Coasters-101: 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, side friction wheels are mounted perpendicular to the road wheels (on either the inside or the outside of the rail depending on the manufacturer) and up-stop wheels are placed under the rail to prevent the vehicle from coming off the track over airtime hills.
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:
- Low rolling resistance
- High load endurance
- Smooth ride performance
- High durability (low maintenance cost)
Let’s examine each of these requirements in more detail.
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. A typical rolling resistance coefficient value could be between 0.009 and 0.018 of the supported load. On a roller coaster, energy losses due to friction must be minimized in order 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.
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.
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.
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:
Convert to miles=18.875/12/5280=0.000297.
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. The speed of the coaster can be affected by mixing and matching wheels, which parks may do in order to keep the ride running within its specified performance window. To achieve a perfect balance between all of the demanding requirements requires a lot of testing.