Power, grip, aero…

Everyone wants to improve their car a little (or a lot). How much do power, grip, and aero improve lap times? There’s lots of anecdotal evidence out there, but not much rigorous study. One reason for that is that every day is different. Lap times in the morning can be more than 1 second faster than the afternoon in California. One way to test these parameters on equal footing is through simulation. So I decided to embark on such an activity with Assetto Corsa. Why AC? Because it’s easy to modify. I can simply edit the power.lut file to change horsepower. Aero is similarly easy. Tires are more difficult, but there are already 2 tire choices for most cars, so I did that.

The baseline car I started with is 2400 lbs, 120 hp, 0.40 CoD, 20 sq-ft. frontal area. This doesn’t represent any specific car. It’s not far off from a Spec Miata though. The track I used was Brands Hatch Indy. I almost always test stuff on this track because it has a very small layout, about half the size of most tracks, which lets me get consistent numbers in a short time. I also think the mixture of turns and straights represents the average race track pretty well.


Let’s imagine changing horsepower from 1 to 300. I didn’t do that exactly. I drove the car with 60, 80, 100, 120, 140, and 160 hp. Then I did a curve fit to smooth the data points and extrapolate from 0 to 300. The extremes may be inaccurate, but they are also somewhat unrealistic given the starting vehicle. In the graph below, the X-axis is horsepower and the Y-axis is lap time. You can see the diminishing returns with increased power. If your car has 50 hp, adding 10 more makes a big difference. However, above 200 hp, there’s very little to be gained. If your car is a momentum car, increasing the power will lower your lap times. If you’ve got a muscle car, you need to search elsewhere.


For this next study, I once again drove the car with 60-160 hp but with harder tires. The take-home lesson here is that the higher the power of the car, the more sticky tires are important. Let’s illustrate that with a couple data points. If you’re on the red tires and have 173 hp, your lap time is 57.000 seconds. Switching to the blue tires will lower you to 56.336 seconds. You could also increase horsepower to 217 to get the same lap time. It’s a lot easier switching to stickier rubber than finding another 44 hp. Let’s look at a low horsepower example. At 100 hp on red tires, the lap time is 60.454. On blue tires, it’s 59.940. You can also get to 59.940 on red tires with 106 hp. So, sticky tires could be worth 6-44 hp depending on your starting hp of 100-173.


Aerodynamics are modeled with frontal area and coefficient of drag. It’s kind of hard determining what these values are for your car. You can look these figures up online, but I’ve found really conflicting figures. You can also try to estimate these at http://hpwizard.com, which is a pretty awesome website. I urge you to check it out. Another great website is https://mycurvefit.com if you want to do some curve fitting. Below is the lap time as a function of CoD. This was actually done at 100 hp not 120 hp.

Fantasy Enduro Builds

Let’s imagine 2 builders, Mario and Ian, who decide to build endurance racers from Spec Miatas. Mario decides to perfect the aero while Ian decides to reduce weight.

Mario’s aero mods add weight, but since he’s no longer playing by SM rules, he is able to remove an equivalent weight. So his car stays at 120 hp and 2400 lbs, but his coefficient of drag is now 0.30 CoD. I kept the 20 sq-ft frontal area. The aero mods are worth ~6 hp.

Ian guts and chops his Miata, turning it into a freakish dune-buggy like thing. Same power and frontal area, but weight is just 2150 and CoD has increased to 0.45. The decrease in weight effectively increases hp by 14. But the CoD effectively decreases hp by 8. Overall gain ~6. (Note: I don’t believe simply summing up the hp losses and gains this way is very accurate, but for small values around the defaults, it’s probably okay).

So what’s the difference between these two builds on track? To test that, I loaded up Laguna Seca and input the new vehicle parameters. The two cars were nearly identical in lap times. The aero car had a very slight advantage at the end of the main straight. The lightweight car had a very small advantage in the infield. But after T6, the cars were neck-n-neck the whole way back to the finish line. It was a little surprising to me that the advantage of shedding 250 lbs, which is over 10% of the weight, could be completely mitigated by bad aero. I guess I had better add an air dam to my dune buggy.


Gems from William Chen

SuperMiata is a really interesting racing series that does things a little differently. Chief among these is that the rules are designed to save money and the competitors must share information openly.

William Chen has written an amazing document called the Unofficial SuperMiata Racing Guide. The latest version is linked on the SuperMiata FaceBook page, but I’ve copied it below just in case it disappears.


There’s so much critical info here from building to racing to towing. Surprisingly, among these 35 pages, there’s less than a page on actually driving. Let me sum what it says.

  • Intermediate drivers are too slow in corners
  • High intermediate drivers are slow in corner entries
  • You must heel-toe shift, and most beginners do it too soon
  • Messing up high speed corners is worse than low speed
  • When in doubt keep it in a higher gear
  • Trail brake: the slower you release the brake, the more oversteer you get
  • Don’t shuffle steer, leave the hands at 3 and 9

Let’s discuss these in a little more detail. I’ll rearrange them for convenience.


If you want to give the impression that you’re just learning how to play tennis, smash your first serve out of a forehand grip and patty-cake your second. You won’t have to tell anyone you suck, we can all see it. Similarly, if you want to look like you’re a novice racer, move your hands all over the steering wheel. You can get yourself out of this habit easily. Drive your street car everywhere with 2 hands on the wheel fixed at 9 and 3. Move them only briefly to shift, not to drink coffee.


First off, shift less. Those corners where you’re not sure if it’s a 2nd or 3rd gear corner? Leave it in 3rd. How about those straights where you shift into 4th for 2 seconds? Just bounce it off the rev limiter. Why? The short answer is it’s faster. Shifting takes time. It also takes action, and that action can sometimes have negative consequences. You’ll be more consistent with fewer shifts and that leads to faster and safer laps overall.

The main problem I see with shifting is people bringing their bad street driving habits to the track. Shifting on the street is very different because the car speed is low and the engine is nowhere near redline. It doesn’t matter if you blip shift or not on the street. However, some people do, and they do it way too early. Early shifting causes two problems (1) the engine is over-revved and may get damaged (2) the engine is being used as a brake, which proportions too much bias to the rear (in RWD cars), which causes spins.


Racing requires trail-braking. Unfortunately, it’s a difficult skill to practice on the street. One reason for this is that the pedal feels firmer at race speeds than street speeds. Also, if you’re going fast enough that brake release causes oversteer, you’re driving illegally and stupidly. The only way to get better at this skill is with track time. Simulators work great if the brake pedal has a load cell. If you can find an autocross where you get to drive for more than 2 minutes per day, that can help.


While it is true that messing up a high speed corner is more costly than a low speed corner, I see more variation in low speed corners. I think people are willing to be more aggressive in the safer low speed corners and this lack of consistency means more time is lost in low speed corners. Ideally, don’t fuck up any of your corners. They’re all important. Anyone can mash the throttle on the straight. It’s the corners that separate drivers.

I team up with a lot of drivers from complete noobs to SCCA regulars. So I see a lot of variation in talent and experience. Sometimes I find that experienced drivers have a lot to learn. Let’s look at some speed graphs. On the approach to T1, both the red and blue drivers get to approximately the same maximum speed. Both hit the brakes hard as indicated by the steep drop in speed. The blue line has a U shape whereas the red line has a V shape. The V shape indicates the driver has gone from full brake to full throttle quickly. If you look at where the bottom of the V is, it’s to the left of the bottom of the U. The red driver gets off brake and on throttle earlier. In slow, out fast, get to full throttle as early as possible. That should work, right? Sure, it gets you around the track, just not very quickly.

The blue driver is trail-braking. That’s why there’s a long gradual deceleration. The blue driver gets on throttle later, but maintains much more speed. Paradoxically, he’s using both less brake pads and less fuel to go faster.

Now let’s talk about T2. It’s a really long corner. Once again, the red driver brakes hard and makes the transition from brake to throttle sooner than the blue driver. The over-braking has slowed the car so much that it can’t get anywhere near the maximum corner speed. A lot of time is lost here.

So the solution is obvious, right? Just brake less and go faster in the corner entries. No, there is no just do anything here. If you take that attitude, you’ll probably run out of track at the exit and wreck the lap if not the car. Faster corner entries require trail-braking, and trail-braking is not a skill one applies simply by deciding to do it. There’s a phrase I’ve heard multiple times when people compare lap times and find themselves wanting. “They’re just driving harder”. Again, there’s no just anything. Certainly, driving faster requires some mental and physical commitment that goes beyond the casual. While you can bully your way out of being scared, you can’t bully your way to precision.


Revisited: Mirror mirror

When I first posted this, I focused blame on the POV driver who doesn’t know how to drive with 2 wheels in the dirt. Looking at this a couple years later, my thoughts are a little different. Most accidents are avoidable. Two forces generally oppose that:

  • I didn’t see…
  • I didn’t expect…

The real crimes here are (a) the slower driver didn’t see (b) the faster driver didn’t expect. The track is safer when drivers cooperate. It’s still a good idea to learn how to drive with a wheel or four in the dirt.

Mirror mirror on the wall, who’s the fastest of them all?

Who is at fault for this horror show? At first glance, it looks like it was an asshat blocking move by the silver Fiero, but most of the fault lies with the BMW (car taking video).

The BMW was much faster than the Fiero and had the luxury of choosing where to pass. On the approach to T6 at RA the normal racing line is to drift out to the left. The slower car is supposed to stay on the racing line and the overtaking car is supposed to go off line. It doesn’t make sense for the BMW to pass on the left. That was error #1. The more critical error was turning the wheel sharply while off track. When going 2 or 4 wheels off, you’re supposed to ease the car back on track. If the steering angle is too steep, this is what you get. In effect, the BMW cut off the Fiero and slammed on the brakes. Done and done-er.

What did the Fiero do wrong? Take a look at the left side of the car. There’s no wing mirror. He had no idea the BMW was approaching on the left. With better situational awareness, the Fiero driver would have expected this pass even with just a rear view mirror. But the Fiero team handicapped their driver for unknown reasons (aerodynamics?), and the team paid the price for their oversight.

Safety first. Learn to drive with a wheel or four off. And keep the mirrors.

Revisited: Tossing the nannies

Today I was looking back at some old YSAR posts and thought it was time to revisit some of my favorite posts and add a little commentary. “Tossing the nannies” is one of my favorites because it’s about braking. Braking is an incredibly important skill. I’d say it’s the most critical skill there is in driving because the brakes are used for turning as well as scrubbing speed. Most cars come from the factory with safe brakes, but when racers start mucking about with pads, prop valves, and ABS-delete, things can go poorly.

Most cars have enough brake power to lock their wheels. So your stopping distance is generally a function of the tires and road surface, not the brake pads and rotors (but of course these matter because of heat and feel). One aspect of braking that is often overlooked is brake bias. When you step on the brake pedal, some of the brake pressure goes to the front, and some to the rear. But it’s not the same amount. As the car slows, weight transfers from the rear to the front. The front tires gain load and therefore gain grip. Not only does it make sense to bias the brake pressure towards the front to take advantage of the added friction up there, it’s also a lot safer. If the rear tires lock up first, it can lead to a spin. Like this.

Why did this car spin while decelerating in a straight line on a dry track? The short answer is that real racers toss the nannies. No self-respecting racecar driver chooses an automatic transmission. It’s a stick or stay home. Real racers do their own throttle blipping too. Traction control? And lose the ability to do burn-outs? Surely you jest. Power steering? For weaklings. Stability control? Might as well have gramps drive for you. Anti-lock brakes? Everyone knows that threshold braking requires the tires to slip and ABS doesn’t allow that. Real racers turn off or permanently delete the nannies.

This racing team acted on their correct understanding that on a wet track, the overall traction is lower, so the weight transfer from rear to front is less pronounced. You can stop in a shorter distance on a slick track with more rear brake. So they adjusted the bias towards the rear. Nothing wrong with that, it’s what the pros do. But then the track dried, they didn’t readjust the bias, the driver locked the brakes, and the car was wrecked (which is not what the pros do).

There are several ways to deal with this situation.

  1. Don’t mess with the brake bias. It comes from the factory with a lot of front bias. Brake a lot earlier on a wet track.
  2. Pit the car when the track conditions change and make the necessary adjustments.
  3. Make the brake bias adjustable from the cockpit (this assumes the driver knows when and how much to adjust).
  4. Keep the ABS nanny. What you lose in braking distance you make up for in safety.

Going from a wet track to a dry track isn’t the only time you may experience too much rear brake. Racing tires have more grip than street tires, so they transfer more load to the front. When you swap your street tires for R-comps, you probably want to reduce the braking in the rear. If you don’t have a prop valve, you can put a more aggressive brake pad on the front than the rear.

Arrive-n-Drive Rules

Every race, my team almost always has someone new to the squad. While there is some risk to this, I have met some really great people along the way, so I intend to keep doing it. What risks are there? Mostly that the driver abuses the car in some way. I can recall one driver whose shifting was so abusive that I literally could not watch the video. Every 10 seconds I would cringe as he over-revved the engine. I could only watch for a few minutes before I shut my eyes and hit command-Q. Should we have done a better job vetting him? Clearly. But he had a decent racing resumé. Better than mine anyway. It’s really hard to determine who is going to be a great endurance racing teammate until you’ve watched video of them driving your car and hung out with them at the track. Some people are so good off track that you want them to be permanent members despite their shortcomings. This week I decided to write a post about how to be an ideal arrive-n-drive teammate on Triple Apex Racing (or whatever team name I happen to be using that weekend).

  1. Safety first
  2. Don’t be a jerk
  3. Save the car
  4. Do more than your fair share
  5. Have fun
  6. Drive fast

6. Drive fast: Why is this #6 on the list? Because a racing team is a complex entity and there are so many things more important than your lap times. Anyone can learn to be fast. It’s easier to learn how to drive fast than to be a decent human being. You can learn the one in a year and the other eludes people for whole lifetimes. Being a decent human being is the core of what makes a good teammate.

5. Have fun: It should go without saying that we’re driving race cars for fun. We certainly aren’t being paid to do it. In fact, we’re paying hundreds of dollars every event, so it had better be fun. But I see people with sour faces all the time. That’s probably because people are focusing on the outcome rather than the activity. You’re in a racecar at a race track! Do you realize how special that is? Not everyone gets to be so irresponsible. What would life be like if there was no racing? Horrible, right? So love every second of it. Both good and bad attitudes are infectious. Spread happiness.

4. Do more than your fair share: Racing cars takes a huge amount of time and money before getting to the track and lots of work once you get to the track. There’s always something to do. Check the tire pressures, lug nuts, oil level, fuel cap, etc. Tug on some hoses to make sure they aren’t loose. Organize the pit space. Practice getting in and out of the car quickly. Bring food and drink for other people. Do whatever you can to make life easier on everyone around you. Don’t stop when you’ve done your part. Meet life more than half-way. And if something happens on track, take more than your fair share of the blame. Don’t seek to lay blame on others. You’ll be the better person for it.

3. Save the car: When you’re out on track, you’re making thousands of decisions every lap. If there’s no one else around, you’re probably deciding on the best way through a corner (at least subconsciously). And when you’re in traffic, you’re trying to decide how to position yourself to greatest advantage. On our team, “greatest advantage” means not crashing the car. Every little bit of contact matters. A little rub can very quickly turn into a cut tire. And there goes any chance of winning. “Save the car” also means driving it in such a way that you prolong its life. That means not running over high berms or over-revving the engine. Thousands of dollars and hundreds of hours have gone into your time on the track. Treat the car like you understand that.

2. Don’t be a jerk: Our team prides ourself on not getting into trouble on or off the track. We drive “courteously-fast”. Racing is competitive, and there’s nothing better than a hard-fought battle between equals. But we do not cheat and we do not purposely put other cars in danger. If you are overcome by “red mist”, get off the track. Our team has no place for that. Drive with a cool head or don’t drive. If someone does something dangerous near you, you will not retaliate in kind or attempt to “teach them a lesson”. But feel free to chuckle after they take themselves out of the race.

1. Safety first: This is a blanket statement that covers many topics from the garage to the paddock to the track. Since this blog is mostly about driving, let’s focus on the that. Here’s a quick list of don’ts from the second you light the ignition.

  • Don’t speed in the paddock
  • Don’t go over the blend line as you go onto track
  • Don’t scare novices by driving too close to them
  • Don’t attempt to go faster by braking later and harder
  • Don’t pass in corners

And here is one thought to put you in a safety-oriented mindset

  • The race is safer with you in it because you look out for the safety of the other drivers

Saving the brakes

If you’ve been following along, you know that last week we placed 3rd in a 24 hour endurance race and that we had to make a set of pads last as long as possible. With 2 drivers left, there was about 2 mm of pad remaining. So the plan was to save the brakes. The next driver in decided to completely ignore those team orders and drive as fast as possible. How do I know? By looking at the telemetry. I can also see that he needs some coaching on how to drive. So let’s take a look at the traces. You’ll probably want to open this in another window to see the details.

The blue and red lines are driver #1 (going as fast as possible). The green and black lines are driver #2 saving the brakes (green) and driving faster on the very last lap of the race because there were no brakes to save (black). The panels are speed, RPM, throttle, and time differential going top to bottom.

  • At 1100 and 1400 feet, the red line speed graph hits the bottom of the graph. Driving so fast that one runs off track (1100 ft) and spins the car (1400 ft) in the same lap were not part of the plan and could get you pulled from the car.
  • The plan was the green line. Notice how the slope of the line goes gradually down at 8500, 10000, and 13500. Driver #2 is coasting into braking zones. As you can see from the RPM trace, he’s using the gearbox to help slow the car. Normally, that’s a no-no, but in this case, we wanted him to do that. My favorite part is that he hits the same minimum speed at 9000 ft on his fast and slow laps. He has planned out how to arrive at the same speed at the most critical corner without using any brakes. Well done.
  • The blue and red traces generally follow the black trace on the speed graph. The speeds of the black line are higher everywhere, which is why the lap time was 2:29 rather than 2:32. The deceleration slopes are similar. This indicates that driver #1 was hitting the brakes hard. Nooooo.
  • Do you see the weird throttle blip at 1300 feet on the red/blue lines? That’s not a downshift, but an upshift. Is that some kind of slam shift technique I’m not aware of?
  • Look how the black line gets to 100% throttle and stays there. That’s how to go fast in a momentum car. There should be very little time at partial throttle in the Yaris. The red and blue lines are often at partial throttle. This is because the driver lacks confidence when the tires are slipping in a corner. Tires are supposed to slip.
  • Driver #1 has some misconceptions about how to drive fast. It’s not about braking as late and hard as possible. The fastest drivers did 2:26 on this track. You can’t make up 6 seconds by braking late. You have to enter faster, back up the corners, get to 100% throttle before the apex, and leave it there until the next corner.

So driver #1 has some learning to do. We all do. The car finished higher than it had any right to so I still wouldn’t change anything about the weekend. Except maybe eat another bowl of tortilla soup. I should have done that.

Race Report: Buttonwillow 24HR

I wouldn’t change a thing…

That doesn’t mean everything went to plan. It didn’t. And that’s probably why it was such a special event.

  • 340 treadwear tires. I was feeling cheap and didn’t want to spend a lot on tires. Having such a slow car, I figured we had no chance at doing well, so why bother with fast rubber? We’re here to do our first 24hr race. It doesn’t matter if we’re competitive. All that matters is we’re there.
  • Unknown brake pads. There aren’t many brake pad options for the Yaris. I’ve had EBC Greenstuff before, and they melted. I had G-loc R10s, but they only lasted 14 hours. So I decided to bring Hawk HPS and EBC Redstuff. There isn’t really anything else unless I get them custom made. The HPS were down to the metal after 8 hours. We thought they would last least 12 hours, but no. So we were a little unprepared to do the brake swap when the car came in. We had no idea how long the Redstuff would last and needed them to go 16 more hours. So we coasted into braking zones, used the gearbox to help decelerate, and scrubbed speed with decreasing radius entries. At the end, we were scraping metal to metal.
  • Inexperience. 4 of our drivers had never driven Buttonwillow before the race. 4 of our drivers had never driven the Yaris. 5 of our drivers had never raced at night. 2 of our drivers were on their 2nd wheel-to-wheel race.
  • Street legal. The car is street legal and smog legal. It has the unleaded restrictor in the filler neck. It takes forever to refuel. I think the fastest pit stop was about 10 min.
  • Radios. I don’t know why but the radios hardly ever worked.
  • Live streaming. The live stream from the car was surprisingly robust and was up most of the race. It was fantastic watching the car from the pit or clubhouse. The video below shows my favorite moment of the race. I had just spent the first 90 minutes saving the brake pads and then decided to chase down some faster cars for fun. A back-n-forth battle with the race-winning Miata, the 949 Racing Honda, and the Death Race pickup ensued. It ended when I hit the 2 hour limit.

Controversy or Compliment?

Lucky Dog rules state that your car has to be at least 15 years old. Otherwise it goes in the Super Dawg class where the really fast cars can play outside the standard classing. The last time we raced with them, they classed us in the C (slow) class. And that’s the class we ran this race… until we got protested multiple times and sent to Super Dawg. Apparently some other C class cars didn’t like that we were doing so well. Wait a second. We have 100 hp, 2500 lbs with driver, 340 TW tires, fragile brake pads, and insufficient lighting. Apparently that doesn’t mitigate our 2007 model year. The design is basically the Echo, which is from 2001 or something, but heavier. FOR FUCKS SAKE, learn to drive your car. It’s not like we’re racing gods on this team. OK, that’s not quite true. We had Pablo Marx driving with us, and he is a racing god. But that was 1/6 of the team, and the rest of us are mortals. Protesting us was probably the highest form of compliment anyone could have given us, so whoever you are THANK YOU. You have no idea how happy this made us.

Oh yeah, we finished 3rd place overall. Congrats to Risky Whiskey on the overall win from the C class. Well deserved.