Dapol LowMac Compensation Units

Airfix / Dapol LowMacs

LowMacs run like pigs. Well maybe not the new Rapido LowMac, but most others do. Be they the Arifix / Dapol kits or the Hornby RTR models.

I have the Dapol (Ex-Airfix) kit and I tried the following approaches to make mine run better.

More weight
Alan Gibson 10.5mm wheels
Brass pinpoint bearings from H&A Models

It did run better, but not by much. Then earlier this year I spotted someone I follow (I cant remember who - maybe Watkins Wharf or East Glasson Port Authority) on Instagram using MJT compensation units for 4mm scale open wagons. This got me thinking. Could I make a kit to make the Lowmac kits run better?

So in my thinking times (when showering mainly) I began to hatch a design for adding some kind of axle articulation. And this page is the culmination of my thinking and design, and the instructions for preparing and assembling the kit.

These kits will shortly be available for purchase via my contact form or via DMing me on my social media accounts.

Below are the following sections

Tools
What's in the kit?
Optional Items
Preparation
Removal of Coupling
Assembly

Tools

To complete this axle articulation / compensation conversion you will need the following tools:

2.0mm diameter drill bit (for drilling interference fit holes)
2.1mm diameter drill bit ( for drilling clearance holes)
Junior hack saw (Or similar to cut the rods)
120 grit abrasive paper with a sanding block or flat surface, emery board
Small flat modelling or needle file.
Back to Back Gauge for OO Gauge (Or digital calliper)

What's in the kit? Variation 1

Variation 1 comes without wheels. You will need to supply these your self or drill out the original wheels.

The kit comes with the following parts:

2x Pivot Frames
2x Pivot Bogies
2x Rectangular Retaining Nuts
1x 2mm diameter x 80mm long, silver steel rod (Note actual rod is longer than the photo!)
1x rod cutting jig

Whats in the kit? Variation 2

Variation 2 comes with wheels.

The kit comes with the following parts:

2x 36" HO Scale wheels (10.5mm diameter)
2x Pivot Frames
2x Pivot Bogies
2x Rectangular Retaining Nuts
1x 2mm diameter x 80mm long, silver steel rod
1x rod cutting jig

Optional Items

You can choose to keep the wheels that came with the Dapol / Airfix kit, but you will need to open the centre hole out to 2mm diameter by drilling very accurately perpendicular to the surface. If you fail to drill accurately then the wheels will "wobble" or will be eccentric and the axle articulation will give you little benefit.

A better solution might be to use some 10.5mm diameter wheels from Alan Gibson or similar. Ensure the axle diameter is 2.0mm and the wheels do not have spigots on the inner edges which will hinder setting the correct back to back gauging. I originally used Alan Gibson - 4007 - 4mm Scale - 10.5mm Plain Disc 'Lowmac' Wheels x 2. 4008 spoked wheels may also work, but I have not yet tried them, although I have some on order.

In addition 36" HO scale wheels may also work. I have some of these on order and will update this page once I know if they will work or not.

⚠️Watch out for Peters Spares PS33 Hornby Replacement Container / Lomac 10mm Plain Wagon Wheels - These have a 2.35 mm diameter axle so will need a 2.4mm diameter drill bit for the clearance hole. They also need a modified version of the bogies which has a thinner width. Please ensure you specify if you are using these wheels when ordering. At the time of writting this I have not designed and tested these.

If you have not already done so, consider adding extra weights. There are some black 5 gram allow wheel weights which fit right in the recesses under the LowMac. using this kit will leave you with three slots for these weights.

Preparation

“By failing to prepare, you are preparing to fail” - Benjamin Franklin

Preparation for anything is important, but it is especially important with this kit. Without good preparation of the surfaces, the articulation may be stiff or notchy and the wheels may not turn freely.

The very nature of FDM (Filament) printing means there will be printing artefacts which require cleaning off by sanding or filing. Also holes will need to be drilled out.

Sand the pivot frames

The pivot frames need to be sanded on all sides until they are smooth and no longer have the 3D printing marks. Use a sanding block, kitchen tile, emery board, or some other flat surface.

Sanding is required as the items will be slightly oversize when printed. We also need smooth sides where any of the parts need to rub against another.

The top item is shown not sanded, and the bottom items is shown sanded.

⚠️ Failure to sand the sides where the holes are could lead to stiff or notchy articulation.

File the pivot frames

Using a flat modelling or needle file, file the inner edges of the pivot block on the sides where the hole is.

These sides need to be smoot as the retaining nut will rub against this.

⚠️ Failure to sand the sides where the holes are could lead to stiff or notchy articulation.

Sand the pivot bogie

Using the abrasive paper sand the sides of the pivot bogies until the printing marks are removed.

Just do a light sand on the side shown in the photo above, as we will file more this at a later stage.

File the pivot bogie

File the inner edges of the pivot bogie to remove all printing artefacts

File the pivot bogie

Using a flat file with good square edges, file the backside of the pivot bogie to remove all printing artefacts and filling in a nice clean recess.

Sand the retaining nut

Sand all sides of the retaining nut until the printing marks are removed.

Drill Pivot Frame Holes

Each pivot frame is to be drilled with a 2.1mm diameter drill bit to make a clearance hole for the pivot rod to rotate freely in.

⚠️ Ensure the 2.0mm rod rotates freely within this hole.

Drill Pivot Bogie Axle Holes

Each pivot bogie axle hole is to be drilled with first a 2.0mm diameter drill bit, and then a 2.1mm diameter drill bit (see warnings below) to make a clearance hole for the axle rod to rotate freely in.

⚠️ If you bought the kit without the HO wheels supplied, ensure the supplied 2.0mm rod rotates freely within this hole.

⚠️ If you bought the kit with the HO scale wheels included you will use the existing 1.9mm diameter axles from the HO wheels, so 2.0mm drill may be enough to get a free rotation of the axle.

Drill Pivot Bogie Pivot Holes

Each pivot bogie pivot shaft hole is to be drilled with a 2.0mm diameter drill bit to make an interference hole for the pivot rod to fit snuggly in.

⚠️ Try to drill perpendicular to the back edge.

⚠️ Do not "ovalise" or "over drill" the whole as this hole needs to grip the pivot rod.

Drill Retaining Nut Holes

Each retaining nut pivot shaft hole is to be drilled with a 2.0mm diameter drill bit to make an interference hole for the pivot rod to fit snuggly in.

⚠️ Try to drill perpendicular to the long edge.

⚠️ Do not "ovalise" or "over drill" the whole as this hole needs to grip the pivot rod.

Cut the Axle Shafts

Using the jig provided, cut the 2mm diameter silver steel rod into two lengths of between 19mm and 20mm long for the axle shafts.

⚠️ Note: If you bought the kit with the HO scale wheels included you will use the existing 1.9mm diameter axles.

Cut the Pivot Shafts

Using the jig provided, cut the remainder of the 2mm diameter silver steel rod into two lengths of between 17mm and 18mm long for the pivot shafts

Removal of Coupling

Depending upon what couplings you have fitted to your LowMac, you may need to remove this and reposition afterwards. It may be best to build the articulation unit up first and see what room you have before you remove it.

Assembly

There are two units to assemble. Assemble both units following the instructions below.

Note: ⚠️

If you are using the existing kit wheels you may need to open the centre hole out to 2.0mm diameter. This will need to be carried out with the drill bit being perpendicular to the back face of the wheel, or wheel wobble will occur and the wagon will derail.

If you are using the 36" HO 10.5mm diameter scale wheels you will need to file / grind the pin points off of the ends of the axle.

Setting out the Parts

Set out the Pivot frame, the retaining nut, and the shorter (17.0 - 18.0mm rod) on a flat surface. The holes in the pivot frame and retaining nut should be closer to the bottom. The slot on the pivot frame whould be on the top.

⚠️ Ensure the rod passes freely through the pivot frame, if not go back to the preparation step for this item.

⚠️ Ensure the rod is slightly stiff to pass into the retaining nut.

Retaining Nut and Pin

Place the retaining nut in the void with the hole closest to the lower side. The holes in both plastic pieces should line up.

Push the rod through the pivot frame and into the retaining nut. Once the pin reaches the nut, ensure hole alignment remains.

It will be difficult to push the rod into the nut as it was drilled to be the same size as the rod, and the plastic will grip the rod.

⚠️ If the rod slides into the nut too easily then you have drilled the hole in the nut too big and you may have to introduce some superglue without gumming up the mechanism. Good luck on that!

Assembly the Pin

Ensure the pin protrudes by approximately 1.0mm

Fit the Pivot Bogie

Push the pivot bogie onto the pivot shaft allowing a little end float.

⚠️ If the bogie slides onto the rod too easily then you may have drilled the hole in the bogie too big and you may have to introduce some superglue without gumming up the mechanism. Good luck on that!

Check Bogie End-float

There should be a small amount of end-float to allow longitudinal movement of the bogie.

When the retaining nut is fully right at (1), there should be a gap at (2).

⚠️ The bogie should also be able to pivot slightly giving the axle articulation required.

Fit the Wheels

Fit the wheels to the 19.0 - 20.0mm shaft

Check the Gauge of the Wheels

Using a 00 gauge back-to-back gauge, ensure the wheels are correctly gauged and rotate freely in the bogie.

⚠️ If the axel does not rotate freely the bogie axle hole will need to be drilled out slightly more.

Fit the Assembly

Finally after a dry fit check, glue the assembly to the LowMac

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