Pony or Trailing Truck

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For most of the photos on this page, you can click the image for a larger picture.

The "2" in the 4-6-2 Pacific locomotive wheel arrangement is the 2 wheel truck that supports the weight of the rear of the locomotive. This truck can be known as a

Pony Truck
Trailing Truck
Delta Truck (as the weight is taken on 3 points)

In some of the British Railways meeting meetings it is also referred to as a Bissel truck

The British Railways drawing use the term "pony truck" so that is the term used here. The Class 7 Britannia locomotives and the original Class 6 Clan locomotives were fitted with a leaf spring pony truck. This had limited room for movement and was reported to give a hard ride. The solitary Class 8 locomotive - 71000 Duke of Gloucester was fitted with a longer travel coil spring pony truck which is reported to assist in providing a very smooth ride. This pony truck was part of the drawing set for the lot 242 Clans.

Philip Atkins in his book "Britannia - Birth of a Locomotive" provides details of the evolution of the pony truck design. The pony truck consists of 4 primary components

1. The radial arm which has the main pivot bush and is bolted/riveted to the other frame components. The pivot bush takes some of the weight of the rear of the engine.
2. 2 off side frame plate assemblies
3. 2 off "Up and Over" I section stretchers fitted between the radial arm and the hind stretcher
4. A hind stretcher fitted between the side frame plates. This contains the side control gear consisting of 2 pairs of coil springs.

This is a rear view of the pony truck. The coil springs are shown, either side of the outside Timken taper roller bearing axle box. The 2 oval plates are the weight bearing areas; 2 bolster cups "float" on here which engage with bolster domes fitted at the rear of the locomotive frames. Between the 2 plates is the centring pin. This pin is bolted to the dragbox at the rear of the frame extensions and engages in a slot on the pony truck. 4 side control springs (2 either side of the pin) control the movement of the pony truck on entering curves and help recentre the truck leaving a curve. The 2 oil boxes lubricate the centring gear bushes. You can see the end of the centring gear protruding below the oil box. We have the oil box brackets in stock

This is a view from the front of the pony truck showing the central front pivot. The triangular front section of the truck is a casting, the rear section is a fabrication.

	James Conway has joined our design team and is working on deciphering the British Railways drawings. His first job was to produce a CAD model for the radial arm of the pony truck. This is a casting and here is a front view.
	A view from the rear showing how it is joined to the pony truck fabricated frame assembly.
	Starting to build up the fabricated frame assembly. The axle, spring beam and spring links are shown.
	Springs and supporting gussets added
	Side control gear added
	A cut away showing the side control springs and gear. The centring pin engages in the bronze bush
	Wheels added together with the central fore and aft strengthening ribs.
	In order to be able to visualise the radial arm, we'he had a 3D printed model made. The outer arms will be bolted to the side frame plates. The fore and aft "up and over" stretchers are bolted to the rear face of the radial arm.
	Another view of the model
	Keith has started work to convert James' 3D model into models for manufacture. First off is the fabrication model that shows all of the separate pltes which will be built up to make the complete side frame assembly. We believe that the plates for the frame keeps (i.e across the bottom of the axlebox horn guides) will need to be machined before they are welded into the structure. The different colours show the separate plates.

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