Saturday, January 17, 2015

Papa Bear, Mama Bear, Baby Bear


As promised in my last post, I have modified the SMART PRT (Suspended Multi-axis Automated Rail Transport – Personal Rapid Transit) design yet again, this time to accommodate featherweight “pods” and enclose the bottom of the track, as well as fine-tuning certain proportions.  
There is also a secondary rationale for considering ultra-light PRT systems, which is the matter of parcel and light freight delivery. Companies such as Amazon are clearly putting heavy emphasis on speedy delivery these days, and the ability to integrate a steady stream of varying vehicles is, unlike days past, essentially a given. Couple this will the fact that such track would be many times cheaper, and a very good case can be made for exploring the matter further. 
In response to this challenge I submit the pictured multi-sized approach. In the top picture, the comparatively tiny profile of the “Baby Bear” track can be seen, which does not even reach the man’s knee. 
Before going any further, though – a disclaimer: The pictures are to illustrate component geometry only. The track is shown as if it can be simply extruded, which it can’t, and I just thickened the top of the larger sizes to show where ribs would most probably go.
Also, I’m sure that there are some newer readers who may have never considered the matter of switching suspended PRT, and who may be confused by the various end-views. To them, I offer this more complete, yet still simplified, end view and explanation.



To switch suspended PRT, the part of the vehicle that rides inside of the track (the bogie) must grip one side of the track or the other, so that when the track routing diverges, the vehicle’s drive unit (bogie) does not fall out of the track’s ever widening slot. In this end-view, the pair of steering-guide wheels on the right “clamp” into rolling engagement with the track, while the left ones spread apart to release that side. In other words the bogie, shown in blue, can be fully supported by either side of the track and so can follow either as they split apart into different routes. The weight forces of the hanging passenger cabin are shown by the three red arrows. 
 

 
It is also noteworthy that certain parts of the track are eliminated where there is no switching. In this picture, the steering guide wheels are not quite contacting anything. Side-to-side guidance is maintained by the large, counter-rotating horizontal wheels; to do otherwise would simply add wear to the smaller wheels for no reason. The actual contact rails are only at switch points and taper into contact gradually for a smooth transition. Also, particularly observant, long-time readers will note that some track parts will need to be modified or removed for tight radius turns, both side-to-side and up-and-down.
Now, back to the top pictures. In these two illustrations, BTW, the steering guide wheels are engaged as if they were approaching a “Y” in the track.

The “baby” track is sized for speeds below 30 mph, (48 km/h) with weights below 500 lbs. including vehicle and passenger/cargo. It has 8” (215 mm) solid drive wheels. (It could handle more with steel wheels and very frequent supports but this would be inconsistent with most applications) The track width is 13.5”. (343 mm) 
   
To the person’s top-right are the “baby” wheels within a larger (mama-bear) track. Directly below are the wheels for the “mama-bear” vehicle within the same, matching track. This size (30” or 762mm wide) is consistent with the 3.5 person vehicles that have been shown in earlier posts, weighing in at about 1300 lbs., fully loaded. The performance would be generally auto-like, with quick enough acceleration to allow fairly short on-ramps, (an often overlooked design consideration!) and speeds approaching 50 mph. (80 km/h) It is not impossible that faster speeds would be practical…Only testing will tell. 

The second picture above  illustrates (left to right, clockwise) the “mama bear” bogie in the “mama bear” track,  the “mama-bear” bogie in the “Papa bear” track, and the “Papa bear” bogie in the “Papa bear” track. This larger wheel and track would be for still higher speeds and/or loads. Not much bigger than the “Mama Bear” combo, (35” or 890mm wide) the steering guide wheels get an extra two inches of diameter while drive wheels become auto-sized. The guide wheels of heavy vehicles can also be made of longer-lasting, harder polyurethane formulation or even steel, as sound is less of an issue because such routing would typically be along highways, more than fronting homes of businesses. Being designed for long distances and for larger (GRT) shuttle vehicles, this larger sized track would not universally assume full 3D routing and would be encased in a heavier structural elements. High speed sections would require some additional interior space to allow air to slip around the speeding bogie. Some very large cities might opt for the “Papa Bear” track throughout, to accommodate faster personal vehicles designed for typically long commutes. It is, after all, only 5 inches wider and height is more a function of span/vs load than wheel size.  I am somewhat dubious about the utility of making this largest track universally “baby bear” compatible, and one approach would be to design such track to be easily convertible for this purpose, rather than to include this feature initially, like the pictures shown above. This is primarily because of the speed differences, since any quiet and smooth running 8” wheel couldn’t tolerate continuous, heavily loaded, high speed use for very long, and so must be confined areas that are appropriate for low speeds anyway. For lighter parcel delivery, however, in, say, the 25 kg load range or less, it is a different story. Here small vehicles could almost fill the track spaces between large GRT vehicles, not slow them down at all, and still be nearly silent when they get into neighborhoods… without needing new wheels every thousand miles. 

What is regrettable is that the Mama Bear track isn’t smaller and cheaper – more of a mid-point between the larger and smaller sizes. Unfortunately once you can physically over-pack a vehicle with weight, (as is unpreventable with the interior cabin space required by disability laws) you can theoretically load each vehicle just as much, and as long as there is the remote possibility of a sudden stopping of traffic, where suddenly such loaded vehicles are bumper to bumper, applying all of that weight to a span, or even all applying emergency brakes at once on a curve, (thus pulling the track forward and sideways and down) … there is the need for some pretty beefy track, at least that is how it will be regulated. 

Ironically, the baby bear track can be small because it is small… If track and vehicles are cheap enough, it becomes easy to pay for, even without heavy ridership, which permits longer headways and slower speeds, so everything can be lighter-duty. Of course the “baby bear” can’t exist as a transit system alone, since it does not meet the criteria mentioned above. The Mama and Papa Bear systems need to really pack the tracks and run as fast as possible to pay their way, especially until the business model proves itself and costs begin to drop. This kind of traffic is best promoted by providing a certain degree of comfort…And comfort adds…you guessed it… weight. In PRT, there is no such thing as a free bowl of porridge!

Next step - Adapt this wheel geometry to actual stock steel, practical fabrication methods and truss design, as well as to finally address the long-neglected issue of “third rail” placement.