Wheel Sizes

[matacaster]

Over the years, there have been many different wheel sizes ranging from around 7 feet downwards. What is the science behind why current wheel sizes for rolling stock or locos have been chosen? Is there an absolute minimum size that might be used?

 

[furnessvale]

Others will be along with more scientific answers BUT

The smaller the wheel diameter the lower the permitted axle load.

Additionally, with very small diameters, the desire of the wheel flange to "go the wrong way" when faced with the gaps in points and crossings increases. So much so, that the very small wheels used on rolling road wagons on the continent are banned in the UK.

 

[The Lad]

Smaller diameter gives a smaller contact area, hence lower axle load.

 

[edwin_m]

With steam locos wheel diameter was a form of gearing, as one turn of the wheels would take you further with a larger wheel. So express passenger locos had large wheels and freight locos had smaller ones, trading a lower top speed for more tractive effort.

 

[Western 52]

Others will be along with more scientific answers BUT

The smaller the wheel diameter the lower the permitted axle load.

Additionally, with very small diameters, the desire of the wheel flange to "go the wrong way" when faced with the gaps in points and crossings increases. So much so, that the very small wheels used on rolling road wagons on the continent are banned in the UK.

Most low floor trams have very small wheels. On groove rail the groove will ensure they go the right way on points, but on normal track presumably points are modified to make sure derailment can't happen? Additional check rails?

 

[edwin_m]

Most low floor trams have very small wheels. On groove rail the groove will ensure they go the right way on points, but on normal track presumably points are modified to make sure derailment can't happen? Additional check rails?

The crossing and the check rail are different on tramway points for ballasted track, to take account of the thinner flange. This also means that less of the flange has to be below rail level for it to negotiate the crossing - as well as the wheel being smaller the flange is shallower.

 

[AndrewE]

Bigger wheels = much more unsprung weight (area of wheel goes up with the square of the radius) and consequent track damage. Look at the size of Mk1 coaching stock wheelsets at a preserved railway, and compare them with what you see under modern rolling stock.
I don't know whether smaller wheels (so centre of axles lower) helps getting all the other suspension components lower down and hence the centre of gravity too?

Also allows a lower floor without having to put the wheels in pockets above floor level (as I think is the case in tube trains.)

 

[coppercapped]

If two surfaces come into contact and a force is applied to them then the materials will distort. If the load is lower than the 'elastic limit' the the surfaces will return to their original condition when the load is removed, if the load exceeds the elastic limit then the material will remain distorted after the load is removed. This is called 'plastic flow'.

Now consider a round rod resting on the head of a rail. If a load is applied then both rod and rail head will distort until the contact area increases sufficiently that the load per unit area is insufficient to distort the parts any more. For a given load the more 'pointed' the rod is (that is it has a smaller diameter) the greater the local distortion in both parts and the sooner plastic flow will occur.

So the trick is to select a rod diameter (i.e. the wheel diameter) so that plastic flow in either tyre or rail does not occur with the axle loads anticipated.

This is very simplified, there are books and books written about Hertzian stresses and the like and what I described is the static case and takes no account of the dynamic stresses which occur when the vehicle is in motion. These have the characteristic that the instantaneous loading of the contact point can be significantly higher that the static case. (And also significantly lower...!)

So the choice of wheel diameter depends on a multitude of factors: axle load; wheel, tyre and rail materials and profiles; size of gear wheels for the drive; minimum size of brakes for heat dissipation; axle and wheel bearing dimensions and so on and so forth.

There is no simple answer!

 

[Wyrleybart]

I do wonder whether those illustrious CMEs of old had the wrong idea !!!!
22x DEMUs scurrying around on tiny wheels at 125mph - it just ain't right !!!