Rolling Resistance Tips

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Greg Elmassian and I were having a conversation about Aristo metal wheels. He is trying to determine what he has to do to pull the longest possible trains on his layout and has come to some conclusions. Check out his web site for the details. This page mostly about Aristo metal wheels and trucks, but look at the bottom of the page about rolling resistance issues.

I had been doing the same thing over a period of years, but not in such a formal fashion, to determine how to get an older Aristo Pacific to pull it's assigned consist of five heavyweight coaches on my own layout. In the end, I determined that the motor in the poor Pacific was just not going to cut it. It would simply overheat with that load. The solution was a newer Pacific with better gearing and a fan in the motor. The older loco got transferred to fast freight service with a lighter consist. Even so, the work that I had done before materially helped the newer loco cope with the load.

I don't typically run long freights on the GIRR because my curves and grades conspire against me. The actual limit on the GIRR is train straightening on the curves provided I've got enough power at the head end. Because I use truck mounted couplers, this is a limit that I won't get around easily, but 40 cars is enough as I usually run a maximum of 10.

Over the years, I've come to my own conclusions about what I can do to lighten the load on my locos and still pull long trains when I feel so inclined. The result of years of experience can be found on this page.

For those of you that don't want to read the whole page, the bottom line is to use short wheel base trucks and condition the trucks to prevent "crabbing." The absolutely worst offenders in the drag department are flange drag and wheel slippage on the rails, especially in curves. Flange drag can be mitigated with shorter wheelbase trucks. Wheel slippage can be mitigated by allowing the wheels on an axle to rotate independently. If you want to pull the most cars with the least head end power, this is where you have to work first.

It would seem that weight would be a significant factor in limiting train length, especially on grades, but this isn't necessarily so. Very light weight cars are easier to pull of a straight grade, but if there are any curves on the grade at all, a long train of lighter cars will be pulled off the rails. This is assuming that sufficient head end power is available to actually pull the train. Train length can actually be increased by weighting all the cars. Metal wheels do that job adequately.

What Is Rolling Resistance?

Rolling resistance is the sum of all things which tend to cause a car to drag. The energy loss can be real, caused by a variety of forms of friction, or in the case of grades, it can be virtual which is the work necessary to raise a car on the grade.

Other than decreasing the weight or grades, not a lot can be done about the virtual losses. For those with flat layouts, virtual losses don't even matter. It's the real losses that need work and those CAN be reduced, at least to some extent.

Real losses are caused by these issues listed more or less in the order of importance. The order can switch around some depending on the characteristics of a particular layout or rolling stock.

Drag Type Exacerbating Factors Mitigating Factors
Flange Drag Long wheel base trucks
Wheel crabbing
Sharp flange profile
Scuffed wheel flanges
Short wheel base trucks
Properly aligned trucks
Radiused flange profile
Smooth flanges
Wheel Tread Slippage Solid wheelsets
Tight radius curves
Plastic wheels
Ball bearing wheels
Wider radius curves
Metal wheels
Bearing Friction Poorly lubricated wheel bearings
Wheels compressed between sideframes
Too much weight on friction bearings
Proper bearing lubrication
Proper wheel shimming
Ball bearings
Coupler Mounting Truck mounted couplers Body mounted couplers
Power Pickups Brush or slider wheel contacts Ball bearing wheels with power pickups
Grades Too steep
Curves on the grade
Shallow or no grades
No curves in the grades
Curve Diameter Tight radius curves (<8' diameter) Large radius curves (>20' diameter)

The Impact of Grades

Grades are the easiest to describe but are often hard to deal with. The drag caused by lifting a weight alone is a straightforward matter of physics. The steeper the grade and the heavier the car, the more force it takes to pull the car. There are no real losses here as the work done to lift the car is converted to potential energy. However, railroads can make little use of that energy because the cars will roll well enough that most of that energy has to be burned off in friction or dynamic brakes when the train loses altitude on a downgrade.

Try as much as possible to reduce the grades and ESPECIALLY curves on the grades. The impact of the curves will be described below, but the extra drag in the curves simply adds to the work necessary to lift a car up the grade.

The Impact of Bearings

The axle bearings are a good place to look for friction. Most large scale train cars use plain friction bearings. A straight extension of the axle rides in hole in the sideframe. This bearing type REQUIRES good lubrication to work properly. A light treatment with a light plastic compatible oil is usually sufficient to produce low rolling friction. The bushing that the axle rides in may be either plastic or metal. Metal bushings are used when the wheels are used to pick up power. They don't appear to have any less bearing friction that plastic bushings.

Plain friction bearings are sensitive to weight. More weight means more friction. The more heavily loaded they are the more drag have. Even on a grade where increased weight may cause the car to be accelerated more downhill, a friction bearing car will often slow down. The acceleration due to gravity does not change with car weight, but the response of a bearing to increased weight does change. Plain bearings have less friction at low loads and higher friction at high loads. Increased weight can overcome the inherent unloaded friction of a plain bearing and allow a car to roll faster, but too much weight will add enough friction such that the car rolls slower.

Metal wheels add weight to a car BELOW the bearings. The added weight, which can improve tracking in long trains, does not impact the bearing friction as added weight in the carbody does. A car with metal wheels will roll on a grade better than a car with plastic wheels because the added weight overcomes the bearing friction without adding more friction.

Needle bearings, such as are used in virtually all small scale rolling stock, don't hold up in large scale. Large scale cars are too heavy and the small contact patch of the needle cannot handle the loads.

friction_vs_load.jpgPlain and ball bearings respond to bearing load quite differently. This notional diagram shows the difference. Friction bearings, under only the load of a single wheelset, have low running friction. However, as the load increases the friction increases rapidly. Ball bearings tend to have higher friction at low loads but their friction increases much more slowly under load than plain bearings. At some load, the friction characteristics cross over and the ball bearing is better from there.

Ball bearings sound like a good deal at first blush but they are very expensive and typically don't free roll better than a plain bearing for a typical lightweight freight car. Ball bearings do, however, have a place. They can be used for electrical pickup without adding the drag of a slider or carbon brush. More importantly, they handle high loads much better than plain bearings. For a heavy battery car or a snowplow that will carry a brick for ballast, ball bearings provide significantly reduced friction at the high loads. For lighter cars, they are either a wash or worse for running friction. There are more parts in a ball bearing, some of them have to slide too.

Ball bearing equipped wheels are not required to slide in curves, typically because both wheels have bearings. Solid axle wheels MUST slide on one or both rails in curves. This source of friction appears to be similar to, and may be larger, than flange resistance.

I have little direct experience with ball bearing wheelsets, I have only one set. I am assuming that the set that I have IS NOT the highest quality bearing available. That set does not roll as well as a plain bearing when lightly loaded. When heavily loaded, the ball bearing does not degrade, the plain bearings degrade quite a bit.

One test of unloaded bearing friction is the finger flip test. A wheel set is spun as fast as possible with a flick of a finger and allowed to spin down. Longer spin time is better. This test can be deceiving because it is a test with almost zero bearing load. However, it is a good comparison test to find axles which have parasitic friction that could possibly be reduced. These two short videos (Quicktime required) demonstrate the difference between a SanVal metal wheel set on a Bachmann car vs ball bearing wheels on an Aristo snowplow.

A better overall performance test is a roll test down a straight grade to see how far a car will roll. It'll take a 1% or higher grade to get most cars to roll at all. My experience is that Bachmann cars with metal wheels will typically roll further than other makes.

The Impact of Wheel Materials and Geometry

Wheel materials and wheel geometry both have impact on the rolling resistance of the wheels. Plastic wheels generally roll well until they get old and scuffed up. Then the flanges and wheel treads will drag more than when they were new. Metal wheels tend to degrade less with wear. When the track is oxidized the difference is even larger. A scuffed plastic wheel will not slide on oxidized track as well as a metal wheel. Sliding friction is important because in curves, one wheel MUST slide.

Wheels with a radiused profile between the tread and flange tend to roll a little better than wheels with a sharp profile. The radius tends to keep the flange away from the rail. Flange friction is a major cause of drag.

Rolling Resistance on Straight Track

Straight and level track is where the metal wheel on metal rail of a railroad shines. The rolling friction is very low unless there is an upset. The most typical upset is a grade, but another can be very important.

If the axle is not perfectly perpendicular to the track, then the axle or truck is said to have "crabbed." Crabbing is bad and MUST be avoided. Most large scale wheelsets have quite a bit of slop in the axles allowing them to slide side to side. This is bad. When this happens, ALL FOUR wheels tend to bind their flanges on the rails. Drag increases significantly and quickly. Truck mounted couplers tend to straighten out the truck frames on straight track and reduce the flange drag provided that the couplers are centered. The axles should be shimmed with washers (I use #6 nylon washers) so that there is just a little bit of slop left in the axle play. A crabbing truck has a pretty significant impact on drag.

Cars with body mounted couplers rely on the wheel profile to help straighten out the trucks. A radiused transition between the tread and flange is usually sufficient to keep the flanges from grinding on the rails.

Rolling Resistance on Curved Track

The issues change greatly on curved track. In this case, neither axle is perpendicular to the track and the flanges will drag on the rails. This is even worse with truck mounted couplers that tend to twist the trucks are force the flanges against the rails. Undergauge rails in curves are a double whammy. The flanges will reach out and touch undergauge rail more easily than properly gauged rails.

Longer wheelbase trucks (such as 6 wheel heavyweight coach trucks) tend to fare worse in curves. The axles get even more canted with respect to the rails and the flanges drag even harder. This drag can be severe, causing a car rolled through a curve to stop dead as soon as it enters the curve. Removing the center axle actually increases the drag of a 6 wheel heavyweight truck because the non-trivial weight of the car is carried in fewer bearings. The loss of flange friction in the center axle is more than made up for by the increase in bearing friction of the remaining bearings and the flange friction is still there. Changing the 6 wheel heavyweight trucks to 4 wheel trucks actually helps some. The trucks have higher bearing friction (due to higher loads in the remaining bearings) but less flange friction due to the shorter wheelbase. Further, with truck mounted couplers on heavyweight trucks, the 6 wheel truck flanges are driven over to the rails even harder than with the 4 wheel trucks. Much larger curve radiuses are the proper solution to this problem but often, there just isn't room for them.

When a 6 wheel heavyweight truck is hand rolled in a 4 foot diameter turn, the scraping of the flanges on the rails will make an audible scraping sound. A regular freight car truck won't do that. With some 6 wheel trucks, without the load of a car on them, the center axle can actually bind up due to the lateral force on it's bearings causing the center axle to stop turning.

On curves, another serious source of friction is wheel slippage. Wheelsets with solid axles cause both wheels to turn at the same rate. However, the outer wheel has more rail to traverse and one or both wheels have to slip. This slippage is results in friction losses. Tighter curves have higher slippage and losses. These wheel slippage losses are quite serious and occur with trucks of any wheelbase. Ball bearing wheel sets that allow the wheels to turn independently virtually eliminate this source of friction. Larger diameter curves reduce the relative wheel speed difference with respct to the rails and help a lot too.

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© 2009 George Schreyer
Created 21 Sep 09
Last Updated October 15, 2009