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LMS Big Bertha

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Hi to all BIG loco modellers

I am thinking of building a model (DJH kit) of the MR/LMS Big Bertha Lickey banker.

Which motor would be suitable?

What is the biggest motor that will go into the boiler/firebox?

Could I use TWO Mashima 1620s in the loco body?

I'd welcome any suggestions.


Thane of Fife

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The limitation on traction is typically not the motor output, especially if given a 40:1 or more reduction, but the weight on the driven wheels. Better to use the decent size motor with good gear choice recommended, and use all the remaining space in the loco body for the densest ballast that can be accomodated if maximum traction is required.


Practical illustration. For a garden railway with a long run and real gradients and occasional rain and frost, I had an early Triang 9F modified to loco drive with an XO4 motor and gear from a Triang Jinty grafted into the chassis block. (The rear of the original block was shaped in the standard way to accomodate an XO4 which is what suggested this conversion.) Weighted up to 800g the XO4 with only 20:1 reduction could still slip the wheels when held back. The gear set wore over time, and the XO4's magnet eventually died. 


Then the chassis was further modified to take the 5 pole Airfix motor and double reduction 30:1 gears from an N2. Quieter and even better. This was a very successful traffic unit until one hot summer day when it failed out on the line. On two axles the insulated side wheels had slipped on the insulating bushes, rods ripped through on the crankpin holes (these wore badly in outdoor service, frequent rebushings), and it was retired in this form.

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