# The Resistance

Before reading this, I recommend reading the previous post – The Traction Force – including its disclaimer.

When a railway vehicle is required to move on track, it needs to overcome a series of resistance forces. We can call the resultant R of all these forces “the running resistance of the vehicle”.

If we consider the duration of their presence, we could classify the resistance forces in two:

Permanent – there is a main resistance force, always present whilst the vehicle is in motion, caused mainly by the rolling and slipping friction forces at the wheels and by the friction forces in the numerous bearings the system has. This resistance is defined through measurements (for conditions that exclude as much as possible the other resistances) and it is a defined parameter of any type of railway vehicle.

This force varies depending on speed.

Temporary – there are also resistances that appear in certain conditions.

Alignment related running resistance

• Gradient resistance – when the vehicle is moving on a rising gradient, the tangential component of the weight opposes the movement; it is a resistance. When the gradient is falling …
• Curve resistance – when the vehicle moves on a curve, there are additional forces that oppose the movement; they form the curve resistance force.

– Structure related running resistance

• Tunnel resistance – in a long tunnel, due to the change in humidity and the piston effect (train pushing the weight of the air facing it outside of the tunnel) a significant resistance is developed. This consume up to 20% of the traction force. I’m not very certain about the figure.

– Environmental running resistance

• Wind resistance – sections of predominant facing wind are sometimes considered in trailing calculations
• Low temperature resistance – the low temperature and snow reduce the wheel rail adhesion and also have an influence in increasing the viscosity of the bearing lubricators.
• “leaves-on-rail resistance – the leaves reduce the wheel-rail adhesion

– Train movement change resistance – caused by inertia

• Starting resistance
• Speeding up resistance

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