I thought we'd had this conversation in the group before, but can't find a reference...

Is there a rule of thumb for adding weight to a loco - enough to make the most of its power without adding unnecessary strain?

I'm actually getting close to finishing a loco; the prototype is a petrol-electric bogie unit.

The model is plastic and with one powered bullant unit. Ideally, I'd like it to pull half a dozen wrightlines WDLR wagons which have some rolling resistance as they're mostly from the non-pinpoint bearing era.

Before adding weight the loco is 125g and barely enough mass to avoid wheels pin on its own. I've enough lead prepared to fit the body she'll to bring it up to 350g. I've no shortage of lead and plenty of space.

What's the mass is similar locos out there- could equally be a Garrett, funkey, Fairlie. And if powered on just 1 bogie, how is the weight distributed?

Cheers
Simon

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Simon,
You are better off trying to keep the weight equal on both ends, that way the loco will behave equally, this means you need to have slightly more weight added to the non drive [powered] end, because you have to allow for the weight of the Bullant drive, now as for how much weight you can add, you need to look at what your overall ratio is on your drive system take a look at the specs. on the motor and calculate based on the ratio as to what rpm. the motor is going to be running when the loco is travelling at the nominal scale speed, ideally your motor is going to be running at or near its maximum torque output at that speed, but if not it doesn't matter too much, the main value you need to take note of is the nominal current draw is when the motor is running at its maximum torque, its is this value that you need to aim for.
Now you need to set your loco up so it has a load to pull and set up your multi meter to read m/amps., try driving the loco to pull the load, if you get wheelslip, add some more weight to each end of the loco and try again, at the sametime keep checking the current draw, keep repeating this process until the loco can pull the load comfortably or you reach that current draw limit, remember it is better to have the wheels being able to slip rather then having too much weight and the wheels not being able to slip, because that will stall the motor and burn it out.



Brian
Rawbelle County Workshops
Qld. Aust.

Hi Simon,

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Thank you all for your good advice. David & John’s weights for similar-ish locos are showing a pattern. Apart from that St*&%@?d gauge thing - which I’d spotted on your blog, and then ferreted out the K1 articles in NG&I to see if that had weights.