Hello All,
In one of my sleepless moments I was wondering if the horsepower rating of a prototypical locomotive actually translates to the pulling power of the same models on my pike?
For example: a GP 40 is rated at 3,000 hp, a GP 38-2 is rated at 2,000 hp; which, for the sake of argument, has one-third less pulling power.
Given that both the GP 40 and the GP 38-2 I own are from Bachmann, I was wondering...If they are assembled from the same components: prime-mover (D/C motor) and traction motors (dog-bone to worm gear to B-B trucks) how can they have differing pulling power as per the prototypicals?
Any thoughts???
Maybe I should just get a sleep-aid!?!
"Uhh...I didn’t know it was 'impossible' I just made it work...sorry"
Yes, just go to sleep.
The only thing that scales is the physical volume that the object takes up. An HO scale GP38-2 doesn't weight a ton and a half, does it? That's 1/87th of the weight of a real one.
I suspect there is some, but little correlation. Most Big Boys will outpul moguls, however...
But more specifically, for example, I have a Genesis GP9, the prototype having 1750HP, and a LifeLike P2K E6, having two engines that total 2000HP. I suspect my HO E6 would outpull two GP9s, as the E6 has 50% more wheels per loco for traction, and it's more than 10% or so (the HP difference per loco) heavier (has a huge weight). And tractive effort is more related to torque and friction factor than horsepower per se (more acceleration related, IIRC).
And of course you get into pulling power (ala torque in an auto) versus accelleration ability (ala acceleration in a dragster which is HP related), gearing, max speed, etc.
I think you should stop drinking coffee before bedtime.
Paul
Modeling HO with a transition era UP bent
Horsepower and pulling power (better known as tractive effort) are, at best, nodding relations. Two steam locomotives could each develop 2,000hp. The high-drivered but light weight 4-4-0 could go like the wind with a three car train. The low-drivered 2-8-0 might reach 35mph, but there would be fifty freight cars behind it.
So-called 'slugs' make use of the horsepower available but not used at low speed to provide more tractive effort for use in shoving a full-length train over the hump of a gravity yard. The horsepower of the prime mover is the same, but the tractive effort is doubled.
As for the mass of a model, and equivalent HP and TE, you have to divide the prototype number by the cube of the scale to get an answer. This is useful if you are tank testing ship hull designs, but I don't think it need concern us as model railroaders.
Chuck (Modeling Central Japan in September, 1964)
The HO GP38-2 might be on a tad on the heavy side. Weight would scale with volume as a cube of the scale. A full size GP38-2 weighs about 250,000 lbs. That's would scale to only 6 ounces (16*250,000/87^3) in HO.
Ray Hamilton
The pulling power of your model locomotive is controlled by four factors.
The first is the number of powered axles. An old AHM with only one truck powered will not do as well as a model with both trucks powered. An E-unit or PA with all 6 axles powered will do better than the prototype which only had 4 of the 6 axles powered. And a model with 6 powered axles will generally pull more than one with 4 axles.
The second is the weight of the model. A heavier model will usually be able to pull more, where a lighter one will have wheel slip.
The third is the wheel tread. Rubber traction tires provide good grip but do not look right. Even metal wheels seem to vary as to how slippery they are.
And finally, the fourth is the efficiency of the motor installed in your model.
You will notice that the comparative power of the various prototypes is not a factor in model form. A 1,000hp RS-1 and a 3,000hp GP40 each have 4 axles. If weighted the same they should be able to pull the same. An E-unit with its 6 axles may outpull them, which is definitely not prototypical.
There's a good discussion here on horsepower and tractive effort, in the Railroad Facts and Figures section:
www.alkrug.vcn.com
For a model with no traction tires or other artificial aids, about the best you can expect is a tractive effort, in ounces, equivalent to 25% of the locomotive's weight.
Here's an example, a twin motor Athearn U-boat, weighing 33oz., its as-tested drawbar pull was slightly over 8.3oz:
Real locos fare about the same, although modern wheelslip control does add some capability. If you're having trouble sleeping, read the info available in the link: it didn't put me to sleep....on the contrary, it was a real eye-opener.
Wayne
E-8s had 6 axles but only 4 powered ones. If a model E-8 has 6 powered axles it already strays from the prototype before you even put it on the track.
This thread is little bit pointless, a model's performance will not match up to the prototype.
It likely that if you have say a GP9, GP38-2 and GP50 from the same model manufacturer, they will have similar drive designs and probably very similar performance, even though the prototypes are greatly different in HP.
And of course, a Bachmann, Walthers/Proto or Atlas GP38-2 will all have different motors and drive trains and have different performance although they're models of the same prototype.
And also, real locomotives have a separate traction motors on each axle (except for specific examples like the E8 that have an unpowered middle idler axle to spread out weight) while most model locomotives have a single large motor in the shell which drives all the axles via a series of gears. Very different sort of concept there.
Chris van der Heide
My Algoma Central Railway Modeling Blog
NittanyLion Yes, just go to sleep. The only thing that scales is the physical volume that the object takes up. An HO scale GP38-2 doesn't weight a ton and a half, does it? That's 1/87th of the weight of a real one.
rrebell NittanyLion Yes, just go to sleep. The only thing that scales is the physical volume that the object takes up. An HO scale GP38-2 doesn't weight a ton and a half, does it? That's 1/87th of the weight of a real one. It is inverse 1/87 squared to get the weight, or 747 lb!!!!!!!!! Still our models weigh nothing like that.
It is inverse 1/87 squared to get the weight, or 747 lb!!!!!!!!! Still our models weigh nothing like that.
No! Still wrong.
You are reducing in ****three**** dimensions. It's a cube ratio. Not linear. Not squared.
Why do people get this so wrong every single time this discussion comes up?
This reply is correct:
Colorado Ray The HO GP38-2 might be on a tad on the heavy side. Weight would scale with volume as a cube of the scale. A full size GP38-2 weighs about 250,000 lbs. That's would scale to only 6 ounces (16*250,000/87^3) in HO. Ray Hamilton
NittanyLion That's 1/87th of the weight of a real one.
That's 1/87th of the weight of a real one.
To be precise it's 1 / 660,776.31 weight wise.
A lot of our locos can actually pull more then their real life counterparts. Problem being is our grades are a lot worse then their real life counterparts, so that extra pulling power is sometimes needed.
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
cv_acr rrebell NittanyLion Yes, just go to sleep. The only thing that scales is the physical volume that the object takes up. An HO scale GP38-2 doesn't weight a ton and a half, does it? That's 1/87th of the weight of a real one. It is inverse 1/87 squared to get the weight, or 747 lb!!!!!!!!! Still our models weigh nothing like that. No! Still wrong. You are reducing in ****three**** dimensions. It's a cube ratio. Not linear. Not squared. Why do people get this so wrong every single time this discussion comes up?
NP 2626 "Northern Pacific, really terrific"
Northern Pacific Railway Historical Association: http://www.nprha.org/
Sorry.
This "scale weight" discussion seems to come up about once a month on different forums and every single time there's the "our models should weigh a literal ton LOL" response, and for some reason it kind of grates after the 20th time reading it. Sorry I lost my patience.
I disagree that it's such an esoteric point though, it's fairly basic geometry, it's just people don't think about it. A 10 inch cube is a 10:1 scale of a 1 inch cube, but it's not ten times the volume/amount of material; it's 1000 cubic inches. If both cubes are made of the same material, the 10" cube will be 1000 times heavier than the 1" cube.
I guess if I found a topic that surfaced once a month for 20 months I would probably pass on reading it again, 19 times!
All my math I learned has dollar signs in front of it and they never bothered make me take high school physics so at least I have an excuse!
Then in college, I took astrophysics which is less physics-y than you'd think
NittanyLion All my math I learned has dollar signs in front of it and they never bothered make me take high school physics so at least I have an excuse! Then in college, I took astrophysics which is less physics-y than you'd think
Au contraire, I took 3 years of high school physics, plus biology, chemistry and a hatful of math - not unusual for the Bronx High School of Science. Then, two years later, I aced my one semester naval architecture course. My major was Marine Engineering, which is almost entirely physics and math.
(My actual major was named Lynn, which is why I'm a retired sergeant, not a retired admiral.)
Not only are our grades usually steeper, our curves are MUCH shrper as well. GOod thing we have more than scale pulling power (typically).
E units have been mentioned as being way off in weight scaled down - at least the Proto ones are, since they have a huge metal casting that makes up most of the interior, so they are even heavier than a properly scaled down representation would be. Another one that far outpulls the prototype are the Proto DL109's - much the same reason, a HUGE slug of metal fills up the interior, making them proprotionally heavier than models of other locos.
Real world example - the Reading tested E units for dieselizing passenger trains. They had problems meeting the schedules though, and in the end they went with FP7's. The FP7 has less horsepower, but more weight per axle so higher starting tractive effort, which on a passenger train that stops frequently is pretty critical for getting back up to speed. In the model world, at least if we compare the Proto E's to say an Atlas FP7 - the E unit will probably outpull the FP7 due to the much heavier weight.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Ah...but they were perhaps not pointless replies Grasshopper... ;-)
Raised on the Erie Lackawanna Mainline- Supt. of the Black River Transfer & Terminal R.R.
here are some numbers for the average weight of a car (70 tons) and the tractive force of an SD40 (82000 lbs).
ho-scaled are these prototype values scaled by 87^3 and then to ounces.
ho values for car-weight are the NMRA recomendation for a 6" car and the tractive/pulling force is just one measurement I found on the web.
car-weight tractive prototype : 70.0 t 82000.0 lb ho-scaled : 3.4 oz 2.0 oz ho : 4.0 oz 4.0 oz
the values show that the weight of a car is roughly to scale but the tractive force of a model (4 oz) is roughly twice that of a full-scale SD40.
Not only do model locomotives have greater tractive force, but they rarely pull a 100 car train (7000 tons full-scale). Not calculated are friction and acceleration rate.
greg - Philadelphia & Reading / Reading
There is another factor for model locomotive pulling (and by the way I have been sleeping just fine, thanks for asking) and that is it seems many locomotives pull better the older they get, as the high gloss of the driver tread wears down a little. This is particularly noticed with brass steam locos. Some of the smaller ones can hardly pull themselves around the layout until the drivers have had a chance to wear a little bit. I'd think the same would be truly of shiny/smooth diesel wheels.
On my first layout my best puller was an AHM Y6b but that was certainly because of its rubber traction tires. I never had enough cars to stall that engine. But the best puller other than that was a Mantua/Tyco 0-6-0T that an older cousin had enjoyed for years before I got it. The wheels showed a bit of wear and that all metal engine (with all weight on the drivers) could pull nearly as well as the 2-8-8-2. In fact it pulled better than the also all-metal Mantua/Tyco 4-6-2.
Dave Nelson
gregc here are some numbers for the average weight of a car (70 tons) and the tractive force of an SD40 (82000 lbs). ho-scaled are these prototype values scaled by 87^3 and then to ounces. ho values for car-weight are the NMRA recomendation for a 6" car and the tractive/pulling force is just one measurement I found on the web. car-weight tractive prototype : 70.0 t 82000.0 lb ho-scaled : 3.4 oz 2.0 oz ho : 4.0 oz 4.0 oz the values show that the weight of a car is roughly to scale but the tractive force of a model (4 oz) is roughly twice that of a full-scale SD40. Not only do model locomotives have greater tractive force, but they rarely pull a 100 car train (7000 tons full-scale). Not calculated are friction and acceleration rate.
BerkshireSteamI think what should be talked about here is how an engine with 4oz of pulling force can pull 30-40 cars, not all of which weight 4oz but you get the idea.
the engine does not need to lift the car off the tracks, it needs to overcome friction to put the car in motion. At least one website suggested that that the coefficient of friction for rail cars is 0.001.
That means it takes a 140 lbs to move a 70 tons rail car, and that it would take only 14,000 lbs to pull a train of 100 cars (7000 tons) on straight and level track. This is well with the limits of a locomotive with a tractive effort of 80,000 lbs. But this jumps to 154,000 lbs on a 1% grade.
assuming that frictions is 10x worst on a model railroad, it would take 0.04 oz to put a 4 oz car into motion and that a locomotive with 4oz of pulling power can pull a 100 car train on straight and level track.
tomikawaTT Au contraire, I took 3 years of high school physics, plus biology, chemistry and a hatful of math - not unusual for the Bronx High School of Science. Then, two years later, I aced my one semester naval architecture course. My major was Marine Engineering, which is almost entirely physics and math. Chuck (Modeling Central Japan in September, 1964)
I don't even recall if my high school even offered two years of physics. As I recall most in my high school tried to meet or exceed the University California Davis (UCD) entrance requirments, which from memory included: Algebra II/Trig and Geometry, 1 year of chemistry and 1 year biology, english composition, and a few liberal arts oriented classes.
Davis Sr. High School did offer physics and up through Calculus too but I only took pre-calc. In university I took only 1 year of physics, 1 year of biology and 2 years of chemistry including physical chemistry (P-chem) and Organic Chemistry, pre-med biology, a year of Calculus and linear algebra - all required for a B.S. in geology. I don't recall any subject material which would have helped me scale down the physics of full scale railroading to HO scale so I'll just defer to the physic's professors!
As for model trains, it's probably most practical to simulate operations in your scale as space allows - which usually requires some selective compression.
Rio Grande. The Action Road - Focus 1977-1983
Agreed. The ethical thing to do when someone asks a question of a modestly technical nature is to correct apparent misconceptions in order to frame the response properly, and thence to actually answer the question. No sense in agreeing that a scale model weighs only 1/87th of the weight of the prototype because it's simply not true! Mass has three dimensions, not just two. So, when scaling mass, you must take into account the material's height, depth, and width. Three measurements (count 'em).
Why just shrug, if you're bothering to compose and type a response here, and let a fault pass? It's not very reponsible or neighbourly.
jjdamnithow can they have differing pulling power as per the prototypicals?
If what you want is to have the GP-40 capable of pulling more than the 38-2, remove some of the weight from the 38-2. Or disconnect/remove the worm from one of the trucks in the 38-2 and let it freewheel. Andy Sperandeo described a version of this in one of his Operators columns. He was having problems with his trains derailing on a hill, and determined that it was because both the road power and the pusher power were each capable of moving the train itself. So if either the lead unit or the pusher stalled, the other loco set would either push or pull the cars until they derailed. When the drives were modified, neither set could move the train by itself. So if something stalled, the other set would just spin its wheels and no derailment occurred.
For those of us who didn't study physics (and barely passed geometry), this example may be helpful:
Imagine you had a real 40' boxcar, and were going to fill it with HO scale models of the boxcar. You couldn't fill the car using only 87 HO models. To fill it you'd have to have 87 rows of 87 cars along the floor, then stack 86 more cars on top of each one...so it would be 87 x 87 x87, or 658,503 cars.
(Yes I know we're using the internal dimensions of the real car and the external dimensions of the models, but I think the example still works to make the point.)