Tuesday, October 30, 2012

147> Pearls of Wisdom

Well, I said I was going to start from scratch, and that is what I am doing. Sorry for the delay in getting a post out. There were some minor family medical emergencies. And even in the best of times, a total redesign involves a lot of time chasing (and drawing) ideas that turn into dead ends. My apologies also go out to any new readers, since it has been some time since I have even shown an actual PRT vehicle. I have been preoccupied with the bogie from which such a vehicle would hang, to the exclusion of everything else. So if you have been scratching your head wondering what you were looking at, now you know. But fear not, dear readers, for I return bearing pearls of wisdom… Well, OK… Let’s just call it food for thought…

One of the most important concepts that I have learned since the inception of this blog (and the many designs and redesigns within) is that it is counterproductive to design a single track profile that is supposed to handle every type of situation. It may not be the case with every type of PRT track, but for track that is contained within elevated beams to be both sheltered and yet have a minimal profile, a single track style is, I believe, a profound mistake. A uniform track profile limits the curves to larger radii, both vertically and horizontally, and limits the angle of ascent and descent. It even limits speed. The result is a system that is less flexible and therefore less able to meet a city’s needs. PRT should not be restricted in terms of routes or performance – Let’s leave that role for light rail!

The instinctual response to performance limitations is to try to design the bogie to allow the most capability within a given track profile. It is quite counterintuitive to have the track change in midcourse as a first choice, but many of the track’s contact surfaces involved in switching, climbing or tight turns are simply not needed in straight sections of track. Why go to the expense of including them throughout?

A prime example is the “cog” method of handling steep slopes. This is my preferred method for climbing with PRT, and it is noteworthy that once so engaged, none of the regular track surfaces are needed. The track becomes nothing more than an enclosed pair of racks and some guides (orange) to keep the teeth engaged. Note that the counter-rotating guide wheels are positioned quite closely beneath the drive wheels. Whereas they would ordinarily hit the underside of the running surfaces when the track curves sharply up or down, the fact is that such abrupt changes in pitch can only happen at very low speeds and a rack could be used instead of the running surfaces in those instances anyway. Therefore the interference between those guide wheels and the running surfaces is remedied by simply removing that feature of the track, not redesigning the bogie.

In the process of trying to achieve higher speeds, I was confronted with the fact that the small surface area of the various guide wheels would tend to wear out rather quickly unless they were quite hard, which is a recipe for a noisy system. (Yes, I know “hard” does not necessarily mean durable, but I am referring here to ordinary, reasonably inexpensive wheel materials here.) The preferred small profile of the beam that would contain them (and restrict their size) seemed to indicate that the system could be either noisy and fast or quiet and slow, unless, of course, one wanted to replace the wheels every couple of thousand miles. But much of the problem wasn’t really a problem at all; the answer lies in disengaging the steering guide wheels when not in use – not by retracting them, but by simply having the track’s engaging surface end.

It is tempting, when trying to enable tight turns, to assume the bogie must be short. After all, our experience in the automotive world has taught us that smaller vehicles are more maneuverable… right? Actually that is a false analogy. With cars and trucks the road or parking spot width is generally limited by surrounding real estate. In a beam-style PRT system, the width of the beam is kept as small as possible primarily for aesthetic and cost reasons, neither of which have much to do with short little low-speed sections of track for particular, close quarters maneuvers. If the bogie doesn’t fit in the beam because of the tightness of a curve, an alternative is to simply widen the beam in those spots. This point, in particular, has taken a while to sink in with me. While there are some other performance reasons to keep the bogie short, the very limited space between the front and rear wheels in my designs has made it a challenge to cram in the steering and emergency brake components. Below is a snapshot of a slightly longer bogie that has enough space for an upper set of counter-rotating guide wheels. A single, pivoting upper steering guide wheel shares the axle. Even this slight lengthening of the bogie would greatly limit the turning radius were the track to have a minimal, uniform profile.

As a final example, there is the matter of air resistance within the track enclosure. Track that is meant for slower speeds is generally also the track placed where people would find it most intrusive, and so it should be as skinny as possible, and indeed can be, because aerodynamic drag at neighborhood speeds is a very minor issue. High speed track, say running along freeways, could be fatter, allowing the bogie to slip through the enclosed air more easily. A fatter track profile has far superior geometry for spanning long distances, and this will probably offset any apparent increase in material costs of such fat track. Again, one-size-fits-all is not the best solution.

It may seem complicated to have track with multiple configurations, but such track would be factory made anyway. Therefore there can be a fixed menu of standard track sections that would be brought in by truck and basically dropped into place. True, seldom used profiles might cost a bit more, but at least the system itself can be adapted to all cityscapes, station types, and storage/staging solutions with minimal engineering effort. Such a scheme also makes sense in view of improvements in design and technology over time. Making the track offer the widest possible range of bogie design choices bodes well for the evolution of better, more capable vehicles over time. Even though these bogies are designed to essentially last forever, that does not mean that they might not be resold on a secondary market or repurposed for freight, opening the way for next generation designs. This won’t be the case if the bogie is so spatially constrained that there is no room for design variation.

In the early days of this blog, I thought the first step in designing a superior PRT system was to design the perfect track/bogie combination by studying the problem from an end-view. Now I have come to see that the best way is to study (in 3D) how to overcome the various limits first and come up with track/bogie for those, and then design for straight runs from there. So for those of you designing your own systems at home, I hope this helps. If you need me, I’ll be at the drawing board.