Knowing what your car will do without turning a wheel.
Here’s a real scenario: You’re new to autocross and want to make your car faster (just like everyone else on the track). You start talking to people in the paddock about modifications; everyone has opinions and recommendations. You start visiting different forums; everyone has their opinion there. You choose who you believe and spend a lot of your hard-earned money on the next mega gadget. And go to the track and your car is undrivable. As racers we need tools that allow us to make these decisions with more certainty than what someone else said.
Laptime simulation is a great tool for understanding how a racecar will behave on a specific track. There are many different options for Laptime Simulation, starting with simple simulations that can be created in Excel and going to the full bespoke programs that professional race teams use at the highest echelons of racing to predict the car's setup on a specific circuit when limited training time is available. The concept is, what if an ideal driver maximizes the performance of the car on the track? What would the data traces look like? What would be the time? This is very useful because it will tell you of the impact of every vehicle parameter on performance. What if I add a rear wing? What if I change the tires? What if I increase horsepower? Obviously, every level of simulation offers its own level of complexity of the vehicle dynamics model and cost.
At the simple end of the spectrum, there are tools like Optimum Lap, created by vehicle dynamics expert Claude Rouelle and his company Optimum G. Optimum Lap is very useful for basic strategic decision-making. It is also a free tool. As of late, the track database and the car database have broken links and downloading cars and tracks is impossible. But you can still create your tracks (I will write a post about how I created my home autocross at the Daytona Kart track) and create your cars (I will also create a post about how I created the model of my Challenger).
Here is a track map of the Daytona Kart track colored by the speed on each section of the track of a car driven by an ideal driver.
In the Middle, you have tools like Lapsim, developed by Chris Van Rutten in 1996, which has been kept up to date and developed. Lapsim has a free version where you can alter limited parameters, and you can also purchase licenses for the Chassis and Engine modules for about $1500 per year. Lapsim is a lot more complicated to set up, and it takes a lot more effort to set up a car, but you will get much more information about the car’s behavior. In the future I hope to set my car up in Lapsim. Lapsim’s engine module also allows you to predict the power curve changes for different modifications to your motor.
Results from two different setups and how they affect a speed trace in a lap around a track.
On the higher end of the commercial tools, there lies Chassis Sim. It was created by Vehicle Dynamics guru Danny Nowlan, who wrote an awesome book on vehicle dynamics if you want to geek out on it. The chassis sim is a dynamic simulator, so it will include the effect of the dampers on the car's behavior and the transient behavior of the car. Obviously, this means you need more car information to get the model to work but if you’re at the sharper end of the competitive spectrum, this is the same tool professional teams use. Chassis sim has an online simulator that will charge you $5 per run, which is affordable but be prepared to spend a lot of time building your car and track model.
Results from a lap of a track in Chassisim notice the resemblance to a data acquisition trace.
How accurate are lap time simulators? The answer as usual, is, it depends. The key is understanding that all models are false, but some of them are useful. You need to know what you’ll use the model for and its limitations. If you want to understand the general direction for developing your car, you need a different model than you would use to determine if harder springs will improve your time. If you have data acquisition in your car (I will write about this later), you can work hard to correlate the data and ensure your results are more accurate every time.
Ok, now let’s see how we can save $2000 USD by using laptime simulation:
Let’s say we have a hatchback that produces 136hp and weighs 1960 lbs. We race it at the Daytona Autocross track. We want to invest in making it faster and have two options, either we buy a turbo kit for the car or we buy better tires. What should we do?
We are using Optimum Lap for this as it’s the simplest to set up, and its batch run tool is super useful.
Let’s say we want to invest in making the car more powerful and want to buy a tame (10psi) turbo for the car; Turbo kits will start at about $3,000 + installation costs. We use optimum lap to sweep the power curve of the car from 80% to 120% (109hp to 163hp) and check the lap time. We obtain this graph:
Here we can see that there’s a point where increasing the power doesn’t improve the laptime at all (144hp), and even improving it to there will only earn you .02s worth of laptime. Knowing that engine mods are super expensive, it wouldn’t be worth it at all. Why? Your car is grip limited not power limited, so increasing the power will yield very little return since you can't put that extra power to the ground, at least not on this track.
Now let’s say we want to change our 400 treadwear tires for stickier tires. Street autocross classes will be limited to 200 treadwear tires. The grip in terms of lateral acceleration of a high-performance summer tire is about 10% better than the all-season 400s and slicks are about 20% better.
As you can see, improving the grip is the gift that keeps on giving. Moving to the 200TW tires will generate an approximate 1.5s improvement in laptime and going to slicks will offer an almost 3.7s improvement. Given that a set of 4 200TW tires is about $1,000, you just saved $2000 and improved your time by 1.3 seconds more than you would have by buying the turbo kit. Also since you have more grip you can now install that turbo and improve even further as the car now becomes grip limited around the 163hp mark so you can use all of your upgrade.
I am fascinated by these tools and will definitely use them greatly while developing the Challenger. If used correctly, they can save you a lot of money and grief. You can email me at conegeek1@gmail.com if you want the track model for the Daytona Kart track. I will also create posts in the near future on how to create cars and tracks for Optimum Lap so you can get this technology working for you. Comment on how you like this post and how you think laptime simulation can be useful to an autocrosser.
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