3 1/2″ Gauge Model BR Class 121 DMU

This topic has 28 replies, 8 voices, and was last updated 17 November 2025 at 20:01 by peter1972.

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17 November 2025 at 17:04 #825005
noel shelley
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@noelshelley55608
I don’t think so Peter. The bogie rotates around it’s pivot point, the hollow pivot point allows the drive shaft to do what it likes, in this case drive the wheels whilst the bogie can follow the track. Pinion drives the crown wheel on a cross shaft mid way between the axles and a small gear driving a larger gear on the axle, so you have double reduction. One might even mount the motor vertically above the bogie. Noel.
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17 November 2025 at 18:16 #825022
Paul McDonough
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@paulmcdonough43628
Hi I’m aiming for 8mph max, this is higher than scale speed, but will hopefully give a pleasing effect. My calculations are based on my class 93 design which is intended to haul several passengers on a 1:100 slope, I recall the power required was around 600 to 800Watts peak so I am considering 1kW as a maximum with a margin.

As a free running loco, non passenger hauling I would expect the power requirement to be considerably lower.
17 November 2025 at 18:25 #825024
Paul McDonough
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@paulmcdonough43628
On 17 November 2025 at 17:04 noel shelley Said:

I don’t think so Peter. The bogie rotates around it’s pivot point, the hollow pivot point allows the drive shaft to do what it likes, in this case drive the wheels whilst the bogie can follow the track. Pinion drives the crown wheel on a cross shaft mid way between the axles and a small gear driving a larger gear on the axle, so you have double reduction. One might even mount the motor vertically above the bogie. Noel.

Hi Noel, the concentric drive a logical approach, I am a bit squeamish about the necessary gearing required to drive the two horizontal axles from a vertical shaft, it also makes coupling and removing the bogies from the chassis trickier for transport. But I agree technically it’s a neat solution.
17 November 2025 at 20:01 #825035
peter1972
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@peter1972
On 17 November 2025 at 17:04 noel shelley Said:

I don’t think so Peter. The bogie rotates around it’s pivot point, the hollow pivot point allows the drive shaft to do what it likes, in this case drive the wheels whilst the bogie can follow the track. Pinion drives the crown wheel on a cross shaft mid way between the axles and a small gear driving a larger gear on the axle, so you have double reduction. One might even mount the motor vertically above the bogie. Noel.

I think in principle you could have a differential arrangement above the bogie so the torque on the hollow pivot is equal and opposite to the torque on the drive shaft, giving no net torque on the bogie.

EDIT: An approach could be to mount the ‘can’ of the motor on the hollow pivot with flexible wires to the motor (ignoring any gearbox). The motor is then effectively part of the bogie, as if it was mounted within the bogie. Does that make sense?
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