# Traction: Wheels or Weight?



## rexcadral (Jan 20, 2016)

I'm building a small loco, the boiler and cylinder donor being a Roundhouse Sammie. As this is my only experience with live steam, I've found that Sammie likes to peel out on 2% grades if he's got more than 3 cars behind him.

Roundhouse says their locos will pull ~28lbs, 6-7 cars by their measure on flat track. (4lbs/car, which is about right for my stuff) Sammie could definitely pull more if he weighed more than 4lbs himself.

What I'm trying to figure out is, for a 14" long locomotive:

How much loco weight can the Roundhouse cylinders handle without reducing car pulling capacity
What works better? 4 drivers with max weight, or lighter weight but more drivers?

My frame can support either an 0-4-2 or a 0-6-0 setup easily. What do the experts say?


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## SD90WLMT (Feb 16, 2010)

Put on thinning cap here...
.. With a fixed weight loco..nuthing changes..more drivers reduces weight per wheel..
Still same Tractive effort created...regardless...
How much weight to add is function of loco design..safety.. Wear n tear on drives..

Don't get in a rush till you learn more..

Dirk
DMSRy.


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## chuck n (Jan 2, 2008)

I've measured the tractive effort, pulling power of a number electric locomotives and I can't imagine that live steamers would be that much different. The average pulling power is about 1/3 the weight of the engine.

That suggests that the Roundhouse engines weigh about 90 pounds. Perhaps the 28 pounds is the total weight of the cars. This has nothing to do with how much the engine can pull. It is the drag of the cars, friction in the journals that is most important. The weight of the train being pulled comes into effect on an uphill grade.

Chuck

Greg Elmassian has posted my tractive effort table on his very informative website.

http://www.elmassian.com/index.php?option=com_content&view=article&id=649&Itemid=894

This will give you an idea as to what typical engines can pull. Remember the weight of the train is far less important than the drag (friction) of the train. No matter how powerful an Large scale engine is, it in not likely it would pull a brick very far.

Ball baring wheels/axles, will greatly increase the number of cars any engine can pull.


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## Mike Toney (Feb 25, 2009)

The more wheels you have, the more weight that will be needed. Lots of factors go into what it can pull up a grade. Wheel material, rail material Condition of the rail, is it wet, has weeds/vegitation growing across it. Is the rail oily from previous steamers running. The heavier the better. Most little steamers like some weight to help tame them on the low end speeds. Obviously if you go overboard, increased wear in the rods and bosses, axle bearing wear and the like will increase. Also do not forget balance. An engine out of balance will also pull poorly, despite possibly being quite heavy. Same goes with a trailing or pilot truck that removes weight from the drivers. That said, the 0-4-0's I have run pulled better. Most Roundhouse 0-6-0's middle drivers are blind and also do not touch the rails hardly at all when on flat track.. Mike


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## Garratt (Sep 15, 2012)

Traction on rails is more about weight than the number of wheels. If you think about it, the surface area contact between a wheel tread and the top of the rail (without considering the finer points of physics) is in theory an infinitely thin line. All wheels have their slipping point.
With prototype railways a larger number of wheels can distribute the weight more evenly on the track which is the main purpose of having many wheels.

Andrew


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## rexcadral (Jan 20, 2016)

*Verdict appears to be weight.*

Ok, so weight appears to be the consensus.

I'll have to make some sort of pannier for my 0-4-0 (Sammie) and load him up to see how much heavier he can get and still pull.


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## rexcadral (Jan 20, 2016)

*Weight corrections for Chuck N*



chuck n said:


> I've measured the tractive effort, pulling power of a number electric locomotives and I can't imagine that live steamers would be that much different. The average pulling power is about 1/3 the weight of the engine.
> 
> That suggests that the Roundhouse engines weigh about 90 pounds. Perhaps the 28 pounds is the total weight of the cars. This has nothing to do with how much the engine can pull. It is the drag of the cars, friction in the journals that is most important. The weight of the train being pulled comes into effect on an uphill grade.
> 
> ...


============================
Hi Chuck,

Weight: Sammie weighs ~4.5lbs with no water in the boiler.
He can pull ~4 cars with ease up any grade. Each car is ~3-4lbs. Based on Roundhouse's estimate, that train is a little short, but 4.5lbs seems really light, even for a G scale switcher.

Re: reducing friction, I've already been all over GIRR and Elmessan's site. Many of my wheelsets are EZ-Rollers, all are greased with a Lithium grease.

Honestly, my best wheelsets are solid metal ones from USA Trains, which are plain bearing type. The ball bearings in the EZ-Rollers probably need to be torn down and re-greased with some seriously nice grease to do better.

I should also mention that my experiments were performed with body-mounted couplers on both wet & dry brass rail.Oily wheels is a perennial problem for live steam I would imagine, but I did a "realistic" amount of cleaning.

Oddly, sand from my kids' sandbox hand-rubbed onto the rails does make a huge difference in tractive effort, at least for a few passes. I don't suppose there are scale sanders out there....


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## Greg Elmassian (Jan 3, 2008)

A slight restatement, traction is about weight ON THE DRIVING WHEELS...

Some people have put strong springs on pilot and/or trailing trucks to keep them from derailing, and have hurt their traction.

Greg


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## rexcadral (Jan 20, 2016)

Greg Elmassian said:


> A slight restatement, traction is about weight ON THE DRIVING WHEELS...
> 
> Some people have put strong springs on pilot and/or trailing trucks to keep them from derailing, and have hurt their traction.
> 
> Greg


@Greg,

Got it, read one of the sticky posts on the subject. Really informative. While we're talking about weight per wheel, does equalization have a noticeable improvement in traction at this scale?


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## rexcadral (Jan 20, 2016)

*What does an average 1/32 live steam switcher weigh?*

@Mike,

USA Trains has a Docksider (0-6-0T) that has to weigh more than 10lbs (I actually lifted it at Charles Ro last weekend) and that's an electric.

I couldn't believe how much it weighed compared to my Roundhouse, and I'm wondering if the Roundhouse loco is light because at 40psi and 5/16 throw, it can't be any heavier.


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## Greg Elmassian (Jan 3, 2008)

I'm not an expert, but theoretically equalization should help. Not all track is perfectly flat, so it should keep all wheels sharing the weight which should improve traction.

Many tests are done on perfectly flat track, so I'd guess that some people would say it makes no difference based on those tests, but perfectly flat track never happens in our hobby ha ha!.

Greg


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## toddalin (Jan 4, 2008)

OK, how about doing something totally different. 

Rather than adding either weight or wheels, how about cutting grooves into the wheels and adding "traction tires."

When George Schreyer did his locomotive tractive effort tests, the original R/C Bachmann was right up there with the best of them.

Certainly this is neighter a heavy engine, nor does it have beacoup driving wheels. The reason it did so well is that the wheels are made of plastic and had fantastic traction.


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## s-4 (Jan 2, 2008)

http://www.trainworld.com/manufactu...36167-metal-ball-bearing-wheelset-30mm-2-pcs/
You might also try ball bearing wheelsets for your rolling stock. It's like the old saying, why work harder when you can work smarter? Its not as cheap as adding weight, but it will be less wear and tear on the engine and probably gives better performance. I use the LGB BBs with my steam stock, but for a bit cheaper, the PIkos seem to work nearly as well.


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## David Leech (Dec 9, 2008)

Two thoughts!
1) I have never really understood this fact.
If you have a locomotive, of a certain weight say with four equally weighted wheels, then ¼ of the weight is on each wheel.
If you fit the same locomotive with 8 wheels, you will only have ⅛ of the weight on each wheel.
Would that not make the 8 wheeler more prone to slipping?
Or does the fact that it has twice the contact area to the rail surface cancel it out and makes it the same as the 4 wheeler?

2) I have two live steam Hudsons - Aster NYC and Accucraft CP Royal Hudson.
The Aster is fully equalized with no springs, and the Accucraft is sprung, although very stiffly to the point I am not sure if they really do anything.
The Aster is somewhat lighter than the Accucraft, and slips like crazy.
I keep meaning to add weight so that it equal to the Accucraft's weight and then try again to see if it is weight, suspension, or wheel material, or what.
One day I will get around to it!
I know that Bednariks have added downward spring to Aster Hudsons to help.
Regards,
David Leech, Delta, Canada


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## CliffyJ (Apr 29, 2009)

To your #1 point David, if you fitted 8 smaller drivers under the same weight as the 0-4-0, it seems to me that, like you say, there would be no tractive difference. 

However, and I'm just pondering out loud, usually the boiler, frame, etc. grow in length, and add that much weight to each additional driver axle. 

So if the driver diameter is kept constant, and the boiler / frame / etc. grow to accommodate them, I'd think that each pair adds (almost, not quite) a proportional amount of traction. 

The benefit of additional drivers probably diminishes with each pair though, because the more constant weight factors (e.g., cab, firebox, couplers, stack...) don't grow in a linear relationship with the number of driver pairs. These more static weights are instead divided into the traction of the growing number of driver pairs, just as you describe in point #1. 

Just musing here, I'm certainly no expert,
Cliff


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## Greg Elmassian (Jan 3, 2008)

David, physics... the friction is proportional to the total weight and the coefficient of friction. 

10 pounds on one wheel or 1 pound on 10 wheels, no difference.

Basic physics.

Greg


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## rexcadral (Jan 20, 2016)

*Traction Tires*



toddalin said:


> OK, how about doing something totally different.
> 
> Rather than adding either weight or wheels, how about cutting grooves into the wheels and adding "traction tires."
> 
> ...



@toddalin - I stand with George Schreyer when it comes to traction tires - one more thing to maintain. Besides, it's kind of cheating. Half the fun for me is having to solve the physics problems the real railroads do. If I want to pull 40+ cars, I'll get a Challenger. Right now I'm just looking for anecdotal evidence.


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## Greg Elmassian (Jan 3, 2008)

Traction tires can put uneven loads on the driving wheels on a loco, I have found more failed axles on USAT locos on the axles with wheels with traction tires.

Makes sense, but if half your wheels have traction tires, you have really doubled the load on those wheels. 

Greg


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## Phippsburg Eric (Jan 10, 2008)

Except for climbing Hills the additional weight will have little adverse effect on the loco's performance. It will tend to accelerate a little slower and stop slower, both good things. And roundhouse engines have enough power to climb pretty good Hills. 

By the way, I see you are in Massachusetts, we have a pretty good in formal steam up club in northeast mass, Southern NH and Maine, shoot me a Pm and I'll put you on the email list. A steam up a month during the warm months.


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## Chris Scott (Jan 2, 2008)

Phippsburg Eric said:


> Except for climbing Hills the additional weight will have little adverse effect on the loco's performance. It will tend to accelerate a little slower and stop slower, both good things. And roundhouse engines have enough power to climb pretty good Hills.
> 
> By the way, I see you are in Massachusetts, we have a pretty good in formal steam up club in northeast mass, Southern NH and Maine, shoot me a Pm and I'll put you on the email list. A steam up a month during the warm months.



Won't it stop faster (come to a stop in a shorter distance) due to the greater friction from the added weight ?


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## Greg Elmassian (Jan 3, 2008)

Yes, when a loco ascends a grade the "normal force" goes down.

Remember that friction is defined by the "normal force", i.e. the component of force straight down, when you go up a hill, that is reduced, until you get to 90 degrees and it goes to zero.

Yes you stop more quickly going up a hill.

Greg


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## toddalin (Jan 4, 2008)

Greg Elmassian said:


> Yes, when a loco ascends a grade the "normal force" goes down.
> 
> Remember that friction is defined by the "normal force", i.e. the component of force _*straight down*_, when you go up a hill, that is reduced, until you get to 90 degrees and it goes to zero.
> 
> ...



Actually, not quite correct.

The "normal force" is the component of force _*straight up*_ on level ground and up at an angle perpendicular to the ground when on an angle.

This is the force that "pushes back" to keep us from sinking through the earth or any other solid object.


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## JPCaputo (Jul 26, 2009)

Traction is coefficient of friction times normal force. Essentially the more weight per driving wheel the more traction. Also the more driving wheels in contact with the rails i.e. Suspension flexibility to let the wheels follow the track, the more traction.


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## HMeinhold (Jan 2, 2008)

JPCaputo said:


> Traction is coefficient of friction times normal force. Essentially the more weight per driving wheel the more traction. Also the more driving wheels in contact with the rails i.e. Suspension flexibility to let the wheels follow the track, the more traction.


Partially wrong. As has been pointed out before, the number of wheels in contact with the rail does not affect traction, at least in our models, where inelastic deformation of rails is a non-issue. This is simple proven physics. There may be second-order effects where isolated spots of oil on the rails may cause individual wheels to lose their grip. In this specific case an equalized chassis is less prone to slipping. Imagine a 0-4-0 where only 3 wheels are in contact with the rail and the isolated 4th wheel gets on a slippery spot. In this case you lose up to 1/2 of your traction. With an equalized chassis the loss would only be 1/4. 
However any non-driven pony wheels which carry weight reduce traction.
Regards


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## Greg Elmassian (Jan 3, 2008)

Maybe the way to say this is that an equalized suspension may allow the traction (friction) to more closely achieve the theoretical maximum.

Or you could say a properly functioning equalized suspension should help maintain the best possible traction.

Greg


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## bille1906 (May 2, 2009)

I think as Greg pointed out earlier, traction is a factor of weight over the drivers and coefficient of friction between the wheels and track. period.
My most efficient loco is my 15 pound 0-4-0 Ruby which has all of it's weight distributed over only four unsprung wheels. It can easily pull 30+ five pound cars. Draw bar pull is just under 4 pounds or about 4:1 to the weight over drivers.
i believe the main reason for the real ones having more than four wheels was to keep from damaging the tracks with too much weight per foot. Not for more traction. Equalization also protected the track.
As Henner pointed out pilot and pony wheel sets reduce traction so i build my engines so that there is just enough spring tension on them to prevent derailments and no more.
I spring my drivers but feel equalization is not necessary at our scale, I also noticed that the Aster equalization is done with solid links vs leaf springs so I am not sure how effective it is, but it sure looks nice..


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## Greg Elmassian (Jan 3, 2008)

I agree, and I was equating springing with equalization, but of course they are different as you pointed out.

I'd guess that springing is enough to help, but it would be interesting to see a study.

Again, it's the Normal force on the drivers, the part of the weight being exerted straight down as it were, so that explains loss of traction up inclines as compared to level track.

Greg


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## toddalin (Jan 4, 2008)

Greg Elmassian said:


> Again, it's the Normal force on the drivers, the part of the weight being exerted straight down as it were...
> 
> Greg


Again, not quite right.

Again, the normal force is the force of a solid object that "pushes back" against an opposing force, typically gravity..., but not always.

Consider that when a train goes into a curve, and super-elevation is applied to that curve, the train "pushes into the curve" under the force of inertia.

But the normal force of track "pushes back" against the drivers, and in this case, superelevation increases the friction, even though the force of the mass is not being exerted straight down.












"In mechanics, the normal force (Fn) is the component, perpendicular to the surface (surface being a plane) of contact, of the contact force exerted on an object by, for example, the surface of a floor or wall, preventing the object from falling. Here "normal" refers to the geometry terminology for being perpendicular, as opposed the common language use of "normal" meaning common or expected.

For example, consider a person standing still on the ground, in which case the ground reaction force reduces to the normal force. In another common situation, if an object hits a surface with some speed, and the surface can withstand it, the normal force provides for a rapid deceleration, which will depend on the flexibility of the surface."


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## bille1906 (May 2, 2009)

All correct but in our slight grades (usually less than 2%), I think the main culprit of slipping is load and not angle of downward force.
On a level track, once the train gets going it only has to overcome fiction and wind resistance. On a grade, it now has to do this plus lift the entire load


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## HMeinhold (Jan 2, 2008)

As Toddalin shows very nicely in his graph, a grade affects pulling in two ways: The traction of the loco goes down due to the reduced normal force and the required force to pull a train goes up. At our grades the second one is more important, as it goes with the sine of the angle compared to the cosine for the normal force.


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## Greg Elmassian (Jan 3, 2008)

By the way, when you slip, now you change over into kinetic friction, which is normally less than static friction. That's why you have to stop the slipping and try again.

Anyway, although this concept of just the weight and the coefficient of friction being the factors seems to go against common sense for most people at first blush. 

Once you realize that an infinite number of wheels would result in virtually no force per wheel, it starts to make sense. This topic comes up on a regular basis, often with people determined to prove Leonardo Da Vinci and all the others that determined this hundreds of years ago.

Greg


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