I have a small brass 2-8-0. I noticed that when it straddled two rails that weren’t exactly level and one was not powered, that the rear wheels were not making contact with the rails. The weight was over the front of the engine.
I shaped some lead into the sides of the body to put more weight over the rear drivers. I increased the weight of the engine shell from 5.5 to 7.5oz.
am wondering what the typical weight per driver is on a good running engine. I assume that the more weight, the better the traction.
I owned a fair number of brass locos over the past 35 years steam diesel and electric. Many of the brass steam locos and some diesel and electric have sprung drivers. First check for this. If the drivers are sprung, they might be sticking at the journals. If they are not sprung and the axles are aligned in a rigid frame then you are likely to have an axle or axles teetering on uneven track…regardless.
As far as weight. Center the weight as much as possible over the drivers. As far as amount of weight. 7.5 oz is nearly a half pound. I won’t say that is a lot, I am well known for weight overkill in locomotives and have toasted many motors. It’s the combination of wheel contact and weight give you adhesion.
Look at refining your track. It’ll be a benefit all the way around
You assume correctly. The best way to add weight is to hold (with no downward force), or butt the engine up against an object and spin the wheels under full voltage. Then add weight until the engine can no longer slip the drivers and then back off on the weight. Usually, but not always, you run out of room in the interior before this happens.
You can do the same thing with a DMM to monitor amp draw. Using the same holding technique, monitor the max slipping current. Then add weight until you see a sudden rise in current. This rise will usually be in sync with the drivers beginning to stall. Again, back off on weight until this sudden rise is eliminated on the DMM.
Ideally, the weight should be centered between the middle drivers.
Add the weight so the center of balance is between drivers 2 & 3. If the center of balance is over drivers 1 or 4 only those drivers will have traction. The engine will teeter on that driver and lift the other drivers off the rail.
That’s it exactly. For a locomotive with metal wheels (no traction tires) pull will be 0.25 * weight on drivers. As long as the motor can spin the drivers should the locomotive get stuck all is well. In HO this isn’t really a problem. There isn’t enough room inside an HO locomotive to pack in enough weight to stall the motor. The motor will be able to spin the wheels with all the weight that fits packed in. At least for HO locomotives. The old die cast steamers from Bowser and Mantua were good pullers 'cause they were heavy.
Brass steamers are usually light and will benefit from more weight. 7.5 oz isn’t all that heavy. My 80 foot cars weigh nearly that much.
The weight of a prototype loco roster (usually measured per axle, not per driver) is limited by the design of the roadbed and the weight/strength of the rails, and will probably be pretty uniform for locos built in the same era. The weight per axle of a model locomotive is influenced by its overall size, the amount of space used up by the drive and onboard electronics and the materials used.
Given that, there is no, “Typical,” model axle loading - and a 4-4-0 with widely spaced drive axles could easily have a higher axle loading than a 2-8-8-0 with roller skate wheels for drivers. Think PRR D16 versus the original N&W X (0-8-8-0) and Y (2-8-8-2.) The D16 boiler had a greater diameter!
My heaviest axle loading is found under a model of a Hohenzollern (German) 0-4-0T delivered to Japan in 1873 - which is basically a block of lead with a wheel at each corner. (For prototypical accuracy, it should be the lightest!) My lightest is a Bachmann Spectrum Alco 1910 'Catalog" 0-6-0T, which is largely plastic and had to leave space for a decoder that was never installed. The 0-4-0T is almost twice as heavy as the 0-6-0T, and has three times the axle loading.
It’s not a matter of pointless, it’s a matter of protection for the motor. If the motor is stalled (prevented from turning) and the power stays on, the motor will overheat. Like overheat enough to damage the winding insulation.
The unlikely scenario is so much weight that the motor cannot spin the wheels, AND the locomotive is prevented from moving by getting snagged on something inside a tunnel, or blocked by a wreck on the track, AND the power stays on, THEN the motor will overheat badly.
I have never been able to cram enough weight into an HO locomotive to prevent the motor from spinning the wheels. And all those other conditions usually cause the locomotive to derail which disconnects the power. But still, the motor ought to be able to spin the wheels if the locomotive gets stuck.
yes. not that i’m anywhere close to having that much weigh. But i’ve had to work to reduce the current when the engine is stopped and the wheels spinning to < 1A in order to be within the limits of the decoder.
I’m still not sure how close this engine is to running as well as it could.
A one amp draw with the drivers slipping is pretty heavy, even for an open-frame motor. You may have some other issues at play here, including binds in the mechanism, lack of lubrication, or a motor in less than good shape.
Weight doesn’t have to be over the drivers, but it should be balanced about the centre of the driver wheelbase. Most brass locomotives, especially older ones, have springs which are much too strong, and this can cause some of the drivers to have little or no contact with the rails. NorthWest Short Line sells replacement springs in “Firm”, “Medium”, and “Wimpy” and recommends the latter for most HO steamers.
This brass loco, bought “used” by a friend, had been fitted with a large can motor and was extremely back-heavy, to the point where the front drivers had almost no weight on them, and it was very derailment-prone.
I reworked it somewhat, and added enough weight (balanced) to bring it up to 32oz. (loco only). While it’s a very strong puller, it is at its weight limit and I’ve offered to remove some of the excess to give it more of a safety margin. My friend declined the offer, mainly because he doesn’t run his locos all that much and certainly not with the type of heavy trains which I often run. [swg]
As mentioned, most locomotives can handle more weight than can be hidden within them - I tested one of the Athearn Genesis 2-8-2s (notoriously poorly-balanced locos) with a 20oz. “saddle” of sheet lead draped over its boiler and it was still easily able to slip its drivers. Unfortunately, not all of that could be crammed into the loco, although I did manage to raise its weight (loco only) from 12.5oz. to 17.5oz.
replacing the magnets with neodymium magnets and some motor adjustments remarkably lowered the current with the wheels slipping, closer to 0.3A.
yes, the springs seem pretty stiff. Maybe the added weight can put them to use so that all wheels make contact with the rail, all the time. But I’m glad you told me about replacement springs available at NWSL, and that whimpy is recomended.
