Number of Cars G scale

[mike miller]

Is there a limit to the number of cars a g scale engine can pull on a flat grade

[Chuck N]

Yes

It depends on many factors; what engine, weight of engine, drag (friction) of the car wheels, diameter of curves.  I have an engine that pulls 32 cars with out any problem.  I have others that top out at several cars.

As a rule of thumb.  The draw bar pull, amount of drag an engine can pull, is about 1/3 the weight of the engine.  That is not the weight of the cars.  Just the rolling resistance.

Chuck

[Kevin Strong]

Sure is, but it varies by each individual locomotive. And--like the prototype--that number decreases exponentially as the grade increases. Also, the radius of the curves on your railroad also impact the amount of cars you can safely pull. The tighter the curves, the more drag they introduce. There's no magic formula. Garden Railways magazine "estimates" every pound of drawbar pull that a locomotive develops to equate to about 13 "average" cars on the straight and level track. Most locos develop between 1 and 2 pounds of drawbar pull depending on their weight.

Generally speaking, our model locos can pull far longer trains than the prototypes they're based on could dream of. I'm a fan of keeping the trains realistic, based on what a loco could pull on "average" grades (2% or so). Something on the order of Bachmann's 0-4-0, I'd limit its train to 3 to 5 cars at the most. These were small locos used primarily for switching or moving strings of short industrial cars around flat yards. They were seldom "road" engines, so you'd not see them pulling passenger cars or things like that. Something like Spectrum 4-4-0 or 2-6-0 (the fancy 1875 locos), probably 4 or 6 cars. These locos looked beautiful, but in reality were not all that powerful. When the "Eureka" runs on the Durango & Silverton, it's usually pulling just a passenger car and a caboose. Locos like the 2-8-0s (both the outside frame and C-19), and 4-6-0 would probably max out at 8 - 10 cars. A large loco like the K-27, probably 12 - 15 cars would be a prototypical consist on a 2%. Know that when the railroads encountered steeper grades (3 - 4%), they had to bring in extra locos to get those trains up and over. The logging locos (Shay, Heisler, Climax) were "geared" locomotives, whose prototypes were designed to move heavy trains up steep grades, but even with those, I'd probably say 8 cars or so for the small Climax, 10 - 12 for the 2-truck Shay and Heisler, and the large 3-truck Shay could probably go to around 15 cars.

If you use the prototypical trains as a guideline for your locomotives, you usually will be able to run those trains with your model locos around most garden railroads without worry. But keep in mind that--just like the prototype--steep grades and sharp curves will affect how well the trains run. If your railroad has curves of 4' radius or tighter, and/or grades of 3% or greater, I'd consider reducing your train length by two cars or so.

Happy Thanksgiving!

K

[Chuck N]

Several years ago I measured the pulling power of some of my engines.  The results were posted in a table over on My Large Scale.  Here is a picture of how I measured the tractive effort.  I used a fisherman's scale and turned up the power on the track until the wheels started to slip.



Here is the table with the results.



You could also measure the drag of a string of cars by pulling them with the scale and noting the weight.

Chuck

[Kevin Strong]

To give an idea of how grades affect the pulling capacity of the prototype, consider this excerpt from an early Baldwin catalog entry for their Class 8-16 D locomotive, which is the prototype for Bachmann's "Spectrum" 2-6-0 mogul.

On the level - 720 gross tons
20' per mile - 335
40' per mile - 210
60' per mile - 150
80' per mile - 110
100' per mile - 90

To equate that to percentages, 100' per mile is 1.9%--just shy of 2%. So at a very mild 2%, this locomotive can pull 1/8th of what it can pull on the straight and level. At about 1/3 of one percent (0.38% or 20' per mile), the pulling capacity of the locomotive is more than cut in half.

The same catalog contains letters from the superintendents of various railroads around the country describing how the locomotives pull. A mogul of this class was rated by the Mineral Range RR as being able to pull 10 empty hopper cars up its 4% grade (211' per mile). They list the empty weight of these cars at 6.25 tons, so that would be 62.5 tons total. These same cars weigh 18.75 tons when loaded, so if this loco were to have to move loaded hoppers up a 4% grade, its trains would be limited to around 3 cars! On the flat, it could pull over 30! At 2%, it would be able to pull around 5.

That same pattern is common to locomotives of all sizes in the catalog. Typically capacities on 0.38% (20' per mile) are about half of the "flat" rating, and 100' per mile are about 1/8 to 1/10th of the "flat" rating. The East Broad Top operated three locos identical to one listed in this catalog (10-22 E), a 2-8-0 rated for 1060 gross tons on the flat. The ruling grade on the EBT is 2.65%; these locos could pull 15 empty wood hoppers plus a passenger car up that grade (80 tons total).

Note, too, that prototypical train length has a lot to do with the capacities of the cars as well. For instance, the early EBT wood hoppers mentioned in the example above weighed in at 9500 pounds empty, and had a capacity of between 9 and 12 tons. The "modern" steel hoppers run by the EBT weighed in at more than 20,000 pounds, with a capacity of 30 - 40 tons depending on the car. So these early locos might have been good for 15 lightweight cars, but they'd be struggling to get maybe half that number of "modern" cars moving.

Later,

K

[mickeykelley]

Kevin, over past year or so I've been looking ahold pics, etc of various logging enterprises and seems to me like I've seen 2 truck Shays with more logging cars than 10-12. Same for minning cars. Not that I can remember the pic but seems like a fare number more. Now I'm curious and will be more observant on it in future as a matter of curiosity.

[tac]

Kevin, over past year or so I've been looking ahold pics, etc of various logging enterprises and seems to me like I've seen 2 truck Shays with more logging cars than 10-12. Same for minning cars. Not that I can remember the pic but seems like a fare number more. Now I'm curious and will be more observant on it in future as a matter of curiosity.

Couple of things to bear in mind here - ALL geared locos, be they Shay, Climax or Heisler - have ALL those little wheels driven by a VERY high revving geared drive.  Typical Shay drive ratios are in the order of 3 to 3.5 to 1.  That is to say, three revolutions of the crankshaft moves the wheels just one revolution.  Plus those wheels are typically very small by comparison with a rod-driven engine with its 1 to 1 power stroke, even given that there are two cylinders working 90 degrees out of phase.

Some Shays were built to work in conditions that were a real trial even for a Shay, and Lima/Portland Iron and Steel who built them, used to fix that little problem in two ways -

1.  Add another driven truck - or even, in the case of the Western Maryland Shays - TWO trucks, and

2,  Increased the gear ratio to 4 - 4.5 to 1.

Rod-driven locomotives went a way to fixing the traction problem by making their drive wheels smaller, and increasing the stroke and cylinder dimensions as much as possible within engineering limits.

tac
Ottawa Valley GRS