Time Trials on 300 Treadwear Tires? Yes!

This guest post comes from Occam’s Racer.

Pineview Run holds a time trial series on Wednesday nights, called the Challenge Cup. This is a great chance for non-members to run the track, and for everyone to engage in friendly competition. The series has a unique classing system which uses the UTQG (Uniform Tire Quality Grading) treadwear value as the sole determining factor. The three classes are split like this:

  • Street: 300+ UTQG
  • Track: 200+ UTQG
  • Race: under 200 UTQG

The UTQG rating is supplied by the manufacturers, and is thus total bullshit, especially in the 200 treadwear (TW) category. But everyone knows this, and so it’s still a level playing field. If you care about winning, just make sure you’re on the best tires in the category.

I’m interested in the Street category, mostly for the convenience of it. “Run what ya brung,” is how the saying goes, I’ll call it lazy and be fine with it. Also, my 1993 street Miata has only 110 hp, and this class is about the only place where high-horsepower cars won’t stomp on me.

My daily tire is the Yokohama S.Drive, but I recently purchased some Continental ExtremeContact Sport for racing in the rain. And I also have my RS4 race tires on hand. So I figured I’d take them all to the track in advance of the Pineview Challenge Cup, and see how the tires measured up.

Yokohama S.Drive

My 195/50-15 Yokohama S.Drives have a bit of use on them. The tread depth measures about 6/32″, from the 10/32″ they started with.

Yokohama S.Drive 195/50-15, TR C1M 15×7 +30, 31.4 pounds.

S.Drives are a popular tire on Miatas, possibly because they’ve been around a long time. There’s nothing exceptional about them, except the ridiculous sale price I got. I ordered them online at Walmart, and shipping, mounting, and balancing was free. Out the door they were $50 each, which is insane. I keep looking back for another sale like that, but haven’t seen one. Maybe this was a closeout. I don’t see this tire size on Tire Rack, there’s a 195/55-15 or 205/50-15 instead.

I’ve tracked the S.Drives a few times at NYSTand Pineview, and while they are on the slow side, I like the way they communicate. You can hear the howl reverberating around the facility. The S.Drives have a 300 treadwear rating, and these were what I planned to use these at the Pineview Challenge Cup races for the Street class.

I set the S.Drives to 29 psi cold, and on track they come up to about 35 psi. My laps are typically in the low 1:24s, but the track can be a lot faster if there’s some rubber down from other cars. We get a lot of rain in Central NY, so the track gets washed clean quite frequently. On this day my best lap was a 1:24.1, and so right in the expected range for what I consider my control tire on a clean track.

Continental ExtremeContact Sport

The Continental ExtremeContact Sport (ECS) are a newer tire that’s supposed to be a great rain tire, and also good in the dry. I had set them aside as my racing rain tires, not really intending to daily these. They have a higher treadwear rating of 340, but as you probably know, that’s not always a meaningful number.

The ECS also start at 10/32″, and the tires were brand new at the test. The other tires have had some use, so factor that into your armchair calculator.

Continental ExtremeContact Sport 205/50-015, Konig Dekagram15×7.5 +35, 31.2 lbs.

I started the Conti ECS at 30 psi, which was definitely too high, as the center of the tire got significantly hotter than the sides. Nevertheless, they were faster than the Yoks, by a full second. I then dropped the pressure 4 psi and gained a half second, putting down a 1:22.6. That’s 1.5 seconds between the Yoks and the Contis. Wow.

The Contis feel a bit vague on turn in, but that could be me just not being used to them. They are also loud, but not quite S.Drive loud. They are on 7.5” wheels because I keep thinking I’ll autocross in the STS class one day. But I’ve said that before, and I just never do it. Anyway, I wonder if an 8″ wide wheel would stiffen up the sidewall some more.

Hankook R-S4

I just finished a two-day aero test at Watkins Glen with my race car and was curious how a 200TW would stack up against the 300s. The 225/45-15 Hankook RS4s are pretty well stretched on a 9″ wheel, but this is what a lot of Miatas use.

Hankook R-S4, 225/45-15, Konig Helix15×9 +35, 33.4 lbs.

My best time on the RS4 was a 1:21 flat, 3.1 seconds faster than the S.Drive, and 1.6 seconds faster than the ECS. I expected the RS4 to be a good deal faster than S.Drives, what I didn’t expect was that the Conti ECS would split the two almost in the middle.

I’m probably not driving the RS4s to the limit yet, as they are only about half as loud as the other tires, and I expected more talking back. Maybe there’s another second in these, but that’s not enough for me to switch over into the 200-TW category against RE71Rs and Rival 1.5s.

Here’s a video of my first session on the RS4s. The 11″ steering wheel and manual rack make this tight course a bit of an upper-body workout.

Simulating G-forces and lap time

Just for kicks, I want to see the approximate lap time all three tires would do on different tracks, and to get that, I need the lateral cornering Gs so I can plug it into Optimum Lap.

I use an AIM Solo for data, which shows me how much grip the car has in every part of the track. But I don’t drive every lap or corner exactly the same, and the values spike here and there, so it’s not easy to get a steady-state value.

So I plugged my lap times into Optimum Lap and started adjusting the grip values until I got the lap times I got in real life. I started with the S.Drives and called that 1.0g, which is about how much Race Studio shows they grip, and also because it’s easier to view other tires as percentages when you start with 100. This makes the ECS 1.04g and the R-S4 1.09g. Another way of saying that is the ECS had 4% more grip than the S.Drive, and the R-S4 had 9% more.

I wondered what the lap times would be for larger tracks, so I used Optimum Lap to simulate two local tracks, NYST and Watkins Glen. I’ve included my simulated Pineview Run laps, which will let me play with other things like drag, lift, power and weight, at a later date, and find out the differences those changes make (a subject of a future post).

Tire Pineview
(real)
Pineview
(sim)
NYST
(sim)
WGI
(sim)
S.Drive 1g 1:24.1 1:23.95 1:50.70 2:33.06
ECS 1.04g 1:22.61 1:22.54 1:49.27 2:31.41
RS4 1.09g 1:21.06 1:20.94 1:47.15 2:29.15

The 3-second delta between S.Drives and RS4s at Pineview Run becomes 3.5 seconds at NYST and 4 seconds at WGI. That’s somewhat surprising to me, I expected a larger gap because the tracks are roughly two- and three-times longer, respectively. But at Pineview Run, you’re on the sides of your tires all the time, so I guess it makes sense.

Conclusions and post-test notes

  • Pineview Run is a really good place to test tires! You can get in a lot of laps to normalize the data, and it’s cheaper than most tracks. I’ll do more tire reviews in the future.
  • This was only my 3rd full day at Pineview Run, and I’m still leaving time on the track. To keep the simulations accurate, I’ll have to fudge the values to reduce track grip in Optimum Lap.
  • Even though the Conti ECS are 1.5 seconds faster than the Yok S.Drives, I probably won’t do the Challenge Cup on the Contis until I wear out the Yoks. Partly because I’m cheap, but I also want to keep the Contis at full tread in case I’m racing in the rain.
  • When the Yoks are all used up, I’ll buy a new set of Conti ECS and use them for everything.

Race Report: Lemons Thunderhill

I’ll be updating this post each day.

Thursday – arrival

In the picture below you can see how simple my race operation is. I flat tow my Yaris behind a 3.0L Ranger. It’s a very flat route so the 145 hp Ranger has no problems towing the car and gear. I arrived at the track at 4:30 the day before the test and tech day to try to get a good pit spot. I wanted something under the awning so I could shelter the pit from sun/rain. Mission accomplished.

Friday – test and tech

Tech was a breeze. The car has raced in several other series and all the safety issues are well sorted. We got into the B class with zero penalty laps. That was what we expected.

We had decided that the full test day was too expensive. $349 for 1 driver and $149 for each additional. We considered doing the half day at $249 + $100 but then decided to play a joke instead. People walked by and  puzzled: “why is the wing on the front”. We dead-panned “it’s front-wheel drive”. The look of disbelief on Daniel and Mario’s faces was worth the effort.

The weather forecast changes hourly. The latest news is that Saturday should be dry all day with a high of 78. Sunday may be wet in the morning. I told the team I get to drive the wettest stint. That may screw up driver order, but as team owner, I’m putting my foot down on that. There’s no way I can keep up with the fastest cars on a dry track, but give me puddles and let’s see who comes out on top.

Saturday – race day

The race day didn’t start the way we wanted. Our first driver got 2 black flags. One of them was for going off track to avoid a collision. I’ll take a black flag over dents any day. But 2 black flags pretty much put us out of contention. Also, there was some blisteringly fast B cars we could never catch. Our second driver didn’t like the way the car was driving. Actually, neither did the first driver. When I asked if the rear had no traction, he said neither end had traction. Puzzling. So we decided to turn the rest of the race day into a tuning day.

Mario went out and came back in after a few laps complaining that the car was oversteering badly. We were running Federal 595 RS-RR 225/45/15 15×9 on the front and Falken RT615K+ 205/50/15 15×7 rear. So we decided to switch the rears out for a stickier compound: Brigestone RE-71R 205/50/15 15×7. This time he stayed out a while and had a great race with a pickup. When he came back in, he said the car was much more neutral now and that I should get in to see what I thought.

The first thing I thought was the brakes are still mushy. The pedal starts hard but just mushes out and goes to the floor. That’s really disconcerting because it gives you very little brake feel. And without a firm pedal, it’s pretty hard to heel-toe shift. Oh well, I just did more straight-line braking and eased in the clutch. Not ideal, but I’m okay working around problems. It’s likely an aging master cylinder.

The next thing I thought was that the 225 RS-RRs 15×9 aren’t that much different from the 205 15×7 I had run in earlier races. The tires don’t actually feel very fast. Part of that is because they are miserable under braking. They slide way too easily. They aren’t a particularly loud tire, like say the NT-05, and in 225 they are definitely on the quiet side. I started to understand why driver 2 thought the car had no grip on either end. The RS-RR doesn’t feel like it stops very well, so it appears to have no front grip. But once you get into a corner, it’s side grip is really good and overwhelms the thinner and harder rear tire, leading to oversteer. Mario said it was a lot of work just keeping it on track. I didn’t get to try the 615K+ rear setup, but the RE-71R rears felt pretty well planted.

While the car felt like it had better acceleration at low speeds, surely due to the weight loss, the drag was noticeably higher. This may be because the cut down doors don’t have mirrors or the wind deflectors I added. So the inside of the car turned into a parachute. It meant that top speed on the main straight was just 90-91 mph, or about 5 mph lower than usual. That didn’t stop me from having fun though. I managed a 3:43 in my few laps on track. You can see the entire stint in the video below (quality is not good because Windows 10 Movie Maker sucks. I may re-encode this on my Mac later in the week).

Sunday – race day

The forecast was wrong. We arrived at the track to find it drying. I was expecting a lot of rain early so I could one-up some fast cars but it just wasn’t very wet. Discouraged, I decided not to drive first. Danny drove first and while he was out we got our pit crew member, Tiernan, a driving wristband. He got in the car next and despite all the warnings about the blind turn 9C that connects the East and West tracks, he did what a lot of people do, and drove straight though. When he got to the penalty box, they decided to throw the book at him. My book. I had dropped off about 15 copies of the book to be sold for the Alex’s Lemonade Stand charity. Tiernan’s penalty was to read a passage from the book while being filmed. If it doesn’t make the Lemons wrap-up video, I’ll post it here.

The rain started picking up and it seemed there was enough rain to have a bit of fun. And fun was had. I got my wish and was able to dice with the fastest cars on track… and beat them.

Mario drove next and also had a blast splashing around (in the muck and the mire). But then the track started drying and he decided it just wasn’t as much fun. We wanted to get Daniel and Tiernan back in the car one more time, so they split the time on a mostly dry track. In the end, we were 56th out of 110 entries, or something like that. After we realized we weren’t in contention, we relaxed and had a lot of fun. This weekend reminds me how much fun Lemons is. That said, Lemons is changing, and not necessarily for the better. I’ll comment on that later.

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).

Lessons Earned

Guest post and 3rd place in the 2018 YSAR Author Contest. This one from my twin brother Mario. Editorial comments in red.

Lessons Learned Earned

I’ve been endurance racing for six years now, which is that middle ground where I’m no longer a noob, but I still don’t know shit. I’d like to think I’ve learned a thing or two, but if I add up the time and money spent, I think it’s fair to say the lessons have been earned, more than learned.

Alignment first – When we got our 1997 Miata it was set up with toe-out in the rear because the previous owner wanted to drift it. I found out really quick in Sonoma by spinning in T4. The instructor with me said “this doesn’t handle like a Miata.” I skipped a session, got everything aligned to zero, and it fixed everything. Another time our MR2 got sideswiped and it would crab sideways through a corner thereafter. Our teammate Ben Dawson could drive it like that, but I couldn’t get through two corners that way.

It wasn’t sideswiped. The axle broke from metal fatigue and the only replacement we could find on the day was from a previous year MR2 whose suspension geometry was slightly different. You had to saw the wheel just to keep it going straight. Mario got out of the car after one lap and said something like “I value my life too much to drive this”.

Don’t be obsessive about tire pressure – 200TW tires seem to work under a variety of tire temperatures. At a Hooked on Driving day I was at the center of a fairly volatile conversation between a Spec Miata guru and a tech from Tire Rack who completely disagreed on what my tire pressure should be. The former said to run my tires at 38-40 psi, the latter at 28 psi. The comments went from “You’ll fall off the track,” to “the tire will fall off the rim,” and back and forth.

Grassroots Motorsports recently did a 200TW tire test and tried a range of tire pressures. They found out that it didn’t matter much.

It was a bit shocking to find that the lap times were all the same. I think pressures start to matter more when you don’t have square setups. When goofing around I pump my rears up absurdly high. Also, off road so that the tires stay on the rims. 

Safety wire your oil drain bolt – I ruined half an HPDE day for everyone because the oil drain bolt came loose. If the corner workers had flagged me there would have been less cleanup, but for sure it was my fault. I now drill and wire the drain plug, and you should, too.

Get an infrared thermometer – If you’re a pro driver then you need a pyrometer to measure tire temps, but if you’re reading this blog, you can use a $20 infrared gun. It’s great for checking tire temps, seeing how hot your rotors get, checking track temperature, and various things under the hood.

MR2s are great endurance cars. Not. – Our MR2 spun every other race. No, it didn’t spin from snap oversteer like everyone wants to tell you, but it spun bearings all the time. And that requires rebuilding the bottom end. If you want to have a couple really great races and then replace the engine, the MR2 is an ideal platform. If you want longevity, look elsewhere.

Black flags matter – After too many black flags and unnecessary pit stops, we calculated the amount of time lost for a single pit stop vs different lap times. I wrote a whole blog post on this subject, but I can sum it up by saying the fastest driver with a single black flag is the slowest driver.

On some tracks where the stewards are outside the timing loop, a black flag can cost an extra lap.

Don’t trust a racing resume – We’ve had arrive-and-drives with impressive racing resumes, but they don’t mean much. One guy was slipping the clutch on purpose to keep the revs up (and bragging about that being the fast way around), and another had his hand on the shifter the whole time. He also downshitted and put our car to 9k revs, fucktard.

You need a coolshirt – We ran our first couple races without coolshirts and could manage 40-minute stints before we were a danger to ourselves and others (it was over 100 degrees and probably 114 off the pavement). One time we used dry ice to super-cool our ice, but haven’t done that since because most of the energy is locked up in the phase transfer from solid to liquid, and so it’s not worth the hassle of cooling ice further. A big block of ice is better than ice cubes.

Pit stop strategy – No battle plan survives contact with the enemy; no pit stop strategy survives the weekend. But it’s fun to plan them anyway. Also, you can make up time in the pits easier than on track. But like black flags, an unplanned stop takes more time than driving slowly.

Miata is always the answer – They handle, there’s always spare parts at the track, and they don’t break too often. They are underpowered and a bit too common, but still the answer.

Wear a diaper and a big hat – It took me a few races to get to where I could comfortably drive a two-hour stint, and when I was finally ready, my bladder wasn’t. Forty minutes into it and I was weighing the pros and cons of a pit stop vs peeing in my suit. I pitted and ruined our race strategy, but at least I didn’t soil the seat for everyone else. Now I wear a diaper (Depends) every time, and weirdly, have never had to use it.

I also wear a big hat because the sun can tire you out as much as anything. We were racing at Willow Springs one day and it was 108 degrees IIRC, and I had just finished my stint, so I was wearing nothing but a diaper and a big hat. I thought it was funny, so I texted my two sons a picture. One said “I can never un-see that,” the other said “Now I have to burn my phone.” It was so worth it.

Pit board > radios – We’ve had terrible luck with radios and headsets. We now run Boefang radios, but at the lower, and legal, 2w setting. And they still suck. Have a pit board on hand.

Bring a skateboard – Skateboards are great pit transportation, and if you have a longboard, you can carry a gas can on the front.

Don’t race on untested components – At the 24 hours of Buttonwillow, Ian put on tires we’d never used before, and the best brake pads we could find, but also hadn’t tried before (the Yaris has a dismal selection of brake pads). The tires sucked. And we ran out of brakes in the 8th hour. Of a 24-hour race. We only had one extra set of brake pads, and so the last 8 hours was pretty much downshifting and coasting with no brakes. Still placed third overall, tho!

Me culpa.

Put your most aggressive driver in last – I flew across the country for a race weekend. Our most aggressive driver went first. Nobody went next.

FWD is great – I love rear-wheel drive cars, especially in slow corners, but most of the time it doesn’t matter which wheels are driven. Some of the most fun I’ve ever had racing has been in my brother’s Yaris and Tom Pyrek’s Honda Odyssey minivan (Ninja Turtles theme). FWD is especially fast in the rain, and at the NJMP Lemons race, Tom’s rain laps were within 1% of our team’s best dry laps.

Quit racing – Ian and I keep talking about the day we sell our race cars. Then we’ll buy a couple brand new Miatas (him a Fiat) and do HPDEs and arrive-and-drives. Maybe we’ll still be saying this five years from now? I don’t know, but my wife was ready for that yesterday.

Mine too. Racing sucks most of the time.

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.

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Hydroplaning

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.

Summary

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).