Coasters-101: Roller Coaster Wheel Design

Welcome to Coasters-101: Roller Coaster Wheel Design! A typical wheel used on a roller coaster is constructed by taking an aluminum hub and bonding a polyurethane tire to the hub’s outside diameter. This entire “wheel assembly” is then connected to the axle through a bearing. There are three main types of wheels which secure the vehicle to the track:

Road wheels bear the load or weight of the train and are subjected to the biggest loads, which is why they’re also called load wheels. The allowed vertical downward gs are greater than what is allowed for lateral or uplifting gs. Load wheels are usually also bigger in diameter as well.

Side friction wheels, or guide wheels, are mounted perpendicular to the road wheels on either the inside or the outside of the rail depending on the type of track. The side friction wheels on steel roller coasters are spring-loaded against the sides of the track. The wheels are forced to steer to follow the track.

Conversely, wooden coaster wheels have historically been on fixed axles. There is a gap between the wheels and side of the tracks so the cars “shuffle” through the turns without exactly following the turning track. This results in more banging around and causing the track to wear because it eventually crushes the wood. New wood coaster trains have now introduced steering into their vehicles to help eliminate some of the roughness.

Side wheels can either be on the inside or the outside of the rails. Having side wheels inside the track allows lowering the heartline as trains can travel partly inside the track, and thus the track shape is easier to calculate by hand. This is partly why older steel coasters by Arrow Dynamics and Vekoma always had their side wheels inside the rails. The most recent major roller coaster in the US to use inside wheels was Kentucky Kingdom’s Lightning Run by Chance Rides (who is also supposed to be opening another major inside wheel coaster in 2023).

roller coaster track design

Inside versus outside the track wheels.

Upstop wheels are placed under the rail to prevent the vehicle from coming off the track over airtime hills or if the vehicle were to stall in an inversion. Even if there are no accelerations that could cause lift-off, a safety device capable of withholding 50% of the fully loaded vehicle weight is required. Some coasters with no lift-up forces use small steel plates rather than upstop wheels.

Roller Coaster Wheel Material

A roller coaster’s wheels may actually be one of the biggest limiting factors of building ever taller and faster rides. To design the perfect wheel for a high-speed roller coaster, engineers must find the best combination of these four main requirements:

  1. Low rolling resistance
  2. High load endurance
  3. Smooth ride performance
  4. High durability (low maintenance cost)

Let’s examine each of these requirements in more detail.

roller coaster wheel

SolidWorks model of a roller coaster wheel assembly.

Roller Coaster Rolling Resistance

Rolling resistance is caused by the deformation of a tire at the point where the tire meets the surface on which it travels-in this case, the coaster’s rails. The lower the pressure and/or the higher the force exerted on the tire, the larger the coefficient. For example, a non-deformable train wheel made of steel riding on a non-deformable steel rail has a very low rolling resistance, hence making it very efficient. To calculate the rolling resistance force:

FRoll = R×M×G

R is the rolling resistance coefficient, m is the mass, and g is the acceleration due to gravity = 9.81 m/s2. A typical rolling resistance coefficient value could be between 0.009 and 0.018 of the supported loads. On a roller coaster, energy losses due to friction must be minimized for the train to complete its circuit composed of complex maneuvers and dynamic inversions. The requirement of a low rolling resistance leads designers toward selecting a harder wheel material.

High Load Endurance

Some record breaking roller coasters today plunge down mammoth four-hundred foot drops, operate in speeds excess of 120 miles per hour and can subject riders to forces more than six times that of gravity. The wheels not only carry the weight of the passengers and the vehicles, but they must also be able to do so up to six times their weight, possibly resulting in a load of 6000 pounds of force on each wheel, all the while the train is moving at a very high rate of speed. This again leads toward the selection of a harder material.

Smooth Ride Performance

The most important requirement from a guest experience perspective is to make the ride as smooth as possible (especially for a steel coaster, maybe not so much for a woodie). Manufacturing large sections of track perfectly within tolerance without any imperfections is very difficult (and expensive) to do. Therefore, the wheels must absorb any deficiencies during manufacture or other conditions such as dirt or debris on the track.

Rough rides are not only unpleasant for the park guests riding the coaster, they can also cause damage to the vehicle over a period of time resulting in higher maintenance costs. The need to provide a smooth ride directly opposes the first two requirements by leading the designer to choose a softer wheel material.

High Durability

A typical roller coaster train may contain over one hundred wheels (12 wheels per car * 9 cars per train = 108 wheels per train) resulting in a significant portion of ongoing roller coaster maintenance being tied up in wheel replacement.  Let’s say we have a coaster with 6 inch diameter wheels traveling at 70mph. To calculate the revolutions per minute (or RPMs), first find the circumference of the wheel:

c=2*pi*radius=2*3.14596*3”=18.875”.

Convert to miles=18.875/12/5280=0.000297.

RPMs=linear velocity/circumference=70mph/0.000297miles=

234968.022 revolutions per hour / 60 min=  3916 revolutions per minute.

After running through the calculation we see that the wheel will be spinning at nearly 4000 rotations per minute. It is an absolute requirement that the wheels last as long as possible. Occasionally, the wheels will wear out. Sometimes an air bubble may develop in the layers of urethane. In this case the wheel must be replaced. When acwheel heats up from the friction there can actually be a blow out. Now, if one wheel blows out on the coaster while it is in motion it can still  make it around back to the station, where the wheel will then need to be replaced.

Most steel roller coasters today generally use two main types of tire material: nylon and polyurethane. There are advantages and disadvantages for each type. The nylon wheel is a hard plastic while the polyurethane is a softer material. Nylon wheels vibrate a little more and put more wear into the track, making it a rougher ride but also results in a little bit faster of a ride. Polyurethane is a softer material and reduces the vibration, providing a smoother ride. It provides more friction and slows the ride down due to a higher rolling resistance.

Urethane tires have certain disadvantages for use on wood coasters. They are less durable than the plain steel tires and are subject to damage from rolling over the track joints. Perhaps more important is the fact that owing to its flexibility the urethane tire can absorb a considerable amount of energy resulting in a slower ride. Yet several operators choose to utilize non-steel wheels on their wooden coaster, possibly due to noise limitations. These rides tend to be taller but shorter in overall length due to the higher rolling resistance of the softer tires.

The speed of a coaster can be affected by mixing and matching wheels, which parks may do to keep the ride running within its specified performance window.  A new ride may use one type of wheels until it is tested and broken in, then will switch to the other kind of wheels. Many hours of testing may need to be performed to achieve a perfect balance between all the demanding requirements.

Uremet is one of the main suppliers of roller coaster wheels for the industry.

Roller Coaster Wheel Size

Roller coaster wheel size varies depending on the type of roller coaster – whether it’s a high speed, high g force thrill machine or a tame family coaster with low forces. Guests at Cedar Point’s annual Winter Chill Out event get a behind-the-scenes tour of the maintenance shop where they can see all kinds of different roller coaster wheels.

 

top thrill 2 wheel diameter

The largest roller coaster wheel at Cedar Point is for Top Thrill 2, the LSM launch coaster that will hit a top speed of 120 miles per hour. These wheels are massive – 20.8 inches in diameter to be exact!

top thrill 2 wheel diameter

Falcon’s Flight, the upcoming world record smashing roller coaster, will have several key unique features, including “extra-large” running wheels with custom designed rims for enhanced cooling, due to high speeds and high ambient temperatures of the Saudi Arabian desert.

falcon flight wheels

Look at Falcons Flight specially design wheels


To read more about coaster design check out Coasters 101: An Engineer’s Guide to Roller Coaster Design.

This article was originally published in October 24, 2011. It has been updated on March 7, 2024.

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