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Motor vehicles come in a huge variety of shapes and designs each tailored for a specific purpose.
Whether that be simply getting you from A to B in comfort and safety, travelling around
a well paved race track in the shortest time possible, crossing difficult terrain, or sliding
sideways around a corner.
Each have been carefully crafted by their engineers to excel in their disciplines.
Last month Ubisoft invited me to the Red Bull Ring in Austria to drive in 4 incredibly different
vehicles.
The KTM x-bow, an ultra-lightweight track car, driven by a professional driver.
An 0ff-road buggy, a Land Rover defender, and a Porsche Cayman S, driven by a complete
idiot.
All this to get a better idea of how these vastly different vehicles perform in real
life, and to discuss with the programmers of Ubisoft’s new game, the Crew 2, about
their process for programming their vehicles how to behave in game.
“So, my name is Stephane Janjowski and I’m producer at Ivory Tower, Ubisoft Studio, and
the Crew 2 obviously.”
Speaking with Stephane, I started to get a much better idea of the relationship between
real world physics and in game physics, and just how developers programme vastly different
vehicles like these to perform like their real life counterparts.
Unlike real life, where boy racers love to destroy their cars with modifications that
negatively affect performance, the in game engine separates the physics model from the
3D model.
“When we do the 3D model that’s one part of the vehicle and then we have the pure physics
model, that is seperate from the 3D.”
The physics model creates a model where things like the wheels are independent physics objects,
with their own friction and mass modelled.
The separate 3D model allows the developer to pick and choose what cosmetic changes to
the vehicle affect performance, for example spoilers are not treated as aerodynamic surfaces
in the Crew 2, so your inner idiot can apply spoilers on front wheel drive cars to your
heart’s content.
One of the few cosmetic features in game, that does affect the performance of the vehicle,
is the suspension.
Suspensions are a system of springs, shock absorbers and linkages that connect the vehicle
body to it’s wheels, with a goal of keeping the tires in contact with the ground while
minimising the transfer of that motion to the vehicle body.
This provides provides comfort for the passengers of the vehicle, not much of a concern for
a video game, but they also determine how the vehicle will handle bumps and corners.
Off road vehicles have incredibly soft suspension.
This allows them to run over rugged terrain at high speed without transferring too much
force to the body of the vehicle, but with one significant drawback.
While cornering, or driving over slanted ground, a large amount of weight of the vehicle is
shifted onto one side of the car.
With suspension this soft the vehicle is extremely susceptible to roll, as the body is fairly
free to tilt upon the wheels, combine that with a fairly high centre of mass and it’s
a disaster waiting to happen, especially when you put an attention seeking australian vlogger
in the driving seat.
And while you can’t roll-over your vehicle in game, as the physics engine applies a torque
to car, when it rolls too far, but it does allows enough roll for the suspension model
to behave realistically, which feeds into traction physics model.
Cars designed for tracks will always have stiff suspensions.
This helps them keep their tires in contact with the ground, ensuring traction is not
lost in corners through tilting, while maintaining a consistent ride height, an important characteristic
for vehicles like the x-bow, as it’s undercarriage forms a significant part of it’s downforce
generation though it’s rear diffuser.
It’s important to maintain a low centre of gravity too, as this minimises that rolling
effect we saw earlier.
The one draw of back stiff suspension like this is ride comfort.
I was in the passenger seat of the cross-bow for a couple of laps, and boy do you feel
every bump.
You can see the effect of suspension stiffness and weight distribution, which is simulated
in the physics model accurately, on traction very well in game with front engined, rear
wheel drives like the Ford Mustang GT, that is particularly easy to drift around a corner.
By lifting off the accelerator and hitting the brake at the apex of a turn, the g-forces
of deceleration causes the heavy front end of the vehicle be pushed downwards, lifting
the rear axle and causing the rear tires to lose grip, thus initiating over-steering.
This is a lot of fun, and the penalties on speed in game are fairly miniscule, but in
real life this is something the designers of racing vehicles want to avoid as you cannot
apply full power if your wheels are slipping and you lose forward velocity by sliding sideways,
so most racing cars are mid engined to maintain grip on the rear tires during braking, and
keep the suspension stiff to ensure the balance between tires is as close to the ideal as
possible.
Keeping the weight evenly distributed between the front and rear tires in corners is important
because it creates a vehicle which does not over or understeer.
Over steering being drifting, where the back wheels lose traction, and understeering is
where the front tires lose traction, diminishing the steering tires ability to dictate the
direction of the car.
Allowing momentum to take over.
In a race you want neither, but this is steer a video game, so you can adjust both your
front axle grip and your rear axle grip to encourage these effects, this slider simply
adjusts the coefficient of friction being applied to the tires in the in game physics
engine, which adjusts the traction force the tires can apply to the ground.
Traction is one of the primary tools the developers will play around with to change the feel of
the game, as this is the one that dictates how the vehicle will interact with the road.
More realistic games, like simulators, will do their best to model traction as realistically
as possible, but may give you optional driving assistance to make the game easier, for example
at higher speeds the max steering angle will be limited to help with overtaking with clumsy
thumbstick controls.
Leading to many cars behaving like understeering cars, where if you take corners at too high
speed you will end up crashing into the barrier.
But without the driving assist the result would have been pretty similar anyways, you
may have just spun a few times instead like I did when driving the Porsche with the traction
control off.
You just need to learn to manage your speed in corners better.
Games aiming for an arcade feel may do away with this mechanic and separate traction from
steering mechanics all together, allowing you to take corners at any speed with no worry.
The Crew 2 lands somewhere in the middle, it’s obviously an arcade game.
You can take ridiculous jumps like this in a Ford F-150 Raptor race truck at 180 km/h
and land without destroying your car, and then take a turn take a turn at 200 km/h on
a dirt track without losing lateral traction.
The traction here has just been boosted to make the game easier and more fun to play
Next up let’s look at how the power of the engines are handled.
Now obviously the computer programmers do not simulate an entire engine and transmission
system to determine vehicle performance.
Take this Porsche 911 GT3 RS, in this clip it does 0-100 km/h in 3.2 seconds, pretty
much identical to it’s real life counterpart, even with the nitrous boost.
The programmers want the relative performance between vehicles to be relatively accurate,
as it would make no sense if a Mazda RX-7 was beating a Porsche 911 GT3 RS in acceleration
and top speed.
To understand how acceleration and top speed are programmed, we first need to examine what
forces are acting to slow the vehicle down.
The top speed of a car is primarily dictated by the drag force acting on the vehicle, which
is something we examined before in my helicopter versus car top speed video.
The equation for drag force is provide by this equation, and the equation for power
is simply force times velocity.
By rearranging these variables we get this equation for top speed, which the physics
engine uses to limit the vehicle's top speed.
The programmers can adjust the top speed of each vehicle simply by adjusting it’s max
power, coefficient of drag or frontal area.
If they really wanted to they could change the air density with elevation on the map
too.
Adjusting the acceleration is slightly more difficult as the acceleration is determined
by several factors which vary with the vehicles speed.
The primary variable that are concerned with is wheel torque, which varies dramatically
for internal combustion engines across different engine rotation speeds and gear ratios.
The force applied by the wheel to the ground, if there is no tire slip, will vary with the
torque applied divided by the wheel radius.
The wheel radius is just one part of the internal gearing system that needs to be considered
when calculating the force the wheels will impart on the ground.
The vehicle will have several gears it can cycle through at different speeds to change
the applied wheel torque and will have one permanent drive ratio that is determined by
additional gearing in things like the differential.
All these parameters will be programmed into the in game vehicle, and the final wheel torque
will be determined by this equation.
Where Te is the engine torque.
The engine torque varies across different revolutions per minute of the engine, and
we define that variation with a torque curve, like this one for a Porsche Cayman S. The
programmers will again actually input the vehicles torque curve into the vehicle parameters
to ensure if performs like the real deal.
Designing games like this is always a delicate balance between realism and fun.
The developers don’t always go for ultra realism, programmers will often play around
with their world’s physics variables like the lateral traction to give a more arcadey
feel with exaggerated drifting, or simply to make the car easier to drive around corners
at high speed.
Or like in this game you can switch from being a plane to being a boat in mid air.
There is no real ad read here, I simply want to say thank you to Ubisoft of inviting me
out to such an exciting event and allowing me to chat with the programmers of the game.
If you are into car video games, The Crew 2 is out now.