# Scaling Loco Power? Beginner Question



## SmokeNBones (Oct 7, 2015)

Hi everyone. I'm new to model trains, in fact I don't even have any models or track yet, but I'm very interested in it and have been absorbing everything like a sponge.

Got a question about scaling for loco power though. For example:

SW8 vs GP7 
GP7 is about twice the HP and starting tractive effort of the GP7.

Is there any way to somewhat scale that. I would like to get my hands on a Proto 2000 SW8 (Algoma Central if I can find one, good luck) I don't want an SW8 that can pull like a GP7. Now since I am new to this; do manufacturers ensure that in their own product line, a loco like an SW8 would not be as powerful as something like a GP7?

I thought what about switching out with a less powerful motor, but damage could happen right? But if it's a switcher and you're always running it slowly or never fast, is that not so big of a deal?

I hope this isn't a dumb question.

Thanks,
Jake


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## Lehigh74 (Sep 25, 2015)

It’s not a dumb question. The only dumb question is the one that doesn’t get asked.

As far as I know, the manufactures don’t adjust power in different engine models to match the prototype. For example: an MTH SW1, C628, GP9, RS3, etc. all have one motor of the same power for each truck. If you have DCS, you can set the max speed to match the prototype.


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## SmokeNBones (Oct 7, 2015)

Would it be "bad" then to swap in a less powerful motor for a switcher? I saw a video of a 25 ton HO switcher... well switching haha, and it was pulling 4 or 5 cars pretty slowly, which probably isn't a big deal in real life, but it got me thinking, there is probably a limit on what that little switcher can do. But bigger switchers (in the model world) probably can pull just as much as big 3000hp loco like a SD40, I was wondering if deliberately making your SW8 or GP7 (or whatever) a little weaker, would be an okay idea. I know you wouldn't want to use a weaker locomotive and try to pull a huge consist fast, that's not good for the motor right?


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## MtRR75 (Nov 27, 2013)

There are differences in motors, but there is no way to predict accurately how motor differences will affect pulling ability. You can increase your chances of getting a stronger motor by buying a better quality locomotive, but there are no guarantees. How much a model loco can pull is mostly a function of: (1) how heavy the loco is -- so that the wheels won't slip on the tracks, (2) whether the loco has rubber traction tires on its drive wheels -- they are not that common, and (3) the rolling resistance of the train that you are trying to pull.

Train rolling resistance is where you can make the biggest difference -- by making sure that your car wheels spin freely. If you have grades, then the weight of the cars also becomes very important -- it is harder to pull heavy cars uphill. (But lighter cars are more likely to derail.)


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## Cycleops (Dec 6, 2014)

I understand your question and so do all of us on here and we take account of it in varying ways. Model trains are not like their full size counterparts when it comes to hauling power. Haulability is mostly limited by wheel slippage as MtRR75 says. If you can make your loco heavier it will pull more, lighter and it will pull less. This isn't always practical. You can simulate it in model form by making your cars heavier or limit the top speed, acceleration rate and momentum if you are using DCC. In fact there is so much you can do by altering the settings on a good decoder that you can entirely change the character and performance of any loco.


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## tkruger (Jan 18, 2009)

Just a thought, I am not sure if this will work or not as I have not tried this. If you are running DC can you just place a resistor on the feed to the motor? If the locomotive should have 50% less power and you limit the current by 50% it is that much weaker. The down side is that speed is also effected. To me this would not be a big deal as the max speed I run my locomotives at is 50% on the mainline anyway, thus the switcher would only be running at 25%.


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## DonR (Oct 18, 2012)

The 'power' of most HO motors is arguably about the same
on most locomotives. It is very seldom that you see an
HO loco motor Stall due to too many cars. When the train gets
to a certain point the loco wheels begin to slip. The real railroads
have the same problem. They, however, drop sand onto the
tracks to give loco wheels better traction. Modellers don't
have that available so, As MtRR says, weight
of the loco has more to do with pulling power than anything
else as does the number of powered wheels on the track. 

Like so many other things, a modeller has to operate his trains
in the way that presents them as realistic as possible, but
the real thing will not be duplicated.

TK

The resistor would reduce the voltage to the motor.
That could be used to afford a speed match for
consist purposes, but otherwise seems pointless.
The speed control on the power pack is what
determines the track voltage, thus the speed
of the loco.

Don

Don


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## SmokeNBones (Oct 7, 2015)

Thank you for the replies.


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## cv_acr (Oct 28, 2011)

Personally, I wouldn't worry too much about tweaking the locomotive characteristics in any extensive way. Most model engines have similar drive trains, so depending on manufacturer, you might find that a model GP9 is actually just as good a puller as a modern SD60 with twice the HP. Maybe better.


Now, the small size and light weight of the SW8 (particularly if you're after one with DCC/sound instead of a plain analog DC model, as the speaker loses some of the metal weight from the older models) should actually make it a slightly worse puller, but keep in mind that one of these engines in real life really could handle at least a couple dozen cars no problem on flat grades at slow speed - the conditions found in a switching yard. They don't need to move a lot of tonnage very fast.

For road trains, you can institute a simple number of cars per engine rule, and possibly make it more interesting by allowing more cars for 6-axle units than 4-axle ones, or go even further and define certain power classes for different types of engines, to really divide that GP7 from the SD60.

For example, we make a rule that a single GP7 can handle up to 8 cars on the road, and an SD60 can handle 16.

Or just not worry too much about it at all and just run what "looks about right".

There's not really a wrong approach either way, but if that sort of thing interests you, I wrote some more detailed thoughts on this subject with a specific example on my personal blog site a while back:

http://vanderheide.ca/blog/2013/09/20/adapting-a-tonnage-rating-for-a-model-railway/


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## SmokeNBones (Oct 7, 2015)

I've read through that post before. Actually I'm the same guy who asked you those recent questions on the Yahoo Group for Algoma Central about the history for Wisconsin Central.

So I can simulate it all pretty well anyways then just by proper simulated weight of the prototype and running it at correct speeds and acceleration.


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## thedoc (Oct 15, 2015)

cv_acr said:


> For road trains, you can institute a simple number of cars per engine rule, and possibly make it more interesting by *allowing more cars for 6-axle units than 4-axle ones,* or go even further and define certain power classes for different types of engines, to really divide that GP7 from the SD60.


Actually this would be very realistic according to real RR practice. I was once in a conversation with a locomotive driver for the real RR and he said that one of the major problems was that dispatchers would assign power to a train based on tonnage and powered axles. One time he might get several 6 axle units and the next he might get 4 axle units. The problem was that the 6 axle and the 4 axle units often had the same HP and the train with the 6 axle units was much slower accelerating and pulling than the train with 4 axle units. It seems that total HP was very important in getting the train moving and moving it over the road.


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