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 Rolling resistance and friction
Cornering
Pros and cons |
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| Pneumatic tyres have a much greater rolling resistance than steel wheels |
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Rolling resistance and friction
If a vehicle has a high rolling resistance, it's harder to move. Rolling resistance involves a combination of factors, including how free the bearings are to rotate and the amount of friction between the wheels and the road. The amount of friction is in turn related to the materials involved, how much weight the wheels are supporting and how much the surfaces deform at the point of contact.
A car's pneumatic rubber tyres have a relatively high rolling resistance for the weight they are carrying. The rubber surface provides a lot of friction and traction (grip), and the tyres bend and deform a good deal as they roll, giving increased traction. But this also means increased rolling resistance, which absorbs a lot of energy. In fact something like 25% of a car engine's power is used to push its tyres along the road.
Rolling resistance and the amount of energy used by a wheel are proportional to the weight that it is carrying. The more weight, the more deformation and the more friction. A car is relatively light, so it is acceptable to have pneumatic rubber tyres, sacrificing a lot of energy for the purpose of roadholding.
However, the weight on a train's wheels is far more than that on a car's. So in order to minimise rolling resistance and the amount of energy needed to move the train, steel wheels are used, which run on steel rails. Obviously solid steel deforms much less that inflated rubber, and a train's steel wheels have a tiny area of contact with the rails – about the size of a 5p coin. This means that a train is about the most efficient way to move heavy goods.
However, the low friction between a train's wheels and the track also means that breaking and acceleration are much less efficient. The higher loads on a train's wheels go some way towards compensating for this, keeping the wheels in firm contact with the rails. Some locomotives also spray sand through a small nozzle onto the track in front of the wheels to improve grip. In addition, whereas a car tyre has to provide grip not only for acceleration and stopping but also to control sideways movement, so the car can be steered without skidding, a train's wheels are guided by the track (see Cornering, below), so they need less traction.
Because the Scrapheap machines are a lot lighter than real locomotives, high-traction rubber wheels can be used to drive them, while steel guide wheels keep them on the track. Rubber on steel can give up to five times more traction than steel on steel. However, in the wet, much of this advantage is lost, because the larger contact area of rubber on steel allows a thin film of water to act as a lubricant. The small contact area and high pressure of a steel wheel on a steel track forces out the water, preserving contact.
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| The conical shape of the wheels guides the train round corners |
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Cornering
A train's wheels have flanges – protruding rims – which sit inside the rails to keep the train on the track, guide it through points and help guide it around bends. But contrary to popular belief, the flanges should not normally touch the rails. The flanges of a pair of wheels on an axle are set slightly closer together than the distance between the rails. In theory, they are a last resort to prevent the wheels leaving the track – a safety feature – though imperfections in wheels or pieces of track mean they come into play more frequently. If you hear a train's wheels squealing, the flange is rubbing against the rail.
What should guide the train around a corner is not the flanges but the conical shape of the wheels (see diagram). As a train turns into a corner, the wheels try to continue in a straight line, so on a right-hand turn, for example, they move to the left relative to the track. This causes the contact point between wheel and rail to change. Because the wheels are conical in shape, the wheel's diameter at the point of contact also changes. The contact diameter of the wheel on the outside of the turn increases, while that of the inside wheel decreases. This means that the outer wheel is covering a greater distance per axle turn than the inner wheel, so the train is guided around the corner. This is a self-adjusting arrangement: the exact contact position of wheels and rails depends on how sharp the turn is. The conical treads are self-centring when running on straight track.
The conical wheels of a train achieve the same effect as a differential gear unit in a road vehicle, which allows one wheel on an axle to rotate more quickly than the other on a turn.
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| The Tunnel Ratz go for a lightweight design based on a motorcycle |
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| The Pit Stop Crew build a heavier, more powerful vehicle |
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Pros and cons
Tunnel Ratz
The Tunnel Ratz' machine is light and can be manhandled by the team.
Their rail guide-wheel system fits easily into place, making for a quick change from road to rail.
On the other hand ...
Motorbike engines don't have a reverse gear. This means the Ratz have to turn their racer through 180 degrees to come back down the track.
Their machine only has one driven wheel, so maintaining traction, especially in the wet, could be a problem.
Pit Stop Crew
The greater weight of the Pit Stop Crew's machine, plus the fact that they have two drive-wheels, gives them more power and better traction.
On the other hand ...
In order to deploy their rail guides, the Pit Stop Crew's machine has to be in exactly the right position, and because of its size and weight the machine is quite awkward to move.
Part of an axle used to make their guide wheels is mounted it so that it trails along underneath the machine, where it catches on an obstruction.
Their machine's weight means it is very difficult to get back on track in the event of derailment.
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