The following formula is for calculating the length of the truck radius bar (or in other words, the position of the centre of the radius) of a radial loco (e.g. on an 0-6-2T or a 2-6-0 loco). This is "J.D.Baldry's Method" from the 1928 Locomotive Engineer's Pocket book, which claims to be mathematically correct, and quite independent of the radius of the track curvature. It does however assume that the front and rear wheels of the fixed wheelbase, and the wheels of the radial truck, are perfectly positionned relative to the track (equally spaced on either side) as they round the curve.
I have used this formular to generate a radius length for both LBSCR E4 and E5 models, which have then been found to run absolutely reliably over model track with the roughest pointwork and most extreme curvature.
First, measure the length of the rigid wheelbase (B).
F is half the rigid wheelbase = B/2
Then measure the distance from the centre of the rigid wheelbase to the radial axle (T)
The length of radius bar required (y) is then:
y = (T - F2/T)/2
The resulting radius is surprisingly small, but believe me, it does work!
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