You Suck at Aero

Guest post from Mario. Editorial comments in red.

Every year I make the same New Year’s Resolution: 1) drink more water, and 2) stretch. It’s simple, free, and I still can’t fucking do it. Same thing every year. At the start of 2019 I had a crazy vision to do a bunch of aero tests at Watkins Glen. I didn’t call it a resolution (because then I wouldn’t have done it), but I resolved to do the tests just the same.

I wrote the story of Real World Testing Miata Aerodynamics on my website occamsracers.com, and so I won’t reiterate that here, but I thought it would be good to give my brother’s readership a fresh angle on the tests, and recap some of the high points. Or low points, as it were.

The resolution I make every year is “talk less, listen more”.

You Suck at Watkins Glen

The three biggest obstacles to testing at Watkins Glen are weather, Armco, and the combination of weather and Armco.

Ian and I grew up 25 miles from Watkins Glen, and have suffered a lifetime of events that have been hampered (or is that hammered?) by weather. Upstate NY weather just plain sucks. Nevertheless, Watkins Glen is an historic track, attains high speeds, and has a great back straight for testing aero. So despite knowing better, that’s the location I chose. Of course I got bit by it.

The night before the test it rained, and the track was still wet track in the first session. I took the opportunity to rush home to get some more aero parts, while the rest of the test team sent the car on track for some initial shake downs. When I arrived back at the track, I see three guys literally hammering on the hood of my car. What the fuck did I just miss?

Apparently they left the hood pins unengaged, and as soon as Anthony entered the track, the hood smashed flat against the windshield.  Thankfully the window didn’t break, so he got to make a full parade lap looking through the gap under the hood. We borrowed a hinge from Evan’s Miata, but we got the hood fixed. (In the tests, the bent hood probably affected the coefficient of drag, but at least it was the same for all of the testing.)

After we got the hood fixed, we had a 90-minute delay due to thick fog. So it wasn’t until nearly noon that we got out first test done, and then the track closed for an hour to break for lunch. So here it is 1:00 pm, and we haven’t done shit yet.

The other thing that’s special about Watkins Glen are the steel guardrails, or Armco. You see these on the sides of roads, and on race tracks that don’t have sufficient runoff. Watkins Glen paints their Armco in a distinctive light blue color that’s not dissimilar from Gulf livery.

Cool little 914 in Gulf Livery at Watkins Glen

Whenever someone crashes at Watkins Glen, it’s a long delay. WGI doesn’t do hot tows, and so they have to close down the track to bring out the tow vehicle and clean-up crew. The Armco virtually guarantees that any crash is a wreck. I know this first hand – I came together with an E30 in an AER race and we bounced off both the inside and outside walls of Turn 6. Both cars were kaput for the rest of the weekend. Armco is not kind.

In a weekend that started with delays, we really didn’t need any more delays. But when you mix a wet track and steel guard rails with impatient drivers in race cars, you get more wrecks and more delays.

Those delays meant we didn’t get to test everything I brought. Most significantly, I didn’t test a stock front end vs R-package lip vs airdam vs airdam and splitter. I also wanted to remove the mirrors and see how much that affected drag, do more open-top tests, and other etceteras. Maybe next time. Maybe never.

If you want all the details, check out my site, occamsracers.com, but I’ll recap the high points of the test here.

Really, go check out the site. This post has only a small fraction of what is there.

Open top

I see a lot of convertibles with rear wings: Miatas, S2000s, Corvettes, etc. I’ve often wondered about the effectiveness of a wing with an open top, and now I can answer that question. On my Miata, the open top generated the least downforce and reduced the effectiveness of the rear wing a lot – to the tune of 2.5 times less downforce than an OEM hard top. But that doesn’t mean you shouldn’t use a wing with an open top. If you run simulations in OptimumLap using my data, the open top with a 9 Lives Racing wing beats any combination without a wing every time.

Chop top

The Treasure Coast “Chop Top” is a partial mold from the OEM hard top. It’s primary purpose is to enclose the cockpit so that you don’t have to wear arm restraints when racing. It also helps aerodynamics slightly by reducing drag and lift. When compared to an open top or OEM hard top, the chop top is slightly faster.

However, once you add a wing, the Chop Top performs barely better than an open top. This is interesting, because you’d think airflow over the roof is considerably smoother than an open top. However, it’s what’s happening on the underside of the wing that’s more important, and the Chop Top roof can’t defeat the turbulence coming from the open sides of the cockpit and going beneath the wing.

I enclosed the sides of the Chop Top, and that makes it about the same thing as using an OEM hard top without a rear window. And so if you have a hard top and you’re not using a rear wing, removing the rear window will make you go a bit faster. Likewise, if you have an OEM hard top and a wing, don’t remove the rear window.

OEM hard top

The OEM hard top generated more drag and lift than expected from published data. This is likely due to the open windows and wide canopy, which turns the cabin into a parachute. The drag is supposed to be around .38 with closed windows, but we measured over .5. Lift is also supposed to be around .30-something, and we measured in the .5s again. Sucky.

All told, if you run simulations in OptimumLap using numbers from the test, the OEM hard top is only a bit faster than an open top, and actually gets beaten by the Chop Top.

However, once you add a wing, the OEM hard top wins by a lot. It’s all about getting clean air to the wing, and most importantly, beneath the wing, and the hardtop crushes them by a wide margin.

Adding AirTab vortex generators reduced the effectiveness of the wing by about 20%, and increased drag substantially. If I can save just one poor soul from adding vortex generators to their top, this test was kinda worth it.

DIY Fastback

My fastback uses the Chop Top for the roof, to which I attached a long sloping back. It’s quite light, weighing about 17 pounds less than the OEM hard top, and when you consider there’s no trunk, it’s lighter still. But weight isn’t why I made a fastback, I did it because it looks cool.

I really had no idea how well it would work, and I don’t mind saying I’m pretty impressed. The fastback beats up every other top and takes their lunch money. Compared to the OEM hard top, the fastback made 20% more downforce with the wing. I suspected that cleaner air to the wing would help, but I didn’t imagine it would be that much.

The fastback also reduced drag by 15%, which not only helps top speed, but fuel economy. Combined, the downforce and drag created a lift/drag ratio that was 50% better than the OEM hard top with a wing.

With the wing removed, the fastback was less impressive. It still beats all of the other tops in endurance racing simulations, but the lap times weren’t that much faster. Also, every top with a 9 Lives Racing wing beat the fastback without a wing.

9 Lives vs Cheap Wing

I tested two wings, a 9 Lives Racing “Big Wang” and a cheap Chinese double decker wing. I had to modify the cheap wing a lot to make it work. Nevertheless, the 9LR wing simply outperformed the cheap wing in every way possible, and it only contributed .03 to the coefficient of drag. The cheap wing, on the other hand, was like dragging an anchor.

There is some reason to use one of the cheap wings, however. At autocross speeds, where drag is inconsequential, the wing helps. I ran a simulation using the 2010 SCCA Solo Nationals West course, and the double decker wing was three-quarters of a second faster than without a wing. The 9LR wing was another .5 seconds faster than that, but still, a cheap wing is better than nothing.

Splitter

I intended to try four different front ends. OEM, R-package front lip, Supermiata style airdam, and the airdam with a 4” splitter extension. I already mentioned the many delays, and so the only front-end test I got to do was airdam vs airdam and splitter. The splitter made more downforce, adding .38 to the coefficient of lift, and reduced drag by .01. It’s clearly a win-win situation, use one.

Simulations

I keep referencing OptimumLap simulations because it’s the best way to use the comparative data. Nobody drives every lap exactly the same, the track changes ever lap, and so does the weather. In the end, it’s hard to quantify real-world lap times. Case in point: we had an 11 mph headwind at one point, and that totally skewed the data until we corrected for it. If we hadn’t run environmental sensors, I’d be telling you to buy a cheap double-decker wing. As it is, I’m telling you not to.

I put the various aero combinations into OptimumLap and ran endurance racing simulations at Watkins Glen. You can find those here. Watkins Glen is a high-speed track and drag matters more here than just about anywhere else. If I re-run these simulations at different tracks, the margin between the various configurations are a bit closer, because drag factors into it less.

You still suck at aero

If I come off sounding like I know a lot about aerodynamics, it’s just me regurgitating various things I’ve read. I still suck at aero, but I’m learning as I go. When I look around at other amateur race cars, I see a lot of other people suck at aero as well.

Here’s a quick recap of dumb shit I see all the time.

  • Exposed front tires are a large source of drag. Cover them.
  • Splitters without dams.
  • Splitters that are too flimsy. They should be able to support your body weight.
  • Wings with too much angle. The roof creates downwash, and if that angle combined with your wing angle is more than about 10 degrees, you’re making the wing stall. That means less downforce and more drag. A wing at zero degrees still creates a lot of lift and not much drag. Check it out, planes fly around like that!
  • Wings assembled incorrectly. I saw a Lemons team that had the wing on backwards. Not intentionally, but because it came from China that way. For realz.
  • End plates on the wrong way. The low pressure zone (the important part) is low and forward on most wing shapes. Cover that part with the end plates.
  • Wings set too low. If air can’t get underneath the wing, it isn’t a wing. Get it roof height, at least.
  • Dual wings with gaps that are too large, don’t converge, aren’t adjustable, or are otherwise defeating the purpose of the second element.
  • Cockpit venting done wrong. This is things like installing vents at the base of the rear window, thinking that air will go out. It goes in.
  • Removing weight at the expense of aero. This one is aimed at my brother, Ian. He was chasing weight and enlarged the openings in his front windows and removed the rear windows on his Yaris. This made his hatchback into a drag chute, and we lost 4 mph on the front straight at Thunderhill.

I admit to being (a) curmudgeonly and (b) skeptical about aero. The curmudgeon in me hates race-inspired cosmetic enhancements from fake factory air ducts to stick-on vortex generators. The skeptic in me wants someone to “show me the data”. It’s difficult to model the cost/benefit of various aero components when it’s so difficult to measure exactly what they are doing. You need really expensive equipment and someone who knows how to operate it. So Mario hired someone with his own money and actually got the data. Fuck’n-A.

I’m no longer a skeptic. I still hate ricers though. And while the benefits of a sorted aero package are absolutely clear, I prefer cars with terrible aero. Back in the 60s, sportscars looked like WWII airplanes, and in my mind that’s what they should look like. I don’t give a shit if the shape has a CoD of 0.5 and generates lift. Now you may be wondering why I race a Toyota Yaris, which has a pretty clean CoD and dog-shit looks. Half of that answer is that it’s the cheapest car to run. The other half is that beating the snot out of other racers is more fun when you have no business doing it.

Track Report: Palmer Motorsports Park

My wife is a PhD student in history. Yeah, that’s right, I’m 52 and my wife is a graduate student. We’re in the Boston area for two weeks, and while she’s studying in the Harvard archives, I’m off playing with cars.

Wait, don’t get the wrong idea. She isn’t a trophy wife. I’m not that old, and she’s not that young. She’s actually 6 months older than me and started her PhD at 50. I think that’s pretty cool, but this is a blog about racing, so let’s go there.

I’m racing in a Lemons event at Thompson this weekend, and it turns out there’s a Track Night in America event at nearby Palmer the previous day. So being the “gotta have the track sticker” kind of people we are, my brother and I decided to hit Palmer the day before Thompson.

The drive from Boston to Palmer is mostly boring, but once you leave the I90, you pass through lots of quaint little villages with historic buildings and bridges with bubbling brooks beneath. I wanted to stop and have some tea and biscuits more than once.

The event was well organized with minimal fluff or hassle. Mario arrived in his RV with Miata in tow at 4:00, just in time for the Advanced/Intermediate drivers meeting. Amazingly, we went to the meeting, unloaded the car, and got me on track by 4:20. While I had never driven Palmer before, I had done some laps in rFactor2 a while ago. Not all maps are created equal and my memory of the virtual track wasn’t all that clear, so it’s hard to make an accurate assessment. So instead I’ll make an inaccurate one.

  • There’s a lot more elevation in real life. This is pretty much true of every virtual track. It’s much harder to sense elevation in 2D.
  • The real-life camber seemed greater in both the on-camber and off-camber turns.
  • Geometrically, it is not a difficult track to learn because most of the corners are pretty tight and very long.
  • Because of the changes in elevation and camber, every corner has a different level of grip.
  • The track is more of a roller coaster than just about any track I’ve ever driven.

I put in two back-to-back 2:03s. Here’s the faster one.

 

In just about every session, someone drove their car into a tire wall. Or maybe it just seemed that way. Palmer isn’t very forgiving of people who don’t know their limits, and the short distance from track to tire wall to boulders means that small missteps become big missteps.

So how do I rate Palmer? I’ll put on my Professor garb and give grades.

  • Location: B – It’s a little out of the way and there are some narrow, low-speed roads. But at least the scenery is pleasant.
  • Facility: C – It’s functional but minimal from the sheet metal buildings to the mostly gravel parking lots.
  • Track: B – I love all the elevation and camber, and there are a few interesting compromises. But the corners are all pretty tight and very long.
  • Safety: B – The course has very little runoff anywhere. The tow trucks were efficient.

Third Gear No Brakes

Cross-posted from Mario’s Occam’s Racer. Editorial comments in red (at the end).

I don’t know a lot of car drills, and in fact I only do one: “Third gear no brakes.” Leave it in third gear (or fourth on a higher speed track) and don’t touch the brakes, that’s all there is to it. I learned this exercise from Keith Code back when I was a motorcycle journalist for Moto Euro. We did an article on the California Superbike School, and Keith made us do this drill for two sessions at Sears Point (Sonoma), in the rain.

Third-gear-no-brakes is a great way to focus on entry speed, and you absolutely have to use reference points. You will eventually scare the shit out of yourself, but after that, you’ll be surprised how fast you can go.

For example, here are two laps, the red is me, the black is my friend Jim. We are in the same car on the same tires, and he is .2 seconds faster than me. But if you look at the traces, he’s shifting and braking, while I’m staying in 3rd gear the whole time, never touching the brakes.

If you look at the time graph along the bottom, you can see I make up most of the time in the middle of the graph. This is the “knuckle”, a triple-apex corner. I have to shut off the throttle right at the end of the corner, and that long sloping line is me coasting downhill, waiting for the blind hairpin. At the same point, Jim’s trace looks like a mountain, with strong acceleration upwards and hard braking coming back down.

Jim isn’t a bad driver, he’s taken racing classes, and has raced wheel to wheel. He has strong inputs behind the wheel, and an aggressive driving style. But he slows down too much and my Miata doesn’t have a lot of power to overcome that.

This is what third-gear-no-brakes looks like from the cockpit. It’s not very exciting. I edited out the part where I went three wheels in the dirt!

One of the most common phrases you hear at the race track is “in slow, out fast”. I like making fun of this phrase because it does more harm than good. While I could mention that again here, I’m going to examine another common myth: “the most important corner is the one that leads onto the longest straight”. At Pineview Run, the “S Trap” leads onto the main straight. This is a super-slow left-right combination that is positioned at about 3600-4100 feet in the graph above. The black driver goes “in slow, out fast” and ends up with superior speed on the main straight. Yay. This leads to a couple tenths advantage at the end of the straight. Big fucking deal.

Now let’s look at something that turned out to be more important than the straight: the S Trap itself. Going into the S Trap, the black driver has built up a nice cushion (you can see this in the time graph as the big red hump). The reason for this is clear, the red driver wasn’t using any brakes on the approach and consequently has a huge speed disadvantage. From 3250-3750 the black driver is going a lot faster. But all the time gained in this 500 feet is lost in the next 200. Why? Because contrary to popular opinion, the slow parts of the track are critical. Going 1 mph slower in the slowest corner of the track is a lot more costly than 1 mph slower in the fastest corner. Why? Simple math. Going 99 mph in a 100 mph corner is only 1% off but going 49 mph in a 50 mph corner is 2% off. Given that slow corners tend to have long arcs, you can spend a lot of time going slowly.

The 3rd gear only exercise is one of the best things you can do on a practice day. With the focus on momentum rather than engine, your minimum corner speed will be higher. Like all drills, this isn’t the end of the story. You don’t want to drive like this all the time. If your minimum corner speed is too high, you will have to lift at the exit. Not only does this make your lap times longer, it’s also dangerous. But you can’t get to the end without getting through the middle, and 3rd gear only is an important part of the middle.

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.

Winning B Class: part 1, fuel consumption

When I built my 2007 Toyota Yaris for racing, I aimed it at the SCCA B-Spec rules. Only after competing in an SCCA sprint race did I realize that sprint racing is not for me. It’s much more expensive per hour, the “win every corner” mindset makes it more dangerous, and it’s lonely not hanging out with a team. With that in mind, the decision was clear: re-build it for endurance racing. Sadly, it’s a little too slow for most applications. In Lucky Dog, it’s slower than most class C cars (when it doesn’t get protested for being too new). In ChampCar, the build is 120 out of 500 points so there are plenty of points to work with. But in order to compete it would take an engine swap or forced induction. Given that I want to keep it emissions legal in California, these options are mostly out of the question. Neither World Racing League nor American Endurance Racing league run events out West, so the target is Lemons. In 24 Hours of Lemons, it would probably be placed in class B. Could we win the B class with a little luck and a lot of planning? Well, this post is the first in a series where we document our efforts.

So what are our advantages? Reliability and economy. Unlike half of the cars in B class, we have a very good chance of running the whole 14.5 hours of a typical race (8 hours on Saturday and 6.5 hours on Sunday). However, we will be competing against much faster cars. We need to be on track as much as possible. This means zero black flags, of course, but it also means as little time as possible in the pits. In fact, we’re hoping to cut out one pit stop.

Most endurance driving stints are 2 hours or less. Lucky Dog and ChampCar actually limit drivers to 2 hours. Lemons has no such rule. However, most cars burn fuel fast enough that they pit between 1.5 and 2 hours. That means that a typical team will run 4-5 stints on Saturday and 3-4 stints on Sunday. I believe our best chance to win means driving only 3 stints on each day. The question is, can a single tank of fuel last 2 hours and 40 minutes on Saturday?

Our previous racing at Thunderhill, Laguna Seca, and Buttonwillow shows that the Yaris burns about 4 gallons per hour. With its 11.1 gallon fuel tank, it should be able to run 2:45. That’s no problem for Sunday but Saturday could be. If our calculation is off by 10%, we might find ourselves running out of fuel, and there would be no chance of winning if that happened. So we need to figure out how to extend our range.

The simplest answer is to install a fuel cell. That would instantaneously solve the range problem but would bring up new problems. They’re expensive. It would require removing the stock fuel tank and fabricating a new structure. The car would also no longer be street legal. The center of gravity would be higher. Too many negatives, so I’m not getting a fuel cell. Lemons does not allow one to modify OEM fuel fillers, so I can’t increase capacity with a fat intake tube either. So if we can’t increase fuel capacity, we’re going to have to increase efficiency.

Economic driving

Who knows how to get the most miles from a tank of gas? Hyper-milers. I’m sort of a closet hyper-miler myself. On the street, I often drive under the speed limit, conserve as much momentum as possible, pump my tires up pretty high, and draft trucks on the highway. I don’t go as far as making aerodynamic improvements though. But we will on the racecar. However, that’s a topic for another day. Today we are going to consider the act of racing more conservatively. The driving can’t change so much that we do more harm than good, though. We have these two connected questions to consider.

  1. How much fuel do we save by changing our driving style?
  2. What style of driving optimizes our chance of winning?

To answer these questions, I’ll be using Assetto Corsa, Brands Hatch, and the NA Miata. While I do have a Yaris model for Assetto Corsa, I don’t think it’s very accurate. The NA Miata is one of the highest quality models and besides, Miata Is Always The Answer. The car is loaded up with 5 liters of fuel, “Street” tires at 30 psi, max camber, and zero toe.

So let’s define a few different driving styles.

  • Hard – Hit the brakes hard. Hit the throttle hard. Steer like a mad man. In slow, out fast. Brake in a straight line. Shift at red line (7k). Lots of amateur racers drive like this, especially those in powerful cars. Clearly we’re not considering this, but I wanted to investigate the efficiency of a typical sucky racer. Intensity 9/10ths. Intelligence 3/10ths.
  • Soft – Conserve momentum as much as possible with early apex lines. Coast slightly before braking zones. Shift at 6K and choose a higher gear if there’s any question. Steering corrections are unnecessary driving like this. Intensity 5/10ths.
  • Enduro – Drive fast but with a lot of margin for error (not much yaw). Shift at 6.5k. Use lots of trail-braking but only a little brake-turning. Intensity 7/10ths.
  • Sprint – Drive faster with plenty of yaw. Still keep some safety in reserve. Shift at 7k RPM. Intensity 8/10ths.

The most interesting finding for me was that Soft driving increased fuel economy by an amazing 40% over Hard driving while having nearly identical lap times. My typical Enduro style results in decent fuel economy and speed. I’m only about 1% off my Sprint pace but my economy is up 13%. Compared to driving Soft, Enduro is 3.3% faster at the cost of 22% less economy. So which style is best for endurance racing? Is it better to drive slowly to get a tank of gas to last 148 minutes, drive as fast as possible while only getting 103 minutes, or something in between?

Style Laps Fast Median Laps Minutes
Hard 10.3 63.96 64.18 98.88 105.8
Soft 14.5 63.84 63.97 139.2 148.4
Enduro 11.9 61.31 61.91 114.2 117.9
Sprint 10.5 60.84 61.29 100.8 103.0

The track is live for 480 minutes on Saturday. But not all of those 480 minutes are hot. Lemons does live towing and when there are several tow trucks on track at once they will fly full course yellows. Sometimes that goes on for 5 minutes and sometimes for 30. I recall one race where they threw a red flag and I waited nearly 20 minutes with the engine off. It’s hard to predict how much of the 480 minutes are green and how much are yellow. So we need to investigate what happens with 10 to 120 minutes of yellow flag time, which results in 470 to 360 minutes of race time.

The next thing to consider is how many driving minutes there are. The car isn’t lapping when it’s in the pits. My calculations use a pit stop time of 10 minutes. It doesn’t take that long to fuel a car and change drivers, but Lemons pit stops occur in the paddock, outside the timing loop on the track. So every time you pit, you lose 1 lap in addition to transit time.

Taking into account lap times, fuel burn, yellow flag time, and number of pit stops, we arrive at the table below. I have highlighted the driving style that produces the most laps in red.

Yellow Hard Soft Enduro Sprint
10 401 412 426 420
20 392 403 416 411
30 383 393 407 401
40 373 393 397 391
50 364 384 387 381
60 364 375 377 372
70 355 364 368 372
80 345 356 358 362
90 336 347 348 352
100 327 337 339 342
110 317 328 329 332
120 308 318 319 323

When Enduro beats Sprint, it does so by 5.67 laps on average. Conversely, Sprint beats Enduro by 3.67 laps on average. The difference comes down to how many stints there are. Enduro sometimes runs one less stint, and when it does, it has a huge advantage. It doesn’t impede lap times that much and has the added benefit of reducing fatigue and the chance of a black flag (which pretty much guarantees we won’t win B class). Driving Soft never wins. It can be as much as 10 laps better than driving Hard, and there are a few situations (highlighted in blue) where it is better than Sprint. But it never beats Enduro. There isn’t much point in driving super Soft. The hyper-miler in me wanted that to be useful, but it isn’t.

Telemetry or it didn’t happen!

The line colors are:

  • Red Hard
  • Blue Soft
  • Green Enduro
  • Black Sprint

The panels from top to bottom are:

  • Brake pressure
  • RPM
  • Speed
  • Steering angle
  • Throttle
  • Time delta

Click on the image to open it in a new window and then follow along with the text below.

There are 4 braking zones (top panel). In Soft style I only applied brakes in 2 of these. Note how low the RPMs are in general. You can also see long periods of coasting in the 5th panel (throttle). But the speed graph (3rd panel) isn’t that terrible. Driving economically is a kind of intellectual challenge, which is why I hyper-mile in real life. I have to do something to make street driving entertaining.

Hard style sees me sawing the fuck out of the steering wheel (4th panel) and mashing the brake and throttle pedals mercilessly. The brake trace (top) shows early and hard application of the brakes followed by no trailing pressure. Just on/off. It’s not a fast or economical way to drive.

There isn’t a huge difference in driving style between Enduro and Sprint. I use more brake pressure in Sprint mode to turn the car and I also choose a lower gear in a couple of places. I consciously take an earlier apex line in Enduro to favor momentum over engine.

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).

Coaching in an Exocet

Have you heard of an Exomotive Exocet? I’ve sort of drooled over photos of these and other kit cars for some time. They look so damn cool. Exocets are built from Miatas, which are a personal favorite. While Miatas are pretty light cars, tipping the scales at around 2200-2300 lbs, an Exocet is only around 1500. At the start of the day, I was trying to choose between an ND Miata and an Exocet. I haven’t been in an ND on track yet, and was eager to check one out. I imagine I might own one some day. But how often do you even see an Exocet? Almost never. So I decided to choose the student with that car. It turns out it’s not his car, but a buddy’s.

My first experience with the car was on the skid pad for the figure 8 drill. Given that the car had the stock 1.8L engine, it wasn’t a tire burner. But the student did get to practice some oversteer and recovery. The car felt safe and fun. Then I went out with the owner in the B session and changed my mind entirely. I had an open face helmet, and the wind was really uncomfortable at 100 mph. It was so loud I couldn’t audibly  communicate with the driver. It didn’t feel at all safe. I made the mistake of hanging onto the frame at one point and got thwacked by a piece of rubber. Ouch.

I decided I couldn’t coach in that car unless I made some kind of change. I switched to a closed faced helmet and put Senna bluetooth radios in both helmets. What a difference. Problem solved. In the first on track session with my student, I drove 2 orientation laps to talk about the flag stations and such. So how does an Exocet feel? Not so different from a Miata actually. It was a little too light in the rear, which doesn’t make for an ideal car for a novice student. The major problem I had with it was the position where the A pillar bar meets the floor. The tube ends up very close to the left foot and there’s no room for a dead pedal. I’m really used to anchoring my left foot on a dead pedal. What was I supposed to do with my left foot? Hover over the clutch? Rest under the clutch? Who builds a frame that prevents proper foot positioning? Exomotive does. Maybe they expect you to reposition the pedals to the right?

Building an Exocet costs in the neighborhood of $20K and takes hundreds of hours of labor. In the end you get a unique car that is a real head turner. But let’s be clear, it’s not the ultimate track weapon. If you’ve already got a Miata, you could run circles around an Exocet with a turbo upgrade. But for the price of a Miata and turbo, you’re also be in the neighborhood of other excellent track cars like Acura RSX Type S, BMW E46/Z3/Z4, Nissan 350Z, or Porsche Boxster.

Over the course of the day, my attitude about the Exocet evolved as follows

  1. Too much wind
  2. Not as fast as I would have thought
  3. A lot of time and money went into this…
  4. It’s like an asphalt dune buggy
  5. This thing is actually kind of awesome

There are a lot of reasons for owning a sports car. I don’t think any of them are about how practical the cars are. In fact, some of their appeal is their lack of practicality. The main appeal of sports cars, in my opinion, is how they make you feel. That feeling should be special. And if you’re the kind of person whose mantra is “built not bought” then an Exocet is going to make you feel doubly special. I would never build an Exocet myself. I’m a driver, not a builder. Heck, I’m not even a sports car enthusiast. But there is a part of me that is 100% on board with what this represents.

Thanks Colin and Mike, the Exocet made my coaching day special.