Not long ago I wrote some software that would predict how fast a car can lap a track given vehicle data (mass, power, grip) and a track model. I was reluctant to distribute it or even work on it because it wasn’t something I wanted to support. Thankfully, I discovered Optimum Lap, which does everything my software did plus more. This free piece of software is really fun to use once you understand the interface. Optimum Lap let’s you design a car with just a few parameters.
- Power (entered as torque at various RPMs)
- Tire grip expressed in G
- Tire radius
- Transmission and final drive ratios
- Drag coefficient
- Frontal area
- Down force
Here’s a screenshot showing how I entered data for my B-Spec Yaris. I’ve selected the driveline model. This shows that the gearing isn’t matched well with the engine. Either the gear ratios need to be closer together, or the engine needs to rev higher. Well, it’s not like a Yaris was designed for racing, so that’s understandable.
Optimum Lap has a track database containing a several common tracks, but it turns out that it’s not very difficult to make your own with Google Maps. That’s good because at least in one case, their tracks are really bugged (Laguna Seca). Here’s an example of a couple tracks I made (New York Safety Track and Pineview Run).
Mario and I have been playing with this quite a bit over the last couple months as a way to imagine what modifications we’ll make to our race cars or fantasize about what my next car will be. Let me take you down that highway a little.
I used to have a 1986 BMW 325es. I loved that car, but it was getting a bit old so I gave it to Ben and his mates to turn it into a racecar. I got to drive it again at Carolina Motorsports Park and it was like putting on an old pair of jeans: comfy. I’ve driven a bunch of cars on track since then, but I just keep coming back to wanting a BMW 3 Series as a track car. But which one? What does Optimum Lap say about each?
Optimum Lap predicts an e30 325is will lap Thunderhill in 2:15.67. The e36 is about a half second faster at 2:15.10. There’s no way I could get an e30 M3, so let’s look at later models. The e36 M3 is at 2:12.45 while the e46 M3 is at 2:10.59. How fast do I need to go? I actually don’t care that much about speed. Any of these will be fine. But what’s interesting is what happens when you change tracks or change parts of the car.
Mario lives in New York not too far from Pineview Run (PVR), which YSAR reviewed last Summer. PVR is a twisty little driver’s track that is an absolute hoot to drive. While the distance is much shorter than Thunderhill, the lap times are even more compressed. The difference between the E30 325is and E46 M3 isn’t the Thunderhill 5 seconds, but 0.63 seconds. Over-rotate and screw up your steering correction and it could cost you 3 tenths of a second. It’s easy to make up for those mistakes with power at Thunderhill, but not PVR. That means lap times at PVR are going to be more about driver than car. Speaking of lap times, PVR recently announced their own time trial series, and it has a unique classing system: tire rating only. There are 3 classes, R-comps, 200+ treadwear, and 300+ treadwear. The differences among the cars become even less when tires get harder. In my simulation, the difference between an E30 325is and an E46 M3 is just 0.31 seconds. If you want to know who has the fastest car, mount up some R-comps and go to Watkins Glen. If you want to know who is the best driver, go to PVR with 300 treadwear street tires.
Final Drive Swap?
After I gave my E30 away, one of the performance modifications the new owners made was to put in a higher final drive ratio. I considered doing the same thing for my Yaris. The immediate change is that you get more acceleration in lower gears. That might help for drag races at the stop light, but the reasoning for doing it on a race track is that you can use the higher gears where the ratios are closer. That way you can be at optimal power more often. In order to do this, I would have to source a Scion xB transmission, crack it open, and take the final drive gear and swap it into my Yaris. Sounds like a good deal of labor. What does Optimum Lap say it’s worth? 0.03 seconds at Thunderhill. That’s the equivalent of losing 9 lbs. It wouldn’t even be noticeable and yet I’ve heard drivers who swear it’s the best thing to do to your car. Whatever.
Weight Matters. Power Matters More
One of my recent performance goals for the Yaris was to lose 200 lbs. That’s worth about 1.15 seconds. Pulling another 200 lbs out of a car isn’t exactly easy, but it would be worth another 1.26 seconds. You know what else is worth 2.41 seconds on this car? 15.3 hp. It’s a lot easier to add power than remove weight. Swapping in a Corolla motor would give me 30 hp and a drop of 4.27 seconds. This is why many of the successful endurance racing teams do motor swaps or forced induction.
Grip Matters Most
One of the best ways to improve lap times is by increasing grip. This can be done with tires or downforce. While both parameters are available in Optimum Lap, I don’t really know how to estimate aerodynamic parameters accurately. So I’ll focus on tire grip. Assuming tires have a grip of 1.0g, increasing to 1.1g reduces lap time by 2.1 seconds and decreasing to 0.9g increases lap time by 4 seconds. The more power you have, the more you need grip to take advantage of it. If everyone raced on 400+ treadwear tires, lap times would be more about driver skill than power:weight ratio. If I was ever going to design a racing series, that’s the first rule I’d make. Cars would be cheaper to run and budget builds would have a better chance against expensive builds.