Barber-Bettendorf leaf spring caboose trucks

[rprince]

I am looking for information/articles on the underlying reasons that Barber-Bettendorf leaf spring trucks were more or less adopted as the standard caboose truck, while coil spring trucks became the norm for most other freight cars. I found a .pdf file on freightcar trucks which seems to indicate they gave a more "passenger car" ride, but I find that claim somewhat suspect after riding in several cabooses! Cheaper? They claimed they were the lowest priced truck available for caboose use. I can buy that arguement.

But I'm looking for better engineering explanations. One explanation that I suspect is true, is that the leaf spring could be designed to function well for a specific, narrow car weight range, such as a caboose, whereas coil springs could function well under a wider weight range experienced by freight cars that would travel both loaded and unloaded. The weight of a freight car could vary but a factor of 2X or 3X the unloaded weight of the freight car itself, whereas a caboose wasn't really designed as a load-carrying car, and therefore had more of a fixed weight that a leaf spring setup was capable of handling.

If anyone has any more information on this sublect I would like to hear your thoughts. Thanks!
Russ Prince
Apple Valley, MN

[Anonymous]

I have a couple thoughts on this.  I believe your observation about the constant weight of a caboose is a significant factor affecting the truck suspension design.  Spring rate is also a factor.  The spring rate is the rate the spring load increases per unit of distance that the spring is compressed.

With a coil compression spring of a given diameter and wire size, the rate varies with the number of coils.  The greater the number of coils, the less steep the spring rate.  In other words, a given applied load will compress the spring with more coils a greater distance than the spring with less coils.  Or in still other words, a shorter coil spring is stiffer and harder riding than a longer spring.  The shorter spring absorbs shocks in a shorter travel distance than a longer spring.  

There is only so much room for the height of coil springs, so if you want a softer riding spring, an option is to revert to a different type of spring that has a less steep rate.  I am guessing that the leaf spring set would have a flatter rate for its height than a coil spring.  If so, it would fundamentally be softer riding than coil springs.

Another factor in suspension is dampening, which is friction to impede the spring movement.  When a spring absorbs a shock, its job is done.  But it tends to keep reacting by bouncing until the reaction to the shock is dissipated.  The friction of dampening impedes the bouncing that follows a shock.  On highway vehicles, the dampening is performed by the shock absorbers.  On freight car trucks, the dampening is performed by a unique mechanism that creates friction between the truck bolster and side frames.

Leaf springs have inherent self-dampening by the friction created as the leafs slide against each other as the spring is compressed.  Closed coil torsion springs also have self-dampening.  However, coil compression springs have no self-dampening, so if dampening is desired, it must be added as a separate device or system.  So the self-dampening effect with leaf springs is an advantage.