Rigid Beam Compensation for Six-Axle Trucks

In yesterday’s post about lunch with Chris, I mentioned my frustration with the three-axle trucks under my passenger cars. As I noted,

I’m fed up with the less-than-perfect performance of the six-wheel trucks under my passenger cars and with Chris’ help I plan to fix the problems. (The passenger cars run well about 90% of the time – but that last 10% of frustration needs to be addressed. There’s no reason they can’t be 99.9% reliable.)

Dan Vandermause wrote in the comments section to ask about our plans. Dan – thanks for writing. As you noted, these trucks are a real problem.

I would love to see someone like Dan Navarre at River Raisin Models offer a set of replacement three-axle trucks for the American Models heavyweight passenger cars. Maybe if enough of us ask? Dan has done runs of trucks before – his website lists a pair of Triple Bolster (four-wheel) passenger trucks for SP, UP, NYC, PRR, and IC. So it’s not unreasonable…

Failing that, though, I’m looking at the rigid beam compensation system many British modellers use for their equipment. This has been written about extensively – Google “Flexi-Chas” and you’ll see what I mean.

There’s an excellent book by Mike Sharman, published by Oakwood Press, that explains the system and offers several examples of how to deploy it. It’s out of print, but used copies can be found fairly easily online.

There’s also a great online resource by Russ Elliot called The Principles of Locomotive Suspension that covers many configurations. The configuration used for an 0-6-0 is perfectly suitable for a three-axle passenger truck – and can be found in Section 15 of the Principles. For American Models three-axle trucks, one outboard axle would be rigid while the centre axle and other outboard axle would be linked with a rigid beam compensation system.

My plan is to create a kit – principally, an etched fret – that builds into a subframe into which the wheel sets would mount. This subframe would also include mounts for the American Models side frames, which look fine to my eye and which would be cosmetic in this application. There are so many mechanical problems with the side frames that I think it’s best to just have them along for the ride.

The stumbling blocks? I have no experience doing CAD work – for etching, or anything else. I’m looking at courses, but in the meantime, Chris has been recruited. That said, Chris and I don’t know when we’ll tackle this project since there are so many other things to do – in the hobby and without. It could be years. Therefore, if anybody else reading this has experience designing photo-etched frets and wants to work with me on this project, I would love to hear from you!

6 thoughts on “Rigid Beam Compensation for Six-Axle Trucks

  1. UK firm Brassmasters make etched bogie compensation units for four-wheel trucks in OO/EM/P4.

    I am still mulling over tweaking the idea to produce a few pair to go under the truck sideframes for some resin HO CPR coach kits.

    A six-axle variant would be very useful, and could be easily produced in many scales, not just S.

    • Hi Steve:

      Yes – I found those while doing some online searches and that’s similar to what I’m thinking of in terms of a kit. As the image shows on the Brassmasters page, the kit builds into a subframe to which any sideframe castings of the proper wheel spacing can be attached.

      And as you say, such kits would be useful in other scales too.


  2. “For … ….three-axle trucks, one outboard axle would be rigid while the centre axle and other outboard axle would be linked with a rigid beam compensation system.”

    Um, not necessarily. The system you have described is rather simple, and the rigid axle means that the ride is a bit lumpy – on a loco built thus, when the rigid axle hits a bump or dip in the track, the whole loco lurches. A better solution is to have one axle pivoted at its centre point (directly bearing on it is fine) with the other two axles compensated by “twin beams”, I.e. arranged longitudinally, in exactly the same manner as the compensation beams on an “American” 4-4-0. However, as we are talking here about two 6-wheel trucks, this may be less of an issue. However, you need to arrange one truck to have a ball type pivot, so that the truck is free to take up any angle relative to the body above it, and or the other to have some form of restraint from rocking side-to-side, a piece of bar either side of the pivot, perpendicular to the track, provides stability to the body. In essence, this is primary three-point suspension, with secondary three-point suspension feeding into it…

    A couple of other thoughts to throw into the pot. Many years ago now I built a British outline 6 wheel brakevan (caboose in the USA, just a van to Canucks) which had (scaled own) 36″ wheels set at a total wheelbase equally divided of 10′. I some what lazily compensated this with a single rocking ale at one end, and a fixed one at the other. The central axle had its pin-points removed, and ran in a piece of tube. The piece of tube was attached to a very light, 0.012″ (guitar string) sping wire, arrange to keep the wheels on the track, but take no weight – on a model, we do not need to worry about the axle loadings. You might try this arrangement in conjunction with springing between trucks and coach.

    Trevor Nunn came up with a very effective solution for his 6 wheel Victorian carriages. Each axle was sprung, using 0.010″ thick nickel silver sheet, cut to a fat-stemmed T shape. The stem was cut down the middle from the top of the T, but not to the full length: for the centre axle, this could be a longer cut, to create a less stiff spring. At the extremities of the T-bar, there was a hole o accommodate the axle, as an inside bearing in this case. The sides of the cross arm, outside of the stem, we’re folded own at right angles, such that the axle holes are lined up. If this unit is fixed at the base of the stem, to a piece of 0.032″ metal, then it has an upper limit on movement with an allowance for the weight of whatever is above it to compress things, but retains the ability to absorb shocks. If the idea of running the axles within thin metal holes does not appeal, then brass bushes or ball-races can be used.

    Personally, I think some degree of spring is a good idea, evn if just to proved a smoother ride. It is also an idea to have a slightly lighter spring on the centre axle, on a model, to prevent the truck rocking about the centre axle.

    Hope that helps. I can provide preliminary sketches, if required.


    • Hi Simon:
      Thanks for the additional info. It shows there are more than one way to skin this particular cat. I’d be happy with a system as I described. And thanks for the offer but I don’t need more sketches – there are plenty in the Flexi-Chas book and elsewhere. What I really need is someone with CAD skills to draw the artwork for photo-etching a subframe. That way, others with the same issues can address them too. That’s why I’m throwing this out to the wider world: I’m looking for someone with the skill set I need to make this happen.

  3. Hi Trevor,

    I have the molds for Commonwealth sprung brass 6 wheel brass passenger trucks that I haven’t offered for some time. At today’s costs a KIT would sell in the $70-$80 range with my 36″ stainless steel RP25 code 110 wheelsets.

    I have seen most of the equalization references mentioned and look forward to any other ideas from this thread. I too am working on a solution for rigid six wheel trucks. I would like to change my Buckeye trucks to one piece sideframes and lower the price which now would be similar to the Commonwealth 6 wheel trucks I mentioned.

    • Hi Fred:
      Maybe we should talk about working together on this. I’m convinced compensation is the way to go. I’ll email you privately…

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