What is the rule of thumb for how many wheels per truck for the amount of radius in the turn
is it 4 wheels per truck minimum radius 18"
or
6 wheels per truck minimum radius 18"
is there a rule of thumb for how many wheels can be used on what radius of track?
Thanks
the rule of thumb is not for number of wheels per se. the determining factor is the total length ot the car or locomotive. 4 axle locomotives are usually around 50 feet in length, they work fine on 18"r. some small 6 axle locomotives such as the alco rsd5 are also appropriate for 18" r. but most 6 axle locomotives are 65 feet or longer and thus should be operated on 22"r-24" r or greater. the same rule of thumb applies to freight cars. anything over about 55' in length should be run on 22"r or greater, less than that will work fine on 18"r......
6 wheel trucks are almost always 22 or 24" inch radius. Larger steam locos are also 22 or 24" radius or sometimes greater.
Let's clear things up a bit. A six-wheel truck, by itself, will go around a very sharp curve, probably even below 15 inches. As noted, the limiting factor is the length of the wheelbase. Now, six-wheel trucks were usually used with large steam loco tenders and under passenger cars. Large steam loco models typically want or demand larger radius curves to accommodate the long wheelbase of the drivers. As for passenger cars, a full-length (+89 scale feet long) may go around an 18" curve but the overhang looks bad and it might not clear equipment on an adjoining track. Again, broader curves are really needed for this type of equipment. Now, there have been some prototype steam-era freight cars built with six-wheel trucks that would still be OK to run on sharper curves because the overall car length is not too bad.
I have gotten the six wheel trucks of my Bachmann 2-8-4 and GS-4 around curves that are most likely sharper than 15". But it does look better to run them on wider turns, as it looks more realistic and prototypical.