You often see members state the drawbar pull capacity of model locomotives. How do you measure it? Does someone sell a scale, or do you modify something that is readily available?
I have an idea on how to make one, but if there’s a better way, I’d like to see your ideas. Thanks Dan
I have an old spring Ohaus scale:
Drawbar_pull2 by Edmund, on Flickr
Then I bought a digital one but I don’t like it as much because it will lock on to what it thinks is the peak reading. Maybe I can “dampen” the display but to me It is easier to see the red needle move on the spring scale rather than try to comprehend the rapidly changing numbers.
Drawbar_pull3 by Edmund, on Flickr
DB_Baldwin2 by Edmund, on Flickr
Good Luck, Ed
Alternate to the spring scale method, if you have a set of weights, lead the line from the locomotive over the edge of the table and hook on a weight. Drawbar pull is the heaviest weight that the locomotive can pull up.
I have a dynamometer car. It started as a Walthers dyno car in this body style.
Since I model the Rio Grande, things are a bit different. They repurposed a troop sleeper, so that’s what I did.
I added some lighting so the gauge was easier to read.
The numbers on the gauge are arbitrary. I obtained an electro-mechanical strain gauge with the hopes of getting a better defined reading, but haven’t gotten back to that yet.
its common to record the number while the wheels are slipping. but i don’t believe this is accurate.
it would be better to record the highest value just before the wheels begin to slip
Weigh the locomotive.
Rig up an adjustable ramp with powered track.
Measure the maximum angle at which the locomotive will still move up the ramp without spinning its drivers.
Do the trigonometry to convert locomotive weight to tractive effort.
Or, do what I do. Couple cars to the locomotive until the drivers slip at the steepest grade on my layout. Then add a locomotive and keep on going.
Three Proto 1000 C Liners will easily pull my 8 car IHC passenger consist of Budd style cars, weighted and with their standard awful trucks up the steepest grade on our layout around the 24" (+/-) radius curve near the summit of a 3% (+/-) grade. Two will struggle but still not spin the drivers.
Put another way: prototype railroaders knew their locomotives were grade limited so we can use the same methodology.
I guess knowing draw bar pull is all well and good, if you feel you need to know that info.
I’ve always added cars until the wheels slipped, than removed 1 or 2 or whatever it took to keep the train moving.
I suppose draw bar pull is just one of those things, that modelers who like to document data, feel is nesessary.
Mike.
I bought a Portable Electronic Scale off eBay and so far it has worked very good. Most of my locomotives are heavy and have a lot of pulling power. On my tests the max drawbar occurs at wheel slip and doesn’t change with the amount of wheel slip, probably because of the weight of the locomotives.
A typical Rivarossi Cab Forward out of the box weighs 11.8 ounces and has 2.8 ounces of drawbar. After remotoring and adding 10 ounces of weight to the boiler the draw bar increases to 5.8 ounces.
My typical EMD E7s have an Athearn SD40-2 frame remotored with a Mabuchi FS280 and a Cary metal body weighing in at 33 ounces with 9+ ounces of drawbar.
I put a screw eye in a Kadee coupler gauge and use that to couple the locomotives to the scale.
Mel
My Model Railroad
http://melvineperry.blogspot.com/
Bakersfield, California
I’m beginning to realize that aging is not for wimps.
MR used to have a pulley mounted in a bracket at the edge of a table and used weights to make the determination. I suspect with today’s quality we will run out of passing siding capacity before reaching engine capability
Perhaps someone will care to build something I thought about years ago. A ‘proper’ analogue to a dynamometer car could serve a number of purposes that a plain old scale can’t, for example finding small faults in trackwork that have the effect of increasing train resistance, varying perhaps by car type and individual truck performance. For this a time-varying radio signal would be transmitted to the equivalent of a chart recorder on a computer (or equivalent) which could then display prototypical charts, give proper average drawbar-pull measurements without artifacts or misleading peak-hold (a la Mallard [:-^]) or difficulty reading scale numbers designed to be read in stasis and hence poorly buffered.
Probably such a scale has been built for some dedicated application. On the other hand it would be reasonably easy to take something like a Rolling Thunder daughterboard and tie it into Mel’s scale’s sensor output, with appropriate trim resistors etc., and receive the result in modified lab software…
Most transducers are available 4-20ma output and with the Arduino Bluetooth anything is possible. I have a wireless 160° panable camera on a HO flatcar and a wireless speedometer in a HO baggage car so a wireless drawbar meter is probably doable too.
Mel
My Model Railroad
http://melvineperry.blogspot.com/
Bakersfield, California
I’m beginning to realize that aging is not for wimps.
While it’s analogue and somewhat arbitrary, my hacked Walthers dyno essentially does this. You have to walk along and observe, and there is no recording unless you bring a camera, but it does clearly show the effects of grade, rolling resistance, and trackwork as the train encounters them.
The parts I bought to digitize things seemed designed to make things like a hanging scale, so I hoped they would be sensitive enough to give relevant readings. However, I found what I needed to know for my own pruposes with a little observation, so haven’t felt the need to fix what’s already working. Obviously, a digital readout and documentation would be helpful in cross-layout and multi-loco comparisons.
In general, on a well-balanced locomotive, drawbar pull is usually equal to about 25% of the weight of the loco, which is similar to the real ones.
I modified three Athearn U-Boats, installing two motors in each, and adding weight until they were each just over 33 ounces. I had no way of measuring the drawbar pull, until a friend suggested that I take one to his model railroad club, were they did such tests to determine how many locos would be needed for trains of various lengths and weights.
The test facility was simply some track on a hinged board, and the procedure was to elevate the free end of the board until the loco’s wheels slipped. They then did some sort of calculation which used the degree to which the board had been elevated, which came out as 8.3oz., pretty-much dead-on 25% of the loco’s weight.
When I returned home, I placed one of those diesels on a train of loaded hoppers and ran it up a 2.8% grade, laid-out on an S-curve. It managed to get about 1/3 of the train onto the level area at the top of the grade, but then slipped and could move no further.
I backed the train down the hill, then coupled another of the same diesels to the train, which easily hauled the entire train up the grade,without slipping. I would have added more loaded hoppers, but had no more. The trailing train was 44 cars, each weighing 8oz., for a total of 22lbs. of trailing weight.
While I still have the hoppers (and quite a few more) the three locos were bought by a friend when I later backdated my layout to the late '30s era.
Some time later I came across this gadget from MicroMark (click on the photo to enlarge it, as the wire is otherwise difficult to see)…
The unit is battery-powered and can show the drawbar pull in either grams or ounces. To use it, I bent a piece of piano wire, forming a hook on e
It’ll be approximately the coefficient of friction of nickel silver x locomotive weight in whatever units suit you.
Of course the load pulled varies with grade, curvature and truck condition for each hauled car. Knowing drawbar pull is of esoteric interest.
Thank you for the replies. I want to use it for the same reason that Mel does, measure improvement in locomotives as modifications are made and weight is added. Mainly curiosity, which is part of the fun. Dan
I bought my scale off eBay.
Mel
My Model Railroad
http://melvineperry.blogspot.com/
Bakersfield, California
I’m beginning to realize that aging is not for wimps.
At my club, we use a spring scale mounted vertically as it was designed to be. The scale measures from 0 oz. to 16 oz. in 1/4 oz. increments. If you use certain kinds of spring scales in the horizontal position, it can induce drag on the internal components and affect the reading.
Our scale has a string attached that’s tied to a piece of piano wire bent into a hook. The string wraps around a teflon pulley on a teflon bearing designed so that the string is at coupler height.
Two important steps to take is to make sure both the track and wheels are clean. Dirt acts like rubber tires and can change the reading by a significant amount.
We run the loco at full speed and check the reading, then we slow the engine down to a stop and check again. In a perfect world, it’d be the same reading but it rarely is. We usually go with the stopped reading.
I also purchased the Micro-Mark digital scale, and when I tested it out using proper test weights, it gave inconsistent readings. Some weight indications were ok and some were not. Worse, it was seemingly at random. I used the same test weights on our spring scale and it was correct for each one which is why we still use the spring scale.
I read and considdered all replies, thank you. The only hanging scales I could find were for weighing fish up to about 100 lbs, I doubt they’ll be real accurate in the tenths of ounces.
The incline idea doesn’t take into consideration tender weight, which I add plenty for current pickup.
Well, the pully and weight system sounds good, but with a twist here. This was the idea I mentioned in the original post, and seeing no easier accurate way around it, here it is:
The little Harbor freight scale I’ve been using for years to weigh locomotives has proven itself sensitive and accurate, so I applied the pully idea to it. I made the pully and frame from aluminum,
and an idler truck from a delrin passenger truck with a coupler. Added them to my existing test track board; done!
Place the 4 oz weight on the scale, push “tare” to zero it. Then let the loco pull the little super free rolling idler truck, and measure “negative weight” on the scale.
The weight showing on the scale at the time I took the pic wasn’t accurate, the little Pocher Inyo can pull an honest 2.6 oz. I’ve added considerable weight to it, the engine (no tender) weighs 8.58 oz. Drawbar pull: 30%, due to traction tires.
I’ve added weight to other locos as well. One example, a Mehano 2-6-0 (yes, I like those!) engine only originally weighed 9 oz, and pulled 1.2 oz
This has been an interesting thread, something I have considered doing myself.
Up this point I have used a different approach to measure pulling power.
I have about 70 pieces of identical rolling stock, same car, same trucks, same weight. I measure pulling power by simply testing how many of these a loco can pull.
The car in question, Athearn 50’ flat car with two vans, trucks are Kadee sprung metal trucks refitted with Intermountain wheel sets.
At 4.3 oz each these are one of the heaviest pieces of freight rolling stock I have, so it is also a good practical measure of motive power requirements for all trains simply knowing how many of these a loco can handle.
And, most of my freight cars have those same trucks, which are very free rolling and track very well.
Sheldon
Nice work, Dan. If you measure more drawbar pulls, please post the results!