Hi Noel,

The pics you have included (both by me!) illustrate the issue very well.

The McGuire truck has a conventional bolster which is located as close as possible to the big (driven) wheel.
It's only marginally closer to the axle of the big wheel than the small wheel - so axle load is shared fairly equally between both.
I have also attached a pic from the "bible" of tramcar trucks - E Harper Charlton's "Electric Railway Car Trucks"

The Brill 22E truck has no bolster - the 'virtual bolster' is mid way between the springs in the foreground.
The rubbing plate on top of the springs runs in a semi-circular channel under the car body.
This means that a larger % of the weight is on the driven wheel - so it slips less.

As far as I am aware, only the Brill 22E "Eureka" truck achieved this level of shift of weight onto the drive wheel.

Mal Rowe - who spent many years puzzling about the extraordinary design of the Eureka truck

On 12/11/2017 8:03 PM, 'Noel Reed'noelreed10@... [TramsDownUnder] wrote:
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> Why is the motor suspension on a McGuire  truck as on Sydney freight car 24s different from that on a BRILL 22E truck as on Sydney F, L, LP and N trams and most other trams with “Maximum Traction” trucks ?

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> Would the traction motor of a McGuire truck mounted _outboard_ of the main driving wheels reduce the axle loading on the pony wheels by comparison with the Brill 22E truck which has the traction motor mounted _inboard_ of the main driving wheels ?

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> Would the pony wheels of a McGuire truck be less effective with braking or more likely to derail on points or uneven track due to reduced axle loading ?

>

> http://tdu.to/i/46211?size=d http://tdu.to/i/46211

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> http://tdu.to/i/11231?size=d http://tdu.to/i/11231

>