Beyond Thompson

At the last Lemons race at Thompson, I only got in 6 laps of practice. However, the AiM Solo was running then and during the race, so I got to do a little comparative analysis afterwards. In the graph below, the red line is my fastest practice lap while the blue line is the fastest race lap on Sunday. Click on the image to open it up in a larger window and then write down at least 3 things you notice that’s different about the two traces.

  1. OK, so the most obvious thing is that the drive down the main straight was very different. I had an extra 200+ lbs of passenger and gear in the car and was driving in 4th gear. There may also have been traffic.
  2. The second thing you probably noticed was the very low speed in T2. I was experimenting with the brakes seeing how good they were, so my braking point was very late and this caused me to botch the corner. No big deal, this is what practice laps are for. I had never driven the car before and I needed to experiment. I tried different lines and gears nearly every lap.
  3. The thing I want you to notice next is that all the red lines are shifted left relative to the blue lines. The braking points are earlier and the acceleration points are earlier.
  4. Because my acceleration is earlier, I tend to have higher speeds on the way to the next corner.
  5. The most important area of the track is the 9-10 combination that sets up the main straight (6000-7000 ft). I take this as a single descending radius corner rather than two corners.
  6. While the 7-8 carousel (5000-5500 ft) isn’t nearly as important, I have a very different line compared to everyone else on the team (who all take a line similar to the blue one).

We could go through each corner talking about the trade-offs of taking different lines. But the differences in the red and blue lines aren’t really about Thompson. We can summarize all the specific differences with two general strategies, which I’ll describe below.

Backing up the corner

In point #3 above, I noted that my driving style involves braking earlier and accelerating earlier. This is called “backing up the corner”. The earlier you can get the car pointed to the exit, the earlier you can get to full throttle. Getting the car rotated early is usually accomplished by trail-braking deep into the corner so that the steering and braking inputs overlap quite a bit. This has the effect of swinging the rear of the car around, and you may have to make a steering correction to prevent the car from oversteering into a spin. There are risks involved when driving with this style. That said, no matter how hard I tried, I couldn’t get the car to rotate. It was set up with a lot of understeer. While I could get on throttle early, the car was leaned over quite a bit, and the open diff caused the inside front tire to search for traction. But even without getting the rotation I wanted, you can see from the telemetry graphs that there are gains to backing up the corner.

Connecting combinations

There’s only one combination corner at Thompson: 9-10. The blue driver “sees” this as 2 corners with a small straight between them. You can see this as the hump in the speed graph. The red driver (me) sees this as one long corner. Why? It’s the most important corner of the track and my goal is to optimize the position, angle, and speed of the nadir (slowest part of the corner). So I focus all my attention on getting to the nadir with the best combination of grip and speed that I can, which means throwing away the first corner. By slowing down early and keeping the suspension quiet, I optimize grip. If I speed up too much, or turn too much, I’ll upset the car and lose grip. This costs me some time at the start of the corner, which you can see at 6100 ft. But the investment pays off as from then on I’m gaining time.

Summarizing

Next time you’re on track, try making a conscious effort to get your shit (braking, turning) done earlier. Stop optimizing the straight you’re on and start optimizing the straight coming up. Also try to get your connected corners more connected. Think about how the first corner affects the second. Try some different lines to see what works and what doesn’t. Make sure to bring a data acquisition device. Not only will it help you sort things out later, it also makes the downtime between track sessions a lot more interesting.

Tire pressures don’t matter

I remember reading a recent article comparing 200 treadwear tires and one of the initial concerns was setting tire pressure. Shockingly, they found that varying tire pressures had little affect on lap time. Whoa there! I did not spend good money on a needle pyrometer for no reason! Did I? Did I?

Clearly this is something YSAR needs to investigate. In theory, raising tire pressures does several things.

  1. Decreases rolling resistance
  2. Decreases grip
  3. Improves steering feel

I can imagine that these forces offset each other to some degree. Straight speed vs. corner speed: it’s 6 of one, half-dozen of the other. It makes some sense that tire pressures might not change lap time by much. But making sense isn’t the goal here. I’m a scientist by profession and passion, so I just have to conduct some experiments. Since I don’t have immediate plans for a semi-private test day, I’m testing this in simulation first. Later in the year I hope to revisit this study on a real track.  Let’s begin with the usual sim testing environment: Assetto Corsa, Brands Hatch Indy, NA Miata.

Experiment #1: Ideal tire pressure

In order to remove any human sources of variability, I’m going to let the AI drive first. Assetto Corsa sets the Miata pressures at 28 psi by default and allows a range from 15-40. I chose to change pressures in 4 psi increments. As you can see in the table below, 28 psi seems optimal. Interestingly, all laps are within 0.25 seconds using pressures from 24-40. If I had seen these numbers in real life, I would probably conclude that all lap times were roughly equivalent. But the AI drives each lap within hundredths of a second, so the differences are real, though small. Overall, I have to agree with the initial premise: tire pressures don’t affect lap time very much.

Front Rear Seconds
16 16 65.41
20 20 64.68
24 24 64.32
28 28 64.09
32 32 64.26
36 36 64.29
40 40 64.34

Experiment #2: Asymmetrical tire pressure

One of the things I like doing at the track is running non-square setups. I’ll mount completely different tires on the front and the rear. The two ends of a car are doing very different things, so there’s really no reason to run square setups. One of my favorite ways of goofing around on a skid pad is to mount sport tires on the front and all seasons on the rear. That’s a good way to train your oversteer recovery skills! Note that I said skid pad not HPDE session. I don’t think it’s a good idea to mess around too much in the presence of other drivers on a fast track.

So what happens when the AI drives a non-square setup? As it turns out, Assetto Corsa doesn’t allow you to have different compounds for the front and rear. But you can change individual tire pressures.

My first thought was to change the psi by 4 lbs on either side of 28. So 24F 32R and 32F 24R. The faster combination was to have more pressure in the rear. It wasn’t much of a difference, so I decided to go extreme and set one pair of tires to the ideal 28 psi and the other to 40. The result is sort of shocking. 28F 40R (64.04) is not only faster than 40F 28R (64.41), it’s also slightly faster than 28 square (64.09).

Front Rear Seconds
24 32 64.22
32 24 64.33
28 40 64.04
40 28 64.41

A stopwatch doesn’t give many details, so let’s load up the telemetry and take a closer look at what’s happening in Experiment #2. Green is 28-28 (because green is in the middle of the rainbow). Red is 28-40 (because oversteer feels red). Blue is 40-28 (because understeer feels blue).

For some reason, the AI chooses a different line on the square setup. The green line shows that the AI attempts to hold too much speed which results in being later to throttle. While initially faster, this ultimately causes the square setup to lose nearly 2 tenths by 1800 feet. It maintains that loss for a little while but then recovers most of it by the end. Apart from one bad decision in one corner, the square setup is actually faster everywhere else. This is why we don’t rely solely on the stopwatch.

What’s happening with the understeer and oversteer setups? The reason the oversteer is faster is that it’s able to use more mid-corner throttle, and it gets to full throttle sooner. It also has more yaw early and requires less steering effort in a few places. You have to zoom way in to see this. These are very subtle differences, but they add up to 4 tenths of a second by the end.

Experiment #3: Human driver

OK, time for me to drive. The first thing I did was run some square setups at a couple different pressures. There’s a little difference in the way they feel but not that much. I’d rather focus on what happens when you run different pressures in the front and rear.

Front Rear Fast Median M – F Cuts
28 28 60.93 61.25 0.32 0
28 40 61.80 62.26 0.46 1
40 28 61.25 61.36 0.11 0

The fastest was the square setup. That’s not really surprising. What is surprising was that the understeer setup was very close. The median lap was only 0.09 seconds off. If you look at the difference between the median and fast laps (M – F) you can see that the understeer laps have the most consistent pace. That was my impression while driving too: “oh well, another uneventful lap”.

The big shock is how bad the oversteer setup was. Its fast lap was 0.55 seconds slower than understeer and the median is even worse: 0.90 (some of the laps were not pretty). I was having to make steering corrections in nearly every corner as the back stepped out under braking and also under throttle. I also had one lap where I went a little too much off course and got a cutting violation.

In the graph below, the panels are speed, steering angle, throttle, and time. I have plotted the top 5 laps of each run. As you can see from the red steering angle trace, the position and magnitude of the steering corrections are quite variable. This indicates that an oversteering car is hard to drive consistently (and possibly also that I suck at racing).

Let’s take a closer look at the fast laps to dissect how understeer and oversteer affect driving style. I’ve zoomed in on the first corner (a fast, descending right-hander) below. Again, the panels are speed, steering angle, and throttle from top to bottom. The area under the blue steering angle trace is relatively large. I’m having to crank the steering wheel quite a bit because the front of the car is sliding (understeer). On the green trace, there is very little steering because the rear is stepping out just a little. This is what Paul Gerrard calls zero steer. On the red trace, the back has stepped out so much (oversteer) that I have to make a steering correction in the opposite direction to prevent myself from spinning. Note that the green trace also has a steering correction (it’s bowed down in the middle), but it is very mild.

Looking at the throttle trace (bottom panel) you can see the disadvantage of the understeer setup: it’s late getting to full throttle. So in addition to the loss of speed from scrubbing the front tires, it has an additional opportunity cost in throttle time. The oversteer setup should get to full throttle first because it’s pointed straight first, but I’m fighting the wheel so much I don’t manage it. A better driver could make this work better than me.

Here’s the whole graph. Note that the understeer setup isn’t always the last to full throttle. Sometimes the initial application is delayed. But once applied, the throttle can be used as an on/off switch. You don’t really have to balance the back end when the back end isn’t sliding. In contrast, the oversteer setup requires a soft foot and live hands to keep it on track.

Tire pressures do matter

The AI was relatively unfazed by non-square changes in tire pressure, but I was not. Having a loss of grip specifically on one end of the car or the other completely changed how I drove. I can sum up the driving experience as follows:

  • An understeering car
    • feels boring
    • requires a lot of steering effort
    • requires trail-braking to rotate
    • requires patience before throttle
    • may see you running off track at the exit
  • An oversteering car
    • feels exciting
    • practically turns itself
    • requires steering corrections to prevent over-rotation
    • requires throttle modulation
    • may see you spinning at the entry, middle, or exit

Why is the AI behavior (oversteer fast) so different from mine (understeer fast)? I’m not sure exactly what to take away from the AI driver. It’s several seconds slower than me and doesn’t even know how to trail-brake (data not shown). The AI sucks at racing. However, it is very good at controlling oversteer. Its steering corrections are always exactly the right amount. I don’t think we should read too much into the AI performance.

Although I set out to determine if tire pressures affected lap times, what I ended up focusing on was how tire pressures affected grip balance. Why? Because the handling of the car is what will ultimately dictate lap times. Too much oversteer not only results in a car that is difficult to control, it’s also slow. But what of too much understeer? It’s a little annoying but can be mitigated by trail-braking. Ultimately, it’s easier to deal with a little extra understeer than a little extra oversteer. For many inexperienced racers, the natural reaction to stuff going wrong is to lift off the throttle. If the car naturally understeers, the stuff is mostly understeer and lifting is the appropriate response. In an oversteering car, lifting is going to make matters worse.

Going Forward

All of the experiments here depended on the Assetto Corsa tire model. How accurate is that? No idea. I don’t think of these experiments as the end of anything, but rather the seeds for the real-world tests I’ll do later in the year. Stayed tuned (pun intended).

How To: panic braking understeer

This is the third part in a series of posts on how to suck at racing. Continuing on the understeer theme from last week, let’s talk about panic braking understeer. It’s one of the simplest ways to suck at racing. Simply hammer your brakes and then observe that turning the steering wheel does nothing while your tires are skidding. Afterwards, you can also observe the detrimental effect on your tires: flat spots. Let’s see it in action. Since I’m currently on holiday in the UK, let’s watch some racing from there. The incident comes from the Tin Tops series from the Classic Sports Car Club. The rules sound like fun: under 2 liters, 4 cylinders, and a mandatory driver change in a 40 minute race. The two videos show different perspectives of the same incident.

 

Ok, so the driver in the 2nd video doesn’t suck at racing. Does he have magical powers? Did a psychic tell him this was going to happen? Alternatively, maybe he expected this to happen and executed a plan born from experience. If you don’t have the luxury of hundreds of hours of track time, mental imagery and simulation can be very effective. Want to know more about mental imagery? Go get yourself a copy of Ultimate Speed Secrets by Ross Bentley. Want to know more about simulation? Click the simulation link above or purchase my book.

How To: power-on understeer

Oversteer scares passengers. Understeer scares drivers.

There are two primary ways to experience understeer, heavy braking and heavy acceleration. Under heavy braking, the tires are using all their grip for braking, and there’s none left for steering. This is a pretty common occurrence when people try to steal an apex and then realize there isn’t room to pull it off. Less common is power-on understeer.

If you want to replicate the driving above, start by taking an early apex line and maintain enough speed that your front tires are sliding. Maintain throttle. Whatever you do, don’t lift. Lifting will transfer the weight to the front of the car, add grip, restore steering, and ruin the fun.

Watch it again, this time looking for the paint on the wall. It’s like a magnet pulling the car to its doom.

The two kinds of understeer

If you turn the steering wheel and the car doesn’t want to turn, that’s understeer. If you find this happening to you, it’s probably because of a heavy right foot. Understeer generally occurs either (1) during hard braking or (2) during hard acceleration. If you’re trying to mix turning with accelerating (positive or negative), you have to modulate your right foot and only apply as much brake/throttle as allowable by the remaining traction.

Braking understeer generally occurs on corner entrances. The typical scenario is the driver brakes too late and too hard, and there’s no grip left for turning. Tires have only so much grip, and you can use that grip for braking, turning, or some mixture (the relationship here is often thought of as linear, but it’s not strictly the case, especially when skidding). If you brake really hard, you can’t turn, no matter how much you wind the steering wheel.

If you find yourself in this situation, you have to release some brake pedal pressure to restore your ability to turn. If you have to think about what to do, it’s already too late. The sound of skidding tires and the lightening of the wheel should trigger your muscle memory to stop braking. If you don’t have that mental programming, the cheapest way to acquire it is through sim racing (see the How To link above).

Power understeer generally occurs on corner exits. Pressing the throttle shifts the weight to the rear of the car, which reduces the load and traction on the front tires. If they loose too much traction, they may slip, causing the car to drift off to the outside of the turn. Front-wheel-drive cars with a lot of power can spin their front tires for an even more dramatic loss of traction.

If you watch that whole clip, you’ll see both kinds of understeer.