August 3, 2024, Second Autocross

After trying autocross, I was back. Now, I needed to get better. For this event, I had two goals; the first one was to improve my driving. I wanted to get a video of my hands during a run. I don’t have data to track my steering angle, so this is the next best thing. I invited a passenger to run with me and record me while I drove. The second one was to start trying to understand the tires. For this, I bought a tire pressure gauge and a temperature sensor. Racing sensors were too expensive, so I got a generic one off of Amazon. This one has a probe that you can pinch the tire with, as well as an IR measurement tool. In this event, I used the IR. I also made a change in my driving; by looking at the speeds from the previous event, I realized that through the whole track, I was within the powerband for second gear, so after launching, I shifted into second and completed the runs in second. I am now having doubts about that strategy (based on my Optimum Lap simulations that show that letting the transmission choose the gears was 0.2s faster) but that is the choice I made. On the next race I will leave some time for a back-to-back test to see which is faster.

I will go in-depth on all the things I did, and then I will share a summary of each run:

A simple explanation of the tire:

The only external forces acting on a race car when it's not hitting anything are the forces on the four contact patches and the aerodynamic forces. As we saw in a previous post, tire coefficient of friction is about ten times more impactful to the performance of the challenger than aerodynamics, so having the best car that I can for these runs relies on pampering those four contact patches and making sure they provide the largest amount of grip. There are two mechanisms for a tire to generate grip. The first one happens at the molecular level when the rubber in the tire adheres to the surface of the pavement. The second one comes from the physical deformation of the tire around the physical bumps and mounds in the asphalt. In order to maximize the grip, we need to create conditions where both are working in synchrony.

Each tire rubber compound has an optimal temperature where it generates the best molecular adhesion and where it is soft enough to deform around the asphalt. If you’re operating below this temperature, there won’t be enough grip, and if you overcook it, you will lose the molecular adhesion. As a rule of thumb, the softer the tire, the smaller the ideal temperature window for the tire will be. The size of the contact patch is also important; having a small contact patch concentrates all of the cornering energy in a very small area, which overworks the tire and makes it lose grip. If a tire is overinflated, it will balloon, meaning the contact patch will be reduced, and we will lose grip. On the other hand, if the tire pressures are too low, the tire will rest on its sidewalls, and we will not be using the center of the contact patch.

This is why the two first things we look at are temperature and pressure. Pressure also has a vehicle dynamics effect with slip angles, and I will look into understanding this in my next autocross by using the tire as a tuning tool. However, in this session, the focus was on ensuring that I understood how my tires behaved in terms of temperature and pressure.

To do this, I did two things. The first one was to measure the pressure before and after each run. The second one was measuring the temperature of the tire (I used the laser, but there is a large opportunity for discussion here) in three points of the tread: the outer, the middle, and the inner. The ideal temperature is taken inside the carcass and actually, racing tire temperature sensors are wireless inside the wheel and measure from the inside, but an IR pyrometer is cheap and easy to use between runs in a tight schedule. These measurements are part of my setup sheet, which you can download here.

A rule of thumb is that if the middle temperature is equal to the average between the inner and outer, your tire pressure is at a point where grip is maximized. If it is below, your tires are under-inflated; if it is above, you are over-inflated. Racers also mark their tires to look at tread wear. I tried this, but the tire polish I used when detailing the car made it so the tire markers slid off the sidewall, giving me an unreliable reading. I will make sure I clean the silicone off of the tires next time and share a little more about that.

This event was run in a very similar layout compared to the previous event, the only difference was the slalom at the start was replaced by an s-turn. Also, the track had a new surface that worsened as the day progressed; we saw a few spins when people got offline and lost grip. The weather was similar to the first session, with an 85-degree sunny day. This time, I could measure the track temperature, which was at 103. I started with 38psi at the front and 36 at the rear

Layout used.

Notes from event transcribed to setup sheets.

Run 1:

My time on this run was 54.124. It was a good run, and I felt like I was able to drive the car smoother than in the previous autocross. I knew I had a few opportunities to break a little later, but I was getting the feel of the track. I am still getting the hang of the launch and spun my wheels a little too much, and I feel like I lost some time there. I could also feel the car was understeering in the corner entry and then I corrected it with the throttle. The open diff made it lose a little on the corner exit when I did this. In the future, I need to make the car turn better and be more patient before getting on the gas. Looking at my tire pressures, my fronts gained 2psi, the rear left gained 3psi, and the rear right didn’t gain any PSI. Looking at the temperature, I noticed the tires cooled pretty quickly; the front left was always the tire I measured first, and that’s why it was the highest. I need to measure the cooling time of the tires in the next event. The front temperatures were significantly higher than the rears, which validates my understeer hypothesis.

Run 2:

My time was 53.308, a big improvement, mostly from a better launch and better braking into 3. I started with 35psi on the rear as I was looking to get the car to turn in better. The car improved, but it can use more rotation. From temperature readings, the front pressures were spot on, with the front right working a little harder. Pressures also remained pretty stable during the run with just a 1psi increase. The video of the run, which shows my hands in several places, shows that I’m struggling to get the car turned in. I will try to be smoother in the future.

Run 3:

Time was 52.319. I think it was mostly due to smoother driving. The pressures increased by 2psi front and back. The front left was worked harder, this is because I leaned on the car more on the sweeper before the last hairpin.I think I have more grip available and can lean more on the car in the corners. I feel like I braked early into 3 and 7.

Run 4:

My time was 52.806, would have been in the 51s for sure, but I overcooked it braking into 3, and locked it up. The rest of the run felt great. I still have a little understeer. Messed up the apple notes where I wrote down the temperatures so lost that data. Will use the setup sheet in the next event to make sure my info is safe from clumsy hands.

Run 5:

My time was 52.297, the best time of the day. I braked early into 3 as an overcorrection from run 4. I feel like the track lost grip as the session progressed, so this was probably the slipperiest run. I also messed up the note-taking for the temperatures and pressures.

Key points for the next event:

Use the setup sheet for complete records
Get data acquisition, particularly throttle sensor and ideally steering angle
Run back-to-back shifting and no-shifting runs.
Get good videos of runs for better analysis between runs
Build better event strategy with questions to answer.

Event results:

I ended up placing 5th in the novice street class, which was a good showing in the top third of the field. I was 3s away from the lead of STX, the class I am targeting to participate in after I graduate from novice.

Here is a link to the complete results.

Photos from the event: