I'm not too convinced about that. Locos with small wheels will often have a higher tractive effort, but that is because the effective gearing is lower, like being in first gear in a car. Like the car, they also can't go as fast.

The actual grip between the wheels and track depends mostly on the coefficient of friction and the weight on the wheel. The civil engineering department gets upset if you increase the weight on any one axle too much, so to get more tractive effort you need to add more axles. Eventually the civil engineers will start to complain about the length of your fixed wheel base spreading the track on curves. so to keep the wheel base reasonable you make the wheels smaller, which will tend to limit the top speed. So hence express locos with fewer but larger wheels, and goods locos with many smaller wheels.

John

There must be some deformation between the rail and the wheel due to applied pressure, steel is not infinitely hard. For a given weight on a wheel a smaller wheel will have a smaller contact area so more deformation will occur as the pressure is higher. The actual friction force will not increase as that is dependent on both pressure and area so I can only imagine that an increase in traction is due to a either a greater deformation taking place with a smaller wheel or the increase in pressure effectively cleaning the rail surface better.

Martin C

Shock/Horror … sloppy use of technical language in a Hobby magazine

… Whatever next ?

MichaelG.

At a graduate engineer BR management training course some 40+ years ago I encountered an M&E engineer and asked him what kind of coefficient of friction was seen between a locomotive wheel and a rail. He stroked his beard thoughtfully and said, "Well, it normally lies between 0 and 1."

We either need to be talking about static friction, where the wheels are not slipping, or dynamic friction, where they are. The coefficient of friction is very different, and usually much lower for the slipping case. The coefficient will also be liable to change with slipping as things heat up, and in any case prolonged slipping is liable to quite quickly lead to catastrophic damage.

For the static case, the area in contact makes very little difference to the friction force. The two main factors affecting the friction force are the force normal to the contact, and the actual coefficient of friction. The diameter of the wheel is unlikely to make much difference. Neither is deformation of the wheel or the rail, at least so long as both remain within their elastic limits. Deformation will increase the area in contact, but decrease the force per unit area.

Note things are a bit different with rubber, which can give a coefficient of friction of more than 1, which is not usually possible with other substances. The point of wider tyres is not in itself to give more area, but to allow more rubber so that softer compounds can be used, to give more grip than harder ones do. But the softer ones would wear faster, so by widening the tyre there is more rubber available for a given tread depth. The downside is of course that the wider tyres will aquaplane much more readily.

Sanding will tend to increase the coefficient of friction, and I doubt if the diameter of the wheels makes much difference to this effect, compared to other factors like the wetness of the rail, the depth of leaves or snow etc. One thing I have been told about using sand is that you don't want to overdo it, as you will have to drag the whole train over the sanded rail. The sand that has been embedded in the rail by the weight of the locomotive will roughen the surface and so increase the rolling resistance of the train. Apparently this is noticeable if you use too much sand, eg leave it on once you are past where you were slipping.

John

One rightly expects the coefficient of static friction to be greater than that of sliding friction.

However, apparently,and very surprisingly, the coefficient is maximum at 15% slip!

This is employed in the low speed ground radar controlled wheelslip locomotives such as the General Motors manufactured BR Class 59, and their Class 66 successors.

Not long after the 59s were introduced I was able to witness the difference between a 59 with 100 ton hoppers and a 47 with 20 ton wagons., when checked by a yellow. The more heavily loaded 59 accelerated without problems while the 47 smoked and had to have the throttle "jockeyed" to pass the same signal post.

Howard

double post, no idea how

Edited By duncan webster on 30/10/2021 18:20:41

Apparently, the ground radar slip control cuts out above 5 mph, which ties in pretty well with Duncan's graph

Howard