Baldry's Rule

Baldry's rule establishes the theoretical pivot centre for a pony truck such that the pony axle will be radial to a curve.
Data has been taken from 'Building Model Locomotives' by Roche and Templer and corrected.



The position of the pony truck pivot can be found by graphical means or by calculation. You will note that the track radius is not involved. The result is correct for any practical radius.


Graphical method, refer to the drawing

First draw a straight line and mark out the positions of the axles, the outermost axles of the fixed wheelbase at A and C and the pony axle at E. Any intermediate axles within the fixed wheelbase can be ignored.

Then establish point B at the centre of the fixed wheelbase such that AB = BC.

Then draw line EG at right angles to AE and of length equal to half the fixed wheelbase, that is EG = BC.

Now join BG and bisect it to find point F where BF = FG.

Now draw FD at right angles to BG and intersecting the line AE at D.

D is the pivot point, the pony truck radius arm will have length DE.


Formula Method, again from the drawing

As shown, Y = DE = pony radius
X = BC = half the fixed wheelbase
T = BE = distance of pony axle from centre of fixed wheelbase

Formula is Y = (T-X²/T)/2


As an example take the Stanier 2-6-0.

Fixed wheelbase = 16.5ft Pony to leading drive axle = 9ft

Therefore X = 8.25, T = 17.25

and Y = (17.25 - 68.06/17.25)/2 = 6.65

Baldry's rule puts the pony axle radial to the curve but is based on pure geometry. There is no allowance for sideplay in the axleboxes or between wheel and rail. To compensate for sideplay the pony radius, Y, should be increased. Otherwise the pony will turn too far and try to climb the inner rail. Compared to the prototype our models have exaggerated sideplay so more compensation is needed. So if you run on sharp curves and have sideplay to suit, you may need to try a longer pony radius, experimentation will be needed.


underline graphic

Return to Modelling Resources