Locomotive weight standards?

Discussion in 'FAQs' started by Gary S., Dec 31, 2006.

  1. Gary S.

    Gary S. Senior Member

    There have been several questions about rolling stock weight lately, and we have the NMRA standards for that, but what about locomotives? Are there any weight standards for those? Specifically, I am speaking of GP30s, GP35s, and CF7s in HO scale.

    My HO P2K GP30 weighs about 17 ounces. My Athearn HO GP35 is 14 ounces. And the Athearn HO CF7 is only 11 ounces.

    It looks like there is room in the Athearn shells to add some more weight. Any thoughts or suggestions?
  2. doctorwayne

    doctorwayne Active Member

    Generally, it is safe to add weight as long as the locomotive can still slip (spin) its wheels when it is not able to move its train. To test this capacity, couple one car to your locomotive, add the weight on top of the loco, and while holding the car (not the loco) apply power to the rails. As long as the wheels can slip, it is unlikely that you will burn out the motor. If the wheels do not slip, remove weight until they do. Most quality HO scale locos don't have enough room in them to accomodate enough weight to stall them. I realise that you're asking about diesels, but here's a link to a way to add weight to an Athearn steamer. My initial tests with an external weight showed that this loco could accommodate in excess of 22 ounces of additional weight, far more than could be crammed into the body shell.

    Boosted output from an amplified mike...

    If the drive wheels slip too much (the engine is being overloaded constantly), you can wear the conductive plating off the wheel treads.

    Here a some Athearn U-boats that I've added some weight to, and with the sintered iron wheels, no chance of wearing out the plating.:D


    Here's a view of the inner workings. The two motors are connected electrically, but each one powers only one truck. Each complete loco weighs about 33 ounces, yielding about 8.3 ounces of drawbar pull, or about 25 ounces for the three-unit lash-up.


    Because I now model the 1930's (and occasionally the '50's), these locos don't get much use. All of my locos have had weight added to them.

  3. Gary S.

    Gary S. Senior Member

    Thanks for the advice, Wayne. And also I commend you on your efforts adding weight to the steamer. Nice work indeed.

    The Athearn Geeps have room in the long hood, under the roof and in the sides of the cab, and in the short hood for more weight, without interfering with the truck/gearing. I'll do some experimenting as you mentioned, but I figure that there isn't enough room to add enough weight to keep them from spinning under heavy load which could result in a burned out motor.

    I don't figure to ever run trains so heavy that I need more tractive power, what I am thinking is that a heavier loco makes a better electrical connection with the track. Reason I think this is my little lightweight switcher seems finicky, it needs clean track to avoid stalls, but on the other hand, the 17 ounce GP30 runs perfectly over the same spots.

    Yeah, yeah, I know, the track needs to be kept clean anyway, the extra weight is just a crutch?:oops:
  4. doctorwayne

    doctorwayne Active Member

    Thanks, Gary

    Well, I think that the extra weight does help with the electrical contact, and all of my steamers have heavy, loose coal in the bunkers and additional weight in the cistern area. But, unless I've just done some scenery work, I don't clean track. And the locos of my good friend cn nutbar, which often run on my layout, and don't have any added weight, seem to run equally well. :confused:

  5. pgandw

    pgandw Active Member

    In addition to increased traction, extra weight helps the wheels stay in contact with the rail a little better, so you get slightly improved electrical pickup as well as slightly fewer derailments. Extra weight also help the wheels to polish/clean the rail heads a little better. The heavier locomotives wear down the metal oxides faster than lighter locomotives.

    Best ways to prevent stalling and improve power pickup with small and light locomotives is 1) make sure all metal wheels are being used for power pickups; 2) add sliders and/or wheel wipers; 3) hard wire sliders and wipers directly to the motor or decoder; 4) power frogs on crossings and turnouts using electrical contacts to get the correct polarity; 5) add jumpers between point rails and stock rails on turnouts (assumes point rails are already insulated from each other, and are insulated from the frog).

    A certain amount of weight is usually required in steam tenders to get power pickups to work well; after that weight in the tender takes away from pulling power.

    my thoughts, your choices
  6. Triplex

    Triplex Active Member

    So, a 25% factor of adhesion. Makes sense - that's within the range for real locomotives.
  7. doctorwayne

    doctorwayne Active Member

    Yeah, that seems to be about as good as I can get, although the U-boats are the only units I have that have actually been tested. It would follow that most steamers would be less than 25%, as not all weight is on the drivers, and there's the additional losses incurred by side rod/valve gear friction. Traction tires might give a higher factor of adhesion, but at the expense of electrical pick-up, and adding pick-up shoes/sliders to compensate for that would only add additional drag.

  8. Gary S.

    Gary S. Senior Member


    I added another 4 ounces to the CF7 bringing it up to 15 ounces from the original 11 ounces. It is feeling nice and hefty, much better than before.

    Doing the spin test, it will start spinning at 7 volts. I added another 3 ounces on top of the loco, and it still started spinning around 7 volts. I took it up another 5 ounces, and it began spinnng around 8 volts. Should I try to stuff some more weight in the shell? At this point, I am starting to worry about making it top heavy, as the majority of the added weight is in the upper portions of the shell.
  9. doctorwayne

    doctorwayne Active Member

    Unless you're planning on running at supersonic speeds, in a locomotive it doesn't really matter if the weight is high. If you look closely at the photo of the U-boat with the "hood popped", you'll see that most of the weight is mounted near the top of the hood. That grey stuff running the length of the hood is a slab of lead, which barely clears the tops of the motors. The only weight down low, other than the frame, trucks, and motors, is that mounted on the underside of the battery boxes. There is room to add about 2 more ounces to these units, but not much point, given their "stored serviceable" status.

  10. Gary S.

    Gary S. Senior Member

    No supersonic speeds here! This baby will be doing lots of switching at a slow pace.

    I'll see about stuffing some more lead in...
  11. doctorwayne

    doctorwayne Active Member

    I think that as long as the wheels can slip, the motor should be okay. Athearn motors are pretty durable, but it's useful to have an ammeter to keep an eye on the current draw. I was recently running a heavy train using four remotored Athearn switchers (You can read about it here: Why do I need a powered "B" unit to my F7"A"?) and the current draw went from just under 1 amp to just over 1.5 amps as the train ascended a grade. Had the wheels slipped, the current draw, I think, would have returned to the lower reading. Had the locos stalled (stopped moving forward and the wheels not turning), current draw would've shot up, followed by smoke effects.:oops: :curse: Neither event occurred, but the trip experienced other "complications".
    Because my layout has so many curves and overly steep grades, it's often necessary to doublehead even heavily ballasted locos, but if two locos can pull a regular train, then they're heavy enough. Of course, if there were fewer curves and gentler grades, I'd probably run longer trains.:D

  12. brakie

    brakie Active Member

    As far as weight I usually add 4-6 pieces of stick on weight to the inside top of the Athearn shell..I did not add any weight to my CF7s due to the fact the prototypes was on the light side as well because these units was rated at 1500 hp and weigh 249,000 lb.
    I do not add weight to my Atlas,my 3 Kato or 4 Walthers GP15s.
  13. Russ Bellinis

    Russ Bellinis Active Member

    Excellent point, Larry. I think sometimes we forget what the capabilities of the prototype really were, and we can add weight to where an f7 will out pull anything on the layout including some modern 6 axle diesel with 2-3 times the horsepower for the prototype. If I have a mix of motive power and a f7a outpulls a sd45 because there was room for more weight in the body of the f7, I have to ask "What's wrong with this picture?"
  14. Gary S.

    Gary S. Senior Member

    My thoughts: I will never pull more than about a dozen cars at a time on my layout, so the maximum output of any given unit will never come into play. Let's say the added weight in the CF7 has now given it more pulling power than my GP35. It doesn't matter because I cannot forsee any situation where either one of the locos would be subjected to a max power load anyway.

    I assume that more weight = better electrical contact to the track, and this perception of mine outweighs the prototypical power comparisons between the locomotives.

    There is a chance I don't have a clue of what I speak.:oops:
  15. Gary S.

    Gary S. Senior Member

    Another thought... How does the output power/tractive effort compare between the prototype and the models?

    How many free-rolling cars can an Athearn HO CF7 pull on level track?

    How many free-rolling prototype cars can a proto-type CF7 pull under the same conditions?

    I am guessing the models are considerably underpowered.
  16. doctorwayne

    doctorwayne Active Member

    I wish that I had enough level track to find out what any of my locos will pull!!:rolleyes: One of the advantages of walk-around control is that you don't often see the whole train at one time, or at least you can concentrate on one area at a time. The "heavy train" that I referred to earlier looked pretty impressive passing any given point (or annoying, if you were a LPB sitting in your car at the crossing, waiting for the darn caboose to finally show up!), but it looked rather ridiculous when you viewed the whole layout. The locos were in Elfrida, the middle of the train was blocking the only crossing in South Cayuga, and the caboose was still in Dunnville.
    There's a sketch of the layout (not a trackplan) HERE, if you're curious. (It's on Page 2)

  17. Gary S.

    Gary S. Senior Member

    So when do we get the photos of the catastrophe with the long train?

    I ended up adding 5 ounces to the Athearn CF7. I ran it tonight... I had to boost CV2 (start voltage) up quite a bit, as in almost double what it was. I also added in a touch of kick start at CV65. I like the way it is running with the extra weight, although it could all just be a placebo effect in my head.

    While I was running it, I got to thinking about stuff I hadn't thought of before. With locomotives, either real or model, the "pulling power" comes from a combination of the horsepower, gear ratio, and friction of the wheels on the track. The friction on the track is certainly related to the weight of the loco. A very lightweight loco with a 2000 hp engine still isn't going to pull much because it will end up spinning the wheels at some amount of load. It would be possible that a 1000 hp but heavy loco could pull more than the aforementioned loco.

    Now I wonder, how does the hp and weight of the prototype compare to the hp and weight of the models?
  18. doctorwayne

    doctorwayne Active Member

    I think that this might be some sort of limit, at least for a simple weight/power formula. Perhaps traction tires on a model, or wheelslip control on the prototype could skew the equation, though.
    I also thought that gearing would have a pronounced effect: my remotored Athearn SW1200RS diesels each have a medium-size Mashima can motor (similar to the ones used in the U-boats). One, in working order, weighs 13 1/4 oz. and has stock Athearn gearing. I also have a remotored Athearn NW-2 (built from the same EMD SW-type switcher) that is equipped with a very large Sagami can motor (so large that I had to use an autobody file to thin the inside of the hood), and has been regeared with an Ernst gearset. This loco weighs 13 1/2 oz., and is also equipped with pick-up shoes, which does have an effect on the pulling power. Until a couple of days ago, both loco types weighed 12 1/2 oz. each, but the ones with the smaller motor would pull a 68 oz. train up a test incline, as opposed to 52 oz. for the loco with the bigger motor. I attribute the difference to the pickup shoes. I haven't retested them since adding the extra weight, and I may hold off, as there's room for at least 2 oz. of additional weight (possibly more) on all units. All of these locos, the U-boats included, are overpowered for their weight, which, I suppose, is better than being overweight for their power.:rolleyes: I may also remove the pick-up shoes from the NW-2. It's the only loco I have that will not m.u. with any other loco, although as a switcher, this isn't really a problem. By the way, all of these locos have the stock Athearn sintered iron wheels.
    I think that the two motors that I put in the U-boats would've been better in an E-unit, or a PA, where I could get enough weight into the carbody to come closer to the capacity of the motors. I'm sure, though, that the 25% factor of adhesion would still apply.
    As for photos of the wreck, I can't even get them out of the camera until I get a new computer, so it could be a while. Surprisingly, there was very little damage: one car had the end broken off the roofwalk, and lost a door, while two cars had their lead weights torn loose from floor, which dislodged their (removeable) roofs. After I repaired the cars, I rerouted the train, and it ran, without incident, from staging to the opposite end of the layout. I took some photos of the trip, which included some back-up moves to spot cars for the camera. The first car in the train, incidently, was an empty flatcar.:thumb:

  19. LongIslandTom

    LongIslandTom Member

    There is just one element you need to add-- speed. A good illustration that puts it all together I think would be to compare two 2000-hp locomotives, the EMD SD38-2 and the GP38-2.

    Both locomotives use the same engine and both generate 2000hp. But the SD is heavier, has 6 axles and is geared for slow, steady speeds, while the GP is lighter, has just 4 axles, and is geared for higher speeds. The SD38-2 can haul very heavy iron-ore and coal trains due to its weight and gearing, but only at very slow top speeds (15mph). The GP38-2 can't haul those heavy iron ore trains, but its lighter weight and higher gearing means it can haul light commuter cars at 50mph+ (which it did on the Long Island Railroad until 2000).

    Also, on the prototypes at least, there is indeed a limit on how much tractive effort an axle can generate... I think I read somewhere that each axle can only generate up to a certain amount of tractive effort before overcoming the maximum amount of adhesion possible, which is why as locomotives get more and more powerful, it is no longer feasible to build them on 2-axle trucks. That's why there are no more 4-axle road freight diesel locomotives after the GP60, and subsequently more powerful freight locos henceforth are all 6-axle units.

    I think this can actually translate somewhat to models.. If you re-gear your Athearn locos with the Ernst gear kit, it will lower the gear ratio and allow it to pull heavier trains, at the price of much lower top speed. Adding weight to a 6-axle unit also seems to be less likely to "release the magic smoke" from the motor than doing it to a 4-axle unit too, since less weight on each wheel will decrease the likelihood of wheel stall.

    Just my late night ramblings.. :D
  20. brakie

    brakie Active Member

    Guys,While working on the C&O under the Chesie banner,I recall 2 GP9s down on their needs lugging 68 cars of coal out of a twisting hollow at 10 mph.The throttle was being jockeyed between run 7 and run 8 the amp meter was in and out of the red,sanders open and the engineer was cussing up a storm.Once we entered the Big Sandy Sub then things got easier and back to normal.
    3 weeks later 2 SD35s had this run and still we had to use the sanders and the engineer used a lot of throttle postilions and I believe the speed was a dashing 12 mph..
    So,you see this mine branch was hard on geeps and SD units.

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