4 ms·

The fixed wheels, plus the bevel create a self-centering effect. As the wheels move off centre, the bevel makes one wheel have a larger effective diameter and

The fixed wheels, plus the bevel create a self-centering effect. As the wheels move off centre, the bevel makes one wheel have a larger effective diameter and the other wheel a smaller effective diameter. Because the wheels have a solid axel, that forces one wheel to start travelling slightly faster than the other, causing the pair of wheels to turn. With the bevel making the wheels larger on the inside, it means that as a pair of wheels drifts right relative to the tracks, the right hand wheel rides up, starts behaving as if it’s larger, and then starts to overtake the left wheel (which has ridden down, and behaves as if it’s smaller), creating a turning effect to the left, correcting the rightward drift. When setup correctly, and travelling at a reasonable speed, the big metal lip on train wheels should never touch the track. The self-centering effect does all the work, the lip is just a backup to deal with overly tight turns, points and extremely slow running.

geokon 4y ago
I get the principle of how it works - though it seems imperfect bc it would vary depending on the weight/load. But why favor such a mechanism to just independent wheels? Or are you saying the whole setup actually tilts the carriage into the turn - helping to prevent it from tipping?
- mulmen 4y ago
  If the wheels were independent the lips on the wheels would constantly ride on the inner sides of the rails. That increases rolling resistance and wear.
- RX14 4y ago
  There are some trains which use independent wheels, but they need complicated mechanical centering mechanisms to replace the self-centering effect, and these are more expensive to maintain.
  - dredmorbius 4y ago
    Any examples / explanations of which you're aware?
- avianlyric 4y ago
  > though it seems imperfect bc it would vary depending on the weight/load. Weight and load have no impact of the effect. The only requirement is that the wheels remain adhered to the track, and don’t slip. Which is expected anyway, because it would be tricky to accelerate and decelerate a train with slipping wheels. With independent wheels you have no self-centering effect, so only think keeping your wheels point in the right direction would be inner flange on the wheels colliding with the track. Such collisions obvious involve quite a bit of friction, noise and track/wheel wear. Not to mention independent wheels are more complex to build. Two wheels directly attached to a single fixed axel is about a simple as you can get. You only need two couplings between your single axel and non-rotating part of your train. With independent wheels you need four, or more complex wheel designs to allow bearings at the wheel/axel coupling. There no need to tip carriages into turns to prevent tipping, gravity, track banking, and speed limits already assure that tipping isn’t an issue. Tilting trains only exist to increase passenger comfort, not safety. Tilting trains are prefect capable of operating a full line speed with their tilting mechanism disabled or jammed, it’ll just be uncomfortable for the passengers.
  - IIAOPSW 4y ago
    One minor correction. Tilt trains exist to allow higher speed service on older lines where tipping would be a safety problem. They can run just fine with the tilt mechanism disabled, provided they go at the lower speed of the older trains.
    - avianlyric 4y ago
      > Tilt trains exist to allow higher speed service on older lines where tipping would be a safety problem. That isn’t true. No train network is going to allow train to operate at speed, if the safety of those trains depends on non-failure of a complex tilting mechanism. Having a tilting mechanism fail or jam before a corner (which has happened) can not result in a high speed derailment. That risk is simply unacceptable on modern railway. Tilting trains make it possible to run at higher speeds on older lines by allowing older lines to have the camber on corners increased to accommodate the higher speeds. But the camber can’t increased enough to hit optimal amount of camber without make the rail unusable to older trains. Tilting trains provide a useful compromise. Increase the camber enough for stability (but not comfort), while keeping the camber acceptable for older trains, then make up the rest of the tilt with carriage tilting mechanism. The total amount of tilt is then comfortable for passengers. There is a minor safety element provided by tilting trains, in that lateral forces in cornering are lower, thus passengers are less likely to fall, and items won’t come of tables. But the tilting mechanism certainly is a safety critical element to running faster on old lines. It’s also worth mentioning that carriages on passenger trains have a pretty low center of gravity, as all the heavy equipment (motors, bogies, breaks, suspension) sits below the passenger compartment. As a result tilting a passenger train won’t significantly move its center of gravity, so impact on total train stability is going to pretty minor.

geokon 4y ago

I get the principle of how it works - though it seems imperfect bc it would vary depending on the weight/load. But why favor such a mechanism to just independent wheels? Or are you saying the whole setup actually tilts the carriage into the turn - helping to prevent it from tipping?

mulmen 4y ago
If the wheels were independent the lips on the wheels would constantly ride on the inner sides of the rails. That increases rolling resistance and wear.
RX14 4y ago
There are some trains which use independent wheels, but they need complicated mechanical centering mechanisms to replace the self-centering effect, and these are more expensive to maintain.
- dredmorbius 4y ago
  Any examples / explanations of which you're aware?
avianlyric 4y ago
> though it seems imperfect bc it would vary depending on the weight/load. Weight and load have no impact of the effect. The only requirement is that the wheels remain adhered to the track, and don’t slip. Which is expected anyway, because it would be tricky to accelerate and decelerate a train with slipping wheels. With independent wheels you have no self-centering effect, so only think keeping your wheels point in the right direction would be inner flange on the wheels colliding with the track. Such collisions obvious involve quite a bit of friction, noise and track/wheel wear. Not to mention independent wheels are more complex to build. Two wheels directly attached to a single fixed axel is about a simple as you can get. You only need two couplings between your single axel and non-rotating part of your train. With independent wheels you need four, or more complex wheel designs to allow bearings at the wheel/axel coupling. There no need to tip carriages into turns to prevent tipping, gravity, track banking, and speed limits already assure that tipping isn’t an issue. Tilting trains only exist to increase passenger comfort, not safety. Tilting trains are prefect capable of operating a full line speed with their tilting mechanism disabled or jammed, it’ll just be uncomfortable for the passengers.
- IIAOPSW 4y ago
  One minor correction. Tilt trains exist to allow higher speed service on older lines where tipping would be a safety problem. They can run just fine with the tilt mechanism disabled, provided they go at the lower speed of the older trains.
  - avianlyric 4y ago
    > Tilt trains exist to allow higher speed service on older lines where tipping would be a safety problem. That isn’t true. No train network is going to allow train to operate at speed, if the safety of those trains depends on non-failure of a complex tilting mechanism. Having a tilting mechanism fail or jam before a corner (which has happened) can not result in a high speed derailment. That risk is simply unacceptable on modern railway. Tilting trains make it possible to run at higher speeds on older lines by allowing older lines to have the camber on corners increased to accommodate the higher speeds. But the camber can’t increased enough to hit optimal amount of camber without make the rail unusable to older trains. Tilting trains provide a useful compromise. Increase the camber enough for stability (but not comfort), while keeping the camber acceptable for older trains, then make up the rest of the tilt with carriage tilting mechanism. The total amount of tilt is then comfortable for passengers. There is a minor safety element provided by tilting trains, in that lateral forces in cornering are lower, thus passengers are less likely to fall, and items won’t come of tables. But the tilting mechanism certainly is a safety critical element to running faster on old lines. It’s also worth mentioning that carriages on passenger trains have a pretty low center of gravity, as all the heavy equipment (motors, bogies, breaks, suspension) sits below the passenger compartment. As a result tilting a passenger train won’t significantly move its center of gravity, so impact on total train stability is going to pretty minor.