Greenawalt on Tires

With Microsoft®'s hugely anticipated racing sim Forza Motorsport™ on the horizon, we decided to sit down with the lead Forza designer Dan Greenawalt and pick his brain on an important component of the upcoming title, tire wear. Here's the dirt we dug up.

Xbox.com: During development of Forza, how important was it to make tires and tire wear a strategic element for the players to manage? In other words, why expend such effort on this particular aspect of the car? What benefits did it yield?

Dan Greenawalt: It's not possible to overestimate the importance of tires. Tire grip affects every part of how a car drives—braking, accelerating, and turning as well as responsiveness and load transfer. Load, pressure, temperature, wear, camber, compound, and size all affect the tire's grip. It's an incredibly interrelated system. If any one aspect of the system is wrong, the whole system crumbles. The physics behind simulating tires makes turbo and exhaust gas physics look like child's play. The bottom line: If you are making a simulator, you have to go "all-in" on the tires.

Xbox.com: Makes sense. Can you describe the tire track development process? What went into the research, and how did the Xbox® hardware make it possible to achieve the kind of realism you were after?

Dan Greenawalt: Other simulators such as Gran Prix Legends use Pacejka's Magic formula (yes, that's the real name) to model the physics of their tires. This was the first option we used for Forza Motorsport. After implementing it, we found that this formula works well for most cases. However, it doesn't work perfectly for the specific tires we wanted to build (tires based on real-world data we collected from manufacturers). It was close, but we could not match Pacejka's results with the specific confidential real-world data we had access to from Toyo, Ferrari, and other sources. We elected to start from scratch with our tire model. In the end, we spent well over a month creating the tire model and at least three months tuning and retuning it to match the real-world data. Our system is more expensive on the CPU side, but it let us tune more predictably and accurately. As a result, our tire model is far more realistic in how it performs and reacts to stressors. We created tires that perform very realistically. The heat, wear, and deformation are based on real-world test data.

Xbox.com: Sounds like the work really paid off! In terms of gameplay, how important are the state of your tires to your car's overall performance?

Dan Greenawalt: In gameplay terms, if your tires are worn or overheated, you have to brake earlier and take corners at a slower speed. Tire wear reduces your average speed around the track and thus lowers your lap times.

Xbox.com: Not to be ignored, then. When managing tire wear, which performance elements are most important?

Dan Greenawalt: As far as the state of the tires, there are five dynamic components that can compound to slow your lap times. These five components are load, pressure, temperature, camber, and wear. Pressure, temperature, and wear are self-explanatory. You want the tires to be at optimal pressure (~32psi for most of the tires), while at optimal temperature (180-210 F) and wearing evenly. You also have to watch the tire's camber against the ground. Load is a bit more complicated. If you don't drive smoothly, the tires can become overloaded and that load can become unbalanced while turning. In turn, this unbalanced load can induce either oversteer or understeer through a drop in coefficient of friction. Since the tires are a system, depending on how you use the tires, all five of these components can hit you at the same time. Even great tires that are overheated, over-inflated, overloaded, worn, and in positive camber can feel like ice.

Xbox.com: Hope you are taking notes, Xbox racers—there'll be a quiz later. Graphically, how is progressive tire wear presented to the driver? What does a fresh tire look like compared to a worn one?

Dan Greenawalt: Forza does not feature open-wheeled race cars. As a result, there's no way for you to see the tire degrade. Instead, we furnished the player with a Tire Wear HUD. Tire wear is integrated into the car damage HUD that appears on the right side of the screen in all views. This HUD can also be called up with the white button on the Xbox controller. Pressing the white button again (when the HUD is active) cycles to the Tire Wear HUD.

Xbox.com: Simple and informative. What are some of the effects of worn tires on racing? How are performance and gameplay affected? How are worn tires reflected in the Tire Wear HUD?

Dan Greenawalt: Well, at first wearing (scoring) your tires makes them perform better. Quickly, though, the wear stops adding grip and just holds at the same grip level for a while. About halfway through the tire's life (yellow on the Tire Wear HUD) the wear starts to diminish grip noticeably. Eventually, grip drops through the floor (when the Tire Wear HUD is red). The peak slip angle is also affected by wear. You'll feel peak slip in a tire's responsiveness.

Xbox.com: Yellow is okay, red is bad (or bald, as the case may be). Got it. What kinds of real-world tire manufacturers licensed their products for Forza Motorsport?

Dan Greenawalt: Toyo, Hoosier, Pirelli, and Bridgestone.

Xbox.com: Big names in the industry. Are brands and tire models important to know for racing in Forza?

Dan Greenawalt: Though we licensed tire brands and based the tire physics on specific brand compounds, we did not want to force players to understand the difference between, say, two different Y-Rated or DOT-Spec tires. Thus, we don't offer different brands as options in the game. The goal was to give the player great tires at all levels.

Xbox.com: No steep learning curve is good news. In terms of realism (both visual and performance), what do you think is the game's biggest success when it comes to the tire tracking component?

Dan Greenawalt: Load sensitivity. Load sensitivity is the phenomenon where rubber becomes less efficient at returning friction as more load is applied. As a result, a tire with more load returns more absolute friction, but less proportional friction than the same tire with less load. In driving terms, a heavy car has more friction going through a corner, but a lighter car turns higher lateral G's. Therefore, a lighter car with everything else the same is able to take the corner faster because of load sensitivity. Very few games (though they call themselves simulators) actually model load sensitivity, that is, load sensitivity in non-Newtonian physics. It's not taught in high school physics classes, after all. However, it's absolutely imperative in simulating a tire. Without modeling load sensitivity, a tire will not function correctly as weight transfers. The car just won't drive "right" and you have to hack in the effects of weight.

Xbox.com: Great insight. Any tips for a driver that finds himself running on bad tires? How can you compensate for negative impacts mid-race?

Dan Greenawalt: Drive slower, don't lock up the brakes, and don't peel out. Use the pit to replace your tires.

Xbox.com: Good to know. On a more personal note, what kind of car do you drive in real life? Any upgrades alaForza?

Dan Greenawalt: Forza upgrades are based completely on the types of packages a tuner shop would put on your car to transform it from stock to lightly modified, from lightly modified to heavily modified, and from heavily modified to race ready. My car is actually in the shop right now getting some more upgrades put on it. I drive a lightly modified 2000 Audi S4 (the B5 platform with the 2.7 bi-turbo engine).

My upgrades are focused on the car's handling rather than power. Any fool can speed down a straightaway and enter a corner fast; it takes skill and tuning to exit a corner fast. When it returns to me, my car will have a short shifter, track coil-over suspension, 18" ultra-lightweight rims, wider tires on all four corners, rolled fenders, 22mm rear sway bar, big slotted brake kit, conical air filter, and some better plumbing.

Xbox.com: Most impressive! Thanks so much for chatting with us today. See you on the Forza tracks!

Comments about this article?