Whether you’re a fan of riding roller coasters, or you have simply played way too much Roller
Coaster Tycoon, you are probably familiar with the clacking, clicking, or clanking sound
that many roller coasters make as they climb the lift hill.
What you may or may not know is that this sound is part of a critical safety system
that is designed to keep passengers safe as they are lifted to the top of the ride.
The system is known as anti-rollback, or ARB for short, and it is used to prevent trains
from rolling backwards in situations where motion in the reverse direction would either
be undesirable or dangerous.
Most conventional roller coasters have stations and tight unbanked turns located at the base
of their lift hills, and these are not usually designed to handle trains moving at high speeds.
It would be quite disastrous if a train were to fall backwards down the lift in this scenario,
which is why carefully engineered anti-rollback systems are used to ensure that this never
happens.
Unfortunately, this wasn’t always the case in the early days of the roller coaster, and
a number of catastrophic rollbacks have occurred throughout history.
The most notable accident occurred on the Big Dipper at London’s Battersea Park in
1972, where the lift rope snapped and the rollback brake failed, sending the train speeding
back down the lift.
5 children were killed, and 13 others were injured, when the train derailed on a curved
section of track at the bottom of the hill.
Ride failures like this one were extremely tragic, but they have also led to a much higher
standard of safety for the modern roller coasters that we ride today.
The ASTM F24 Committee was established in 1978, just 6 years after the Big Dipper accident,
and they have developed a set of standardized guidelines for the safe design and operation
of amusement rides.
These guidelines are revised and updated annually by a team of more than 800 members, and they
are referenced by ride manufacturers and operators all over the world.
ASTM Standard F2291 relates specifically to the design of amusement rides, and it includes
a number of requirements for anti-rollback systems on roller coasters and other ride
types.
For example, it states that when anti-rollback is required, no less than 2 independent devices
must be provided for redundancy, and at least one of these must be engaged at all times.
This greatly reduces the risk of an accident because two safety devices would have to fail
simultaneously in order to cause a rollback, and the probability of that happening is very
low.
Let’s say we have ARB device A with a probability of failure of 1 in 50,000, and ARB device
B with a probability of failure of 1 in 200,000.
If both devices need to fail at the same time in order to cause a rollback, then the associated
probability can be calculated by taking the product of the individual failure probabilities,
which is 1 in 10 billion.
The risk of an overall system failure is far lower with 2 independent safety devices rather
than just 1, and getting a rollback with a redundant ARB system is virtually impossible.
Now before we take a look at how anti-rollback actually works, it’s worth mentioning that
not all roller coasters require these systems.
Vekoma boomerangs, for example, start by lifting riders up a reverse incline before releasing
them back down the same hill.
It would not make sense to use ARB in this case since the train needs to travel in both
directions, and a premature failure of the lift system would not result in any damage
or injuries.
Launched coasters don’t typically use ARB either, as they are specially designed to
handle rollbacks, and are often equipped with fail-safe braking systems to bring the trains
to a stop.
Where you will find anti-rollback being used is on roller coasters with conventional lift
hills, where the trains are meant to travel in the forward direction only.
These rides typically use a chain or a cable to pull the trains up the hill, and the lift
mechanism itself is often the first line of defense against a rollback.
If the lift is brought to a stop for any reason, then the drive system will engage a holding
brake to prevent the chain or cable from slipping backwards, which will keep the train stationary
on the hill until the ride is started back up again.
If the holding brake were to fail, or if the chain or cable were to snap unexpectedly,
then there is a secondary anti-rollback device that would act as a fail-safe.
For chain and cable lifts, the secondary ARB usually consists of a ratcheting system, which
produces that classic clicking sound we are all familiar with.
The way it works is basically the same as a handheld ratchet… except on a much larger
scale.
There’s a steel rail called a rack that is mounted on top of the lift track, and it
has asymmetric teeth that are sloped up towards the top of the lift.
Metal pawls hang down below the train which slide over the teeth as the train is pulled
up the hill, and they are pinned at one end so they fall into the grooves under their
own weight.
These pawls are commonly referred to as anti-rollback dogs, since they are designed to catch against
the rack when the train moves backwards.
The shape of the teeth allows them to easily slide over the rack in the forward direction,
but they will lock into the grooves in the reverse direction to prevent the train from
rolling back down the hill.
This mechanism is extremely reliable because it operates entirely under gravity, and it
will always be engaged whenever a train is present.
As you head up the lift hill on a roller coaster, you can usually see the rack running parallel
to the chain or cable, and the clicking sound that you hear is the pawls falling into the
grooves between the teeth.
Many parks prefer to minimize the noise as much as possible, though, and soft pads are
often installed on the pawls to dampen the sound.
On rides that have heavier trains and steeper lift hills, like B&M dive coasters for instance,
you will typically see 2 racks installed on the track instead of just 1 in order to handle
the additional load.
Each car has its own set of pawls to distribute the weight across multiple teeth, which also
protects the individual cars from a rollback in case they become separated from the rest
of the train.
ASTM F2291 specifies that every car must be equipped with its own secondary anti-rollback
device for this very reason, unless the cars have a redundant safety mechanism such as
a cable or chain holding them together.
In this photo of the Griffon dive coaster at Busch Gardens, you can see that each car
has 4 pawls with polymer sound dampers that align with 2 racks on the lift hill.
The device suspended in the middle is the chain dog, which connects the car to the lift
chain.
The slotted cutouts that you see in the pawls and chain dog all have pins running through
them, and this limits how far the dogs are able to drop below the bottom of the car.
B&M dive machines have some of the heaviest trains out there, so this is pretty much the
most robust anti-rollback system that you will see on any roller coaster.
For a quick comparison, the Leviathan giga coaster that we saw earlier was built by the
same manufacturer, and it only has 1 smaller pawl on each car.
For rides that have trains positioned below the track, such as inverted or suspended roller
coasters, the secondary anti-rollback device needs to be slightly different because the
ratcheting system cannot rely on gravity.
Many of them still use the same mechanism that we have already seen, however the rack
is positioned upside down above the train instead of below, and the pawls must be spring-loaded
so that they are pushed up against the teeth.
Some inverted coasters such as Vekoma SLCs use a non-ratcheting system instead, but it
works in a pretty similar same way.
There’s a flat steel fin that is fixed to the track which runs parallel to the lift
chain, and two spring-loaded cams are mounted to the top of each car.
As a train is pulled up the lift, the fin pushes the cams apart freely, but if the train
begins to move in the reverse direction, then they will wedge against the fin and prevent
the train from rolling backwards.
The cams have small teeth that bite into the metal to increase the frictional resistance,
and they will clamp together harder and harder as more force is applied from the train.
This type of mechanism is known as a unidirectional bake, and it is used extensively as a fail-safe
in the elevator industry.
One of the benefits of a non-ratcheting ARB like this one is that it produces less noise,
however some roller coaster manufacturers have also developed clever ways to make their
ratcheting systems silent.
Intamin, for example, utilizes the principle of electromagnetic induction to hold the pawls
above the rack when the train is being pulled up the lift.
There’s a small wheel that runs along a separate rail on top the track, and this spins
a circular conductive plate as the train rolls forward.
A lever with a permanent magnet is positioned just behind the plate, and it is connected
to the pawl, which is able to pivot up and down.
When the wheel spins clockwise, the magnet induces an electric current in the plate which
creates an opposing magnetic field.
This produces a counter-clockwise torque on the lever, which in turn lifts the pawl away
from the rack.
As long as the train continues moving forward with enough speed, the pawls will remain disengaged
and the anti-rollback system will be silent.
However, if the train comes to a stop and starts to roll backwards, then the torque
from the electromagnet will reverse direction and push the pawls down into the teeth.
If the ride is running on a rainy day, then it’s also possible for the pawls to engage
while moving forward because the wheel may slip when the track is wet.
Sound dampers are rarely used on these electromagnetic systems because their primary purpose is actually
to reduce wear and tear rather than noise, and so they tend to be extremely loud when
this happens.
Each type of anti-rollback device has its own advantages and disadvantages, though,
and they all achieve the same goal of making roller coasters as safe as possible.
Modern ARB systems are engineered to be completely fail-safe, and the probability of failure
for any of them is extraordinarily small.
With standardized design rules like those provided by ASTM F24, the roller coasters
that we ride today are safer than they have ever been in the past.
With that being said, I still prefer the simpler anti-rollback designs with fewer moving parts
because there is even less that can go wrong, and I’m also a big fan of that iconic clicking
sound.
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