Question is about trailing steamer trucks. What determined the number of wheels used on a trailing truck? Ex. 2-6-2, 2-8-4, 4-6-0. I know passenger steamers typically used 4 wheel leading trucks and freight steamers used 2 wheel leading trucks, but a recent post about MLI 2-6-6-2 Union Pacific brought up this quandary.
size and weigh of the firebox and boiler were determining factors. as locomotives grew the fireboxes became huge. 100 square feet of grate area was not uncommon and some were much larger. to put this in perspective, 100 square feet is the size of a small bedroom.
grizlump
Larger fireboxes gave greater steaming capacity, thus increasing the horsepower of the locomotive. This resulted in higher, sustained speeds than otherwise obtained with the same weight on the drivers (tractive effort).
Mark
[:O][:O] My layout area is only 60 square feet!!! Ok, so, 2 wheel trailing trucks it is. After that info all my little RR idea’s probably wouldn’t even need any trailing wheel trucks. Which is cool, I like the look of 2-6-0’s.
For “logger” type locomotives and others where good reverse speed was desired, the trailing wheels functioned as pilot wheels in reverse and didn’t necessarily support an enlarged firebox.
Mark
I really don’t think size of the fire box was a determining factor on trailing truck axle count. Some 2-10-0 fire boxes were huge. There are a number of factors to determine how many axles are needed.
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Speed of the locomotive. 1 axle gave more stability the faster it went. The 6-8-6 turbine was built to sustain 100 MPH while running. Too bad materials at the time did not hold up and the loco was down more than up. There are some discrepancies on this. For example the 2-10-0 PRR I1sa was a 50 MPH drag freighter and the 2-10-2 N1s was rated a 35 MPH drag freighter.
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Weight on axle loadings. Heavier locos needed more weight distribution than lighter ones. Instead of more drivers they would put more pilot wheels/trailing wheels. It was hard to fit an 80 inch driver under the cab and fire box but still needed to take weight off the rear driver so a 1 axle trailing truck was used instead. I am sure the rail roads would want all drivers under their locos but facts are they would not fit.
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Late in the steam era there were some rail roads that dabbled in starting boosters. A small steam engine geared to a trailing truck axle to help in starting a train.This engine also needed another axle so a booster truck was made with 2 axles.
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Rail roads are cheap. If they could get away with fewer wheels to maintain they would. More axles = more maintenance and cost.
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Driver size. 2 locos with the same size boiler and fire box with different driver size. Like one with 50 inch drivers and one with 80 inch drivers. one would be something like a 2-8-0 or 2-8-2 while the other would be a 2-6-2 or 2-6-4 or even a 4-6-2. About the same weight and same axle count.
Pete
Considering the rather light loading of the N&W Y- class trailing trucks, it was probably more useful as an aid to tracking in reverse. Most of the firebox was above the rear drivers - easy to do when those drivers are only 56 to 58 inches in diameter.
The anthracite roads had huge fireboxes, but carried them high enough to live without trailing trucks on most of their Camelbacks. Only locos with tall drivers needed a trailing truck for clearance reasons.
OTOH, some locomotives with larger trailing trucks needed them to carry the weight of the firebox. Check out the geometry of the N&W class A 2-6-6-4. That 4-wheel trailing truck had a lot of weight above it. Likewise the 2-6-6-6. (Lima’s chief designer was a fan of huge fireboxes supported by huge trailing trucks!)
The JNR had a couple of classes of 2-8-2 that actually carried more weight on the trailing axle than they did on the driving axles. Late in the game, some of them were given 4-wheel trailing trucks and had their springing re-calibrated to make them a little lighter on the rails. (And they always used to tell us that you can’t make a Mike into a Berkshire just by jamming a four-wheel truck under it!)
Chuck (Modeling Central Japan in September, 1964)
Although it’s true some steam engines, particularly tank engines, had a trailing truck to help it running in reverse, it really does go back to the firebox.
Before trailing trucks came along, the firebox had to be narrow enough to fit between the drivers of the engine. The only way to make a wider firebox was to make the engine longer at the cab end, so the firebox could then be behind the drivers. But to hold up the weight, a trailing truck had to be added.
Eventually “SuperPower” engines came along with four- or even six-wheel trucks holding up huge fireboxes - so big that they generally required a stoker as one (or sometimes even two) firemen couldn’t provide coal fast enough to feed the huge fire.
As this discussion shows, this is a complicated question. The issue of providing a large firebox for greater horsepower is definitely a factor, but there are also questions as to the length and depth of the firebox. That is, can it be both wide and deep and located entirely behind the drivers, or does it have to be shallow enough to be carried partially above the drivers? Big engines following the the first approach included the N&W 2-6-6-4 and the C&O 2-6-6-6. Big engines using the latter type of firebox included the NP 2-8-8-4 and the UP 4-8-8-4.
In this comparison, the type of coal burned was also a factor. The C&O and the N&W had access to some of the best steam coal and designed fireboxes to use it. The NP and the UP burned poorer grades of coal and needed larger grates (the firebox floor was a grate that supported the burning fuel) to achieve equivalent heat output.
Also compare the N&W Y-class 2-8-8-2s, which had large, wide fireboxes carried on the last four relatively small-diameter drivers of the rear engine and two trailing wheels.
Four-wheel and especially six-wheel trailing trucks were usually employed to support a large firebox, but weren’t necessarily the only support for the firebox.
So long,
Andy