How the trucks work on our models - rolling qualities and the value of equalization

Model Railroader General Discussion (Model Railroader)
1

In several recent threads the topic of sprung trucks and various brands of wheelsets once again was discussed.

I have gathered some engineering information in an attempt to explain how model trucks actually work, why some roll better than others, why equalization matters, and hopefully dispel a few myths.

Myth #1 - The car rides on the points of the axles.

Myth #2 - Our cars do not weigh enough to make sprung trucks “equalize”.

Myth #3 - Rigid trucks track just as well as equalized trucks.

First, some basic engineering standards for model trucks and their components, from the NMRA:

Interestingly, the axles on Intermountain wheelsets are nearly identical to the design shown above.

The key feature being the small axle diameter outside the wheel, resulting in a smaller axle cone.

Myth #1 - The car rides on the points of the axles.

Well no, actually the cone of the truck journal contacts the cone of the axle tip, but not on the “end” or point of the axle, but rather like this:

Notice from the NMRA specs that the journal cone is 60 degrees minimum, while the axle is 50 degrees maxinum and the max axle length is less than the minimum journal span, creating a minimum standard for side to side play. Interestingly there is no maximum spec for side to side play.

It is assumed that as long as the axle stays in the truck, gravity will center the axle.

This leaves a 5 degree difference on the weight bearing top side to minimize the contact patch of the axle onto

2

Wow, Sheldon, you put a lot of work into this! You make a good arguement for Intermountain and Kadee (equalization), And the two combined together. And, cleared me up on Myth #1.

Running about 15 car trains on my layout doesn’t push my trucks limitations enough to sweat it. But I have a string of IHC ore cars that have a lot of drag, despite using a truck tuner and metal wheelsets. I’ll have to see if intermountains will help those. Thanks for the work. Dan

3

As well as HO and On30 I also have some G scale, and there at least some cars do indeed weigh enough to compress springs a bit.

4

Gidday Sheldon, thanks for putting in the effort.

In MY bigger picture, I guess it really doesn’t matter, but my initial thought is, does the smaller bearing surface of the Intermountain axles cause any more wear to the bearing surface of the trucks?

Cheers, the Bear.[:)]

5

Sheldon, well done! It is great to see a definitive explanation on this from an expert with experience and (icing on the cake) the information to back it up. Thank you.

6

No, because under “ideal” conditions the contact patch is the same size with both types of axle. And the weight vs hardness is not really a wear issue of much concern.

It is only as the sideframe moves during equalization that the larger axle is more likely to have more contact, and thereby more friction, and more possiblity for binding.

If, as in my case using Intermountain wheels in Kadee trucks, you have metal to metal, some form of lubrication is desirable.

I use a small drop of light oil. It soaks into the porous cast metal of the sideframe journal and acts like an oilite bearing, not needing relubrication for many years, and causing no issues commonly believed about lubricating trucks.

If either the axle of journal are slippery plastic, wear concerns are even smaller.

Sheldon

7

Yes, as you get into larger scales, the physics move closer to the prototype.

And equalization can become that much more important.

Sheldon

8

Well done, Sheldon, and quite convincing! [Y]

Takeaways: Sprung or equalized trucks are superior to rigid trucks. Intermountain metal wheelsets are the best wheelsets. Any other takeaways to note?

Rich

9

Good info for novices!

10

When I was in Scale Rails Of Southwest Florida, we had a demonstration board showing why sprung trucks (HGC did not exist yet) were required on all cars that ran on the club layout.

It was just a piece of flex track with a couple of staples over the track. Rigid trucks would always derail, but a string of freight cars with sprung trucks would glide right through the obstacles.

The fact is that when a sprung (equalized) truck has one wheel lifted, the other three stay on the rail. When a rigid truck has one wheel lifted, another wheel is also lifted, and there is a derailment.

Great work on the post Sheldon. THANK YOU!

-Kevin

11

Very good discussion. This ought to be a ‘sticky’ somewhere…

I want to draw attention to the intentional ‘blunting’ of the tips of these axles. This defines the ‘bearing’ area on the axle slightly differently: it is now where the curved radius of the blunted tip intersects the 50-degree cone machined or formed in the axle end.

In my opinion, it follows that this very small region is the ‘only’ part of the axle that needs to be carefully trued and polished. But there is another implication of this radius: it maintains a good contact area on both ends when the two sideframes equalize…

Naturally there is a small deformation between the near-point contact of the relatively hard axle and the softer sideframe material – the latter will, ideally elastically, deform slightly to give the required bearing area. I suspect you’d need very good instrumentation to measure this.

A Delrin sideframe will distort ‘more’ than a brass one, but the inherent low surface activity of acetal will ensure low friction of the ‘deformed’ area even without lubrication.

Something I have not read about is the bearing arrangement used for those trucks with rotating simulated roller-bearing endcaps. Those usually involve a small ‘wire’ extension on the axle end that penetrates the sideframe. Now if you remember your Hot Wheels cars, those use a Delrin tube bearing with line contact to a thin wire axle… and roll very well. The question is whether the rotating-bearing trucks ‘bear’ on those pins, or on a contact with the axle taper inboard of the pin end.

12

Sheldon, thanks for your informative post.

I find your first diagram interesting regarding axle length. When I decided to go with metal wheelsets, I measured the original axle and also the Intermountain and Proto metal wheelsets I had acquired. I had noticed that another vendor offered a variety of axle lengths and a chart to chose the best length item for a particular freight car, which made me wonder how important axle length is.

I found that the Intermountain wheelsets did very well in downhill rolling resistance as long as the IM axle length was not much longer (it was often shorter) than the original. So that became my default. I used the Proto in cases where I wanted the ribbed back wheelsets for older cars.

Your explanation firms up my guess that shorter axle lengths than original are usually not an issue because the axle point does not need to ride in the very end of the cone. Of course, too long an axle length, exerting horizontal force into the cone ends would not be desired, as that would creating a binding force.

13

Threads like this benefit from a “sticky” feature. Also, a “like” feature is beneficial. To indicate thanks for a very useful post without having to actually reply. This post gathers together information scattered around many places and then consolidates and explains how the various bits of information relate to each other. It’s taken me more than a year to get even halfway to where I am now after reading the initial post, thanks very much.

The explanation of the mystery of why otherwise apparently identical Intermountain wheel sets roll so much better than any other is convincing and can be the only explanation. There is no doubt the Intermountain wheel sets do roll noticeably more freely than any other brand. I looked at the different brands of wheel sets, observed the difference but failed to notice the difference. Explains why Intermountain charges a bit more, they need to because their product is a bit more expensive to make. Kadee should change their wheelset supplier. Today.

Sprung suspension is very interesting and doesn’t work quite the way most people think. I’ve read a lot about car suspension, especially performance and racing car suspension, but all sprung vehicle suspension systems work in similar ways. (For those sceptical of my expertise I can confirm I modified the suspension under my sportscar using this expertise and it works just fine, far better than the Subaru factory version. And I know exactly why. Subaru actually did what I did for their 2017 and later models, although my setup is slightly better for a supercharged car than Subaru’s current setup.)

First of all, there’s a difference between spring rate and spring length. The weight bearing capacity of a spring depends on both variables. Weight bearing capacity is irrelevant to the value of sprung trucks under model railroad cars. It’s the compliance that matters.

Even the lightest model railroad car will compress the springs of a sprung

14

Thank you for the great analysis. Are there any studies or opinions as to the use of lubricants on the axel tips as to rolling qualities or incompatabilities of materials? These lubricants would include everything from the Kadee “Grease-Em” to the line of Labelle products and anything else (i.e powdered graphite or Teflon type product) being marketed. I model On30 using the stock Bachmann trucks and have never lubed or adjusted any of their trucks as they seem to roll just fine. Any opinions on the “truck tuner”? Or “truck tuner” and a lubricant? Just something I’ve kind of wondered about but never tried.

Again, thanks to Sheldon for the great information.

Mark B.

15

Truck tuning often makes good sense. Note that it works by smoothing the 60-degree cone in the sideframe. Even though the ‘active’ bearing area might be only from about 5 degrees before and behind ‘top center’ and only a few thou long, it’s easiest to cut and smooth the whole cone at one time.

The lubricated area on these is small and any hydrodynamic bearing wedge likely slight at most. I think the use of lube depends on the sideframe and axle materials – and only the smallest amount. Dust will preferentially get up in there over time if you overlubricate, and it can be surprising how little constitutes 'over’doing it.

16

Well, this novice sure learned something.

Thanks for your work in putting this together Sheldon, I found it very informative.[Y][B]

17

I appreciate all you went through to provide us with your informative post, Sheldon. It’s good, solid, information, well thought out, and well-presented. Thank-you.

18

It is not that things don’t mater (spung trucks, etc.), it is just that if enough other things happen (bolster rub or missing exact center at attachment point, etc,), then it negates the effectiveness of the item. The reason for extra weight on sprung trucks is it counteracts the dozens of things that negates their effectivness (in other woods, all things do not scale due to gravity etc.).

19

As Overmod points out, the truck turner is useful to insure a smooth bearing surface in the journal.

I use Labelle 108, I prefer it over graphite or Teflon powder.

Another guy I ran into years ago who also puts Intermountain wheels in Kadee trucks, uses 108 as well.

One other note about Overmods earlier comments, yes, no matter how sharp it feels to you, the axle tip is rounded off, to the benefit of the actual contact point.

Sheldon

20

Your presentation of that information was very well-done, Sheldon. I am not at all surprised.

Wayne