So the old post is cut short, and my pictures would not copy and past, and I don’t have time now to restore them. Here is the text.
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 the journal.
Varying amounts of side play are allowable, but the axle points are not all the way into the points of journal cones, so the load is not on the axle tip, but on the side of the axle tip near the point.
Myth #2 - Our cars do not weigh enough to make sprung trucks “equalize”.
Well, here is the thing, your freight car does not need to actually compress the springs in sprung trucks, in fact there is no expectation that the springs will provide that sort of “suspension”.
We don’t need that. But what we need is equal loading of the car weight onto all the wheels, typically 8 of them on a freight car.
For that the to happen the only thing that the truck needs is independent flexing of the sideframes at the truck bolster. The springs allow this and very little weight is required to cause this flexing.
Each sideframe needs to move as the red arrows indicate, one end up as the other end moves down, the springs need only compress the smallest amount while still holding the top of the bolster in contact with the sideframe - which is unlike the prototype.
Myth #3 - Rigid trucks track just as well as equalized trucks.
Rigid trucks track “OK” for most peoples needs, no question.
BUT, equalized trucks track better. They will navigate grade transitions better, work better on super elevation with less chance of string lining, and result in smooth running and less slack action because the “dynamic friction load” will not change as much as track conditions change.
All this is pretty small numbers wise - until - your trains get longer, 30 cars, 50 cars, 100 cars.
Much of the time, with rigid trucks, the actual weight of your fright car is only on 6 wheels. No different than a chair on an uneven surface.
Equalized trucks also travel thru turnouts better because no wheels are ever “floating”. Those wheels that might be floating on a rigid truck can more easily ride up on frogs and points.
Why do the Intermountain wheelsets work so good?
Why do the Kadee sprung trucks roll OK, but not great?
Why does the Intermountain wheelset fix this?
The sideframes of a sprung or equalized truck have to move relative to the axles, this changes the relationship of the axle to the journal - some would say this is justification for the rigid truck and “settling” for the 6 wheeel effect.
Kadee, and many others use a large cone axle rather than the small cone in the NMRA Recommended Practices.
They look like this:
As the sideframe moves, the larger cone has more surface area that can come in contact with the journal cone, with more friction and more opportunity for binding.
Admitedly this is less of a concern with rigid trucks, but tests show, and owners report, that small axle wheelsets like Intermountain work better even in rigid trucks.
Here is a photo of the Intermountain axle end (right) vs the Kadee axle end (left):
I started in this hobby at a time when most better equipment came with sprung trucks, and names like Kadee, Central Valley, Lindberg, Walthers and Silver Streak were synonymous with quality sprung trucks. That was 54 years ago.
Today, and for the last two decades, I have been refitting Kadee sprung trucks with Intermountain wheelsets to achieve the most free rolling equalized truck possible.
The goal - derailment free operation of trains in the 40 to 70 car range.
Another side benefit, the weight of metal trucks adds weight where it works the best, down low at the track. Often making it unnecessary to add additional weight even the cars are slightly below NMRA recommended practice.
Questions and comments welcomed.
Sheldon
