Leapfraud

One of the things that I espouse is that when you go to the track, it should be to train. While it’s important to have fun, you should also spend some time working on your technique. So when YouTube suggested this video of cars doing a leapfrog passing drill, I nodded my head in approval. The video shows 4 cars taking turns passing each other as they work around Laguna Seca. The video is sped up 2x probably because it was sprinkling.

And then I started watching the video and the head nodding turned to head shaking. The drivers all seem to think that you’re supposed to crawl around the outside of the track and let the faster cars pass on the inside. I checked my library and the Internet to find the source of such wisdom. Finally I found the source. NOBODY FUCKING EVER. Apparently they’re trying to perfect this technique because they keep dive-bombing each other again and again. In some corners, they are driving around with their point-by hand still out the window. For crying out loud, there could be kids watching!

And what of the POV driver? The last time I saw this much pinching was at a shitting convention.

Virtual Rally Training

Driving with low grip is a great way to improve your racetrack driving skill. That’s why the Kenny Roberts Ranch is a dirt track. It’s also why the Skip Barber Formula 2000 rides on BFG T/A Radials. If you want to get better at driving, leave the sticky tires at home and drive on all-seasons. The same is true of virtual training. Drive on loose surfaces and with hard tires if you want to improve your feel for vehicle dynamics and develop your car control skills.

DiRT Rally

My favorite rally sim is DiRT Rally. When I first discovered it, during the Steam Early Access release in 2015, I knew nothing about rally. But I soon became such a huge fan that I built my Yaris to do double-duty as a rally car. Truthfully, I haven’t done much rallying in real life. I attended the Primitive Rally School at the Santa Rosa Fairgrounds and goofed around a bit at the Prairie City Off Highway Vehicle Park. Those experiences told me 2 things: that rally driving is the best kind of driving, and that DiRT Rally felt pretty realistic.

So why don’t I do more rally? I only subscribe to a few YouTube channels, but one of them is “Racing Fail!”. I like it so much that I donate to it monthly via Patreon (you can even see my name at the start of the videos). Every week, Racing Fail! shows motorsports crashes from the previous week. And every week there are multiple rally drivers wrapping their cars around trees, driving off cliffs, and rolling through fields like mechanical tumble weeds. Occasionally they catch on fire. That’s sort of terrifying. Racing Fail! is a weekly reminder to stay safe and not to wreck my Yaris (or burn myself crispy).

Back to the sim world. DiRT Rally is old enough that it can be picked up on Steam for as little as $10 when it goes on sale. There are newer rally sims from the same developer, Codemasters, but neither DiRT 4 nor DiRT Rally 2.0 is actually better. One of the downsides of DiRT Rally is that there is no community-created content. While DiRT Rally has some great vehicles (by great I mean lower performance cars similar to what I drive) the collection cars and tracks are fixed. There’s nothing new coming. Community-content is what makes Assetto Corsa great. So that begs the question “how good is Assetto Corsa as a rally trainer?”

Rally Training in Assetto Corsa

While you won’t find much official (Kuno Simulazioni) rally content, the community has created plenty of cars and tracks. While the choice of rally cars ranges from the modern WRC Polo to the historic Lada VFTS, you don’t need a rally car for rally driving. For tracks, there are rally stages on gravel, dirt, and snow, as well as hill climbs, street races, and stadium rallycross. As with all AC community content, the cost is mostly free and the quality highly variable.

For training purposes, it’s a good idea to drive both RWD and FWD layouts because they behave differently. For RWD I go with the NA Miata because Miata Is Always The Answer. I say this even though I no longer own a Miata. The Assetto Corsa NA Miata is such a great model that it’s the first thing I turn to, even on dirt. For FWD, I like the Chevy Monza. The motor is on the weak side and the suspension is on the plush side, just like the cars I drive. The Miata is faster on asphalt but the Monza is faster on dirt. But they are very close on any surface, and make a great set of cars to play with for any occasion.

One of the things that makes rally driving unique is the co-driver. In DiRT Rally, you can have visual or audio cues, and you can specify how early or late you want to hear them. Personally, I use audio only and have them announced as far forward as possible. I really enjoy having a co-driver, but for the purposes of training it’s not necessary or even desirable. So while you can download a co-driver app for AC, and you can drive long rally stages, the best way to use AC for low grip training is on a small, closed course. Below are three tracks I recommend and some target times for a Miata/Monza.

  • Karelia – This is a fantasy rally circuit with a good mix of low and high speed corners as well as compromises. It’s probably my favorite rally trainer. Fast laps: 1:04.
  • Gentlemen Rallycross – Although the graphics are sorely outdated, the track is a great mixture of turns and surfaces. There is a joker section. Fast laps: 1:12 (non-joker).
  • Kouvola Rallycross – This is a stadium rally cross that alternates asphalt and dirt. The graphics on this track are much better than the others. There’s more than one fast line, so experiment. The lap features a joker. Fast laps: 0:50 (non-joker).

Conclusion

While DiRT Rally is the king of rally sims, there are a few things Assetto Corsa does very well. It gives you a HUGE selection of cars and tracks to play with. And if you want to change the grip of any track, simply edit the surfaces.ini file. I’m not going to pretend I’m an expert on force feedback, but the Miata and Monza feel really good. Good enough to program your muscle memory anyway, and that’s the whole point of virtual rally training.

Postscript

Some people say that Richard Burns Rally (RBR) is the king of rally sims. That platform is so old that you can’t even buy it anymore. That said, there are people making content for it, even though the game never supported that. The only way to get RBR is by violating copyright, which I try not to do, so I don’t have personal experience with it.

Just 1 mph faster

I think most people who read this blog would like to go just a little bit quicker around a race track. In fact, that may be your New Year’s resolution in a couple weeks. Rather than trying to make a huge leap, like 5 seconds, focus on something more realistic, like averaging 1 mph faster. How much faster is that in terms of lap times? It depends on the car and track. For example, in the Global MX-5 Cup at Laguna Seca, lap times ran about 1:40 in the ND2 Cup car. That’s a nice round number because it’s 100 seconds. Anyway, it turns out that 1 mph amounts to about 1.3 seconds.

As a complete aside, if you’re wondering how much faster the ND2 MX-5 is compared to the ND1, both models are raced in the Global MX-5 Cup (in different classes of course), and the answer is about 2 sec at Laguna Seca. That’s a pretty significant gap, but there’s a lot more gap to be found among the drivers. The top ND1 driver runs about the same speed as the middle of the pack ND2 driver. The difference between the two cars is 26 hp. It’s kind of amazing that even among very good racers, some drivers are effectively 26 hp better than others. Among HPDE drivers, the gap can be huge.

So back to that 1 mph faster. How are you going to go about averaging 1 mph faster? It turns out there are two ways.

  1. Enter the corner with more speed
  2. Enter the corner with more yaw

1. More Speed

For most people, more entry speed is the low lying fruit. That’s because most people brake too much and enter the corner several mph too slow. To go 1 mph faster, just enter every corner 1 mph faster and everything should sort itself out, right?

Let’s take a look at some real data from my team at a Willow Springs race a couple years back. The driver on the red trace is braking way too much, on the order of 8-10 mph in T1 and T2. That results in a lower speed all the way to the next corner and a lot of time lost. You might think the red driver is a novice, or this isn’t his fastest lap, but he isn’t a novice and this is his fastest lap.

If you’re over-braking your corner entries, as do most drivers, then there’s certainly room to enter with more speed. But how can you determine if this is the case?

  • The best way is to compare your driving to someone in an identical car with identical setup and identical weather. That’s easy to do in the sim world, but hard elsewhere.
  • Have a coach or local hotshoe drive your car so you can compare data between drivers.
  • Compare your data to someone else driving a similar car. Perhaps you both have an GT86/FRS/BRZ.
  • Compare your data to someone else in a different car. If you’re on similar tires, your entry speeds should be similar.
  • Compare your data from different laps. You might find some laps you go in faster than others.

Perhaps you’ve noticed a theme here? You’re going to need some data acquisition gear and do some comparative telemetry analysis on the speed trace. Phone apps like Harry’s Lap Timer, RaceChrono, CMS Lap Timer, Track Addict, etc. work well enough. What if you can’t use a smartphone app? I’m not sure what world you’re living in where you’re worried about lap times and can’t use a phone app, but here’s my simplest advice.

  • If you can get to 100% throttle immediately, without any kind of maintenance throttle mid-corner, you probably entered too slowly.

One of the reasons people enter corners too slowly is that they’ve heard the phrase “in slow, out fast” too many times. Another reason is that going faster would scare the shit out of them. In any case, one of the problems of entering slowly is that being under the limit gives you an invitation to add a lot of throttle mid-corner. Here’s a pretty common sub-optimal control input sequence that’s very common among intermediate drivers.

  1. Mash brake pedal – leads to low entry speed
  2. Mash throttle – leads to mid-corner understeer
  3. Lift throttle – to prevent running out of room at the exit

One of the misconceptions of the intermediate driver is that they should mash the throttle mid-corner. That will get the car to rotate, right? Somewhere in their past the driver not only heard “in slow, out fast”, they also heard “loose is fast”. So they think mashing the throttle will get the car to loosen up. Spinning the rear tires isn’t the same as transferring weight to the front. Drifting greatly reduces the overall grip of the car. Transferring weight does not.

Too much speed

As you get better at optimizing your entry speed, you will eventually run into another problem: you can’t actually enter any faster. Let’s assume that 66 mph is the limit for a specific vehicle in a specific corner. What happens if you try to go 67 mph? The corner radius has to get bigger. The equation that relates speed, grip, and radius is: speed = sqrt(grip * radius). If you decide to enter a 66 mph corner at 67 mph, the radius of the corner will have to get larger to compensate because grip is a constant. In other words, you’ll fall off the track at the exit. If you don’t want that to happen, you’ll have to lift off throttle to tighten the radius and now you’ve basically done the corner backwards (in fast, out slow).

The intermediate level of driving is a mixture of too little and too much entry speed. In both cases, drivers are fighting understeer at the exit, but for different reasons. In either case, if you have to lift at the exit, you’re killing your lap time. The whole point of the typical late apex racing line is to optimize the power of the car in the second half of the corner. Lifting ruins that.

Even if you’re not lifting at the exit, you might still be in the “too much entry speed” category. Some drivers have enough discipline not to mash the throttle, so they don’t have to lift later. Instead, they spend a lot of time coasting in the mid-corner and are late on throttle. The time to add throttle is actually before the apex, but mid-corner coasters add throttle at or after the apex.

The high intermediate performance plateau

There is a very natural performance plateau associated with optimizing entry speed. Eventually you can’t go any faster and you learn the exact entry speed that maximizes every corner. If you accidentally enter 1 mph slow, you add a little extra throttle mid-corner, but not so much that you run out at the exit. If you accidentally enter 1 mph too fast, you coast a bit mid-corner, and end up a little late to throttle. This style of driving, where you modulate mid-corner speed with the throttle can be pretty fast and consistent. It isn’t actually the fastest or safest way around a track, however. Breaking out of this style of driving can be difficult, especially if you’re good at it. If you’re a racer whose been hard stuck 1-2% behind the front runners, this is probably the reason.

Brace yourselves, another tennis analogy is incoming…

One of the greatest tennis players of all time was Steffi Graf. She had a huge serve, killer forehand, tireless legs, and a consistent slice backhand. But no matter how good your slice backhand is, it is a liability against a serve-n-volley player who loves slow rising balls. In order for Steffi Graf to beat Martina Navratilova, she had to learn how to hit a topspin backhand. It’s a completely different stroke requiring changes as fundamental as how she held the racquet. Eventually she learned the stroke and the rivalry ended shortly thereafter. A similar situation existed with Ivan Lendl and John McEnroe. In case the analogy isn’t crystal clear, slice backhands are like intermediate driving. If you want to get to the advanced levels, you’ll have to learn how to rip a topspin backhand.

2. More yaw

The other way to lap 1 mph faster is to enter a corner with more yaw. There are two main advantages to this technique.

  • The front wheels do less steering
  • The drive wheels are pointed towards the exit sooner

Steering slows the car. The phrase “in slow, out fast” is not nearly as important as “the driver who steers less wins”. Having the drive wheels straight sooner leads to opening throttle sooner. Entering a corner with more yaw means less loss of speed and more gain of speed. It’s a win-win scenario. So why don’t more people do it?

  • Yaw leads to spinning

That’s reason enough. Spinning is dangerous. It wrecks cars, injures people, and gets drivers kicked off track. Lose-lose-lose. So why bother learning how to do it? Safety, paradoxically. A driver who can deal with yaw can deal with other adverse conditions such as rain, dirt, oil, and off track excursions.

How are you supposed to learn to drive with yaw when practice may endanger people or property? Thankfully there is sim racing. Your body can learn how to drive with yaw without breaking stuff. All you need is a sim rig and the motivation to unlearn your bad habits. But wait, what about that blog post a couple weeks ago where I was giving 12 reasons not to buy a sim rig? Those reasons are good reasons. But training your muscle memory to automatically correct for oversteer? That one positive is worth a few dozen negatives.

The basketball analogy

I’m constantly trying to come up with analogies between high performance driving and some other sport. Here’s my attempt at using basketball.

I go to the gym 5 days per week before I go to work. Partly this is a personal commitment to general physical fitness, but more and more it’s about taking care of my weak back. For my cardio, I often ride a bike or use the rope climbing machine, but if my back feels okay, I’ll shoot baskets for a half hour. I’ll warm up shooting around the key and then spend the bulk of the time hoisting 3-pointers. I used to be a jump shooter, but in order to protect my back I’ve become a set shooter. I’m too lazy to count up every shot I make and miss, so instead I just remember the longest streak each day.

3, 4, 4, 6, 5, 3

That’s my recollection of my longest streak in the last few sessions. I consider 4 to be a good day, 3 to be an off day, 2 to be a bad day, and if I happen to get more than 4, it’s sort of a lucky day. I’ve hit as many as 8 in a row. Overall, I estimate I hit about 35% of my threes from the college line. At the NBA line it probably falls to 20%.

What does this have to do with driving? Each corner is like a 3 point shot. So if there are 10 corners on the track, what are the odds that I hit each one perfectly? Not that great. Out of every 10 3 point shots, I might hit anywhere from 0 to 9. Let’s say I’m having a shooting contest with Steph Curry and have that rare moment that I hit 9/10 and he has an off day and hits 8/10. That one time I beat him doesn’t matter much when he wins 99.5% of the time. Similarly, getting fast time of the day doesn’t mean much when you’re counting overall laps.

More importantly, I think it’s important to realize that you can’t have your best performance every time. In the same way that I can’t expect to hit 4, 5, 6, 7, 8, etc. 3-pointers in a row the next times I go to the gym, we can’t expect our lap times to improve every time we go to the track. If your lap times are improving every time, it’s an indication that you’re still very early in your driver development. Getting faster is hard work.

To get to the point where I can hit 35% of my 3-pointers, I’ve played well over a thousand hours of basketball. I haven’t driven on track even 100 hours. It stands to reason that I’m a better basketball player than driver. And yet, while I probably could have played on my high school basketball team, there’s no way I would have made it on my college team. Given that most people can’t count their track time in the thousands of hours, much less hundreds, it stands to reason that most drivers are actually really bad at driving. It’s not their fault. There simply isn’t enough time and money to drive an hour a day for several years.

How do I get better at basketball? To improve my shooting, not only do I need to go to the gym more often, I also have to fix some bad habits. I don’t actually know how many bad habits I have because I’m a self-taught player. That said, I think my form is better than a lot of the other self-taught players I see at the gym who are still shooting with 2 hands. Most self-taught players have much unlearning to do. I think this is true of drivers. More and more, I think self-taught = bad habits.

Speaking of driving hours, the actual number of hours I’ve driven on track is 89.5. This doesn’t count coaching hours. Here’s the break-down year-by-year.

  • 2012 – 5.5 hours
  • 2013 – 6.5 hours
  • 2014 – 18 hours
  • 2015 – 18.5 hours
  • 2016 – 16 hours
  • 2017 – 12 hours
  • 2018 – 8 hours
  • 2019 – 5 hours (8 expected)

If you’re reading this blog thinking I’m some kind of driving expert, note that I have less than 100 hours of track driving. I’ve driven several hundred hours in iRacing, Assetto Corsa, DiRT Rally, etc. but how much do those cross over to the real world? I’m probably a little unusual in how serious I take my sim racing time. I don’t goof around on the thing. I train. I also spend a lot of time researching and writing about driving. I’ve spent more time in the library than on virtual tracks, and that counts for something in my driver development. Taken all together, it still doesn’t make me an authority. The fact that I can dominate a wet race in an econobox with 4 year-old tires speaks more to the overall low level of the sport than to my own ability.

Let me wrap up this rambling with a couple thoughts.

  • It’s okay to suck at racing because there isn’t enough time/money not to
  • Being faster than someone who sucks at racing doesn’t mean you’re actually good at it
  • You can improve your real driving in the virtual world or by opening a book

 

First Year Seminar and Turn 2 Racing

For the first time ever, I’ve decided to teach a class on “High Performance Driving”. This is a First Year Seminar at UC Davis. FYS courses are a mishmash of topics designed by professors on virtually any topic. They are meant to be a fun diversion away from the more stressful courses. Since they are taught voluntarily, the content is usually something the teacher is passionate about. The courses are actually open to students of all levels, but first years and transfers get first choice I think. For the last few years, I’ve been teaching an FYS course on “Nanowrimo”. I’m doing that again, but that is quite literally a whole other story. For some reason, I thought I should try teaching a course on driving without actually doing any driving.

Serendipitously, a driving simulation business, Turn 2 Racing, just opened up in Davis. I applied for a mini grant for the course and the university gave me $500 so that the students could get some seat time on the simulators. Thanks UCD! The students are going to love this. Here’s a picture of the setup. See farther below for a brief review.

Course Details

Learning Objectives

If you’ve never designed a course of instruction, an excellent place to start is with the learning objectives. This should be a short list of things you want the students to remember 1 year later. Here are mine.

  • Communicate using the vocabulary of drivers and engineers
  • Describe the racing line in mathematical and conversational terms
  • Identify common driving errors from watching video
  • Interpret telemetry traces to diagnose driver and car problems
  • Dispel common performance myths using data

Syllabus

Here’s a brief description of the content for each week. On the first day of class, I’ll discuss with the students what things they most want to learn. After that, I may adjust the syllabus to make sure the most popular content is covered. Well, except if people want to know how to modify their cars to make them look cooler. I have zero patience with ricers. RICE isn’t an ethnic slur. While many of the cars with Race Inspired Cosmetic Enhancements are Japanese, the stupid shit wannabe racers do to their cars transcends country of origin, ethnicity, gender, etc.

  1. Introductions, the racing line
  2. Power and grip
  3. Oversteer, understeer, balance
  4. The unusual properties of rubber
  5. Getting started in simulation driving
  6. Understanding telemetry traces
  7. Common errors
  8. Advanced driving techniques
  9. Getting started in the real world

Turn 2 Racing

A couple days ago I took a trip to visit Turn 2 Racing. I had been chatting with the owner via text to plan out the FYS visit and guest lecture, and I thought I should finally meet him and look at his shop. He let me try a couple of his rigs. Here are some random thoughts.

  • The owner is a really nice guy who is very passionate about sim racing, karting, and technology. He’s definitely the right person to be venturing into this area.
  • Nearly all of his business comes from Sacramento rather than Davis. But UC Davis school isn’t in session yet, so the large population of car enthusiasts on our campus have yet to arrive. I hope his business thrives in Davis, but I wonder if he’ll end up in Sacramento.
  • All of his rigs are custom built with high-end equipment: direct drive wheels, load cell pedals, triple monitors, external and headphone speakers, etc. Two of the rigs have platform motors that rock, roll, and rumble the seat to give you a feeling of driving a real car. His kids rig is sort of like my home rig.
  • Since most people who drop into his shop have no idea how to drive a race car in simulation or the real world, he has to make a lot of setup choices to reflect that. By default, all of the cars are set up with nannies. Also, the brake pedals are all mushy.
  • Each rig is set up slightly different from the others. I tried 2 of them, and they drove differently from each other and much differently from my home setup. I prefer a really firm brake pedal and a pedal geometry that allows me to heel-toe while keeping my heel planted on the ground. It wasn’t possible on his rigs. But just like the real world, you have to adapt to the car you’re driving.
  • I didn’t try the Formula rig or kids rig because they are 2 pedal systems with the brake pedal way over on the left. While I have done a few karting sessions, I haven’t learned to left-foot brake.
  • I’m really looking forward to bringing the class here. I think they’ll have a blast and the physical experience will improve the theoretical work we do in the class.

Intermediate Topic #1: training wheels

Here in California, there isn’t much off-season, but for a lot of YSAR readers the driving season is just starting. Each year, I have specific driver development goals. I hope you do too. With that in mind, I thought I would do a series of posts aimed at the intermediate driver who wants to improve their craft in 2019. Let’s identify and fix some common errors. If you’re not an intermediate driver, fake it.

So many tires

What tires should you bring to an HPDE track day? Popular choices include Hoosier R7, Nitto NT-01, Toyo R888R, Maxxis RC1, etc. There are literally dozens to choose from. Myself, on a race track, I’ve driven on a bunch of different compounds made for sporty driving and several others that were definitely not. For those who like lists, here they are to my best recollection: BFG Rival; Bridgestone RE71R, RE11A; Continental ECS; Douglas Xtra Trac II, All Season, Performance; Dunlop Z1, Z2; Falken RT615, RT615K, RT615K+; Federal 595 RSRR; Goodyear Eagle Sport; Hankook RS3, RS4, H724; Hoosier SM7; Nitto NT01, NT05; Pirelli P6; Riken Raptor; Toyo RR, RA1; Yokohama S.drive.

Which stops best? Which turns in best? Which has the lowest lap times? Which feels best? Since the ‘E’ in HPDE stands for education, what we really should be asking is which one is most educational? In other words, which tire will make you a better driver? If you’re trying to improve your driving skills, your primary goal is to learn how to sense and control traction. As a student of driving, it’s literally your job to find out what’s on the other side of the slip angle curve. You know, the part where it dips down and gets less grippy.

Compliance

Tires are effectively part of your suspension. In the “it’s raining lies” series, we discussed why you soften the suspension in the rain. In a word, compliance. Drivers need time to adapt to changes in grip. Intermediate drivers, who aren’t comfortable sliding their car around, need time to explore the traction space. Your job as an improving driver is to play around on the unfamiliar side of the slip angle curve. If you’re not making steering corrections, you’re not sliding enough, not exploring enough, not learning enough. Am I telling you to spin out on track? No. If you find yourself spinning, you’re getting surprised by loss of traction. To combat that surprise you need tires with more compliance.

Slip angle

How much difference in compliance is there among different kinds of tires? In other words, how much does traction change with slip angle? Take a look at the following graph. Racing tires have the most grip, but they also have the most change in grip. Once the optimal slip angle is exceeded, grip falls away very quickly. On the opposite end of the spectrum is the street tire. It has low grip, but a very gradual loss of traction. As a result, intermediate level drivers are better served with street tires than R-comps.

Some of you are probably thinking “But I want to drive on R-comps some day, so I ought to be driving on them all the time”. I can see the logic in that, but your muscle memory does not. There’s a reason why the best motorcycle racers have dirt racing backgrounds. If you want to improve your driving, you need to experience more sliding, not less.

Sound

A critical part of your grip-sensing toolkit is your ears. The sound of your tires is a language you will eventually understand at a higher resolution than the following quote I got from my racing buddy Ben.

A squealing tire is a happy tire. A screaming tire is a screaming tire.

The fastest way around a corner requires balancing tire grip throughout the corner. Use too much too soon and the tire will lose traction in the second half of the corner. How do you monitor that? Got an APEX Pro? Its lights tell you how much grip you’re using. Don’t have one? No problem, your ears do the same thing. Street tires tend to be narrower than R-comps. The extra load and open tread means that street tires are louder. If you want to hear what your tires are doing, and really you do, you should be training on street tires. Again, some of you are thinking, “but some day I want to use R-comps, so I should be training myself for that sound”. You know who’s talking? That part of your ego that doesn’t want to run slower laps. Don’t let your ego hold you back from actually improving.

Don’t believe me? How about Skip Barber?

Despite recent financial problems, the Skip Barber racing school is the most famous racing school in the USA if not the world. They have been training drivers in Formula Fords since the 1970s. Guess what tires they mount on their school cars? Street tires. How streetable are we talking about? 400 treadwear BF Goodrich T/A Radial at last reckoning. If the #1 racing school uses street tires on its Formula cars, maybe you should consider the same on whatever you happen to take to the track.

As a side note, when I was researching the T/A Radial, I read a bunch of reviews to see what people thought of them. You know what the #1 complaint was? No, it wasn’t problems with durability or grip, but rather the white lettering on the sides of the tire. Apparently they aren’t white enough and if you scrub them too much it rubs off. Oh for fucks sake, who the hell buys tires because of the lettering on the sides? Apparently lots of people. This reminds me that there are two kinds of car people, (1) the kind that wash their cars (2) the kind that drive their cars. If you’re the first kind, thanks for stopping by a blog about the second kind.

Which street tire?

On my Yaris I’ve used everything from Hoosiers to Hankook runflats. One of my favorites is Douglas Performance in 195/55/15. The Douglas Tires brand is probably not one you’re familiar with. They are actually made by Goodyear in their Kelly Springfield subsidiary plant. Douglas currently makes only 2 models of tires: All-Season and Performance. Both come with 420 treadwear ratings and a 45,000 mile warranty. They cost about $40-45 each. On track, I’ve found them to be more heat resistant than some performance tires. How do they perform? Like a 420 treadwear tire, so perfect.

 

My brother has his street/track Miata set up with Yokohama S.drives. At 300 TW, that’s a bit sportier than a Douglas, but a great choice because it’s loud and has a nice balance of grip and slip. I think 300 is a good compromise, but if you’re not sure, here’s a suggestion: OEM tires. That’s what your car was designed to use. And as my friend Harkamal used to say, you should always run in jeans because if you ever have to run for your life, you’re probably going to be wearing jeans.

Are you ready to leave your ego in the paddock? Are you willing to make short-term sacrifices for long-term gains? Sadly, most drivers are not. Let’s face it, even though HPDE sessions don’t allow racing, it’s always a race, and nobody wants to be slower than the next guy. No problem, just bring 2 sets of tires to the track. You’ll be grinning ear to ear when you pass people in faster cars with your training wheels on. Don’t be surprised if you find yourself leaving them on the whole day.

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).