Ever Wondered Why Locomotives Have No Tires? Here's Why

 Drivers of Formula 1 vehicles, which are regarded as the best in the business, accelerate their cars four to five lateral gs during each lap. This is because an F1 car exerts a force of more than 4 tonnes between the car and the track and weights 800 kg. One of the most crucial aspects of Formula One racing and the main factor limiting the vehicles’ top speeds is traction. After thousands of hours of testing and simulations, Pirelli was left with eight compounds in the end: two wet tyres and six slicks with different hardness levels.

Even so, a single contemporary diesel freight locomotive is capable of exerting up to 50 tonnes of forward force (also known as tractive effort) onto the tracks; yet, this force is applied through the minuscule points of contact between two stiff, smooth surfaces. There’s a lot of engineering between those steel wheels and steel rails, even though it looks impossible. Grady talks about the lack of necessity for tyres on locomotives in this episode.

The primary function of locomotives in railway engineering is to overcome train resistance by exerting all the effort necessary to drag the train and deliver it to the rails, hence maintaining the motion of the entire system. The majority of contemporary freight locomotives are powered by a diesel-electric drive, which in turn drives an electric traction motor that turns the wheels. This configuration has several advantages, one of which is that the engine and wheels don’t require a very complex gearbox to be coupled. Locomotive power is still constrained by the electric traction motors’ power rating, which is determined by multiplying force by velocity.

The adhesion limit controls tractive effort when moving slowly because, even though the motors could produce more force at low speeds, the amount of force that can be mobilised is limited by the friction between the rails and wheels. This presents a significant obstacle for a railway since the friction at the wheels prevents them from even using the engine’s maximum power. Before the race, dragsters perform a burnout to get the tyres warmed up for increased friction.

The “last frontier” of vehicle/track interaction is friction, which is crucial to almost all facets of railway engineering. Railways function because of the absence of friction, which enables relatively small locomotives to overcome the rolling resistance of massive loads. But for trains to be able to brake and accelerate without sleazing on the tracks, there must be some friction.

Weight, or the normal force between the two surfaces, and the more complex coefficient known as the coefficient of friction are the two numbers that determine friction. By multiplying both the locomotive’s weight and the friction coefficient, engineers can control tractive effort, but they don’t always have much control over that second knob.

The coefficient of friction is lowered by environmental impurities such as oil, grease, rust, rain, and leaves, which makes it more difficult to keep wheels attached to the track. Turning the knob requires dividing the force necessary to climb the steepest piece of track requiring the most tractive effort by the “dispatchable adhesion,” or the friction coefficient that can be relied upon for the particular locomotive and operating conditions. There is a limit to how much weight you can put on a single wheel for longer and heavier trains before the tracks break or a bridge is damaged.

Sandboxes, pipelines, air, water jets, chemical combinations, and lasers can all be utilised to increase the friction between wheels and tracks, giving some control over the friction coefficient. But there’s no clear distinction between a wheel slipping on a rail due to insufficient friction and one adhering to it due to friction. Between the two, all locomotive wheels under traction exist.

A rug and a round brush can be used to show how a steel-on-steel surface deforms under hundreds of thousands of pounds. Pulling beneath the rug provides very little traction and no slippage. The rug moves at the same speed as the brush. It is much harder to remove the brush from under the tyre, though, because it is completely slipping and exerting a lot of traction force on the surface. Since the wheel is stationary, the relative movement between it and the rail is essentially unlimited.

The bristles of the brush undergo deformation as it rolls between these two states, exerting a traction force. The bristles adhere to the rug initially, but they slide backwards when they break apart from the rug at a location in the contact region. This is also precisely what occurs to locomotive wheels. The rail stretches the wheel’s surface layer forward, but as the elastic stress is released, there is insufficient adhesion towards the rear of the contact region, causing them to separate.

A wheel’s behaviour under different circumstances demonstrates that in the absence of traction, there is neither slip nor creep. An increase in traction causes a larger area of the contact patch to slip, which increases the contact patch’s creep—its relative motion to the track. The traction force eventually levels out when the entire contact patch slips.

This graph represents a fictional scenario under perfect circumstances. A wheel that is completely sliding on the rail has less traction than one that has at least some stick.

Stick-slip occurs when a material’s dynamic friction coefficient—which applies to sleds and locomotives—is lower than its static friction coefficient when there is no relative movement. Because of the varying traction force at the wheel rim, this effect can create corrugation of the rail and undesirable noise, which was an issue for steam locomotives. In reality, though, this distinction between static and dynamic friction produces a traction versus creep curve that has a maximum and decreases as one moves beyond it in the direction of more slip. To take advantage of this, modern locomotives use highly developed creep control systems that monitor each wheel separately and adjust the tractive force to maintain the curve’s peak. By maximising the friction coefficient, you may maximise your power and potentially reduce the number of locomotives needed, resulting in fuel, expense, and wear and tear savings.

There is a trade-off in the difficulty of employing various materials with greater friction coefficients, such as rubber tyres on automobiles. While locomotives can have their steel “tyres” removed and changed, certain passenger rail vehicles have rubber tyres. In order to maintain position without wasting too much time in the pits, F1 tyres are used to acquire track position and then swapped out for harder, more durable tyres. If you continue this thinking to more resilient tyres that can handle multiple tonnes of weight across hundreds of thousands of kilometres, you end up with a steel wheel on a steel rail because pit stops for goods trains are expensive.

In order to generate sufficient friction to drive a whole train, locomotive weight is necessary, but the weight of the locomotive must be supported by the tracks, particularly when utilities like water lines need to pass below. A recent film featuring Integza, Scotty from Strange Parts, Sam from Wendover and Brian from Real Engineering showed how a water line is constructed beneath railway tracks in a way that can endure tremendous stresses and enter without interfering with train activity.