what size radius is considered broad size in z scale want to run auto racks and sd70 locomotive
Radius guideline developed by Joe Fugate
Length of longest equipment
x2 Some equipment may track reliably, but 2x is generally considered pushing it.
x2.5 Most equipment will track reliably if everything is of similar length.
x3 All equipment should track reliably; coupler performance adequate if altered to allow 50% car width swing.
x3.5 Equipment will look less toy-like when viewed from inside the curve.
x4 Equipment will look less toy-like when viewed from outside the curve.
x5 Most reliable coupling on curves with body-mounted couplers and near-scale draft gear boxes.
I tried to sell my two cents worth, but no one would give me a plug nickel for it.
I don't have a leg to stand on.
To put this in rough perspective, the SD70 is just under ⅓ foot, or 4 inches, LOA. That translates into minimum 10", optimal minimum 12", and best appearance 20" by the rules given.
The proportions would change if running prototypically long passenger equipment or 'modern' auto carriers. Then x5 is 23" radius for 85' passenger and a smidge more for 89' TOFC flats or single enclosed autoracks. (The articulated ones are the same effective length per 'section' as the SD70.)