Ghosting the Aliens: part 1, data

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As you may know, this is November and therefore national novel writing month. A few years ago I wrote a novel about racing called Ghosting the Aliens. No, you can’t find it on Amazon or anywhere else for that matter. A novel written in one month isn’t worth reading! The title is a phrase I use for being able to virtually ride along (ghosting) with the fastest drivers (aliens). This is the first in series of blog posts that will focus on reading telemetry data.


You can improve a lot just by comparing your laps to each other. But in order to take your driving to the next level, you have to examine the inputs and outputs of faster drivers. Warning: this can be pretty humbling. You may find yourself 10 seconds behind the leaders. But don’t worry, they started out just like you. All you have to do is learn how to drive. And if you’re 10 seconds behind, let’s be frank, you really haven’t learned how to drive.

So where do you get these embarrassingly fast laps? Real life simulation data is hard to come by because not everyone runs telemetry in their car. And a driver’s telemetry data is sort of their intellectual property. They might not want to give it to you. But even if you do have real life telemetry data, the variation from car to car and from day to day means that analyses may not be very robust.

One of the great things about simulation is that we can lock down the weather, the car, the setup, and focus completely on the driver. You can prowl the forums at iRacing, Assetto Corsa, rFactor 2, etc. and ask people to upload telemetry data somewhere but you probably won’t get many takers. Just like other forms of racing, almost nobody willingly shares their secrets. Thankfully there are a few sources that sort of enforce sharing. Let’s introduce a few of them.

  • iSpeed is a digital dash that you can run on another monitor, a tablet, or your phone while you drive iRacing. When you run the dashboard, it records telemetry and sends your fastest lap of the session back to a server. Later, you can use iSpeedLapAnalyzer to examine your laps. The cool thing is you can also look at other drivers’ laps. iSpeed is used by a lot of iRacers and there are lots of fast laps to compare. The analysis software is not as pretty or complete as some professional programs and you can only compare 2 laps at a time. iSpeed doesn’t annotate the weather, which can affect laps by more than 1 second, so when you’re comparing laps, it’s important to make sure the laps come from the same season and week of racing (which all have identical weather). iSpeed is donation-ware, meaning you can use it for free but $15 removes some annoying messages. I’ve paid because it’s worth a least $15 to me.
  • Z1 Analyzer is a digital dashboard that supports every major Sim title. If you check the box to share your laps, you can also download other peoples’ laps. There aren’t nearly as many laps stored as iSpeed, but you can find laps for sims other than iRacing, such as Assetto Corsa and rFactor 2. The free demo runs for 15 minutes at a time and the full version is about $25. I haven’t bought this yet, but the cross-sim-platform support is really attractive. The user interface is slightly better than iSpeed I think.
  • iAnalyze Racing is a dashboard, telemetry suite, and database of fast laps. The database has nowhere near as many drivers or laps as iSpeed and is poorly organized. It’s $15 for an annual license, but doesn’t appear to be supported anymore. I have a license, but I don’t suggest you get one.
  • Pure Driving School and Virtual Racing School are iRacing coaching businesses. For about $10 per month you can get their weekly guides and download their telemetry. For about $50 you can also buy one-on-one coaching sessions. PDS uses the professional Motec analysis suite while VRS has their own tool that runs on their website. I prefer PDS to VRS but VRS supports a lot more series.

You may have noticed the iRacing bias here. That’s because iRacing currently has the largest market for competitive online racing. That may change soon when Assetto Corsa Competitzione is completed, but for now iRacing is where we’re going to find telemetry data.


Go to the iSpeed website and download the latest build. After installing you will have two new applications, iSpeed and iSpeedLapAnalyzer. We’re only going to be using iSpeedLapAnalyzer. Launch the application and then click on the File menu to load laps into the Lap 1 or Lap 2 slot. We’ll put a fast driver in Lap 1 and compare some slower drivers to that.

Use the menus at the top to select the Global MX-5 Cup series and the 2018 Season 3 season. This series uses the same fixed setup as the time attack from two weeks ago. The laps are sorted by lap time. The fastest lap, 1:39.681, is from Alex Czerny. Let’s load that.

Now click on the Driver column to sort by driver. Scroll down to Bastian Stirl and load in his fastest lap, a 1:46.248. Once you have a local copy of the file, it shows a floppy disk icon like the image below.

Now let’s take a look at the squiggly lines. Drag your mouse over one of the traces and it will highlight that portion. In the image below, I’ve highlighted T3, which you can see on the track map. You can resize the various columns if the map is too small or large.

Examine these traces using the various tabs at the bottom of each pane. See if you can figure out what Bastian needs to fix. Note that Bastian’s account is pretty new and he’s a rookie, so he’s not expected to be fast yet. He’s going to have to put in his hours just like Alex did. Unlike a lot of rookies who don’t even know what telemetry is, Bastian is using iSpeed and is therefore a step or two above the others. Over the next couple weeks we will be dissecting inexperienced and experienced drivers to get a closer look at the anatomy of speed.


Heraclitus Redux

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Out of every one hundred men, ten shouldn’t even be there, eighty are just targets, nine are the real fighters, and we are lucky to have them, for they make the battle. Ah, but the one, one is a warrior, and he will bring the others back. — Heraclitus

Below is some Time Attack data from iRacing. The car is the MX-5 and the track is Laguna Seca. 281 drivers have taken part in this particular challenge. That’s actually a small number compared to the number of people who race weekly in the MX-5 series (the last race week at Laguna Seca saw 3,148 drivers). Time Attack isn’t as popular as racing right now because it’s part of a beta UI (and maybe other reasons). But the data for fast laps is easier to mine from TA than races so that’s where the histogram comes from.

Following Heraclitus, the top 10% are the real fighters. This corresponds roughly to the 1:39-1:41 segment (actually only half of the 1:41s). The bottom 10%, who shouldn’t even be here, are lapping at 1:50 and above. Only the top 3 drivers are his warriors. If you’re looking at these lap times and comparing them to your own iRacing lap times, make sure that you’re using the exact same weather conditions (78°, late afternoon, partly cloudy, 55% humidity, wind 2 mph N) and setup (baseline, just like in the MX-5 rookie series with fixed setup). Otherwise you may conclude you’re slower or faster than you really are.

In the world of racing, we don’t call the best drivers warriors, we call them aliens. In 2009, Colin Edwards used that term to describe Rossi, Lorenzo, Stoner, and Pedrosa, the 4 riders with a strangle hold on MotoGP. Since then, the term has spread well beyond MotoGP and it’s one of the more common accolades in virtual racing. As long as we’re labeling driver skill, let’s put labels on various levels of driving because Heraclitus’ targets isn’t a very descriptive term for the 80% of the drivers in the middle.

  • Alien: top 1% of drivers, the benchmark
  • Expert: top 5% of drivers, around 1% slower than aliens
  • Advanced: top 10% of drivers, around 2% slower than aliens
  • High Intermediate: top 50% of drivers, 4-5% slower than aliens
  • Low Intermediate: top 75% of drivers, 7-10% slower than aliens
  • Novice: bottom 25% of drivers

So how does one become an alien? Not being one, I can only say so much. My best time under these time attack conditions is 1:40.3. While I’ve been an iRacing member for 5 years, I haven’t used it much for the last 3 years. So I’m actually pretty pleased that I was able to pull out a 1:40.3 after a few sessions back from a long hiatus. I know I lack the precision and consistency to be an alien. I might be able to get there one day, but it would be a lot of hard work and I’m not sure that takes priority in my life. More importantly, I generally understand how to drive a car fast. But what about those people who have been on iRacing for 10 years and still haven’t figured out why they are 3 seconds off pace? What are they doing wrong? How can they fix it? These are two very different questions, and something we will be exploring in detail via telemetry analysis as we finish out 2018 (and we’ll continue in 2019 as well).

Didactic vs. socratic teaching

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On Facebook, I belong to the HPDE Instructors group. I like this group because nearly all of the content is nice people genuinely trying to make a positive difference in the world, in as far as improving high performance driving instruction makes the world a better place. One of the topics that has come up several times is the difference between coaching and instructing. The dictionary doesn’t make much of a distinction, but the group members do. Instructing, they say, is like lecturing or demonstrating while coaching is more active and probing. I believe most of the group thinks they are coaches. The distinction between the two types of teaching is actually very old, at least 2,500 years. Given that this is the case, I will use their proper names, and not those of the HPDE Instructors group.

  • Didactic method – Presenting information to the student with materials prepared ahead of time. Examples include books, track maps, videos, seminars, etc. In the didactic method, the student is a vessel into which knowledge is poured. Most classroom education is didactic because there is an efficient student to teacher ratio.
  • Socratic method – Challenging the student with questions about their own beliefs and experiences. Examples include asking students where and when they brake, where they are looking, how they think they can go faster, and which corners they think are most dangerous. In the socratic method, the instructor and student engage in a dialog in which the instructor provides prompts. This is relatively labor intensive as it is difficult to parallelize for multiple students.

Last week, one of the coaches wrote in with the following problem.

I’m in the middle of the toughest instructing day I’ve ever had in 15 years of doing this. My student, with trailered-car autocross experience, and go-karting experience, is driving a 2014 GTI and cannot grasp the concept of tracking the car out on exit. He says he understands what I’m telling him, but he simply won’t do it. He’s also divebombing corners with his shitty-ass HP+ pads despite agreeing wholeheartedly that we would spend the session focusing on line instead of speed. Lap 2, he’d warped his rotors. I have never not been able to get through to a student and I’m about at my wits’ end. So…tips/tricks/advice?

So what kind of advice did the group give him? Here are some ideas, some of which were mentioned several times.

  • Take him as a passenger in your car to show him the line
  • Make the student put a tire on the exit curb (even if it means going out of the way to get there)
  • Let him make mistakes if he’s not endangering others, mistakes are learning opportunities
  • Narrate every aspect of the track while he drives it
  • Tell him, “you’ve paid for the track, use all of it”
  • Warn him that if he doesn’t do as you say, he’s done for the day
  • Don’t be afraid to get out of the right seat, it’s your life
  • Give him maximum RPM and MPH limits
  • Have him draw the track on paper from memory with his eyes closed, he probably doesn’t know it
  • Slow him down and make him stare at the exit
  • Explain track-out with the string method (an imaginary string is attached to the steering wheel and throttle pedal – no throttle without unwinding)
  • Establish an end-of-braking point
  • Send him home?
  • Agree on what you’re working on before the session and if the student deviates, take them back to the pit and discuss
  • Autocrossers have a different driving style… he may be too set in his ways to change
  • OSB – other sports beckon, as in, some students aren’t worth the time
  • Trade students with another instructor
  • Smack him in the back of the head

How much of this advice is didactic vs. socratic? Or in the groups’ words, how much of the advice suggests instruction vs coaching? Drawing the track on paper from memory is definitely socratic but the rest? Not so much. For a group that is keen to provide coaching, their advice is mostly to bully the student into submission. I don’t subscribe to that way of teaching. Here’s what I wrote.

I suspect the problem is that he thinks performance driving is about mashing pedals. Two drills that might work are (1) drive some laps without using the brakes except for emergencies (2) drive some laps in 4th gear only. In both these cases, mashing pedals doesn’t make you go faster. You have to think about line and momentum. Instead of telling him what to do, you can make him figure it out by posing a different kind of problem.

I would never throw up my hands and say “other sports beckon”. One part of my job is to be a teacher, but another part is to make sure my student is having the best day of his life. I’ve had a few students who couldn’t drive for shit and didn’t improve at all from one session to the next. I can only think of one time where we didn’t have a great time, and for that I blame myself. I think the day could have turned around but he left early and I never got a chance to make up for my early impatience. It’s a learning process for the coaches too.

Endurance tire testing: part 2

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Last week I blogged about how my brother and I tested a few tires to find out which one was the best. What exactly does best mean? Lap time certainly matters, but also consistency. We do endurance racing, not time trials. So our ideal tire is one that gives the driver the confidence to lap consistently fast. I’ll summarize Mario’s impressions from last time.

  • RS-4 is the best tire because it feels best and records fastest laps
  • RE-71R is a good tire, but hard to drive consistently fast
  • RT615K is a good rear tire, but it lacks grip and feel on the front
  • R1R is soft and feels weird

This week, let’s take a look at telemetry traces and see if we can get a little more resolution on the differences among the tires.

RS-4 (first run)

The two laps in the graph below have nearly identical lap times (1:37.4). But they are really very different laps. Looking at the time difference in the bottom pane, you can see that one lap gets ahead of the other by about 0.25 seconds. Then it loses all that time and ends up 0.12 seconds behind. While the fastest lap recorded was 1:37.4 it could very easily have been a high 1:36.


Here are 3 laps on the R1Rs in the same 0.2 second span (1:38.3 – 1:38.5). Again, while that sounds pretty consistent, it isn’t. Mario drives the first and second halves of the course very differently. The first half of Thunderhill West has more compromises and high speed corners. The second half features several hairpins. He drives the hairpins very consistently, but not the high speed corners. Why? Maybe he has better braking markers in the hairpins? In any case, when trying to figure out which tire is best, we have to take into account each corner, not the lap time.


The 3 laps graphed are all 1:38.1X, so amazingly close together. I haven’t plotted the fastest lap here. Overall, the laps look more consistent than the R1R laps. Is that because the driver is getting more accustomed to the track or because the tires give better feedback?

RS4 (second run)

These 3 runs are within 0.1 seconds of each other. The fastest lap was not plotted. Overall, consistency is much better.

All runs combined

Overlaying all the runs, you can see just how much variation there is in first half of the track. Mario felt much more confident on RS4s, and this translates into braking much later. This produces a transient time gain that is partially lost by braking a little too deep. These are bumps in the time graph at the bottom relative to the fastest blue lap. The corner where RS4s appear to help the most is T6 through T7. Here, the blue lines pull away from the others (see bottom time lost). Once in the hairpins, RE71Rs appear to be just as good as RS4s despite having narrow tread and wheel widths.


So what did we learn? For one thing, laps that look the same from the perspective of a stop watch can be really different in detail. There’s too much variability in the first half of the lap to say much about the relative grip of the tires. We can say, however, that the feel of the tire matters very much to the driver. In the second half of the track, where comparisons are more robust, RE71Rs may be slightly better than RS4s. Despite having only one session on RE71Rs, he drove them at least as well as the RS4s.

Let’s finish this off with a few bullet points

  • Because feel is such an important characteristic, you really should try a few tires rather than settling on what is cheap or convenient.
  • It’s probably easier to fit tires to the driver rather than asking your driver to change their style to fit the tire.
  • Don’t rely on a stopwatch to tell you which tire is best.
  • Doing tire tests on a 5-run HPDE day with a driver who hasn’t driven the track in over a year isn’t going to get the most consistent data.
  • It was a great day of driving and data mining, and I can wholeheartedly recommend taking tires and timers to the race track.

Endurance tire testing: part 1

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I had an MRI recently that shows I have a herniated disc. So my back problems are pretty fucking real as well as being debilitatingly painful. As such, I can’t do any performance driving for a while. Good thing I have a twin brother who can step in and drive for me. In this case, it was a tire test day at Thunderhill West (follow link for instructional video). The car was my Yaris in mostly B-Spec trim but I recently upgraded the calipers to increase the brake pad choices. The Yaris has such a huge cargo area that we were able to fit several tool kits, 2 people, and 6 tires inside even with a full cage. I also have a custom tire rack that fits on a mini hitch that increases its capacity to another 4 should the need arise.

Here is a brief list of the tires we were testing. A more thorough description is given below.

  • Falken RT615K+ 205/50/15
  • Bridgestone RE71R 205/50/15
  • Hankook RS4 225/45/15
  • Toyo R1R 225/45/15

Five 20-minute sessions isn’t an ideal way to test tires. There aren’t that many runs, the runs are a bit longer than necessary, and there isn’t enough time to make a tire change in the middle of a run with a pit crew composed of exactly one gimpy 51-year old (me). So how do we figure out which is the best tire when the test conditions are so volatile? We have 3 methods.

  1. Lap times
  2. Feel of the car from the driver’s perspective
  3. Telemetry analysis

In part 1 of this post we’ll look at lap times and how the tires feel from the perspective of the driver. Next week I’ll present the details and show why telemetry is so important. Now let’s hear from Mario, whose contributions are in blue text.

I tried to stick with a pace that I would do in an endurance race, and not to find faster lines or experiment with driving style too much. The point was to go for consistent laps, and measure all the tires on a level playing field. Naturally the air and track temperature changed throughout the day, and as this was a HPDE session, I had to throw away some good laps to manage traffic. Also, I don’t know the track that well, and so my driving was certainly going to improve throughout the day.

Let’s talk about the tires in the order they were run.

Falken RT615K+ 205/50/15 15×7

Fast 1:40.9, Median 1:41.6

Our team has used a lot of different performance tires over the years (Bridgestone RE11A, RE71R; Dunlop Z1, Z2; Falken RT615, RT615K, RT615K+; Federal 595 RSRR; Hankook RS3, RS4; Hoosier SM7; Toyo RR, RA1; Nitto NT01, NT05, Yokohama S.Drive). Historically, some version of Falken RT615K has been our go-to tire. The reason for this is that it strikes a nice balance in expense, life, and grip. It’s not the fastest tire, but it is one of the more durable in the 200TW category. Given that we’re more cheap than fast, we like Falkens. We almost always mount these on 15×7 rims even though 15×8 (or even 15×9) are supposed to be faster. Why? No good reason. Possibly because Spec Miata uses a 15×7.

I’ve heard that the RT615K+ is made in the same plant as the Dunlop Z3 Star Spec. The difference is the tread pattern. Falkens are typically $10 cheaper per tire. If they truly are the same thing, the choice comes down to camber wear. Dunlops have a symmetrical tread pattern, which means you can flip the tires on the rims, which may extend tread life quite a bit. If you read reviews on the RT615K+, people say they get greasy if you run them hard. That’s not a bad thing if you ask me. If you’re driving the limit, any tire will get greasy.

The tires we had for the test day were mounted on 15×7 Kosei K1 rims. They had seen quite a bit of track action, but only as rear tires on the Yaris. That means they were hardly worn at all. For the rears, we had a set of old RE71Rs (see below). Let’s hear what Mario has to say about the RT615K+..

I’ve driven the Falken Azenis 615K probably more often than any other tire, and they are the gold standard which I measure everything against. I hadn’t tried the 615K+ yet, so I was happy to go out on the Azenis first and see what all the plus was about.

My experience with them is that they warm up quickly, and usually the fastest lap is the second lap, but then they get a bit greasy when hot. Traction drops off a bit then, but stays at that level forever. And so they felt the same as always, with good audible feedback and a generous traction limit that doesn’t suddenly go away.

However, after trying the other tires, the gold standard is now the old standard. By comparison, the turn in was vague, and they simply don’t have as much stick. Perhaps on a 8” rim they would have worked better, but I doubt an inch of rim width was going to be the night-and-day difference I’d experience with the other tires. Later in the day we’d put these on the rear, and for that, they are my tire of choice.

Hankook RS4 225/45/15 15×8 (first run)

Fast 1:37.4, Median 1:38.4

The RSR has become one of the most popular endurance racing tires. One reason is that much of the competition has reengineered their tires for the larger autocross market where grip is more important than longevity. RS4s are a more traditional 200 TW tire that last a long time.

The tires used in the test were mounted on 15×8 TR C1 rims. The tires had been used in a previous Lemons race and had about half of their tread remaining. The rear tires were the same as the test above (old RE71R).

The second time I turned the wheel I knew I was on a totally different tire. I was initially a little bit nervous because the steering was so different than the Azenis, but the tires warmed up quickly, and I to them: super accurate turn in, great feedback, and you can hear them working. I put down a few consistent laps and was surprised to see they were over 3 seconds faster than the Azenis!

That’s pretty astounding considering these were back-to-back sessions an hour apart. The track and weather conditions were probably as similar as they were going to be, and I don’t think my driving line or technique changed much from the previous session.

The RS4s seemed to take a bit longer to come in than RT615K+, and were fastest on the 4th or 5th lap. After that they seemed to fall off about a quarter second.

Toyo R1R 225/45/15 15×9

Fast 1:38.3, Median 1:39.3

Originally a 140 TW tire, Toyo later rebranded the R1R as a 200 (probably to get more sales). Magazine tire reviews consistently report that the R1R is a pretty soft tire that wears quickly. It’s supposedly really good in the rain. It has one of the more interesting tread patterns. The brand new tires in the test were mounted on 15×9 Konig Dekagrams. Rear tires were as above.

I thought these were going to be the fastest, because they were the only ones on 9” rims, and they are basically rebadged 140 TW tires. But I just didn’t have much confidence under braking. They didn’t have as much audible feedback, and even through the wheel and pedals I never knew what they were doing. Any corner which required some initial braking cost me time, and while I thought these were the fastest tires around T2 (I could floor it all the way around), it was simply because I didn’t have the confidence to go through T1 faster. The run up to T7 requires the longest and hardest braking, and I felt like I was going to flat-spot them every time.

When we pulled the tires off, they looked totally different. The way the rubber was melting off the tire made them look like they were much softer than the other tires. I’d worry about the longevity of these in an endurance race. However, they were also brand new tires with full tread, and probably got the hottest because of that. I hear that R1Rs are good rain tires, and that’s probably where these will be used now.

Bridgestone RE71R 205/50/15 15×7

Fast 1:37.1, Median 1:38.2

The RE71R is well known as a cheater tire because it’s more like a 100TW than a 200TW in grip and longevity. I’ve heard some stories of them lasting only a few hours. My experience is that they are actually more durable than other tires on my Yaris. Despite its low power, my Yaris has caused blistering and chunking on most of the tires it has seen. It’s pretty frustrating to see a tire with very little wear except the shoulder has been completely chewed away. RE71Rs don’t do that because they can handle the heat.

We had planned to use these on 8” rims, but due to some fitment problems, we had to use the ones that we’d been using as rear tires, which were on 7” rims. That meant we had to move the Azenis to the rear, and so this wouldn’t be an apples-to-apples comparison with the other rubber.

But that turned out to be not such a bad thing, as the RE71R front and RT615K+ rear made a balanced combination with neutral handling. I was able to rotate the car much easier and play with balance more effectively.

As such, the RE71Rs set down the fastest time of the day (so far), but I didn’t feel they were as consistent. I felt like the one fast lap was an outlier, and that I couldn’t drive them that way lap after lap. These were also the oldest tires, and I’m not sure the effect of that.

Hankook RS4 225/45/15 15×8 (second run)

Fast 1:36.7, Median 1:37.3

We went back to RS4 front leaving the 615K+ on the rear, and again the handling was very neutral, similar to the RE71Rs. Taking some traction away from the rear definitely helps me.

Compared to the RE71R, the RS4s instilled more confidence, and this might be down to simply the sounds they make. A better driver might go fastest on the RE71R (or maybe even the R1Rs), but I felt better on the RS4s. And they were more fun.

So much so that on the last two laps I decided to screw consistency and up my pace. I immediately dropped half a second and did a 1:36.4. On my second flying lap I looked at the RumbleStrip and saw I had another .3 seconds in hand and thought I had a sub 36 lap in there… But they threw the checker on me in T8, and so I didn’t get a chance to find out.

Conclusions Part 1

Although we had come to the track to determine which tire was fastest, one of the most important lessons we learned was how much feedback is important to the driver. A simple tire swap can make a huge difference in the way a car feels and consequently what performance a driver can extract from a car.

Mario drove the RS4s faster and more consistently than any of the other tires. He also felt more confident with RT615Ks on the rear rather than the stickier RE71Rs. Moving forward, we’re now considering new combinations of tires. Perhaps 245 width RS4s in the front? Maybe something in 195 width for the rears? More tests will follow, but not until my back heals or my brother visits again.

Next week we’ll take a higher resolution look at the data using telemetry and see why you should always run telemetry.

It’s raining lies: part 3

Are we finally going to end the “It’s raining lies” series? Yes, yes we are.

Screamer vs. Big Bang

Before we begin, let’s take a brief tour through a seemingly unrelated topic in the motorcycle world: big bang vs. screamer engines. A big bang engine is one where all the pistons fire at the same time (or very close together). A screamer engine spaces out the ignition pulses as much as possible. From an engineering standpoint, it shouldn’t matter much, but the screamer is a little more powerful because it vibrates less. However, from the rider’s perspective, the firing order makes a big difference. Bikes with screamer engines tend to send their riders off the high side. How the heck does piston firing order affect the rider?

In a big bang configuration, the tire gets a big kick in the ass every 720 degrees of rotation. But it also gets a long rest period before the next kick. In a screamer, the tire is getting kicked every 180 degrees (assuming a 4 cylinder motor). Apparently the downtime in the big bang configuration gives the rider more time to sense the level of grip and adjust accordingly. In a word, the big bang gives compliance.

Softer Suspension

Before getting to the objective stuff, let’s be subjective and talk about how driving in the rain makes us feel.

  • How does a car feel on a wet track? Unpredictable.
  • What are we afraid of? Crashing the car.
  • How does that make you drive? With a large margin for error.

It’s fine if you don’t want to admit it, but I will. Racing in the rain scares me a little. The tires don’t make the same sound. The steering wheel doesn’t have the same tug. The throttle pedal feels like an on/off switch. When things go wrong, it seems they go wrong suddenly and without warning. That said, I actually really like driving in the rain. The extra stress makes it extra fun.

The reason why we soften the suspension in the rain is to slow down weight transfer. A car with a stiff suspension is sort of like a bike with screamer engine. It is theoretically the faster configuration. Stiff suspension leads to less weight transfer which leads to more grip. Lap times should be lower with stiffer suspensions. This is true regardless of the wetness of the track. However, there is also the human element to consider. The weight transfer in a car with stiff suspension is much more abrupt than a car with soft suspension. A human driver needs time to make adjustments to grip, and a suspension that is too stiff does not give the driver enough time to sense and react to changes in traction. So what are the physics underlying this phenomenon?

Basics of Friction

The coefficient of friction (CoF, or µ), is a ratio of the downward force of gravity divided by the frictional force. In the old days it was thought that you couldn’t get more than 1G of frictional force, and that the CoF was limited to 1.0 (this was due to blindly following Coulomb’s Law, which doesn’t really apply to viscoelastic compounds like rubber). Racing tires can generate over 1.0G, and much more with downforce.

Tire grip comes from the interaction of the rubber with the road. These interactions occur at a variety of scales from invisible molecules to stuff the size of tires themselves.

There are two separate properties that account for tire friction: adhesion and hysteresis.

  • Adhesion – Microscopic contacts between the tire and surface. This is also called mechanical keying.
  • Hysteresis – Macroscopic contacts that deform the rubber. The energy used to deform the rubber creates grip.

Adhesion and hysteresis sometimes compete with each other. As a tire gets hotter, it increases its adhesive properties but loses hysteresis. Adhesion likes a smooth surface while hysteresis likes a rough surface. The optimal operating temperature of a tire is therefore a complex function that depends on the properties of the rubber and both the microscopic and macroscopic texture of the surface.

To simplify matters, one usually talks about the optimal friction and relates this as the CoF. The CoF of a steel plate doesn’t change, so it’s a convenient simplification to think of the CoF as a single value. But the CoF of rubber actually changes and therefore can take a variety of values depending on the situation.

Load is sub-linear

It is well known that friction increases with load. But the grip of tires with respect to load is sub-linear. That is, if you increase the load on a tire by 2-fold, it gives less than 2-fold more grip. As a result, all things being equal, a lighter car will have higher corner speeds than a heavier car. One reason for this may be that there are physical limits to hysteresis. Colloquially, once a tire has been sufficiently mashed into a surface, it can’t be mashed any further.

Optimal slip

Whenever a tire is asked to do anything other than roll freely, it will have some slip. We’re not talking about slip angle here. Imagine braking instead. There is a continuum from freely rolling to fully locked. At 0% slip, the tire has a CoF of nearly zero (there is some rolling resistance). At 100% slip the tire is locked into some amount of grip, but that grip isn’t optimal. The peak friction occurs at a relatively mild amount of slip.

Speed affects grip

A tire that is moving across a surface a high speed cannot press into the surface as well as it can at low speed. This means that tires have less grip at higher speeds.

The optimal slip ratio also changes with speed. The faster you go, the lower the optimal slip ratio. We often think of the CoF as a fixed value, but it isn’t. Given that you have less grip and a lower optimal slip ratio, it’s not just self-preservation that should make you drive more reservedly at high speed.

Water affects grip

Water affects grip by getting between the tire and both the microtexture and macrotexture. It can therefore reduce adhesion and hysteresis. Grooves or other kinds of texture in both tire and surface can help evacuate water.

The amount of water on the surface is really critical. If the water film is thin, slick tires grip better than grooved tires. But if there is too much water to be evacuated by the macrotexture, the grip of a slick tire becomes terrible.



Under certain conditions, a tire may hydroplane. In the figure below, the dashed line represents a constant CoF while the solid line represents a variable CoF. The actual stopping distances are given in the inset, which match the variable CoF. The take-home message here is that the grip of wet tires depends on speed. Presumably that’s because of hydroplaning.


Water interferes with microtexture and macrotexture. It can also cause hydroplaning. As a result, the coefficient of friction of a wet tire is anything but constant. A dry tire is easy to drive because it has a very broad band of traction in which the CoF doesn’t change much. You can over-drive the hell out of it and it will still perform okay. This is not true of a wet tire, whose CoF depends on the amount of water, the grooves in the tire, and the speed of the tire. Push a wet tire too far and suddenly, you’re spinning.

The reason why one softens the suspension in the rain is because the coefficient of friction of a wet tire is variable and volatile. By slowing down weight transfer, we give the driver time to adapt to an unpredictable CoF.

Let’s finish off this series of posts with a few key points about driving in the rain.

  • The reason why traction loss feels sudden in the rain is because it actually is. So be careful out there.
  • You may not notice much difference in braking in wet vs. dry but it is substantial.
  • Be extra careful at higher speeds where hysteresis and hydroplaning effects seek to rob you of traction.
  • When applying throttle, make sure you do so gradually because once a tire starts spinning, the loss of traction is catastrophic.
  • Grip in corners is pretty good as long as you don’t upset the traction with too much throttle, too much brake, or jerky inputs.
  • The more water there is, the bigger the tire grooves need to be. If you don’t have grooved tires, pump them up so they have a crowned profile. If you do have grooves, decrease tire pressure.

It’s raining lies: part 2

Where Were We Anyway?

If you recall, three weeks ago I did the following:

  • Called Ross Bentley a liar
  • Committed career suicide (see above)
  • Claimed that wet tires have 9/10 braking grip, 3/4 cornering grip, and 1/4 accelerating grip
  • Showed telemetry traces that support said claims
  • Calculated the G-forces in a Car and Driver tire test and found that braking loses much more grip than cornering
  • Lied that I would resolve the mystery the following week

In my defense, the series is called “it’s raining lies”. So let’s get back to our watery tale and see how this story resolves.

The Braking Mystery

Why do I feel like the car brakes equally well in the wet and dry when the data shows dry grip is so much better? I believe this is pretty simple. When the track is dry, we aren’t braking as hard as we could. Thinking back a couple weeks, let’s be Paul Gerrard and see if we can get to the root cause. It’s certainly not physics holding us back. Do we fear excessive Gs? Not exactly, what we fear is flat-spotting a tire. Our team races on a small budget and tires are the largest expense. Flat-spotting a tire is a huge no-no. Everyone on the team is acutely aware of that. Because we are afraid of destroying tires, we don’t brake as hard as we could.

If the car had ABS, we would probably brake harder in general because ABS prevents flat-spotting. Braking is so much easier with ABS: just mash the pedal and let the computer take over. Surely the Car and Driver tests were done with an ABS-equipped car. It makes the testing procedure much more repeatable if you minimize the human element. And why not use ABS? ABS systems probably brake better than you do. Nannies in cars are getting better and better. For most drivers, having various nannies on is faster and safer than driving fully analog.

Have you ever noticed that WWII fighter planes have wings that slope up and modern fighter jets have wings that are straight out or even slope down?

A dihedral wing, one where the wing tips are higher than roots, is inherently stable. But an anhedral wing is not. Build a paper airplane with an anhedral wing and it will flip over and fly as a dihedral. It’s very difficult to fly a plane with an anhedral wing angle. Human pilots can’t do it. There isn’t enough compliance. They need a fly by wire system that makes hundreds of tiny adjustments per second to keep the plane flying level. So why have anhedral wings? Because the inherent instability makes the plane want to turn, making it more agile. Could cars be tuned the same way, so twitchy that no human could drive one without nannies? Surely. The evolution of performance driving will someday see computers outperforming humans at every level of the sport. When that happens we’ll become even better drivers as we learn from computers whose AI can explore the parameter space more deeply than we can.

Let’s return from my crystal ball and recap: we under-brake in the dry because we are afraid of flat-spotting our tires. I’m happy to make this compromise for 3 reasons.

  1. A flat-spotted tire is a waste of money
  2. A tire that fails on track could cause a crash
  3. Time spent fixing things in the pits is time not lapping

Brake Bias

When tuning a car for driving in the rain, one parameter that is often changed is the brake bias. Since there is less overall weight transfer on a wet track, there’s less weight on the front wheels. With more weight on the rears, more braking is possible out back. In my old E30, I installed a manually operated prop valve. To adjust the bias, you lift the hood and twist a dial. Real race cars put the bias adjuster in the cockpit so the driver can make changes mid-race. Until you’ve tried an adjustable prop valve, you probably haven’t experienced how much it changes the handling of your car on corner entries. If you don’t trail-brake, you won’t notice much at all, but if you do, it’s basically an oversteer tuning dial. Want more oversteer? Add more rear brake. It’s really that simple. If you don’t have a prop valve, you can still tune your brake bias with different pad compounds, but the resolution is much lower.

The main problem with adjustable bias is forgetting to dial it back when the track dries. This can lead to disaster. The rears will lock up first, causing the back of the car to wander when braking in a straight line. This can even happen on a wet track if the brakes are horribly out of proportion. Early ABS systems were kind of crappy and just kept the rears from locking up. If you’ve got such a system and the ABS computer is defeated or the fuse is blown, the bias is dangerously out of whack. Watch below as the fast POV is destroyed by a slow BMW that loses control while braking in a straight line.

How did the BMW team not realize their brakes were so horrible? Probably because they usually brake very gently. The rain moved the lock-up G-force threshold lower and the driver found himself in unfamiliar territory. How do you mitigate this? That’s a very good question. On the one hand, you can tune the brakes appropriately with a prop valve, pad compounds, or ABS. That fixes the problem with the car. But there’s another problem, which is how to fix the driver. Should the car be good enough that the driver doesn’t matter? Or is it the responsibility of a driver to work around problems with the vehicle. Probably a little of both don’t you think?

So how do you get practice driving a car with horrible brake bias? How do you get experience with locked up rear tires? If you want to train yourself for disasters, you have to put yourself in disastrous situations… without wrecking other peoples’ property, your car, or your body. The answer, which you can guess if follow this blog, is simulation. Not every car has adjustable bias even in a sim. Find one that does and then experiment with brake bias. Once you experience how useful and fun it is to tune your corner entry oversteer, you may want to install a prop valve in your race car. It takes all of 30 minutes and costs less than $100.

There’s still more to come in the “it’s raining lies” series. We still haven’t discussed why you soften the suspension in the rain. Check back next week for the resolution (or possibly more lies).