Weight balancing a steam loco ?

[mmagliaro]

Adding weight to a steam loco (well, ANY loco) is an obvious way to increase its pulling power.  But as I've always observed, there's more to it than just weight.  *Balance* is very important.

In general, I have always tried to just "feel" how heavy the engine felt at the rear and the nose, and tried to avoid making it
lop-sided.

Now, in my current project, I'm puzzling over a way to do this more scientifically.
It's a 4-6-2, so the obvious thing might be to somehow balance the engine on the center drivers, placed on some sort of
tiny roller or wire, and get it to balance (like being on a see-saw).
Or is it?  After all, if you do it like that, the weight of the pilot and trailing trucks affect it, yet when the engine is on
the track, those add or subtract nothing in tractive effort (unless they have lift springs in them that are really over-strong
so that they provide some significant lift on the engine.

Then I tried this.

I'm asking. 
Is this legitimate?
 Basically, the rear drivers are on a prop while the front are on the scale.
Then I flip it around and do the same, looking to get the weight on each end about the same.
While not 100% accurate, this does seem like a decent way to get it close.

Is there some physical property I'm overlooking that makes this totally bogus?
(I know it is off by the relative weights of the pilot and trailing trucks, for the same reason stated above,
so I suppose it would be more accurate if I took those off before I weighed it.).

And note, when I sum these two weights, I get within about 1.2g of the total weight of the engine.  (31.2 + 35.1 =66.3,
and the total weight of it on the scale is 67.5)

[C855B]

Not a steam engine expert, but just thinking from a physics approach...

You're looking for balance, or, rather, CG (center of gravity). The scale approach is going to be dicey at best. I would take a bit of track that covers the drivers and improvise a see-saw with the pivot under the center driver. CG is centered over the drivers when it balances on the pivot. If you're interested in total weight on the drivers, put the see-saw on the scale.

[Chris333]

I've always though weight balancing was done to locomotives with sprung drivers.

[mmagliaro]

Not a steam engine expert, but just thinking from a physics approach...

You're looking for balance, or, rather, CG (center of gravity). The scale approach is going to be dicey at best. I would take a bit of track that covers the drivers and improvise a see-saw with the pivot under the center driver. CG is centered over the drivers when it balances on the pivot. If you're interested in total weight on the drivers, put the see-saw on the scale.

You are correct.  That's what I'm after. I've puzzled over ways to make such a see-saw scale, but the obvious idea of using a piece of track may be the key.  I am envisioning soldering a small piece of brass tube under the bottoms of the rails, after cutting a tie
or two out of the way, and then just letting it balance on the tube.
If I wanted to be fancy, I could slip the tube inside two ball bearing races mounted in little blocks to the see-saw would be really low-friction.  But for the purposes of this balancing act, I really don't think that's necessary.    I am interested in getting
the balance point close, perhaps off by a few grams either way, not to fractions of a gram.

Thank you for your observation and suggestion.

[peteski]

I've always though weight balancing was done to locomotives with sprung drivers.

Truly spring drivers would be difficult to achieve in N scale.  But weight distribution is very important (if the model is specifically designed to take advantage of it).  Kato GS-4 is a perfect example where the model is very specifically designed to utilize the weight distribution to maximize its traction (or is it adhesion)?    This was discussed on the Atlas forum where I drew a diagram showing how cleverly Kato approached this problem.  This design is the reason that this loco can easily pull the full 12-car passenger consist.

[up1950s]

And just to make everybody launch their lunch . As a trains drag pulls on the drawbar , and where the drawbar is attached vertically and what vertical slop it has , the static weight on drivers may differ when under load .

[mmagliaro]

And just to make everybody launch their lunch . As a trains drag pulls on the drawbar , and where the drawbar is attached vertically and what vertical slop it has , the static weight on drivers may differ when under load .

Absolutely.   In very simple terms, it's easy to envision how pulling on the back end of the engine under the cab, under a lot of load, could make the engine want to pop a wheelie and lift the front drivers, and in fact, many steam engines do start to lift their front drivers when the load is severe, if they don't slip, that is.   

So if the traction tire is on the rear driver, you may want to load up the weight there, but then again, if that makes
the front and middle drivers start contributing almost nil traction because of the lift, you do NOT want to do that.
That's why balance is important to really good pulling.

[mmagliaro]

After further consideration, I'm not so sure my idea with the scale isn't valid.




How is it not true that if I do what I did at the start of the thread, I don't get an engine that is
balanced over the center driver?    And remember, putting it on the see-saw, you still have
errors in getting the center driver exactly over the center of the see-saw, and making a see-saw
that truly balances on one point.  I think within practical limits of error, weighing each end under
driver #1 and driver #3, while the rest of the engine is supported only by the other driver,
should give me a good balance.

[SkipGear]

I don't aim for equal front to rear weight. I try to get the CG somewhere over the TT driver, if there is one. If there is no traction tire driver, then the CG should be in the center of the main driver wheelbase. Since getting perfect ballance is usually a fairly difficult task, then typically I will shoot for slightly heavy in the nose to improve tracking on curves. I prefer to have the TT on the rear most driver and weight the loco so the CG falls around that point.

As an example, my 2-10-2 is ballanced around the #4 driver. The #5 driver on my loco floats and is just along for the ride and does no real work so that it won't remove any weight from the #4 driver, similar to the way the GS-4 is designed except I did not spring the last driver.

[peteski]

Like Tony stated, it isn't always beneficial to evenly distribute the weight over the drivers (at least in N scale locos).  Here is a simplified diagram of weight distribution of the Kato GS-4 I mentioned earlier.



The arrows show amount of the chassis weight being supported by those particular wheels.  The transparent wheels are for show only: they do not support any significant portion of the model. The 2nd driver set freely moves up into the frame (so it is basically cosmetic). The 4th driver set is sprung very gently (to keep it stable on the track while traveling through curves). The leading truck is also gently sprung. The springs make them support a miniscule part of the chassis mass.  Because most of the model's weight is supported by the traction-tire equipped driver, the loco is a great puller.

[mmagliaro]

Tony, Peteski, and all,
I agree, that the optimal spot is probably to bias it for a little more weight on the traction
tire driver  (which is the rearmost driver in this case).

My plan is to get it as balanced as possible, at this stage, because it will be easy to add lots
of weight in the back, but not so easy in the front.  I added about 8g of tungsten blocks
in the front open area to get it to balance out evenly.   And it is balanced now as near as I
can tell by my scale method.   In fact, it's a little nose heavy.   So later on, when I start tuning
this thing for performance, I won't have to worry that a little more in the back will be going overboard.

It start at 59g, is now up to about 67.   (A Kato F7, old school type, is about 120g !  I wish I could
get weight like that into this engine).

[LV LOU]

 I don't worry about CG at all.I  look at it as both end drivers being fulcrums,so I look at a steam locomotive as having two balance points.You can add weight anywhere outside the fulcrums as long as you don't add so much that you start to overcome the weight of the entire locomotive beyond the closest set of drivers..If the locomotive is heavy enough,and the pulling point is below the axle centerline,engine power shouldn't lift the front,although I usually make the front heavier..

[Ed Kapuscinski]

I think this is the perfect controlled opportunity to test out which thinking is correct.

[mmagliaro]

I think this is the perfect controlled opportunity to test out which thinking is correct.

Ed,  you mean I should build it and test my engine, balanced evenly as I now have it, see how many cars it pulls, and then add weight to the rear a piece at a time, taking pulling measurements to see if it gets better, and if at some point it actually gets a little less better?

I was planning on doing that. (And how 'bout that run-on sentence!)

I am guessing that more weight in the cab will increase the pull, but at some point, if I keep adding it, the pulling power will drop if balance is as important as I think it is.

[Mr Z]

I have an older Bachmann 0-6-0 that I did some experimenting with. While I added weight to the front end, the center of gravity was still closer to the third driver than I would have wished.

I then removed weight from behind the third driver which moved the CG close to the middle. I found that it actually increased pulling power.

Martin Z