A new installment of Microsoft's marquee car franchise, Forza Motorsport, is set to arrive in October. We've not had a chance to turn any laps in Forza Motorsport 4 yet, but in anticipation of a full review in the coming weeks, I spoke with Dan Greenawalt, Lead Game Designer at Turn 10, about the studio's latest arrival and the nuts and bolts of making a driving sim.

The Forza Motorsport franchise has been the Xbox 360's answer to Sony's Gran Turismo series. The first installment arrived in 2005 on the original Xbox, and Forza Motorsport 2 and Forza Motorsport 3 arrived at two-year intervals. The series has done a great many things to innovate the world of racing games, and that trend looks to continue in the newest release.

So, what goes into making a racing game?

One of the reason's for Forza's success has been its ever-evolving physics model. Drive an actual car on an actual track, and the information you're given through the steering wheel and the seat of your pants is the result of thousands of variables: the stiffness of the car's body, its mass (both of the whole thing and the unsprung bits), the amount of power to go and stop, the design and geometry of the suspension, the ever-changing relationship between the four tires and the track surface, and so on.

With over five hundred cars in the game, Turn 10 had to somehow turn all of that into data into something that you feel through a fancy steering wheel setup or the Xbox 360 controller to give the player the feeling of turning into a corner, of tires losing adhesion and beginning to spin, or of moments of inertia.

"We do have extensive information for example on the [Ferrari] Enzo," Greenawalt tells me. "By having the Enzo, and knowing the cars we are simulating, we're able to say 'this car's suspension geometry on the front is more similar to the Enzo than the Corvette,' for example."

They started with five cars, going as far as to take them apart to measure the several hundred points of research data, from simple things like weight distribution to measuring inertia on the driveline. Over the years more cars have been quantified, some to a greater degree than others. This has built up a database that provides the starting ground for each of the cars in the game.

Turn 10 doesn't use any data submitted by the manufacturers, who might not be above sending them a ringer. But their system is surprisingly accurate. "Even though we don't have the real numbers, we've actually been able to find errors in cars through this simulation," Greenawalt claims. All this work results in around 9,000 parameters for each car that are used by the game's physics engine to simulate everything from a tiny European hatchback to a Le Mans winning prototype racing car.

For Forza Motorsport 4, the team has thrown away all its previous tire data and started from scratch. Forza Motorsport 3 had data from Toyo, Michelin, Pirelli, and FAE, and it had to be manipulated so that it would all work together, despite being gathered at different testing facilities and with different testing methods. This time the game is built with new data: custom tests from Pirelli across a huge range of variables (tire height, width, compound, temperature, pressure, camber angle, etc) that was imported directly into the game. "That was the only way we saw to be as up-to-date as possible. We weren't copying the textbooks of racecar engineers from a couple of years ago but putting in the data that would write the textbooks of the coming years," Greenawalt explains.

This attention to detail shows up elsewhere too. Racing games have been getting better at recreating the sounds of cars, but usually only on full throttle. Now, as you build your cars up from shopping-spec to highly strung race monsters, you can expect backfires, burbles and pops on over run. Other aspects have also been worked on to add to the feeling of reality; cars sound different under load—up a hill on full throttle, say—"you can hear it straining, frankly," Greenawalt tells me.

User generated content

Forza Motorsport has been famous for the strength of its user-generated content, and it truly is amazing what some people are able to achieve with what seems like pretty basic image editing tools and a lot of layers. It's been a focus of the series for a long time, and that's set to continue in Forza Motorsport 4. Greenawalt is proud of what the livery editor has allowed gamers to achieve: "A lot of people want the tool to work like this, or work like that... we made a tool that was very powerful, but that doesn't mean that anyone can just go in and slap a texture on it. The important bit about that is we're creating a class of creative that's able to show off their skill with the tools."

One of Gran Turismo 5's new features was the introduction of a track editor, and I asked Dan if we might see anything like this in the future. Imagine being able to take GPS or Google Earth data of your favorite driving road, for example, and sharing it online. Sadly, this just isn't possible right now, mainly because of that perennial Ars topic, intellectual property (IP).

It's something they've considered. Greenawalt has spent time studying the various PC games and communities that have allowed it, and there's very little interest in anything other than virtual recreations of real tracks, both former and current: "Those are things that we really can't condone—the stealing of other people's IP. The first thing that gets made is Spa, or Bathurst, or Brands Hatch. When people make original tracks, they don't do well in these communities."

As to whether Spa or Bathurst or Brands Hatch will appear in the game, I was told enigmatically "we haven't announced all the tracks yet." Cue Internet speculation in 5, 4, 3

I also asked about whether we might see easier ways of getting user-generated content out of the game. Getting pictures out of GT5 is a lot easier than Forza Motorsport 3, as you can save them locally rather than having to upload them. Unfortunately, security concerns mean that Forzamotorsport.net will remain the portal for exporting images or movies from the game for now.

While we're on the subject of intellectual property, it is sadly true that Porsche will be absent from Forza Motorsport for the first time. Electronic Arts (EA) have an exclusive license with the Stuttgart manufacturer, and in the past has allowed Turn 10 to include them, but with their move towards a more sim-based game, the studio that everyone loves to hate has just given me another reason to curse their name. When asked if the German cars could reappear as DLC if EA relented, Greenawalt told me he's said everything he's really able to say on the matter, but the situation will be continually evaluated.

It's frustrating that digital rights issues have to raise their head, but they don't dampen my enthusiasm for the forthcoming game. It's due out in the US on October 11, which means I have to go out and find a Kinect in the next few weeks; for the head-turning, mind you, not the driving-with-no-steering-wheel. And it goes without saying, you'll be able to read a review of the game here at Ars.