DAVID FORTNEYIt seems you guys are more interested in discussing the physics of real vs model then actually building or running your railroad.
Tell you what: you can boast all you want about your unwillingness to fix problems or learn about why your diesels sometimes don't pull ... just do it in your own thread, m'kay?
gregc Paul3 While friction doesn't scale, we (usually) use a different kind of bearing in our rolling stock models. The needle point bearing offers a lot less starting friction due to the very small surface area in contact with the rotating axle. i agree that the needlw point bearings in models is a good solution. however, when i tested my trucks on an incline, i found a lot of variation (i.e. grade they rolled at). some seem to roll uphill. it was a challenge to get others to roll on a 2% grade. when we measured pull force needed for a train on a layout, we measured ~2%. this is 10x the value from the Armstrong chart i wouldn't say the rolling resistance (i.e. friction) of rolling stock doesn't scale. i'd say the mechanism is different, resulting in different performance. understanding this will help modelers have realistic expectations.
Paul3 While friction doesn't scale, we (usually) use a different kind of bearing in our rolling stock models. The needle point bearing offers a lot less starting friction due to the very small surface area in contact with the rotating axle.
i agree that the needlw point bearings in models is a good solution.
however, when i tested my trucks on an incline, i found a lot of variation (i.e. grade they rolled at). some seem to roll uphill. it was a challenge to get others to roll on a 2% grade.
when we measured pull force needed for a train on a layout, we measured ~2%. this is 10x the value from the Armstrong chart
i wouldn't say the rolling resistance (i.e. friction) of rolling stock doesn't scale. i'd say the mechanism is different, resulting in different performance.
understanding this will help modelers have realistic expectations.
Agreed.
Our world in not the prototype world. Note my post above regarding free rolling trucks. Remember my goal is long trains, which must be pulled up 2% grades.
Sheldon
(following on from Overmod's...) AKA, a straw man. Maybe I should follow Sheldon and get outside. My lawn needs mowing. There's always something else that some spending time reading in on these discussions think is a better use of time.
ATLANTIC CENTRALOur world in not the prototype world. Note my post above regarding free rolling trucks. Remember my goal is long trains, which must be pulled up 2% grades.
so if by my assessment decent rolling stock has a rolling resistance of ~2% (2.4oz to pull 30 cars at ~4 oz/car) then pulling a train on level grade is the equivalent of a ~2% grade and ~4% resistance up a 2% grade.
greg - Philadelphia & Reading / Reading
ATLANTIC CENTRAL Remember my goal is long trains, which must be pulled up 2% grades.
My goal is short trains on a 0% grade. That is why the Athearn Genesis 2-8-2 was a complete disappointment.
There is no excuse for not meeting my needs in the pulling-power department. If you can't please me, you can't please anybody.
-Kevin
Living the dream.
ATLANTIC CENTRAL Some plastic trucks equaled it, but none could consistantly beat it. And the benefits of equalization are lost with the rigid plastic trucks.
Some plastic trucks equaled it, but none could consistantly beat it. And the benefits of equalization are lost with the rigid plastic trucks.
Sorry, off topic, but out of curiousity, do you find a benefit from using sprung trucks? Does the equalization help the cars navigate/track better? Do they help the car roll more freely? Is the difference noticable?
I cant tell why a train of cars with equalized trucks could have less drag than a train without.
I've always been curious, and if so, I might have to invest in some kadee trucks.
Charles
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Modeling the PRR & NYC in HO
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gregc ATLANTIC CENTRAL Our world in not the prototype world. Note my post above regarding free rolling trucks. Remember my goal is long trains, which must be pulled up 2% grades. so if by my assessment decent rolling stock has a rolling resistance of ~2% (2.4oz to pull 30 cars at ~4 oz/car) then pulling a train on level grade is the equivalent of a ~2% grade and ~4% resistance up a 2% grade.
ATLANTIC CENTRAL Our world in not the prototype world. Note my post above regarding free rolling trucks. Remember my goal is long trains, which must be pulled up 2% grades.
My real world numbers are not quite as good, but similar. But they include possible small grades in the "level" track, curves, etc. And they suggest the improved suspension of the Spectrum Mountain does improve adheasion slightly.
Bachmann 2-8-4 converted to 2-8-2 and weighted to 18oz.
31 cars (tender) x 5.2oz x 2% = 3.22oz of drawbar pull required.
18oz x 25% = 4.5oz, 18oz x 20% = 3.6oz, 18oz x 18% = 3.24oz
Bachmann Spectrum USRA Heavy 4-8-2 - 20oz
39 cars (tender) x 5.2oz x 2% = 4.05oz
20oz x 25% = 5oz, 20 x 20% = 4oz
This would suggest that the better driver suspension of the 4-8-2 does improve adheasion slightly.
Trainman440do you find a benefit from using sprung trucks? Does the equalization help the cars navigate/track better? Do they help the car roll more freely? Is the difference noticable?
I run over 99% Kadee trucks and wheels on my freight car fleet. Most are sprung, but a few are "HGC" equalized trucks.
The main advantage is that they are less likely to derail. If one wheel gets lifted up, the other three can stay on the rails. On a solid truck, if one wheel comes up, another comes up as well.
Take a piece of track and put some staples over it and do a test. The equalized truck will be able to bump over the staples and only derail ocassionally. The solid truck will derail every time.
That experiment convinced me.
Trainman440 ATLANTIC CENTRAL Some plastic trucks equaled it, but none could consistantly beat it. And the benefits of equalization are lost with the rigid plastic trucks. Sorry, off topic, but out of curiousity, do you find a benefit from using sprung trucks? Does the equalization help the cars navigate/track better? Do they help the car roll more freely? Is the difference noticable? I cant tell why a train of cars with equalized trucks could have less drag than a train without. I've always been curious, and if so, I might have to invest in some kadee trucks. Charles
Charles, not really off topic, it applies to rolling resistance.
Our track, and prototype track is never perfectly level and we have vertical curves into grades, super elevations, etc.
Equalization keeps all the wheels on the rail ALL the time. Ever sit in a chair on an uneven floor?
Evenly distributed loads offer less resistance simply by being consistant. Loads that change create jerky movement, loading up, then unloading.
I pull long trains, there are lots of forces at work, equalized trucks minimize the chances that cars will string line in curves.
Additionally the trucks I use are metal, with metal wheelsets, this added weight is at the lowest possible point, making the car more stable, and allowing the gravity to center the wheelsets better.
Pulling ten cars, none of this will matter. Pulling a 50 car train, or 80 car train, of my 5.2 oz piggyback cars, it matters.
My tests show my trucks to be consistantly free rolling, as much so, or better than, any plastic truck/wheelset combo I have tested.
Out of the package, the Kadee truck is not real free rolling. They use a cast wheel on a plastic axle. The axle has a large cone which can bind a little in the truck.
I replace the Kadee wheelsets with Intermountain metal axle wheelsets which have a smaller cone diameter. I oil them with a small drop of light oil.
That combination is very free rolling, and a little expensive......
Ah, makes sense!
I've always been a fan of metal trucks with metal wheels.
Never understood Walthers/Kadee's choice of using plastic axles. Axles are just as important as the wheels, why cut it halfway, and cheapen the axles?
Plastic axles also mean it is nearly impossible to get a needle-tip edge on the axle ends. Instead, the blunt "cone" is used.
Kadees choice of using cast metal as wheels instead of turned metal never made sense to me either. Oh well...
ATLANTIC CENTRALOur world in not the prototype world. Note my post above regarding free rolling trucks.
Looking at this thing from first principles, once you reject the idea of clock-like cone-tip guidance, some interesting things follow, a couple of them involving 'truck tuning'.
If we presume ("posit" is the famous pedant's word; 'stipulate' mike's) that the alternative explanation about bearings is correct -- as I think it is -- then we have two broad cases: one, that the axle is made of much harder material than the conical 'journal' area, or two, that it is plastic and perhaps low-surface-energy like acetal/Delrin. In both cases we also have the 50-degree axle cone and 60-degree 'journal' cone.
The first thing that jumps out is that "theoretically" you still have point contact at the tip if both materials are 'inelastic' as a couple of earlier posters assumed. The junction between two cones of different angles, whether or not centered, is only at the tip. Of course what this implies in practice is that depending somewhat on the weight there will be some point deformation, and this will result in the very slight flank bearing that Sheldon and others accurately describe. An interesting thing that follows is that, within what may be fairly broad limits, axle length becomes irrelevant to bearing effectiveness: the axle will tend to center between the two 'journal' cones and continue to have 'optimized' bearing characteristic (assuming only that its tips don't permanently deform the sideframe material if they wander a bit with lateral loading) provided the 60-degree cone in the sideframe is consistent and smooth its whole depth. Perhaps this explains why truck tuning as a practice is so good and produces so consistent results when executed; it certainly seems to have implications for 'best design' of truck-tuning jigs and equipment.
Now in the first case, with axle much harder than 'journal', we can quickly realize that the contact area of the bearing will be defined by the (elastic) deformation of the sideframe material against the very end of the flank of the cone, probably a very shallow oval approximating a short (probably less than .003) line contact. In a plastic sideframe we could calculate this from hardness data on the sideframe plastic, but for practical purposes there is a clear advantage in superfinishing, perhaps even burnishing, the extreme tip of the axle cone -- but not most of the cone. This in contradistinction to the value of superfinishing, or hard-coating, or providing a better surface quality or chemistry, to the cone of the 'journal' in the sideframe (especially when it is part of a sprung truck with cross-flexibility).
The immediate thing this suggests for metal axles is that, if there were some way to make acetal inserts with a precise consistent 60-degree cone angle -- in fact, any consistent cone angle that clears the axle cone on cross-articulation of the sideframes -- and insert them into sideframes, you might well have the very best bearing this combination offers. Microfinishing of the axle tips will give you a little something more, so long as the extreme tip doesn't bend when loaded -- something that would appear to favor a blunt axle-cone angle over a long thin one.
Now, a fun consideration is 'what about those axles with rotating cap provision'? These are the ones for which I was playing around with watch balance hole-jewel bearings (where the cylindrical shaft is superfinished and extremely hard, and the 'cone' is in the knife edge of the bevels in the extremely hard hole jewels), but the emergent conclusion is that it would be better to machine these axles to be 'cone-bearing' a bit away from the extended pin for the rotating end-cap, in low-surface-energy plastic, then relieve the cone angle out of contact with the 'journal' and have clearance -- and more than just slight clearance, if the trucks are equalizing -- for the pin all the way through the sideframe or journal-box construction. The cap has to be made so it isn't touching anything, either... [There is also the Hot Wheels alternative, where a relieved Delrin bearing rides with minimal line contact on a hardened and polished wire axle -- anybody remember the contests in the '60s on who could jigger this to get their car to roll the furthest? all things made new again! -- but in a railroad application this isn't self-locating laterally and might suffer weird wear if schmutz gets into the soft plastic and starts "machining" down the thin cylinder of axle material...]
If the axle is 'softer' and made of a low-surface-energy material the question that would have to be answered by experimentation is how much the tip of the cone deflects as well as deforms to create the "bearing contact patch". This may be something that only occurs beyond a certain imposed weight, perhaps something just as critical as the point where in the prototype plastic deformation under the work-hardened martensite layer in the railhead starts to occur with HAL (one of the reasons 315K axle load isn't catching on even with head-hardened rail structure, but I digress), but if the axle tip is deflecting you're essentially doing work on it as the wheel rotates, 'lifting the weight of the car' ever so slightly uphill just as a car does lifting itself out of its own tire depression (see why the original Audi Quattro got better mileage above ~50mph in AWD than in just 2WD) and the number of cars you could pull with a given nominal drawbar TE might "mysteriously" decrease... BUT on the other hand, the deflection will also reduce the mutual deformation, hence the size of the contact patch, hence the interfacial friction effects of bearing against axle, and this effect might improve rolling more than the effect of the deflection. Only careful and informed testing with the 'other variables controlled for' would give you a practical solution, but the results would instantly inform a wide range of practical implementation.
a less esoteric mystery (to me) is why there is less resistance (per ton) with greater weight
coefficient of friction, rolling resistance
This may be valuable in the kind of practice that many people reading this thread want to see, and it's certainly a couple of techniques that are easily used. The catch is that any 'macro' sliding involves a very different mechanism than either actual adhesion from rolling contact or the sort of 'stiction' seen in creep control. For the latter to work at model scale you'd have to pulse the drivetrain reliably in and out of slipping torque at the same rate that produces the angular rotation of the wheels observable at prototype levels... remember the modulation produces pulse noise at audio frequency which will give you an idea how much the wheels actually rotate per 'cycle'... and I suspect even full epicycloidal gearing with instrument-grade backlash compensation wouldn't give that in a model.
You will note that I carefully beg the question whether the difference between 'sliding' and 'static' friction is that significant compared to the value easily observed for sliding wheels or locomotives. Large numbers of serious model railroaders won't care and I'm not going to tell them they are wrong.
LastspikemikeJust slide the unpowered locomotive using your spring scale. You get "stiction" friction as well as sliding or kinetic friction. Modern DCC locomotives might exploit stiction.
some of us have measured the pull of locomotives with wheels spinning as well as at just the onset of spinning. knowing the pull (oz) and weight (oz) of the loco gives use that number ~25%.
~25% can be used to estimate the drawbar pull of another loco simply based on loco weight and along with an estimate for rolling stock wheel resistance (~2%) explain why some locos don't pull as many cars as the owner expects. sometimes the loco is light or could be balanced better and sometimes the trucks are the problem.
weight distribution doesn't affect drawbar force.
it affects when slip occurs
slip occurs when the force on the driver exceeds the friction force. while all drivers have the same force, the friction is lowest on the driver with the least weight.
gregcweight distribution doesn't affect drawbar force.
If a model locomotive were effectively equalized like the prototype, most of the pernicious efforts of individual-axle loading would be solved just as on that prototype: the equalizers would move until all axles were balanced-loaded, within the limits of suspension travel. That would have to be very carefully laid out and made, minimizing any friction or interference causing the equalizing to hang up, but it would require little lubrication other than perhaps at the pedestals to function efficiently if the contact surfaces in the equalizers were sufficiently hard and radius-formed and polished.
Instead, we get drivers that are sprung nominally to increase electrical contact, and while (as with locomotives in England built consciously without equalized drive) the spring pressure can be carefully adjusted to give 'equal' weight apportionment for adhesion to all the driver pairs, this will become wrong if the sprung driver pair is then floated up or down. Much more likely, I think, is the use of lighter springs pressing these down without much effect on actually helping suspend the locomotive itself in reaction -- I suspect that in some engines the actual vertical suspension is carried (hopefully very unprototypically but you never know) on the lead and/or trailing truck(s). Now in my opinion if all the drivers were sprung (with the corresponding results on drive design) you would still have to be very careful balancing the locomotive fore-and-aft and then accounting for weight transfer, as all the springs will compress until in force balance with gravity, but this is not dictated by axial tilt but by weight distribution, and stopping the balancing short of equilibrium (e.g. by supporting the ends with solid or sprung trucks) will necessarily result in some axles having less imposed load than others, and hence lower factor of adhesion. (You would then also need to balance any spring pressure putting tracking weight on the engine and trailing trucks, but that follows logically.)
The issue of lateral accommodation is critical in prototype steam design, although probably less so in models, where dynamic augment is not commonly recognized as an issue (even in the Arbour Models build!) -- my personal suspicion is that when drivers are sprung there's enough play that the cross-level articulation in suspension will 'take care of itself' and the difference in adhesion between drivers in a particular pair would be slight. ("Springing" over the axle center would likely solve any issue concerning this for even exaggerated superelevation in model curves or cross-level defects)
I'm sure that someone, somewhere, has carefully studied the practical effects of traction tires, and how much extra 'friction' they provide for adhesion. Certainly the Michelin experiments in the '20s and '30s established that a wider contact patch with a conformal elastomer increases some measures of adhesion; to an extent they also indicate that even small quantities of 'lubricant', including water, destroy much of the advantage when present. I have not seen data or microphotographs about the conformed contact area of a traction tire with a "scale rail" head profile, but on typical rail with a square gauge corner and near line contact with tapered tread, it could only be considerably larger, quite possibly outweighing the effect of some or all of the other drivers together.
A little complication is the contribution to slip of decreasing adhesion on some wheels that are driven by rodwork. Since no driver actually breaks into slip conditions (full coefficient of sliding friction) you need to consider what is actually happening when a given driver loses some of its effective adhesion in your theoretical 'model' of friction effects -- there is clearly a loss of adhesion vs. applied torque, but not something measured by a sliding or slipping/skidding test until the actual slip starts and propagates.
The statement that "slip occurs when the force on the driver exceeds the friction force." is more correct in the aggregate, but this is something of a truism. The greater concern is, as far as practical, ameliorating any particular driver's or pair's adhesion so that all drivers 'depart' only at the highest applied torque -- which may involve suspension, compliance, and weight distribution, but the range of corrective action may be comparatively slight.
It might be interesting to consider the likely methods of action of 'traction increasers' like Bullfrog Snot in these models. Think of the ways this is like or unlike a fixed elastomer traction tire, or a tire in which the surface activity has been greatly increased (as in 'Gecko Grip')
Do we need to discuss the effects of weight transfer or methods to correct it (a couple of which already appear in the Arbour Models thread)?
LastspikemikeTwo comments: it isn't useful to consider the behaviour of rubber contact patches in a pneumatic tire when assessing how steel on steel develops drawbar force. Anyone who has driven an indoor only forklift intuitively understands why.
Why would this be intuitive for a forklift operator, since "indoor only" forklifts are (conventionally, by an overwhelming majority of standard practice) neither equipped with pneumatic tires nor steel tires?
Please do explain.
-Kevin (formerly an OSHA qualified forklift operator instructor)
LastspikemikeModel traction tires also no doubt creep around the groove in the metal tire which complicates things somewhat.
I am saying that weight distribution cannot be one of the relevant factors, at least at 1/87 scale.
Given the very low contact area pressures of a model locomotive I'd be surprised if the metal to metal creeps appreciably but obviously I've never measured it.
I agree with you that the 'null hypothesis' is that little if any observable creep (in the sense that effect is relevant to prototype traction control) takes place, and I will raise the hypothesis by saying that its 'scale' equivalent would involve hopelessly minuscule modulation of driver rotation even with very fast, somehow effective pulse control of motor rotation.
Jeepers, you need instruction to drive a forklift? They steer just like a boat, mind you lots of people are also confused by a boat tiller.
There's some other stuff, too, along the same lines that while experience is the best teacher, some lessons are better imparted in a classroom or as advice in training ...
(Helicopters are easy to fly, too ... until they aren't. )
I'd like to add that the contour and burnish method has allowed any locomotive I have tested to pull considerably longer trains
I don't know if the adhesion is improved but at the other end the overall drag is significantly reduced and that puts The forces in the locomotives' favor.
that being said there is reduced Shudder on wheel slipping and overall consisten operation
Semafores
semaforeI'd like to add that the contour and burnish method has allowed any locomotive I have tested to pull considerably longer trains
I've had locomotives that were definitely "slippery" when new. I'm thinking of the Broadway Limited PRR P5as as my most recent example.
The six drive wheels and three axles are pretty rigid in the frame. I've been running three of them on a sixty-car freight over the past month or so. In the beginning it required all 3 motors to move those sixty cars up a 2% grade.
Recently I removed one of the motors and the two remaining P5as are walking right along with the same sixty cars.
There may be a coating (darkening agent) or manufacturing oils on the wheel treads but I believe as the engines are run-in that the bearings and wheel treads even out and pulling power improves. Ideally, each drive wheel would bear exactly 1/6 of the weight but without equalization this is not easily attainable.
I believe most engines require at least five hours and maybe more of actual run time before they find their "sweet-spot" and get down to business.
I would have much prefered BLI would have designed these three-axle locos with sprung journals but maybe they will be OK afterall.
Regards, Ed