Why do some locos have pilot trucks while others do not? Is the truck there to carry significant load and/or does it have a steering function? Is a puzzelment.
Geohan
The pilot trucks, IIRC, were used for two purposes:
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To provide guidance for the locomotive into the curve - hence, passenger locomotives usually have 4 wheels on the pilot, to provide for a smoother transition than a 2 or 0 wheeled pilot.
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To help spread the engine’s weight over more wheels - for bridge clearances, etc. You’ll notice that switching engines, which require as much tractive effort as possible, do not have any pilot or trailing wheels. This is to put all possble weight on the driving wheels.
Their main purpose is to ease the entry into a curve. In contrast to our models the prototype pilot truck is horizontal spring loaded by several tons. Therefore the side force entering the curve first engages via the front truck. As they are smaller in diameter they have a smaller tendency to climb on top of the rail than the main wheels.
Without the pilot truck you end up with a switcher that can barely exceed 25mph without becoming instable.
The pilot truck eased the locomotive into curves. The top speed of steam locomotives was actually limited by the common sense of the crew. Speed could be increased until the ride got so rough the engineer feared derailment. Switchers, which never went very fast, had no pilot trucks to put all their weight on the drivers for best tractive effort. Freight locomotives, in an era when 30 mph was considered a good speed, had two pilot wheels. Passenger locomotives, which could do 100 mph by 1890, had four pilot wheels. The fast passenger engines needed more weight on the pilot wheels to give the pilot wheels the grip on the rails needed to yank a heavy locomotive into a curve. With few exceptions, the four wheel pilot truck was the mark of a fast passenger locomotive.
Funny, I never thought of UP’s 4000 class as passenger power. What kind of passenger train needs 16 drivers?
OTOH the N&W A class, with only a single pilot axle, sometimes handled trains that were usually fronted by Js.
The pilot truck has to support the cylinders. Without it, the asymmetric thrust of the pistons is handled well behind the cylinder saddle by the first pair of drivers. The usual result is called nosing, and can be a hazard to the rails as well as the locomotive at speeds as low as 10mph. That’s why some early Mallets, built without pilot trucks, were rebuilt with pilot axles to assure stability at higher speeds.
"Way back when, our host publication had an article on rebuilding a single locomotive (an early Tenshodo Big Boy) which included working centering systems for the lead and trailing trucks. That’s the only really serious one I’ve ever encountered. The light centering springs found under some brass locos aren’t in the same league.
Chuck (Modeling Central Japan in September, 1964)
To help guide some long locomotives around corners.
This is my definitive guide to things steam, and in this case, about the trucks.
http://www.sdrm.org/faqs/boilers/page115.html
-Crandell
The poster said “With few exceptions, the four wheel pilot truck was the mark of a fast passenger locomotive.” He didn’t say every engine with a four-wheel pilot truck was a passenger engine. Challengers sometimes were used on passenger trains, but that wasn’t their primary duty.
Similarly, it wasn’t all that unusual for 2-6-2 or 2-8-2 engines to be used on passenger trains, but at speeds of over 50 MPH or so, the stability of a four-wheel lead truck going thru curves was greatly preferred.
Now there is one cool web site! Thanks for the link.
I may not get anything done on my layout for another week now.
Jim