Locomotive Chassis Construction

[hartleymartin]

I am looking for some advice from people who have experience in scratch-building or kit-building 2-4-0 locomotives.

 

I am looking to build a 2-4-0. I would like to know if anyone has previous experience or recommendations.

 

At the moment, my "plan" is to build a chassis where the rear axle is solid, driven through a suitable gearbox. My thoughts are that the leading axle is not on a pony truck or radius arm of any sort, but only allowed to rock, using a central beam for support. The centre axle of the locomotive is to be sprung either with hornblocks, or springing wire bearing down on the bearings.

 

Any comments or suggestions are most welcomed.

[PatB]

I'm currently in a bit of a hiatus in building a brace of Midland 2-4-0s from Mercian kits. I'm going with the solid rear axle but have yet to decide between a combination of independent sprung leading axle and rocking centre axle (treating the coupled wheels as a four wheeled vehicle in its own right) or proper 6-wheel compensation using leading and centre axles rocking on a fore and aft compensating beam which is, in turn pivoted to allow it to rock longitudinally. The latter is probably superior, giving true 3-point support to the loco and keeping all six wheels on the track. Proportion of weight borne by leading and centre axle would be fine tuned by the position of the compensating beam pivot.

[Izzy]

I don't much favour rocking axles/single beam compensation on any loco because there is a tendency for the arrangement to make the vehicle 'wobbly' around the pivot point. It's much more prone to this than other arrangements and means carefull weighting is needed and some of the advantage of the weight can be lost on just balancing the loco rather than giving added usable traction. A rocking front axle with just a downward sprung centre one wouldn't work together very well, and allowing the front driving axle upward movement to overcome this would tend to loose traction capabilities. You really need to either compensate the two together, or spring both (just downwards).

 

As apart from P4 & S7 compensation/sprnging is more about ride and current collection than just keeping a vehicle on the track, I'd suggest twin beam compensation. Beam compensation can be just having the beams resting on top of individual hornblocks. Depending on the scale being used, 4mm/7mm etc, it is then possible to arrange for chassis screw height adjustment if desired to obtain the correct ride height much as you can with sprung hornblocks. Twin beam compensation can have the advantage in many situations of load transference so that the drivers get as much traction as possible.

 

There is no right or wrong way, some just work better in particular situations than others, and often depending on what is required for the particular loco concerned. I say this because I am not sure whether it is a 2-4-0, or a 2-4-0T that is being considered, although the basics remain the same.

 

Izzy

[John_Hughes]

I know this is heresy, but over the years I've detected a tendency in locos with the Sharman 3-point suspension for the leading axle to lift on the outside of curves and cause derailments, and I can't believe I'm the only person who's found this. (It may be a result of poor weight distribution, but of course it isn't always possible to get the weight into the right place, nor indeed to locate the pivot in the theoretical best location.)

 

Accordingly I now add springing to the mix, with either ph/b strips bearing just inboard of the bearings on both of the compensated axles or by using some other springing system in addition to the compensation.

 

Back in the 1980s, Sir Keith Joseph - then Maggie's Minister of Education, and also known as The Mad Monk - was reported to have agonised over some new policy on the grounds that although it worked in practice, he couldn't see how it could be made to work in theory. Same here - it works fine in practice, but the 3-point suspension theory's screwed.

 

Of course, a lot of North American locos used both springing and compensation beams, so at least I have prototype practice on my side!

[billbedford]

Any fixed axle in a compensated chassis will exactly follow imperfections in the track. Much fun can be had at shows watching locos judder as wheels of their fixed axle passes over point crossings.

[Torper]

I built a 2-4-0 GE E4 using a fixed rear axle and a compensating beam between the front drivers and the leading wheels which are in hornblocks in the main chassis.  The loco runs beautifully, but haulage is pathetic - one free-wheeling Bachmann coach or two wagons is about all it will manage.  It's virtually impossible to add any more weight where it matters and I keep putting off doing anything about it (possibly because I'm not at all sure what to do).

 

DT

[Izzy]

I built a 2-4-0 GE E4 using a fixed rear axle and a compensating beam between the front drivers and the leading wheels which are in hornblocks in the main chassis.  The loco runs beautifully, but haulage is pathetic - one free-wheeling Bachmann coach or two wagons is about all it will manage.  It's virtually impossible to add any more weight where it matters and I keep putting off doing anything about it (possibly because I'm not at all sure what to do).

 

DT

I presume this is a 4mm E4, Gibson perhaps? Whatever, the easiest answer here is to weight the tender and hang it onto the loco via the drawbar. Make the tender float on it's chassis towards the rear, pivoting it somehow, so the majority of the weight falls onto the rear of the loco. It is a technique used in several scales, most notably 2mm, and one answer when space isn't present in the loco to add more weight. Of course it does pay to make sure you have a free running tender/chassis in the first place, it's amazing the drag and loss of traction a poor running one can produce with a tender loco

 

Izzy

[wagonman]

I suppose Continuous Springy Beam suspension is another option see www.clag.org.uk website for (much) more information...

[Torper]

Thanks Izzy - yes, it's the Gibson kit.  I think you're probably right and the tender hanging on the drawbar is the answer.  It's something I've always tried to avoid in the past because I've always thought it would probably be difficult, but I guess I'll have to bite the bullet and get on with it - or put the E4 on the one-coach branch train.

 

DT

[bigherb]

I built a 2-4-0 GE E4 using a fixed rear axle and a compensating beam between the front drivers and the leading wheels which are in hornblocks in the main chassis.  The loco runs beautifully, but haulage is pathetic - one free-wheeling Bachmann coach or two wagons is about all it will manage.  It's virtually impossible to add any more weight where it matters and I keep putting off doing anything about it (possibly because I'm not at all sure what to do).

 

DT

That is going to be true of any loco with non driving wheels taking the weight off the driven wheels (real or model) the coefficient of friction is dependent on weight not area. An 0-4-0 loco weighing 50 tons will have the same adhesion as an 0-8-0 loco weighing 50 tons, add a set of non driven wheels supporting 25 tons of the loco weight would half the locos adhesion.

If you want it to pull something best to keep all the weight on the drivers and the additional wheels nonsupporting or nominally sprung.

[Kenton]

 

If you want it to pull something best to keep all the weight on the drivers and the additional wheels nonsupporting or nominally sprung.

But if those are allowed to "float" with no weight they will trip over pointwork. The no weight option isn't true on the prototype and certainly shouldn't be on a model. It has much more to do with the centre of balance.

[jwealleans]

I've seen a D11 built with the motor and driving axles in a rigid pair of frames but allowed to pitch about their centreline by means of a slightly loose fixing to the body. Haulage was very impressive and the riding wasn't noticeably compromised. Yes to weight in the tender and such like as well, but may be worth considering?

[hartleymartin]

Well, my proposed project is a 2-4-0 tank locomotive. Not entirely sure how it will work just yet, and the whole project may have to be put off until the start of next year, as it looks like the lease on my place is not being renewed and another move is on the cards.

[billbedford]

Just remember that the whole idea of having a fixed axle was, at the time compensation was first proposed, motors were generally of the XO4 type fixed to a frame block and gearboxes were non existent. Now there are many varied motor/gearbox designs all of which, because they are one unit, are able to maintain gear mesh whatever the orientation of the axle. To get maximum traction on a four coupled loco the weight carried by the drivers has to be equalised, therefore the optimum solution is going to evolve either twin side beams or CSBs

[Torper]

My E4 (without tender) weight 167 grams.  My experiments this morning show that a 35g lump of lead balanced on the cab roof improves performance considerably but, sadly, detracts somewhat from the appearance of the loco.   Anything less than 30g isn't very effective.  As I obviously don't want to stuff the nice big cab full of lead it leaves me with three choices, i think - first, balance a heavier tender on the drawbar, secondly rebuild the nice sweetly running chassis for CSBs, or thirdly install twin side beams as I do indeed have a "one unit" (High Level) gearbox and motor.  The main problem with CSBs is that balancing the weight of the loco is crucial and as there's not much room for weight in the rear I won't be able to put much in the front which will probably lead to a much lighter loco which is not what I want.  I'm a bit apprehensive about attempting the third choice but as I'm perfectly capable of installing single beam compensation I don't see why twin beam should be beyond me.  Having said that, what do i then do about the leading pony truck (in fact, incorporated into the chassis) - that would easily be incorporated into a CSB setup.

 

DT,

 

P.S.  There's also the Gibson and Brassmasters systems of individually sprung adjustable hornblocks.  I've never got on with them (at least the Gibson ones) in the past but in this case they might just be worth trying although it is difficult to see how they would solve the weight distribution/lack of adhesion problem..

[hartleymartin]

I know that most people go for suspension in the chassis primarily to ensure that all wheels are picking up power for smooth running. In an age where we have things like Keep-Alive/Stay-Alive DCC units, I'm not sure if this will remain the primary reason, but if we're looking for smoother running and reduced problems with derailments. One model magazine I read back in the 1990s suggested that if derailments were a problem that the solution was to fit wheels with deeper flanges! In some ways I think GOG Fine standard is a good compromise, as I have seen locomotives with near-scale wheel flanges have a terrible time trying to stay on the track. When you've spent quite a few hundred hard-earned units of currency on a model that can't stay on the track, I imagine that you get quite frustrated.

I'm starting to wonder if I should just have the leading and trailing axles rigid in the chassis with a sprung centre axle to make things easier.

[hartleymartin]

The trouble was that we were using near-scale wheels on GOG Fine standard axles and track. For a while there was a real push for super-fine-scale in Australian O gauge, but it quickly became apparent that GOG Fine standard was "standard" because it worked. The particular locomotive was a 6-axle diesel which would run through the main circuit, but would not run on diverging routes on points.

 

The track was okay (I built 90% of it!) but the baseboards on that particular layout developed a slight sag which made things quite problematic. That particular layout was retired only after about 3 or 4 years and the same group built a new layout where we used considerably more bracing and made sure that everything was stable and lined up properly. No such problems since!

[hartleymartin]

I suppose the moral of the above story is to pick a standard and stick with it, or be prepared to rebuilt/refit a lot of things to one standard.