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Austrains C30 Class Tank Loco Suspension Issues


Background

In early 2018 Austrains released their HO scale model of the NSWGR 30 class tank locomotive. Austrains offered thirteen road numbers, three bunker variations, and headlight/no headlight models. They are supplied "DCC Ready" and fitment of a DCC decoder is the responsibility of purchasers.


Austrains C30 straight from the box


Wheels are NMRA standard RP-25/88 profile with bright finished tyres. A feature of this model is electrical pickups on all wheels. The pony trucks have sprung suspension and are fitted with undersized wheels to provide clearances that allow free transit around 18 inch radius curves. Larger, prototypically sized wheels are also included in the box, and these can be easily exchanged by those modellers who do not require sharp curve operation.

I purchased one of these models and tested it, straight out of the box, using the undersized pony truck wheels fitted at the factory. Almost immediately, it became clear that the pony trucks did not track correctly, and the locomotive was prone to derailment rendering it unfit for service.

Investigation

Careful inspection of each derailment revealed that pony truck wheels were lifting on the smallest of track undulations, where no other loco or rolling stock had problems. With the loco inverted on a workbench, signs of binding in the front pony truck suspension were evident. Further examination revealed that the front suspension spring was not sitting squarely over the pony truck, and appeared to be skewed off centre. See photo, below.

Austrains C30 skewed suspension spring

Issue 1: Spring binding

Next step was to unscrew the bogie pivot and inspect the spring. This revealed that the spring was not wide enough to straddle the pivot slot through the bogie: the spring can partially drop into the slot, skewing it off-centre and binding the suspension. The factory fitted spring is only 3.4mm diameter but the top spring seat can accept a larger diameter up to 4.4mm. The diameter difference allows the spring to move sideways. I reckon the factory has fitted wrong sized springs.

Issue 2: Wheel diameter and pony truck ride height

The prototypically sized C30 pony truck wheels supplied with the model are 10.8mm diameter, but Austrains has fitted optional smaller 9.4mm diameter wheels to improve clearances and allow the locomotive to transit 18 inch radius curves. These smaller wheels lower the pony truck ride height by 0.7mm. Unfortunately at this lowered height, the truck is very close to its lower limit of travel which is restricted by the bogie pivot screw. If the locomotive enters a dip in the track, the truck bottoms out on the pivot screw head, and wheels may be lifted off the rails, resulting in derailment.

What if the loco is fitted with large pony truck wheels?

I haven't investigated this set-up. In theory, the raised pony truck ride height should allow the wheels to track a little deeper into dips. I don't know whether the spring binding problem would be affected. It may be that some modellers using the large diameter wheels do not have problems with the C30 suspension.

Solutions

The spring binding issue can be addressed by fitting a larger diameter spring of similar length and stiffness. Century Springs S-1093 is such a spring. It does have a higher stiffness and will decrease weight on the driving wheels, slightly. A softer spring that might work is the Century Springs NN-82. I have fitted S-1093 springs to my locomotive, it still has ample traction, and the binding issue is resolved. The springs cost about $4 each. NOTE: I have been contacted by another modeller who has confirmed that the softer NN-82 spring works, and may be preferred if maximum tractive effort is required.
DISCLAIMER: Fitting replacement springs to your Austrains C30 may void the warranty.

Pony truck vertical travel could be extended by fitting a longer bogie pivot bolt. Finding a suitable, longer bolt may be difficult, however unscrewing the pivot bolt several turns has the same effect. The thread could be locked (using Loctite or similar) but a better solution is to fit a small bush or washer to the spring seat screw hole, so the bolt is extended slightly. The bush must be small enough to fit inside the suspension spring and this might be difficult with the original, undersized springs. I found a suitable Kadee coupler bush that fits neatly inside the Century S-1093 spring, and does the job. See pictures below.
NOTE: Austrains advises that the electrical pickup wires are delicate. Take care not to damage them.

Austrains C30 pony truck suspension pivot
Parts that make up the modified suspension pivot. S-1093 replacement spring, additional packing bush, original pivot bolt and insulating collar.

Austrains C30 pony truck suspension pivot
Bush fitted to pivot bolt, showing how it extends the bolt. The spring is first placed under the truck, the bush is removed from the bolt and placed on the spring seat, inside the spring.

Austrains C30 pony truck suspension pivot
With the spring and bush in place, the pivot bolt and collar are then screwed in to secure the truck.

Austrains C30 modified pony truck suspension
The modified suspension in place, with S-1093 spring fully extended. Note the position of the pony truck, which can now swing down below rail level to safely track through undulations and changes in gradient.
Compare this to the picture showing the skewed spring near the top of this page!


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