Wheel widths. Hypothetical question

[shakey1961]

Hi all.

Was out driving today and in front of me was an Austin 7. Its wheels and tyres were so thin it looked like it had been fitted with bicycle tyres. All modern cars have tyres that are thick giving more contact on the road surface which in turn gives better handling, safety etc.

As far as I'm aware (and have seen) train wheels appear to be a standard thickness.

If rolling stock wheels and rails were thicker, would this help or hinder the ride, comfort, safety, speed and adhesion for a train?

Just curious.

Many thanks.

 

[158747]

Rail wheels are different to road wheels. On road wheels the wider they are there is a greater surface area in contact with the road, increasing the tyre's grip on the road. On rail wheels, although they have a much smaller surface area in contact with the rail the width of the wheel is dictated by the width of the rail.

 

[AM9]

Road wheels need to resist lateral forces by friction. The moments of force try to pull the compliant tyres off their wheel mountings. Wider rims and tyres provide greater road contact and better resistance to the tyres rolling off the rims.
Rail wheels are guided by their profile and track cant to maintain a consistent position. The flanges are really there to limit the extremities of lateral displacement. Traction is by friction where the small surface contact area results in much higher pressure and (usually) grip. Thicker wheels would:

a) increase the lateral friction which does nothing for stability but increases rolling resistance
and
b) increase the train's deadweight.
​

 

[Trog]

Is friction proportional to load?

So if you double the area of contact for the same load do you roughly end up with twice as much friction area at half the rate per unit area.

 

[DelW]

Is friction proportional to load?

So if you double the area of contact for the same load do you roughly end up with twice as much friction area at half the rate per unit area.

In theory, the frictional force available is proportional only to the coefficient of friction between the two materials, and the force applied perpendicular to the friction surface. The contact area has no effect on the calculation.

Road tyres have got wider in part because there is a limit to the lateral stress the rubber itself can carry, so wider tyres can resist more lateral load without overstressing the rubber. If you watch F1 racing you will know the track gets littered with small balls of rubber which have scrubbed off the tyres.

A quick rough and ready calculation suggests the contact patch between a steel wheel and a steel rail is only around the size of a 10p coin, so widening wheels and rails would have no effect.

 

[shakey1961]

Simple when you know how. Wish I knew a lot more about engineering.

If the contact patch on a standard train wheel is about the size of a 10p, would it therefore be increased along the width say to two 10p coins if the wheels were twice as wide?

 

[DelW]

Simple when you know how. Wish I knew a lot more about engineering.

If the contact patch on a standard train wheel is about the size of a 10p, would it therefore be increased along the width say to two 10p coins if the wheels were twice as wide?

The contact patch size is a function of several variables. Because the wheel tread surface is a curve (the wheel being circular), and the rail top is a curve in cross section, if both were infinitely stiff the contact patch would be a point (or possibly a line if the rail top is locally flat). Because neither is infinitely stiff, the steel deforms locally under the applied load until the contact patch has reached the size needed for the applied stress and deformation to be in proportion to each other.

I'm a civil not a railway engineer, and the subject of wheel / rail interfaces is complex and probably still not adequately understood even after nearly two centuries. The Americans run regular conferences on the subject:
http://www.wheel-rail-seminars.com/2014hh/schroeder_smak-abstract.php

and there are probably similar events here.

 

[Flying Phil]

Interesting thread, but another factor is that the full width of the railway wheel "tyre" is not in contact with the rail as the tyre is slightly conical to give a self centering effect. Looking at the rail head after an overnight rusting and first train/few wheels,shows the contact width to be about two or three cms.

 

[pdeaves]

If the contact patch on a standard train wheel is about the size of a 10p, would it therefore be increased along the width say to two 10p coins if the wheels were twice as wide?

No, because the rail is curved profile and the tyre a sort-of 'curved cone' profile. Make the tyre wider and there will just be more metal hanging over the edge not doing anything useful.

 

[daikilo]

Simple when you know how. Wish I knew a lot more about engineering.

If the contact patch on a standard train wheel is about the size of a 10p, would it therefore be increased along the width say to two 10p coins if the wheels were twice as wide?

No. Another element is the profile of the rail and of the wheel and they are optimised for directional stability.

The extremity of the wheel's rolling surface (the bit outboard of the flange) only ever touches the rail during a derailment.

The only way to increase the rolling surface is to increase the wheel diameter (and thus perimeter). That is why steam locomotives produce high tractive effort by having a few large wheels.

 

[DelW]

Interesting thread, but another factor is that the full width of the railway wheel "tyre" is not in contact with the rail as the tyre is slightly conical to give a self centering effect. Looking at the rail head after an overnight rusting and first train/few wheels,shows the contact width to be about two or three cms.

That tends to back up my rough and ready calculation, a 10p coin is 2.4 cm across

Making a range of somewhat uneducated guesses at the input variables gave me results in a range about 18mm - 25mm.

 

[Flying Phil]

No. Another element is the profile of the rail and of the wheel and they are optimised for directional stability.

The extremity of the wheel's rolling surface (the bit outboard of the flange) only ever touches the rail during a derailment.

The only way to increase the rolling surface is to increase the wheel diameter (and thus perimeter). That is why steam locomotives produce high tractive effort by having a few large wheels.

A bit more complex than that as Tractive effort will depend upon boiler pressure, cylinder dia, number of cylinders, stroke of piston and wheel diameter. In fact freight engines (8F/9F) have more , smaller diameter driving wheels.

 

[pdq]

There may be some interesting reading from the University of Huddersfield's Institute of Railway Research: http://www.hud.ac.uk/research/researchcentres/irr/

 

[Flying Phil]

In theory, the frictional force available is proportional only to the coefficient of friction between the two materials, and the force applied perpendicular to the friction surface. The contact area has no effect on the calculation.

Road tyres have got wider in part because there is a limit to the lateral stress the rubber itself can carry, so wider tyres can resist more lateral load without overstressing the rubber. If you watch F1 racing you will know the track gets littered with small balls of rubber which have scrubbed off the tyres.

A quick rough and ready calculation suggests the contact patch between a steel wheel and a steel rail is only around the size of a 10p coin, so widening wheels and rails would have no effect.

Re the road tyres, as DelW says the theory shows contact area does not have an effect, but in reality the coefficient of friction (Mu) can have a value greater than 1 for rubber on roads so it becomes an "Adhesive" effect which means more area = more grip. Hence wider tyres and - for drag racing, "Wrinkle-wall" tyres which put even more rubber in contact when accelerating.

 

[Deepgreen]

No. Another element is the profile of the rail and of the wheel and they are optimised for directional stability.

The extremity of the wheel's rolling surface (the bit outboard of the flange) only ever touches the rail during a derailment.

The only way to increase the rolling surface is to increase the wheel diameter (and thus perimeter). That is why steam locomotives produce high tractive effort by having a few large wheels.

Be careful there - the highest tractive effort steam locos are often freight examples, with much smaller wheels. Also, freight diesels with small wheels can produce huge TEs. It is a complex combination of wheel/axle loading, wheel diameters and other, lesser, factors. To answer the OP though, the steering function of conical flanged wheels on rail means that the tread contact will always be much as now. However, it's worth noting that some steam locos with many coupled wheels (2-10-0 and upwards) have flange-less centre driving wheels of a flat profile to avoid binding on curves.

 

[snowball]

The only way to increase the rolling surface is to increase the wheel diameter (and thus perimeter).

Or, presumably, use a softer grade of steel - but then it won't last as long.

 

[edwin_m]

A smaller wheel will have a marginally smaller contact area and therefore a higher stress if the axle load is the same. However a freight steam loco will have more (smaller) wheels than a passenger loco of the same size, so the axle load will be less and although the maths is too complicated for me I guess the stress in the contact patch will end up roughly the same. There is an upper limit on the amount of stress when the wheel or (more likely) the rail exceeds its elastic deformation limit and starts to change shape permanently. This is most usually seen near rail joints where the impact of wheels on the joint increases the forces and batters the rail.

The reason smaller wheels mean more tractive effort on a steam loco is because the ratio between the piston stroke and the distance travelled is less with a smaller wheel. Hence, assuming the piston area and steam pressure are the same, each stroke of the piston moves the loco a shorter distance but exerts a greater force on the rail.

 

[R4_GRN]

At an interview for a job, not with the railway, I was asked why steel railway wheels were tapered, the only thing I could think of at the time was that this was to give the effect similar to a differential in a car to compensate for the greater distance travelled of the outer wheel.

Was my quick think correct? I was never told at the interview whether I got it right!

 

[SquireBev]

At an interview for a job, not with the railway, I was asked why steel railway wheels were tapered, the only thing I could think of at the time was that this was to give the effect similar to a differential in a car to compensate for the greater distance travelled of the outer wheel.

Was my quick think correct? I was never told at the interview whether I got it right!

It's more to keep the train centred on the tracks. I don't think railways ever have bends sharp enough to warrant any kind of differential effect.

 

[Flying Phil]

At an interview for a job, not with the railway, I was asked why steel railway wheels were tapered, the only thing I could think of at the time was that this was to give the effect similar to a differential in a car to compensate for the greater distance travelled of the outer wheel.

Was my quick think correct? I was never told at the interview whether I got it right!

I think you are pretty close with that, as it also provide the self centering effect ie if the axle goes to one side of the track then the slightly larger diameter of the cone will "steer" the axle back to a central position.

 

[snowball]

I don't think railways ever have bends sharp enough to warrant any kind of differential effect.

They have bends sharp enough to cause squeal, so there's a problem that the taper does not entirely solve.

 

[edwin_m]

The taper does indeed do both things - the wheelset steers itself across the track until the difference in diameter at the two points of contact matches the difference in distance travelled. On tighter curves either the difference in diameter isn't enough or the dampers impede bogie rotation, so the flange comes into contact on one rail and one or both wheel treads are slipping.