That early prototype eventually
evolved into 2005’s acclaimed Forza
Motorsport, an Xbox exclusive that
ultimately became one of Microsoft’s
flagship franchises. But it wasn’t until
2017, with Forza Motorsport 7, that it
finally made its debut on PC, along
with the spin-off Horizon series.
Over the years the game has changed
in a number of ways, but the core
driving physics that underpin
everything are still based on the
design philosophy established by
that original prototype – and
Greenawalt and his team have never
stopped refining it.
Over the years Turn 10 have
conducted research projects in
collaboration with car and parts
manufacturers to reveal more about
how their products behave on the
track. “We’ve worked with partners
such as Calspan, Pirelli, Audi, and
McLaren,” says Greenawalt, “and
through them we’ve learned new
things about physics that aren’t in
those reference books.
“Nobody else is looking into this
level of simulation. Even the F1
simulators are researching one car,
not the broader set of physics
required to make all cars.”
“We commission these research
projects and it makes our simulation
better every time,” he adds. “So when
you’re driving a car in Forza Horizon
or Forza Motorsport, you’re driving a
physics engine that has over a
decade’s worth of research and work
put into it, with some of the top
manufacturers in the world.”HEAVY LOAD
One of the most satisfying, but
difficult to quantify, aspects of Forza
is the feeling of weight you get when
you drive one of its cars. The coders
at Washington-based developer Turn10 are somehow able to take a
light-touch interaction, nudging a
stick on a controller, and make it feel
like you’re moving a heavy object
around a track. I ask Greenawalt how
they create this illusion.
“In physics we have a thing called
the moment of inertia,” he explains
after warning me that he can talk
about this stuff for hours. “When a
skater is spinning they spin quickly
when their arms are close to their
body, but slow down when their
arms are outstretched. Their weight
didn’t change, but the distribution of
their weight changed, and that
determines how quickly they spin.
“Cars are heavy objects, and the
longer the car is, the heavier it is on
its outside axis, determining whether
they feel light or heavy.”
The traditional method of making
cars feel weighty in driving games
involves adding to the moment of
inertia. A larger moment of inertia
means the object is more resistant to
changes in its motion. “But that can
make the car feel like a battleship,
understeering more than it should,”
says Greenawalt.
“Most cars understeer naturally
with how they’re set up on the
suspension, but this largerBOTTOM: There are
over a thousand cars
to choose from
across the series, all
with individually
tuned handling.
MAIN: Whether
you’re in a hatchback
or a monstrous F1
car, every vehicle in
the series feels
unique to drive.Books like this, explains Greenawalt,
are the foundation of rigid body
physics simulation, which is the basis
for every kind of driving-based sim,
from videogames to the bespoke F1
simulators used by McLaren and
Ferrari. “We started from that same
foundation, but we also wanted to
make a game that anyone could play.”
Greenawalt has been working on
driving games for most of his career,
from Project Gotham Racing to
Midtown Madness. To make the
driving in those games fun
developers would go into the base
physics layer of the game and adjust
it. “But when we were prototyping
Forza we didn’t think you had to do
anything to that base layer,” he says.
“We could just add things on top.”
During testing, Greenawalt
noticed people liked to tap the
analogue stick of the controller in the
opposite direction of the turn to
incrementally adjust their angle. “If
you go full opposite lock with the
steering wheel every half second like
this, the car is gonna be out of
control,” he says. “So we put a buffer
between the controller and the
physics layer.” This buffer
compensates for the player tapping
the stick, ensuring the car doesn’t
spin out when they do.
This design philosophy extended
to assists; options that make the game
easier to play, such as automatically
braking the appropriate amount at a
turn. “Some of our assists are based
on real-world cars,” says Greenawalt.
“Today, a Toyota Camry or a Honda
Civic is probably over 200
horsepower, which is how powerful a
top of the range Ferrari was back in
the 1960s. But they’re totally
controllable because of the assists,
and we believed this would work the
same way in our simulation.”Forza Motorsport
FEATURE