The Test Track Test

This endeavour began when I was thinking about the testing processes we went through on the FSAE car. Many test runs were conducted using very specific corner radii, sequences of corners, targeting specific roll velocities or frequencies, etc. The purpose of this was to more efficiently provide validation data for specific vehicle scenarios and to fill out performance envelopes (GGV, yaw moments, shock velocity/frequency histograms, etc). Usually this extra effort meant vastly increased testing efficiency by focussing on the areas of interest and repeating them until sufficient data was collected. 

My idea then was to design a test track that fills out these envelopes in this same efficient way from the start, while also promoting good racing, being fun to drive, and being demanding but rewarding of good setup work. I specifically wanted the track design to target medium-high downforce cars (GT3, Prototypes, F4/F3) which I find the most fun to drive in simulation. My checklist for each of these four criteria was as follows:

Performance envelope:

• Mix of tight, medium, and open corners

• Opening and tightening radii corners to fill in combination acceleration cases

• Fast direction changes on throttle, off throttle, and on-brakes to test stability and the interaction of yaw moment, steering angle, and lateral acceleration

Good racing:

• Avoid short straights that serve no purpose

• Difficult corners leading onto longer straights

• Extend braking zone distances using tighter corners and/or downhill sections and/or convex elevation changes

• Reward deviation from the conventional racing line

Fun to drive:

• No short/sharp corners that require awkward, brief braking

• Space out low-speed corners around the lap, but avoid breaking up flowing high-speed sections

• High-speed "thread the needle" sections

•  No off-camber corners

• Undulations and deviations to long straights

Demanding setup work:

• Convex track shape into some of the braking zones, requiring rearward brake bias shift and promoting understeer

•  Lateral-grip limited sections during full throttle application out of slower corners to highlight power understeer or oversteer issues

• Bumpy sections leading immediately into high lateral acceleration to test damper setup

• Direction change while braking to test sensitivity to roll/pitch and promote understeer

• At least one high speed corner that can be taken at full throttle, but only with a perfect line and well-balanced car

Plugging all of these requirements together into 5-6 km looked like this:

The easiest way to test such a track was in Assetto Corsa, where a simple Blender model could be almost directly imported and work as intended straight away. My brief time learning blender to create my concept LMH McLaren helped here. The track layout above was the final result after some fine-tuning test drives, and once I was happy with the exact shape of every corner I added some basic scenery.

Track Guide

To see if the design really is good as a test track, I drove a flying lap at each of Project 05, Silverstone, Spa, and Monza. Silverstone and Spa for their variety of corners, and Monza for it's frequent use as a test track for many race car series. 

Without diving into actual performance analysis and breaking down the car's behaviour through every corner, bump, and direction change, the simplest way I can think of measuring the efficiency and completeness of the track for testing purposes is a GG or GGV plot. The results from the 4 tracks are shown below, sampled at 50 Hz. 

As well as velocity, COG acceleration values are also influenced by other factors affecting individual tyre grip, such as steering angle, slip angle, slip ratio, and yaw moment forces. Plots of various combinations of these factors revealed that for all tested tracks (apart from Monza in some cases), the frequency and distribution of various combinations of these variables were all very similar. It became apparent that to improve the diversity further, tracks would have to start sacrificing straights for more corners instead (or otherwise be a lot longer).

One specific area where Project 05 did appear to fall short was in the variety of yaw moments experienced in a lap. The distribution was much more concentrated around 0 than Silverstone and Spa, and looked similar to that of a lap at Monza. I thought I was simply missing some variety in high speed corners, where the higher downforce would be able to provide higher yaw moments, but stratifying by velocity revealed a similar trend independent of downforce. I suspect this must come down to the track surface again. Clearly, most moments at corner entry and exit lie within the 50000 range, up to 100000 for faster, sharper instances (arbitrary/unitless values in this instance). That some corners see a value of up to and beyond 500000 is inconceivable from typical cornering forces alone, and I suspect most if not all of these super-high yaw moments are from track bumps/curbs creating noise. Without going back and painstakingly modelling realistic bumps on my track, this is never going to be a fairly comparable metric. 

Based on the results, I'd say the track is certainly fun to drive, seems promising for good racing, and does a pretty decent job of being a test track to the point of being at least on par with some of the most famous existing circuits that are frequently used for real-world test sessions. 

For it to be the perfect test track, I'd want to add some elements from Spa:

• Some more significant elevation changes to test stability at atypical suspension positions. 

• Some more data in the lateral/negative longitudinal area would be nice too, (combination braking/turning) but it's already matching if not outperforming all three other tracks here. The sort of corners that lead to this aren't good for promoting overtaking either, so it's a compromise.