Model vs. prototype adhesion

Model Railroader Prototype information for the modeler
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This discussion is starting up, off-topic, in the detailed-RTR-operating-model thread. It deserves its own named thread for discussion.

Some of the discussion thus far is as follows:

[quote user=“ATLANTIC CENTRAL”]

Lastspikemike

Yes, thanks, I’ve actually previously book marked that extensively detailed description. [This is the link to big blue trains.com on improving adhesion for light models.] Thanks.

The disparity in pulling power for these steam locomotives has me pondering coefficients of friction on nickel silver…

However, I noted an extensive thread on locomotive drawbar force as it may relate to the coefficient of friction of steel on steel as well as steel on sanded steel comparing prototype to HO.

In light of the significant disagreements evident there I decided not to make my own contribution. The topic seems surprisingly controversial, given that the physics have been well understood for over 100 years.

Interestingly, diesel models seem quite predictably uniform in their pulling power and roughly correspond to prototype patterns: bigger and heavier pull proportionately better. This is not the case for steam locomotive models which is frankly a bit weird.

It is not weird at all. Model steam locos, even with spring drivers suffer from a lack of even weight distribution on the drivers, as well as other traction losses on our sharp curves, etc. This problem increases with the number of d

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Roughly, model diesel locomotives have about 25% adhesion (just like the real thing historically, not counting modern traction controls). That means if a loco weighs in at 16oz., it should have about 4oz. in tractive effort, more or less.

At my club, we test all registered locos using a spring scale that measures force in 1/4oz. increments. Right now, we have about 2000 engines on the roster, and since I test most of them, I have a lot of experience in measuring tractive effort.

Things that make an engine pull less:
Less weight on drivers
Vibration
Clean wheels
Clean track
Unbalanced on drivers
Nickel-Silver wheels
Nickel-Silver track
Less powerful motor

Things that make an engine pull more:
More weight on drivers (up to a point)
Smoothness
Dirty wheels
Dirty track
Balanced on drivers
Sintered wheels
Steel track
More powerful motor

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my understanding is that prototypical adhesion is ~25% of the balanced weight on the drivers and this is also roughly true for models: 20-30%. Of course wetness, leaves, dirt, frost, … can affect this.

my understanding of friction is that it depends primarily on weight and less to do with surface area.

i don’t understand why the value is as small as it is if the coefficeint of steel-on-steel is ~0.74-0.8.

“balanced” refers to the desire to have an equal amount of weight on each driver. since friction depends on weight, the first driver to start slipping will be the one with the least weight. Once it slips, all the force is now imposed on the non-slipping drivers which very likely will exceed their friction limit and also slip.

balancing the weight on drivers appears to be easier with diesels than steam.

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Other minor things that are technically covered in the list, but could be expanded upon:

more traction can come from:

  1. weaker/no springs on the leading/trailing trucks

  2. even springs on wheels*

  3. less blind drivers

  4. a more rigid wheelbase**

  5. drawbar on engine should be (ideally) at the height at which the centers of axles on the drivers are***

Many of these have minimal effect, only apply to certain engines, and potentially hurt the operating characteristics in other ways(for ex sprung leading/trailing truck helps tracking) but they do contribute to pulling power.

The most common cuprits are usually unbalanced drivers, and a general lack of weight.

This is a very interesting thread, I will be keeping track of it.

Charles

*often times on brass engines, the wheel with the gearbox has stiffer suspension than the rest, creating a sort of seesaw effect, where not all wheels are putting equal force on the track.

**=flanges scraping against sides of track will provide more traction,thats why engines pull more on curves. rigid wheelbase=more horizontal force on flanges against track.

***=too high and the load will pull the front of the engine up.

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That’s why I model steam!

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I assume it has to do with having enough weight with the correct distribution to have the steam power appropriately transferred to the rails (not totally unlike like getting a car engine’s torque/hp transferred to the tires) in order to get the maximum pulling power.

In the prototype the design reality of the innards were pretty consistent from producer to producer compared to our models…IMO, and the bigger the loco the more it weighed proportionally to a smaller loco.

Adding ballast mattered some, but the weight of the mechanicals and “shell” of the prototype alone was proportionally heavier than the weight of the mechanical innards and shells of our models.

With the models, weight (or added ballast) is a much larger component of the overall weight of the loco…IMO… and with different producers have different innards, there has to be enough space to add the weight in the right spots.

Probably not consistent from producer to producer, or even small loco to large.

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I have little mechanical engineering in me, only personal experience with my locomotives.

I totally agree with the blind drivers being less adhesive. My Bachmann and Bowser 4-8-4s are not good pullers, run and look good but puny drawbar for such a large locomotive. All have stock weight and motors.

My Rivarossi articulateds have very nice drawbar, 5.8 to 6 oz. Most have 8 to 10 ounces of added weight and at least one new can motor, many have dual can motors. They weigh in at 19 to 20 oz. each or around 30% traction, more wheels higher percentage?? Dual motors don’t have any effect on drawbar they just run better. The original Rivarossi drive locking the two driver assemblies cause them to have a slight wobble, the dual motor floating driver assemblies don’t wobble.

Most of my diesel fleet are Athearn three axle truck, a few Proto 2K three axle truck and a few Model Power E-7s, again three axle trucks most with a lot of added weight. My Athearn E-7s have Cary metal shells on SD40-2 frames with can motors, weighing almost two pounds each and have 8 oz drawbar or 25% as mentioned above.

If someone wants to up the drawbar on their locomotives they can add weight but make sure the motor will handle the higher current. A single can motor in a 4-8-8-2 with the added weight is around 560ma at wheel slip, with dual can motors they run about 275ma each.

My remotored E-7s draw a bit over 800ma at wheel slip, 8 oz drawbar, a pair will pull your sox off.

Mel

My Model Railroad
http://melvineperry.blogspot.com/

Bakersfield, California

I’m beginning to realize that aging is not for wimps.

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Friction is a coefficient. Unless steel on steel develops the same coefficient as whatever model drivers are made of on nickel silver the model cannot match the prototype.

I suspect but don’t know that nickel silver alloy (mostly copper actually) has a pretty poor coefficient of friction. I’m very curious about whether the choice of metal coating on the tire surface of a model locomotive significantly affects tractive effort. The nickel silver part is a constant. Bronze rails likely would be stickier, as used in G gauge, but that’s just a guess. What is the ideal metal for drivers though?

As for weight distribution on the drivers that is not relevant to total tractive force because friction is a coefficient. Uneven weight produces proportionally differing traction force for each driver but the total is the same (not the case for pneumatic tired vehicles just btw, but air filled rubber tires are really weird structures) . For prototypes weight distribution matters only because of rail carrying capacity, not a concern for our models.

Weight resting on undriven wheels does reduce total traction capability, in models and prototypes. Steam switchers are 0-4-0 for very good reasons. Road locomotives would not use pilots or trailing trucks if they could avoid them. Diesel trucks with idler axles are rare for the same reasons. You might add an idler axle for load distribution reasons where the prime mover can’t overdrive all the drive axles anyway.

You only need one rubber traction tire on a model locomotive to improve traction significantly.

The locomotive, prototype or model, will develop the same tractive effort whether the drive wheels are equally loaded or have wildly different loading, until the rail gives. Coefficient.

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friction is a force (lbf). the coefficient of friction is a scalar

the coefficient of friction depends on both metals. No need to hypothesize, there are tables: friction and friction coefficients

doesn’t matter until one wheel starts slipping and all the rest follow. max tractive effort therefore depends on the max force of the wheel with the least weight

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The drive wheels are all locked together. They can’t slip just because one wheel develops less tractive force. They all slip as if they were one big wheel.

When we speak of friction we are actually speaking about the coefficient and not the force. People think we speak of the force and that misleads them into thinking that it is understanding the force that produces an understanding of friction. Friction is a property of materials. It is not “a force”.

Friction also doesn’t scale, it remains the same across a very wide range of weights and forces.

Another aspect of model behaviour not matched by prototype relates to possible leverage effects on the locomotive drawbar. For diesels the coupler is the drawbar and always at a standard height. The Diesel drawbar is always pulled from below by the elevation of the drive wheel axles. Not so for steam locomotives equipped with a tender. They use a separate drawbar not always at standard coupler height plus the drive wheel axles are much higher than on a diesel.

Working on my Genesis Mikado I noted the drawbar was a relatively crude stamping out of possibly stainless steel. The holes has been cut from the top face. The underside face had sharp edges which tended to catch on the connecting pin, pumping the connection point up or down out of sync with the locomotive and tender movements. The same effect can be caused by those wire harnesses between locomotive and tender. The lifting effect on the tender is obvious because it is usually light. Not so for the locomotive. The forces that act on the tender come from the locomotive (and vice versa) but the locomotive won’t show movement because it is heavy, relatively. It is possible the locomotive may get lifted in terms of weight on the drivers without there being any observable lifting at the drawbar. Any weight transferred from drivers to tender trucks will reduce tractive effort.

Adding weight will always increase total friction force.

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the force applied by the cylinders is evenly distributed on all wheels because they are locked together, but the friction on each wheel depends on the weight.

when the force exceeds the friction on that one wheel, the force then shifts to the remaining wheels which usually exceeds the friction on those wheel and they immediately slip.

huh? the coefficent of friction will be the same on different types and sizes of locomotives but the friction determining the max tractive effort of a specific locomotive depends on the weight of the locomotive and the number of drivers.

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Well, the list of variables here is pretty long, and lots of people have touched on many of them.

And, with steam loco models, what is a major factor for one model may be of no concern for another.

I have a pretty good sized steam fleet, I pull long trains, I have done some testing. And I have added weight and made other modifications with considerable success in most cases.

I am not going to try and offer all my thoughts on this topic in one post.

In this converstation some will try to rely completely on known prototype engineering theory, some of that has already come up. I wish it was that simple…

A few basic observations based on my 53 years at this:

Balance of weight on drivers does matter.

Springs on leading or trailing trucks will often reduce tractive force.

Sprung drivers sometimes help, sometimes not.

Vertical curves in track are a problem…

Sharp radius curves are a problem…

More axles mean more potential losses and problems.

Yes, drawbars can complicate the issues, in several ways.

Now, example one:

Bachmann Spectrum USRA Heavy 4-8-2 vs Bachmann Spectrum USRA Light 2-10-2

These two locos share the same boiler, just like their prototypes.

The 4-8-2 is driven by the second driver, and has a sprung third driver, all others fixed with minimal vertical play. This puts most of the weight on the second and forth drivers, with the others more “floating”. These locos pull well, mine will pull 35 of my 5 oz piggyback flats on level track.

The 2-10-2, same basic weight, same cast boiler, is driven from the third driver, and has no driver springing. Driver three is blind, like the prototype. The first and second driver have minimal vertical play. Drivers 3 thru 5 have noticable vertical play. This has the effect of putting most of the weight on drivers 2 and 5 on level track, but potentially shifting that weight to drivers 2-4 on so

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The added weight doesn’t all need to be within the drivers’ wheelbase. As long as the additional weight is balanced at the mid-point of the driver wheelbase, it will be effective in increasing traction.

Wayne

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I agree with greg. friction is not a coefficient, but rather a force. In modeling, we generally are less worried about that, but rather the coefficent of friction, which determines how much the weight affects friction, and is different for all materials/situations.

I believe friction between metals is likely more due to how soft each metal is. Even so, I’d argue that the finish on the metal is more important than the metal itself. In theory if you were to put diamond tread on a steel engine wheel, and placed it on a track that has a similar finish, that will run the sh!t out of any nickel silver plated wheel.

weight distribution is most certainly relevant. I may not know the math behind it all, but I know from experience that if the center of balance for the weight of an engine is off, that it will pull less than one with proper weight distribution.

In fact, I presume that’s why Sheldon placed additional weights near the front of his Bachmann 2-8-4. I dont ha

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I agree. Leverage effects are more likely to cause derailments than traction problems. Rigid driver wheelbase increases this effect. Polar moment might make a difference.

It is important to consider that, unlike the prototype,model pilot and trailing trucks are not functional, even if sprung. Also, just like the prototype, wheel flanges don’t work the way you might think.

To understand why weight distribution doesn’t matter (where the coefficient doesn’t change, which I have of course assumed) you just need to ask yourself why adults don’t need grippier shoes than children. Or why people with larger feet don’t need specially grippy shoes. Understanding how a high heeled shoe can possibly work is trickier. Just walking proves that friction force is not dependent on the number of feet touching the ground (or drivers touching the rails).

As for friction being a force, well that is really just semantics for non-mathematical discussion purposes. Since the coefficient is a number that is derived from friction force they clearly can’t be the same thing. Is friction “there” if nobody tries to slide it?

Finally, it is very

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Observe a locomotive with blind drivers that don’t touch the rails. It will still move and pull a train.

I don’t think it is correct to say a particular material has a particular coefficient of friction. The coefficient is a derived number (obviously) relating to a mated surface. Only if the touching materials are exactiy the same will the derived coefficient be for “that material”. Steel on steel, for example, is a useful number to put into a table.

The tires on my Genesis Mikado appear to be chromed. Inde

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Any actual physics involving friction will note it as a force, with units of a force, and in vector analysis corresponding to a force even though the mechanics that produce it are acting at a substantial angle. The reason we use coefficients is that they are dimensionless numbers; see Cd in streamlining for a comparable example.

There certainly were tables carefully comparing the coefficients of different materials at different surface finishes (which is an important characteristic so far missing from the present discussion). As I recall ASTM has standards for how to conduct meaningful research in ‘undocumented’ combinations and perhaps geometries, and it would be relatively simple for someone with, say, access to a college lab to do careful analysis of various materials on nickel-silver (perhaps even against nickel-silver rail of various states of ‘gleam’) as part of the investigation I keep hoping to see.

There are some other concerns here. In the prototype, there is clear deformation and surface-welding in the contact patch, less deformation and wear in the wheeltread than in the railhead by design. I very much doubt anything of the kind occurs in nickel silver rail, particularly that which is extruded with a sharp shoulder in the contact patch/gauge corner area as so much commercial product is. On the other hand I have never read an analytic discussion of model wheel wear, which is an obvious effect and often noted as an obvious problem – I’m sure there are experienced people on this list who can tell us the operative mechanisms that produce it on both driven and non-driven wheels. In my slim experience with steam-locomotive models that have severely worn wheels, the drivers appear to be little if any more deeply worn than the ‘carrying’ wheels in the trucks, which I expect to be significant in appraising the various wear mechanisms.

Note that wheels ought to be both precisely machined and tread-hardened (as all

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I don’t know much, but I do know some things from experience. I run short trains (6 to 12 freight cars), so pulling power really does not mean much to me.

This I know: My Oriental Powerhouse Light Mikados would pull a 12 car freight train around my layout, but the Athearn Genesis Light Mikado would not even pull a six car train around the 24 inch radius curves.

Physical experimentation and real world experience mean more to me than theory. Any model locomotive that does not live up to my needs must be a total lemon in the drawbar pull department.

The Athearn Light Mikado was junked and its nicer-looking tender is now coupled to one of the Powerhouse locomotives.

-Kevin

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As they should – in experienced practice; it’s no different than all the empirical factors and constants that plague steam-locomotive design to this day.

Part of the issue, though, is that you have nothing but black magic and trial-and-error to account for why the stuff you do works – you cannot, for example, account for why some locomotives appear ‘slippery’ while others don’t. One of the great trends in engineering becoming an actual profession is developing the models that can actually serve as good predictors for performance … or to explain anomalous or seemingly-mysterious results (or achievements) in reproduceable terms.

There is very little objective reason to junk a model as a ‘lemon’ when you can’t even tell me why it doesn’t do what its wheels, and basic physics, say it should. Not that there aren’t plenty of people who do, and not just in models – and not necessarily for expedience as in the likely case of the PRR T1s… in a world where practice matters, and time is valuable, we have come to specialize design engineering and proof testing as a profession, as nobody else has the time to waste learning to re-invent the watch before learning to build it when the object is to know the time. (Fortunately or unfortunately, someone will need to know a great deal about how watches are designed and constructed if they have a timepiece that is observed ‘not to be keeping time’ and it is then important to have watchmakers instead of ‘parts changers’ finding correct solutions to guide and inform practice.

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Certainly not what you would consider an objective reason, but in my world, tools and equipment that do not meet my needs are unceremoniously disposed of and something that meets my needs is obtained.

I will not waste my time and energy fixing what a manufacturer should have done right in the first place. Especially if there is something else available that will get the job done.

Why it does not work does not interest me. The designers should have a keen interest in that, not kick the can to the consumer to deal with.

My needs for pulling power are so low, that this should be a non-issue for me.

-Kevin