gregc,
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.
For an example, the Rapido NH 8600-series coaches came with a plain bearing truck. The axles had blunt ends and fit into brass strips that had holes punched in them. These cars do not roll well, to be kind. The next design, the NH parlor, came with a redesigned truck that had needle point axle ends that fit into dimpled brass strips. These parlor trucks roll extremely well to the point you can’t park these cars on any kind of grade or they’ll roll away.
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.
I’m pretty sure bearing friction is a prime candidate for not scaling.
Terminal velocity comparisons and investigations into the flight of the bumble bee will reveal the scaling problems of “viscosity effects” such as in air which apply equally to other fluids. Model oils and greases are likely very viscous compared to prototype if scale is considered, just for example.
Material characteristics also don’t scale at all. Prototype locomotives and trains significantly depress the supporting ground under the rails. You can see it and feel it if you are close enough. Prototype trains are always climbing up a grade in that sense while models likely are not. Soft foam underlay might mimic this effect, mind you. My impression is that model rail is orders of magnitude more rigid than prototype in scale terms. Several kg of force is required to significantly bend and depress model rails (kg weight not mass in this instance).
Wind tunnels and marine architect test tanks (not to mention movie special effects) meet this problem frequently. For wind tunnels the scaled size of the model makes a huge difference. For test tanks the hydrodynamics are easier to adjust for although skin drag problems abound.
The load imposed on the model drawbar isn’t related to the drawbar force developed, which is the topic at hand. The concept for the thread originates with the idea that some model locomotives seem to have less drawbar force than they oughta.
Well, the discussion of physics, etc., etc. didn’t really do much to alter the facts that about the best we’ll get from a locomotive, real or model, is a drawbar force roughly equal to about 25% of the loco’s weight.
That’s certainly one option, but the origin of the thread was a reference to improving the pulling power of our model locomotives, hence the somewhat divergent inputs, most of them interesting in their own right, at least to some of us.
Guys, guys, what I posted is not what I really do but I did to get the responses from a couple of you guys that are just plain silly come on Charlie your response is like it is from a 5 year old. I have no understanding of the entire thread. There are some things you can’t scale down even if the numbers say you can. For instance the low end of sound, you cannot move enough air to get the rumble of a diesel engine. From the size of our speakers in ho.
Maybe one day when I’m dead and buried, we will but for now, NO.
Let’s just keep in mind, that while it is different from the prototype, our “needle point” wheelsets do not ride on the point, they ride on the top of the cone that makes the needle point, no matter how small that contact patch is.
Considerable testing brought me to the Kadee sprung truck/Intermountain wheelset, drop of light oil combination as being the most free rolling setup.
Some plastic trucks equaled it, but none could consistantly beat it. And the benefits of equalization are lost with the rigid plastic trucks.
Another guy who has to be reminded that there is a reason there should be no particular surprise that we guys are more interested in discussing the physics of real vs. model in … a thread about the difference between real and model adhesion.
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?
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.
(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.
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.
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.
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.
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.