|View single post by Herb Kephart|
|Posted: Fri Dec 18th, 2009 04:47 pm||
Very interesting! This gets into the same problem that street cars (trams) encountered in this country. It was desired to extend the wheelbase of two axle units to give less fore/aft pitching, and also to accommodate more passengers. Most cities had curves as sharp as 36 foot radius which meant that the was a limit of about 8 ft wheelbase for two axle cars. There were several ideas tried- one the Robinson Radial truck (bogie) a three axle unit which used the offset of the middle axle on a curve to steer the two end axles radially. Robinson's advertisements showed how the axles conformed to the curve perfectly--but they fought each other when passing from tangent to curved track. and vise-versa.
Another try was made by the Brill company which stayed with two axles, that could displace radially, but in doing so lifted the car body slightly because the axles moved up a slight ramp when they were other than parallel- thus the weight of the car tended to keep them parallel.
This was the crux of the problem-to allow the axles free enough movement to conform on a curve, and yet try to keep them from "hunting" on the straight.
At one point in time, my sons and I built a 7 1/4" gauge railroad behind the house. One curve was particularly sharp, out of necessity. A "diesel" very much like the current Ingersoll-Rand had no problems with this, pulling 40 (scale) foot cars. I got the itch to build a model of a Brill railbus- a gasoline powered unit with a bogie in the front, and a single fixed axle in the rear. I built a powered "mock-up" to see if there was a problem with this configuration, and was surprised, and pleased, to find none. When I built the chassis for the model I noticed that I had made the mock-up wheelbase one (real) inch too short, which I corrected, and the chassis would derail on the outside of the curve every time- the rear wheel would climb the outside rail. I cured the problem by making the front bogie steer the axle through linkage.
I wonder how Hornby has tried to keep the axles parallel? Seems like all there is is a pivot.
Fix it again, Mr Gates--it still works!"