2-8-0, 4-6-2, 2-6-4... so many variants but why?

[GunnerBill]

2-8-0, 4-6-2, 2-6-4... steam engines come in so many wheel arrangements but why?

OK I figured out that the "6" in the x-6-x is the drive and I guess the more the better for heavy freight and of course the amount of big drivers limits the radii they can operate on but what puzzles me the most is why the front & back bogies vary so. For example why were the 2-8-0's not 4-8-2's or 4-8-0's etc...

Can someone give me a clue to how the front and back wheel arrangements were arrived at?

Thanks!

[Zunnan]

Its mostly to do with weight distribution and axle loading to achieve certain route availabilities. In other countries it was also used as a way to reduce rail poundages, and therefore reduce construction costs for actual routes.

A 100 ton 0-6-0 (3 axles) will have an axle loading of 33.333 tons, which is pretty high and will require strong rails and infrastructure to support it. Add on two axles and build it as a 4-6-0, 0-6-4 or 2-6-2 with the same 100ton weight and the axle loading can reduce to 20 tons, therefore it can run on lighter rail/routes and causes less damage to the track. The exact wheel arrangement will depend upon the balancing of the locomotive to make it more stable when running. This 100 ton design may be required to pull heavier loads at lower speeds so is then likely to be redesigned with more driving wheels of a smaller diameter, so it emerges as a 2-8-0 or even 0-10-0, but still maintains the same axle loading so can run on the same route as its 4-6-0 cousin. You also want to maintain as much weight on the drivers as is possible to keep the tractive effort as high as possible, so the fewer non powered axles the better, especially where high starting tractive effort is required. It gets a whole lot more complicated than this, but that is the basic idea.

[GunnerBill]

Nice reply, thanks!

[b308]

To add to the reply, the reason you have a trailing bogie (as in the -6-2 or -6-4 arrangements) is to allow for a bigger firebox on tender locos. The reasoning behind it is that on a loco with no trailing bogie the firebox has to go down between the frames and is thus smaller and narrower, whereas if you put a bogie behind the drivers it allows the frames to be modified and a much larger firebox to be fitted, thus increasing ease of steaming and more power... The best one to look at as an example is the Stanier Duchess, which has one of the biggest fireboxes on a British loco. When used for the tests they had to use two firemen to keep up with its appetite for coal!

In addition to the spreading of weight using bogies front and back the size of the driving wheels is also critical, if you want high speed then you need bigger driving wheels, so freight locos tend to have driving wheels of around 5' (8fs, etc), mixed traffic locos (such as Black 5s, Halls and B12s) around 6' and Express Locos over 6' 6" (Stirling singles were, I think, 8'!, though singles were known for slipping and as trains got heavier were discontinued)...

Initially diesel and electric locos had similar layouts, but it was found that they could have smaller wheels in bogies so now they are usually 3' to 3' 6"... though, to me, the wheels and bogies on 220 always look far too small for a 125mph train!

[bike2steam]

To add to the reply, the reason you have a trailing bogie (as in the -6-2 or -6-4 arrangements) is to allow for a bigger firebox on tender locos.

Or as the GWR, the frames on tank engines were extended to take a bigger, longer range, bunker, such as their 8-coupled tanks. Some 2-8-0T's became 2-8-2T to help spread the extra weight.

[b308]

Or as the GWR,

And the rest of them!

[GHOSTFAN]

It was all part of a conspiracy between the Railway Companies and the Model Railway manufacturers to exploit us poor folk and to squeeze every last penny piece out of our already depleted pockets.

[Dysonsphere]

Just for extra info if you look at the 9F 2-10-0 the centre driving axle is flangeless due to min radius problems.

[Random]

I hadn't heard that any real locos had flangeless wheels on the middle axel before. It's very common in models, but we often use a lot tighter corners than real life.

[b308]

I think one of the 19th century "Singles" had a flangless driving wheel... also several German narrow gauge engines had some wheels flangless as well, its not that uncommon... Another variation which could look similar is a reduction of the depth of the flange, "reduced flanges", which has a similar effect.

[son of triangman]

there's also the GER Decapod, I believe the centre drivers were flangless on that. It's pretty common on long wheelbase locos to have a centre set of flangless driving wheels.

[GunnerBill]

I hadn't heard that any real locos had flangeless wheels on the middle axel before. It's very common in models, but we often use a lot tighter corners than real life.

Cheers - that's answered one question I was thinking about asking!

[UrbanHermit]

In addition to the spreading of weight using bogies front and back the size of the driving wheels is also critical, if you want high speed then you need bigger driving wheels, so freight locos tend to have driving wheels of around 5' (8fs, etc), mixed traffic locos (such as Black 5s, Halls and B12s) around 6' and Express Locos over 6' 6" (Stirling singles were, I think, 8'!, though singles were known for slipping and as trains got heavier were discontinued)...

You don't necessarily need big wheels for high speed. 9Fs (5' drivers) were timed a couple of times at up to 90mph on passenger trains, and the Norfolk and Western J class 4-8-4s regularly exceeded 100mph in service, with 5' 8" drivers - or so it is claimed.

But yes, smaller wheels are better for slow heavy pulling (the locomotive is effectively "lower geared") and bigger ones are better for high speed (less wear and tear on the mechanism for a start, which is why the 9Fs' speedy exploits were curtailed by a 60mph limit being slapped on the whole class).

Just as important for high speed is good valve design - basically big valves in relation to the size of the cylinders, to get lots of steam in and out of the cylinders (automotive engineers call it "good breathing"). The sprightliness of the 9Fs was due to their big valves.

[b308]

I think that you will find that with steam engines driven in the conventional way (via coupling rods) that there is a limit for each size of wheel which takes into account both the max speed the rods can rotate, and, more important, the hammer blow effect on the track caused by the rods going round too quickly for comfort... the latter was the reason they put a stop to high speed runs with 9fs.

When you introduce direct drive to each axle like on an electric and diesel electric then you can have smaller wheels... hence those tiny ones on the 22X classes that can do in excess of 125mph!

I think it was Webb that tried divided drive late in the 19th century but it wasn't a success... it was tried again in the late 30s in Germany for a High Speed steamer, but whilst it was successful (like the Turbomotive), WW2 got in the way and when the dust settled it was obvious that diesel and electric traction was the way forward so was quietly dropped.