Test Day

This post is about a test day at Thunderhill with the Yaris. I’ll update it a few times as data comes in.

The day before

Tiernan came over today to help get the car ready. There were lots of minor things to do like make sure the radios, cold box, and telemetry were all working. Also the usual packing of jacks, tools, tires, etc. The Lemons Race is about 2 weeks away, and making sure everything works now will save us time and worry in the future.

So what exactly are we testing? Mostly tires and wings.


In the past, I’ve set the car up the way I like it and it turned out that nobody else liked it. So this time, I want to get several different drivers in the car to determine what setup everyone else likes. What we’re talking about here is the tendency to oversteer or understeer. I like a car that oversteers. That can be tuned with tire pressures, but even more drastically with tire compounds. There are 3 configurations I want to test.

  • Hankook RS-4 front and back (225/45/15). This is the “baseline fast setup”. To be sure, there are faster 200TW tires. But the RS-4 strikes a great balance between speed and longevity. It’s also the spec tire for Lucky Dog Racing League. I don’t plan on using tires stickier than the RS-4 because I don’t want the cost of switching out stickier rubber. I have rims in 15×9 and 15×8. I may test out both configurations to see if there’s a difference.
  • Hankook RS-4 front (225/45/15), Federal RS-Pro (215/40/17) rear. When I tracked my Elantra, I used Federal RS-Pro tires, which melted horribly. I still have 3 that are hardly used, and I want to try using them on the rear, where they might survive. I’m not sure what the grip balance is between these and the RS-4. Let’s see what my teammates think.
  • IRIS SEFAR front (215/40/17), Federal RS-Pro (215/40/17) rear. This is the Lemons Class C setup. It will probably understeer a lot as the IRIS tires are 300TW and the Federals are 200TW. I don’t want to go the other way around because too much oversteer in a race might not be very safe.


I wanted to test 2 different wings: eBay vs. 9 Lives Racing. I wasn’t able to get the 9LR wing mounted in time, so it looks like the test will be wing vs. no wing. In my previous testing, I thought the wing improved handling in fast corners. I’m curious what the other drivers think.


I’ll be running an AiM Solo DL and video camera all day. Check back for to see that data.


I may have as many as 4 drivers, but I’m not 100% sure who is showing up. I’m not planning on taking many laps myself, as I know the car pretty well. But I am curious to see how the RS-Pros feel on the rear and experience how bad the Lemons setup will be.

Test day

As they say, no plan survives contact with the enemy. In this case, the plan to test rear tires somewhat failed. The rear wheels were rubbing on the wheel wells, so we weren’t able to try the larger rear tires. I’ll have to cut the metal to make enough clearance. However, we were able to test a variety of tire pressures with the RS-4s and also test the IRIS SEFAR tires up front.

Weather or power?

There was a head wind on the main straight all day. This caused our top speed to be about 88 mph. I’ve seen 95 before. I don’t know if this was entirely due to the head wind or it was also a loss of power. I’ve experienced a loss of power before and found that changing the IAT solved it. I’ll have to get in there and clean it just to be sure.

Ideal tire pressures

Running RS-4s on all corners produced too much grip in the rear. Everyone complained that the car was pushing. We ended up overinflating the rears and that made a car that was pretty neutral. The final hot pressures were 34-35 front, 40-42 rear.


The $53 Lemons tires were nicely loud and produced okay grip. I took a few laps and posted a 2:26.01. This was about 5 seconds off my fast lap on the RS-4s (2:20.90). I don’t know if they will survive a whole race. Or even a whole day. But we’re going to try running these as a way of getting into C Class.

Here’s a picture of the abuse the SEFAR took after about 1 hour of hard driving.


My very expensive Li-ion battery died. It had been acting weird lately and it stopped working entirely right before the last stint. This is why we have test days. I’ll be replacing that pronto.


The first thing I want to discuss is the difference in performance when switching out the RS-4s for SEFARs. Where do you lose speed when you switch from the Lucky Dog spec tire to an obscure executive tire? Everywhere. It’s a pretty constant slope of time lost. When the corners are slower, the straights are also slower.

Now let’s have a look at the drivers. Let’s take a look at Turns 1-4.

Turn 1

Turn 1 is a fast left hander that turns up slightly. It’s a corner that requires confidence to approach at high speed. Mastering this corner is really about mastering your fear. As you can see, the drivers have different ideas about how much braking is necessary. Clearly, one can drive through it with a minimum speed of about 82 mph at 1.12G as that’s exactly what the black driver has done. However, it’s probably possible to go even faster because there are other corners where the car manages 1.15G. So why aren’t all the drivers going that fast? It’s mostly about belief. The green and purple drivers, in particular, believe the corner requires a lower entry speed. It doesn’t. How does one change a firmly-held but incorrect belief? Partly by observing data. But also through delibeate practice. I don’t suggest that someone tries to break through a perceived speed barrier in one go. Use your speedometer and gradually raise the speed as you get more comfortable.

Turn 2

Turn 2 is a large, flat 180 left. Mastering this corner is really about how you feel the grip of the tires. There are a lot of different lines one can take. Examine the black, blue, and red traces. They all have a different idea of where the minimum corner speed should be. However, they all drive through at around the same average speed and there isn’t much time difference. On the other hand, the blue and purple drivers are well off pace. Why? Because the drivers don’t have a good feel for how much grip they have. They feel the car slipping and their thought is “I don’t want to lose control”. Experienced drivers trust their car control skills to save them when (not if) they go over the limit. If you don’t have trust in your car control skills, you will always drive under the limit. Having confidence in your ability to control a sliding car is pretty much what advanced driving is. If you have the confidence without the skill, you’re a danger. If you have the skill without the confidence, you’re slow. If you don’t have either, don’t despair, because there’s a lot of fun learning ahead.

Turn 3

Turn 3 is off camber, which changes the grip from around 1.15G to 0.95G. Oversteer is very likely, especially if you brake a bit late and end up braking on an off-camber turning track (I spun in T3 in my very first track day). Again, we see a similar pattern where the purple and green drivers slow the car too much and then drive it through the corner slowly. This corner is a little bit of a mixture of T1 and T2. You have to have the confidence to drive in fast and the skill to keep it fast while the car squirms around under you.

Turn 4

The speed through T4 depends a lot on your path through T3. You really have to sacrifice the exit of T3 to set up T4. As you can see here, the red driver botched it a little, but if I overlayed a bunch of laps you would see that the black and blue drivers also make little errors every lap. We’re not professional drivers you know! I can put 5 laps within the same 0.2 seconds, but it’s not like every corner is exactly the same.


If you’re driving a momentum car, like a Toyota Yaris, you cannot afford to give up speed in braking zones because there’s no way to make up for it with the motor. And even if you had twice as much power, you would probably be understeering as you tried to make the front of the car do both the steering and accelerating. The key is optimizing your entry speeds. “In slow, out fast” is literally one of the worst things to think as a driver. It should be “in on the limit, out on the limit”. However, just because that’s the way advanced drivers navigate a corner doesn’t mean novices or intermediates can. Improvement in high performance driving is like any other sport: it takes deliberate practice and coaching. You’re not going to become a scratch golfer by playing 10 weekends a year. Not everyone needs to be a scratch golfer. I never got better than bogie golf and I’ve always had a great time. As long as your high performance driving experience is safe and enjoyable, you’re doing it right. If you have the burning desire to be an advanced driver, I offer my condolences to your time and money. However, the journey will probably be worth it in the end.

2 thoughts on “Test Day

  1. Any difference due to wing? It could add confidence in 1? I’m still working on that very thing – confidence in 1.

    I like the idea of winning in every class, but class A would likely be hard. Maybe a winter race in the wet??? that’s likely Sonoma and though fun a bit more dangerous.


    1. The wing definitely gives confidence in T1. I’m not sure it has any effect anywhere else on track. Worth it for T1 alone.

      Winning any of the classes is hard. A is going to be pretty near impossible with the fast cars that are there now. Rain would certainly help, and I agree about Sonoma.


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