning turn 7 exit.
I can identify two factors that can make slick spots:
- Pavement sealer on the surface - this seems to be the problem with said turn 7 at Lightning.
- Standing water.
A track walk is very helpful in identifying the cause of slick spots. Even without walking the track one can often see slick spots as they would have different color from surrounding pavement, but visually locating the slick spots this may not always be easy.
Once a slick spot has been identified, you want to alter your line to drive around it. Such alterations may mean running a significantly earlier or later apex, missing the apex completely or using curbing.
Paint on flat surfaces, such as pavement itself, is often slicker than pavement and worth staying away from.
Standing water is not ideal but not always a showstopper. With some logic and experience you can often use the track under standing water to good effect.
The first thing to keep in mind is that water moves. If you drive over a lake, the lake will get smaller. Conversely, driving over a flowing river would not change this river much at all - by the time you get back to it one lap later, new water would occupy the same spot. Therefore the first thing I do when I drive in rain is identify which water is standing (puddles or lakes) and which water is moving (rivers). Rivers generally cannot be changed until the rain stops. Puddles can be made smaller, and perhaps removed completely, by persistently driving over them. You need to keep track of whether your driving is making more of a difference in water level than incoming water from above, if any.
Driving over puddles comes in handy in drying conditions as there is no incoming water, and puddles are guaranteed to be getting smaller with every passing lap.
The second thing to keep in mind when driving over water is that there is a difference between hitting water with loaded and unloaded sides of the car - very similar to driving on curbs. Hitting water with unloaded side often does not change trajectory of the car significantly. What this means is you can often go through a lake at the apex with very few ill effects. There is still drag from traversing water, but what if going into a lake gets you around a slick spot on pavement?
Curbs in rain are sometimes good and sometimes bad. If the curbs are painted, the paint becomes very slick. This is especially problematic on exit curbs, and most drivers should stay away from exit curbs in wet conditions.
The flip side of curbs is that their surface is often angled, which promotes water drainage. Depending on how hard it is raining the curbs may well be drier than asphalt. In such cases curbs will offer more grip than pavement would.
A good example of this is curbs at Pocono North - they are unpainted concrete, elevated and angled, and in my car were worth a full second of lap time versus keeping all four tires on asphalt.
In some turns water pools at the edge of pavement around apexes. Going over curbs in these corners would cut the line along with keeping the car out of standing water - double benefit.
In a banked or cambered turn, water moves from the top of the bank down. This creates two concurrent changes in pavement adhesion: the top of the banking is dryer than flat track, but the bottom is wetter. In turns that are called "banked", such as Lightning turn 9, the outside would be drier and the inside would be wetter. When track has a crown in the center, the center would be dryer and both edges would be wetter.
There are many turns which are cambered but not banked. With small enough camber water movement may not be significant, but at some point it will be. Each track would require experimentation in this regard.
Rimshots - Or Not
Rimshots are a popular wet driving technique. They often combine the concepts of banking and driving around slick spots (patches on pavement are where the dry line is, not off line as the off line pavement normally sees very little wear). But rimshots are not automatically the fastest way around a turn in the wet.
In a rimshot the car is turning more (larger steering angle) for longer time or track position. The assumption is that the difference in pavement grip from the dry line is big enough that the car can accelerate harder in a rimshot line, thus justifying traveling more distance. This assumption is not true for all cars and tires. I often run dry lines in the wet because there is not that much more additional grip in a rimshot line but that line is much sharper, and therefore slower, than a flatter line hitting the apex.
An advice often given to drivers is to drive one gear up from normal in the rain. It's good advice but do not forget that you can modulate the amount of power the engine is making with your right foot as well. Staying in a higher gear is not automatically the fastest way around a wet track.
Another common suggestion is to offset the car half a car width from the dry line. The idea is to drive around polished track (i.e., slick spots) that happen largely in braking zones, and also to drive around grooves in the track where cars are normally braking.
For best results remember why the offseting suggestion is being made and look at whether it is necessary in each turn.
In most cars being on any dry line will beat being on any wet line. When the track is drying, or when the rain is just starting and much of the track is dry, it pays to drive on the line that everyone else is driving on, even if this is not your normal dry or wet line. This line will be prominent in some corners and may not exist in others. Corners where cars take a variety of lines, such as Lightning turn 9, would not have a drying line; corners where all cars take roughly the same line, such as Lightning turn 7, would have a pronounced drying line.
Tagged: intermediate, advanced, rain driving, using curbs