How Coaster Cars Work (or don't)
Dave Althoff
Dave Althoff, Jr.
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There are many types of coaster out there. Those coasters are running
many different kinds of trains. As riders, we usually only think about
what is going on above the floorboards, but an understanding of what is
happening underneath can be very useful in showing why coasters perform
the way they do (or don't).
Housekeeping--
For the purposes of this document, directions and motions will be
expressed as follows:
X axis: A transverse or lateral axis running left to right from the
rider's point of view.
Y axis: An axis running vertically bottom to top from the rider's
point of view.
Z axis: A longitudinal axis running in the direction of travel.
roll: A rotation around the Z axis
pitch: A rotation around the X axis
yaw: A rotation around the Y axis
Got that? Now that you know what the directions and the motions are,
we can examine some coaster cars, their limitations, and the problems
of each.
Straight cars--
Of course the simplest cars are the four-wheel-truck boxcar models.
The oldest wood coaster trains are this type. At each corner of the
car there is a wheel assembly rigidly mounted to the car frame. The
cars are then tied together with a drawbar assembly which allows the
cars to pivot relative to each other with some independence.
Perhaps you can see the trouble with this arrangement. When the car
approaches a curve, the car cannot exactly follow the track. The
wheels must be able to slide sideways at a tangent to the curve,
because the car frame is rigid. This requires some slop in the track
gauge through the curve to keep the cars from binding, and the sliding
action of the road wheels means the track must be lubricated, otherwise
the wheels will wear out the road steel.
The other problem with this design is less obvious, but in some ways
more problematic. The front and rear axles operate in a single plane,
because the car frame is rigid. So the track can only be banked very
gradually. You see, when the train enters a bank, the front wheels hit
the banked portion of track first. Say the track banks to the left.
The right wheel is lifted first by the banked track, and the whole car
is rolled to the left. The rear wheels must remain in the same plane
as the front wheels, so even though the rear wheels have not yet
reached the banked track, the car still rolls. The outside (right)
rear wheel lifts off of the track until the right rear upstop hits.
Either that, or the inside (left) front wheel will lift up until its
upstop catches. Either way, road wheels are lifting from the track,
and if the bank is severe enough, the difference in elevation between
the front and rear axles will be enough for the car to bind up in the
track. The amount of roll that the car can handle between the front
and rear axles is determined by the total clearance between the track
and up-stop on opposite corners of the car. Usually, that clearance
isn't very great...perhaps an inch or two total.
In addition, the car body can twist a little (okay, so the cars on the
Kennywood Racer can twist a lot...) to take up some of the variation.
But this complicates matters a bit, as twisting the cars is something
that can tear them apart. PTC alleviated the problem by engineering
some flexibility into the cars, for example, the seat boards are not
rigidly fastened to the car frame...instead, they are held in place
with sleeved bolts running through oversize (well, if they aren't to
begin with, they will be by the end of the first season) bolt holes.
This lets the (rigid) seat boards shift relative to the frame instead
of splitting and breaking.
While that is all well and good, wouldn't it be better to just move the
wheels so that the car doesn't have to flex?
Articulated Cars--
PTC came up with a solution to the problem of banked track some years
ago with the development of the articulated car. Most 2-bench PTC cars
are of this type. Instead of attaching the rear wheels directly to the
car chassis, the rear wheel assemblies are attached to a subframe
assembly. This subframe assembly forms a rear axle, and it is attached
to the car by means of a longitudinal shaft. If you sit in the back
seat, the end of this shaft is right below you, just above the car
floor. The subframe is only able to pivot a few degrees, but that's
all that is needed for a short car. Tilting the back axle increases
the possible differential between the front and rear axle enough that
the car can take banked curves without lifting any wheels. The wheels
are still rigidly aligned with respect to the car's longitudinal
centerline...that is, the wheels point straight forward...and when the
car is going around a corner, they are pointing at a tangent to the
direction of travel. This is why track lubrication is still needed
even with articulated cars. The wheels still can't follow curves
in-line with the direction of travel, and must slide sideways around
the curve.
Flanged-Wheel Cars--
Much has been written about flanged-wheel cars, but I suspect that the
most important points have been overlooked. Let me try to explain.
The most common flanged-wheel car operating today is the junior car
from PTC. These cars have four road wheels and four up-stops, but no
guide wheels, as the wheel flanges make the guide wheels unnecessary.
PTC's junior cars are articulated in the same way as the larger cars,
in that the rear axle is mounted on a sub-frame which can rotate on a
longitudinal axis relative to the car and its front wheels. But there
are a couple of other differences. First of all, in order to increase
the roll clearance, the running boards...which form the frame of these
cars...are notched to allow the wheels more play. Second, the rear
seat is not attached to the frame and running boards, but rather to the
rear axle subassembly. So these cars can handle a fairly significant
bank transition. But they have a problem. Because of the wheel
flanges, the road wheels cannot slide laterally on the track steel. If
the wheels are held rigid with the car, the wheel flange will cause it
to attempt to derail on the first curve. The flanged wheels must be
able to follow the curve exactly.
This means that the wheel must be able to pivot relative to the
centerline of the car. The simplest way to do this is with a simple
pivoting axle. The problem is that a pivoting axle requires space
which is not available under these cars. And the pivoting axle is
needed at both ends of the car, as all four wheels need to be able to
follow the curves, since all four wheels are flanged. Well, another
way to accomplish this is to do what your car does. Instead of yawing
the entire axle, we can pivot the wheels to follow the curve. With
flanged wheels, it is easy enough to do. But there are two pitfalls to
watch out for. First of all, if you locate the pivot point directly
behind the wheel spindle, then as the leading edge of the wheel flange
hits the outside of the curve, the leading edge will be directed
inward, away from the rail...but it won't be able to go anywhere
because the trailing edge of the wheel flange will already be tight
against the rail. This will tend to keep anything from moving, thus
defeating the purpose of the pivot. You can see this on your car if
you turn the wheels to one side. Notice that on the outside of the
turn, the back edge of the tire protrudes from behind the wheel-well.
On a car, this is no problem, as there is no track involved. But on a
coaster, this can mean trouble. The solution is to move the pivot
point to a location behind the road wheel. That way, when the wheel
pivots, the whole wheel shifts sideways, clear of the rail. The rear
wheels , which, you will recall, are also mounted on a roll axle, have
their pivots mounted forward of the wheel, for much the same reason.
Moving the pivot point back also creates a new problem. So far, we've
concentrated on what happens to the outside wheel when the track
curves. The new problem is what happens to the inside edge wheel.
Because the pivot point is behind the trailing edge of the wheel
flange, the inside edge wheel will not even touch the rail as the car
heads into the curve. There is nothing to 'encourage' the wheel to
follow the track as happens on the outside rail. Instead, the wheel
will plow straight forward until the road surface falls right off of
the rail. That is, of course, a Bad Thing. Fortunately, the solution
is simple. A tie-rod is attached to the wheel assemblies on the end
opposite the pivot. That way, when the track pushes the outside wheel
inward, the tie-rod will also push the inside wheel inward, insuring
that it stays on the rail.
There is one final complication. The rear wheels must also be allowed
to pivot, in a mirror image of the front wheels...that is, the pivot
points are forward of the wheels and the tie rod is behind. The reason
for this can be seen by looking at the tire tracks of an automobile,
with its steerable front wheels and fixed rear axle. The problem is
that as the car goes around the curve, the rear wheels will follow a
path inside the track of the front wheels. This is fine for an
automobile, but on a coaster it means the rear wheels are not actually
following the track. Allowing the rear wheels to steer solves this
problem.
Runaway Train--
It's worth noting here that Arrow solved almost all of these problems
40 years ago when they developed the Bobsled which later became the
Runaway Train. That car has two beam axles, both of which can yaw, and
one of which (the rear) can also roll. They refined the setup a bit by
using dual guide wheels on each wheel assembly. Because of the guide
wheels (rather than flanged road wheels) the wheel position can be
slightly inexact because if the road wheel slides sideways slightly,
the penalty is significantly less than with the flanged wheel. Also,
by using dual guide wheels, the axle is steered from both ends...as the
leading guide wheel strikes the outside rail, the trailing guide wheel
on the other side should catch the inside rail. Allowing both axles to
steer fixes the "back wheels inside" problem, and allowing the rear
axle to roll solves the banked turns problem.
Premier Rides, Pinfari, Chance...
Most of the steel coaster manufacturers are using a variation on
Arrow's Runaway Train for their two-axle cars. The usual configuration
is to attach wheel carriers to the ends of a straight axle, and connect
that axle to the car with a spherical bearing. A spherical bearing can
swivel in any direction. To control body roll, one of the two axles is
fixed in the roll axis relative to the car body. Premier does this by
attaching a large disc to the end of the axle just inboard of the wheel
carrier. The bottom of the car body then rests on top of thaose discs.
Simple and effective.
Bolliger & Mabillard--
(Technically, B&M cars are trailered, but it's worth mentioning
here...)
The B&M coasters (except for the inverted coaster) use a wheel
asssembly which has a lot in common with the PTC junior, except that it
uses two road wheels and two guide wheels, and is pivoted on an axis
centered between the two road wheels. They can do this, again, because
they use guide wheels instead of road wheel flanges. Also, the use of
double guide wheels and the position of the wheel pivot on the center
of the road wheel assembly means that the wheel carrier will be steered
even on the outside rail (note that B&M steer curves from the inside
rail because their guide wheels are outside...just the opposte of the
PTC and Arrow designs), so the tie rod is really not needed, though B&M
use it on all of their designs except the inverted (which works fine
without tie rods).
Trailering--
All of this is fine and dandy, but what if we want to make the turns
tighter, the banks sharper and the drops steeper? At the moment we are
limited by the length of the car, and by the length and action of the
drawbar between the cars. One thing we can do to clean things up a bit
is to trailer the cars. Each car has only one pair of wheel assemblies
under the back end. At the front, the car is supported on the back end
of the car ahead. Done properly, the hitch is a 3-axis assembly,
capable of pitching, yawing, and rolling. This essentially makes each
car independent of those around it. The car is supported in front by
the hitch, located in space above the center of the track. In the
rear, the car is supported by its own wheels. As the train starts
around a curve, the hitch assembly (traditionally just a ball and
socket arrangement) follows the direction of travel, and the car pivots
on its rear wheels to exactly match the train's path. The arrangement
is mechanically simple, and should allow for track configurations that
no other type of car can easily handle. But there are some caveats.
First of all, the lead car has to be supported by something. And, the
lead car needs to have at the very least a roll-axis articulation.
None of the other cars needs any axle articulation because the roll
axis pivot is in the hitch, and the whole car (sitting on a single
axle) can pitch and yaw. Ideally, in the lead car, the roll pivot will
be on the axle away from the hitch, since the hitch end has the
articulation with respect to the adjacent car. That will provide
consistent performance through the whole train. It is not necessarily
desirable to have yaw-axis articulation on the lead axle because
additional articulation on that axle would enable it to fail to follow
the track properly...to twist sideways and derail. This can be avoided
with the use of steerable wheels with tie-rods, or by using dual guide
wheels, and in fact Arrow and B&M both use fully-articulated lead axles
in this fashion.
Note also that my use of the term "lead axle" may be a bit misleading,
as the 'lead' axle can be at either end of the train, in fact Arrow,
Vekoma, and Miler (at least) put their 'lead' axles at the back of the
train.
The other problem with trailering has to do with the position of the
hitch and the wheels. To demonstrate, let's consider a simple trailer.
It has two wheels under the back end, and a leading tongue to support
the front end. Now, if we lift up the tongue and start moving it
about, we will see first of all that the trailer will pitch back on its
wheels. If we move the tongue left or right, the trailer will yaw
about a vertical axis centered on the axle. So, effectively, as we
move the trailer tongue around, there is a point on the trailer which
rotates, but does not translate...does not move...in space. That point
is the center of the axle, in line with and centered between the center
spindles of the road wheels. We could attach another trailer's tongue
to that point, and the second trailer's exact alignment would be
unaffected by the first trailer. If we put the whole arrangement on a
curved section of track, and we pull the first trailer around the
curve, the second trailer will simply follow the centerline of the
first car, tracking independently around the curve.
This is, of course, what we want. We still need to put a lead axle on
this setup, but we will end up with a train which will handle just
about any curve we throw at it. This is how trains built by Prior &
Church, Harry Traver, Carl Phare, Dana Morgan, and Mike Boodley are
constructed. Cars built by Anton Schwarzkopf, Bolliger & Mabillard,
Vekoma, Miler, and Arrow Dynamics expand on the concept a little
further by adding wheel or axle articulation to the cars and using dual
guide wheels. This enables the axle to follow the track exactly, even
if there is a slight misalignment as the car starts into a curve. The
system works remarkably well, but it is not without its problems.
The problem is that coaster cars are very heavy, and the entire weight
of the car...and remember, this weight must include the weight of the
passengers and is multiplied by forces exerted by the ride profile,
which may be in excess of 4G at times...must be carried by the road
wheels. Of course, by trailering, we have reduced the number of axles,
so now the whole load of the car has to be carried on the single axle.
Well, not exactly. Remember, the cars are tied together, so in theory
at least, part of the load is carried on the hitch which supports the
front end of the car. If that hitch is positioned at the axle, then
the axle is carrying about half the weight of the car it is attached
to, and about half the weight of the car attached to it. This means
that not only does the axle and wheel assembly need to carry that full
load (a back of the envelope computation says that can exceed 12,000
pounds) of the car, but the hitch needs to be able to carry at least
half of that weight reliably.
Well, carrying a heavy load on a couple of wheels is dead easy. If you
are using soft wheels, you can decrease the load on each wheel by using
tandem wheels (Arrow, B&M, Schwarzkopf, and Vekoma all do this) on the
axle. But engineering the trailer tongue to carry all of that load and
still provide sufficient articulation is more of a problem. The
solution? Reduce the tongue weight. That is easy enough to
do...simply move the axle closer to the car's longitudinal center of
mass. That is the point where the loaded car would balance on its axle
if nothing moved. Since cars generally have footwells and seats at
opposite ends and are therefore heavier at one end, the center of mass
will usually be somewhere between the center and the back end of the
car. Balance the car in this fashion, and the tongue load is
significantly reduced. National Amusement Device, Philadelphia
Toboggan, and Ben Schiff all did this. It works, but there is a
trade-off: This configuration, especially on a longer car, will put
the axle some distance from the back end of the car. This is a
problem, because for trailering to work properly, the coupler to the
next car needs to be aligned with the axle. The distance can be
significant; on the PTC trailered car, for example, the axle is under
the floorboard ahead of the back seat...actually ahead of the wheel
position used for the 2-axle cars. This means that for the assembly to
work properly, there needs to be enough clearance under the back seat
to accommodate the movement of the hitch tongue...movement which is
amplified by distance.
Where PTC and NAD went wrong--
Philadelphia Toboggan and National Amusement Device have both produced
trailered cars. NAD's trailers are on the Lil' Dipper at Camden Park;
to the best of my knowledge the only remaining PTC trailers are on the
Raging Wolf Bobs at Geauga Lake and on the Predator at Six Flags Darien
Lake. The fact that such cars have been removed from (at the very
least) Texas Giant, Hercules, and Thunder Run should indicate that
there might be a design problem. And indeed there is, with this
design.
What PTC did was to balance the trailered car on its own axle. As
noted, this is well and good, and there are good reasons for doing
that. But they also mounted the hitch point to the very back of the
car, providing a point for the next trailer to rest on. The trouble
with that arrangement is that when the car pivots on its axle to go
around a corner, the hitch point, instead of remaining in the center of
the track, slides sideways. If the car turns to the left, for
instance, the hitch ball gets pushed to the right. This, in turn,
drags the tongue for the next car to the right, causing it to become
misaligned with the track. It can only go so far, though, before its
wheels hit the side of the track, causing it to try and realign with
the track. This tends to pull the tongue back the other way, taking
the hitch ball with it, and causing the leading car to misalign. Add
to this action additional cars trailered in back, each one going
through this oscillating motion, and you can see that the whole train
is going to shake back and forth all the way through the curve.
Perhaps the best demonstration of this is the Lil' Dipper at Camden
Park. Sitting in the last car of the train, you can see all the cars
shuffling back and forth as the train goes through the third turnaround
at the back end of the ride. I have a video clip which shows this
action on Thunder Run, along with another clip which shows the
difference with the articulated train.
But what about Molina?--
Molina is a special case. On the Moli-Coaster, which is a faithful
copy of the Ben Schiff kiddie coaster, the single-bench cars are built
like the PTC trailers, with the weight of the car balanced on the one
axle, and the coupler at the rear of the train. And yet, these trains
do not shuffle when they go around an extremely tight curve. Why?
The answer lies in the hitch position, and a little fact about the
Moli-Coaster. If you take a close look at one of these gems, you will
see that the hitch is positioned off-center, almost all the way over to
the left wheel. You see, when the car pivots on its axle, the center
of the back end slides sideways. But closer to the edge, there is a
point which does not move laterally, because the back of the car is not
merely translating sideways, it is rotating. The problem is that there
are two such points, and which one isn't moving is entirely dependent
on which direction the car is turning. But that isn't a problem for
Ben Schiff, because the coaster has an oval layout. All of the turns
on the ride are to the left, and therefore the hitch is positioned so
that it is optimized for left-hand turns. Of course the upshot of all
this is that the cars on a Moli-Coaster are incapable of turning RIGHT.
Not a problem so long as the ride only turns left!
Incidentally, Wisdom has a similar "one-way" arrangement on their
Dragon Wagon, but rather than being related to the hitch position, it
is a matter of different guide wheel arrangement on the lead axle so
that again, the train is optimizd for turning left.
I think that pretty well sums it up. To recap...
o Straight cars have no wheel articulation at all and MUST slide
laterally in order to take curves.
o PTC and Gerstlauer articulated cars have only one dimension of
articulation, which allows them to take banked curves without lifting
the inside front wheel, but they must still slide sideways on curves
o The sliding can be eliminated by using fully-articulated axles or
wheel sets as on the PTC junior trains or the Arrow Runaway Trains.
o To eliminate the sliding, both axles must be articulated otherwise
the rear wheels will attempt to track inside the front wheels.
o For more flexibility, trailering allows cars to move independently.
Trailering reduces mechanical complexity.
o Trailered cars need to have a lead axle.
o Trailering requires either fixed axles or tandem guide wheels.
o Trailered cars should have the hitch situated right in the middle of
the axle.
o Improperly trailered cars will run poorly.
Comments, questions, corrections, and other insights on this subject
are encouraged. Eventually, this information will be available on a
web page or series of web pages, so there are good reasons for getting
it right...
--Dave Althoff, Jr.
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Locoboy
interesting, especially for dorky mechanical engineers like moi. :-)
Todd Long
>How Roller Coaster Cars Work (or don't)
>Dave Althoff, Jr.
Another must read, must save post from Dave. I am never less than
impressed by his knowledge and always overjoyed that he cares to share
it with us.
Todd "Never did 13th grade" Long
Wolf
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Dave Althoff wrote in message <7q2ar6$g...@acme.freenet.columbus.oh.us>...