33" vs 36" wheels on 70T freight cars

rdunniii posted:

Is there a technical reason why some 70T freight cars have 33" 70T trucks and others have 36" either 70T or 100T trucks?

Yes.

I think the bigger wheels are more common on the 100T trucks.    I think they might have more capacity.

For AAR standard wheels, 40, 50, and 70 ton trucks use 33" wheels,  100 ton trucks use 36" wheels and 125 ton trucks use 38 " wheels.  The 28" wheels used on Trailer Train low deck flat cars, most are used for auto racks, have a 55 ton capacity.  These rating comes both from the load caring capacity of the wheel and the wheels ability to dissipate heat generated from tread braking.  As I recall a 33 inch wheel can dissipate about 20 horsepower continuously. 

Would height be a factor?   Tunnels ?

Gregg posted:

Would height be a factor?   Tunnels ?

Probably not since the couplers/draft gear all have to have the same height in order to comply with FRA regulations. Then there is always the allowance for wheel wear, and subsequent replacement with new wheels.

Richard,

I have a pdf that I downloaded off the internet, explains about the trucks.  I would post it but I just Googled "Modeler's Guide To Freight Car Trucks" and came up with a bunch of good hits:

https://www.google.com/search?...o+freight+car+trucks

These may say the same thing, hopefully one of them will answer the question.

It amazes me how many HO and N scale trucks are available, and also how scarce O scale truck types are.

I could sure use a couple pairs of Dalman 2-level trucks, SAL and GN used they quite a bit.

The late great "Professional Iconoclast" John Kneiling was in favor of using MUCH larger wheels for freight cars, claiming that the wheel in use then and now artificially limit loaded weights and caused more track wear than the larger wheel would.  I believe he was advocating wheels at least 42".

http://mrhpub.com/2013-05-may/land/#/69/zoomed

This is a really good article about freight car trucks 1900 - 1960 with photos.