Wednesday, June 17, 2015
Creating a PRT system is getting easier every day. Increasingly, it is a matter of employing off-the- shelf components in ways that have already been established for other purposes, using software that basically provides a “fill in the blank” framework, and communications that work reliably at blazing speeds using ordinary protocols. These days each individual PRT vehicle will have far more computing power than yesterday’s entire system, and what used to be worrisome safety issues can now be addressed by a redundancy of cheap processors and sensors that can check and recheck everything under the sun hundreds to times per second, using self-diagnostic AI software that is ever more capable.
When I started this blog one of my primary worries was that cities would never accept a whole new infrastructure that would be dependent on all kinds of exotic, proprietary technologies and specifications, understood only by the PRT system developer. At the time, only a precious few corporate giants had the technological credibility and depth of resources to give much political cover and comfort to local transit officials. Those days are fading fast. While any project can turn south, particularly when attempted by an untested company, at least the next generation of PRT systems will be understandable and serviceable by ordinary professionals, no doubt with considerable free help from component vendors. Indeed, the idea of an automated taxi service, what with self-driving cars already roaming our streets, only seems truly futuristic in regards to the uncertainties of the street – the darting dog, the patch of ice and so forth. Such a service within the controlled confines of a track hardly seems like a technological challenge anymore, just a monetary, logistical and political one.
With that in mind, there is a case to be made for setting aside any remaining system components that are overly exotic. Although self-turning wheel motors are absolutely perfect for robotic transportation, at present the selection of such motor/controller packages in the appropriate power range is still quite limited. Therefore, it is not a great idea for me to tightly couple my open-source designs with these motors, as though the designs are dependent upon them. Mechanically, a self-switching bogie for carrying a suspended load (such as PRT passengers) is almost ridiculously simple, even with off-the-shelf motors. I have therefore used four ordinary BLDC motors in the above design. SMART (Suspended Multi-axis Automated Rail Transport) designs require a motor for each wheel because switching from cruising to climbing (and vice versa) requires the front and back wheels to momentarily rotate at different speeds, and turning in very tight quarters calls for the left and right wheels to rotate differently as well. The redundancy of four motors also dovetails well with reliability, continuing a philosophy of completely eliminating any possibility of a single vehicular breakdown, let alone anything systemic.
Shown are Golden Motor’s 5000w offerings, for a combined total of about 26 hp. It is reasonable to assume that these motors would be powerful enough for full “SMART” functionality, such as even climbing straight up if necessary, while still being “geared” to reach highway speeds, or close to it. (Contingent on final aerodynamics/permitted load.) Continuing with the “You can build it in your garage” theme, I decided to use trailer tires, which, despite their small size, are designed with tougher sidewalls and higher inflation pressures than automotive tires while still being load and safety tested. In production solid tires would be preferable, although there are various strategies that could allow this vehicle to limp along, even with multiple flat tires. Note – although I picture a 16”OD tire, as is clear from the end view, the raised steering guide wheel assembly ended up considerably higher, defeating the reason for that size wheel. With a few modifications, a 20.5 OD drive wheel could be used that will have a greater (1050 lb. each) load range.)
From my 3D simulations, it appears that this bogie can achieve turning radii (horizontal and vertical) of under 60 inches, keeping with a design philosophy of enabling PRT to go essentially anywhere with minimal conflicts and expenses regarding right-of-way or station placement. A whole PRT vehicle could easily pull a U-turn within the floor space of a small two car garage!
Briefly, a description of the drawing - Wheels A only come under load when stabilizing a turn, unless encountering gale-force side winds, or helping to support vehicle in the event of a deflated or over-weighted tire. (running slowly on only the two left wheels enables the switching side of the track to be modified or even removed, such as to add a junction, without completely detouring or halting all traffic, but this would tend to compress pneumatic tires until wheels A are sharing the load.) Wheels B have opposite rotation because they hold the bogie down, (engaging the bottom side of the channel that A uses) especially at high speeds into gusty head winds, where the aerodynamic “pushback” would otherwise lift the back wheels, or when climbing “cog” style, using the pinion gear (or sprocket) C. Note that the pinion gear C and the drive wheels F are driven in unison while wheels B are free-turning. Normally engaged, oppositely rotating wheel pairs D keep the bogie centered. Raised steering guide wheel E does not normally contact the track because the guide rail is discontinuous and tapers into contact at junctions only. Note that steering guide wheel E gets raised and lowered with the upper wheels A of the OPPOSITE side, not the same side, as in previous designs, since the steering guide mechanisms (shown in violet) now crisscross each other. This more securely holds the bogie to one side or the other by basically scissor-clamping that side of the track, while hooking the top guide rail, so as to completely support the vehicle, if need be. The channels that hold these upper guide wheels (A) would, by the way, have a liner (shown in blue) that would only be thick enough to make forceful engagement at sharp turns and junctions. Although this bogie might, at first glance, seem to be bristling with various wheels, most only see use for a tiny fraction of the time. Their various sizes are proportional to the durations and load of such engagement. Not shown are the electrical and communications interfaces between bogie and track. Contacts for third rail” electrical feeds would be side-mounted. “Leaky cable” Ethernet could go almost anywhere. I have not yet worked on emergency brakes for this design. Also, the track is shown as a simplified representation of the various contact surfaces: It is not some miracle extrusion!
Finally, I would like to reiterate the rationale for the “Mama Bear” design. I can sympathize with readers who have, for years now, seen design after design come forward from this site, only to be replaced by yet another, presumably better one. Hey! The Model T was a great design as well, but that does not mean it was the “last word” in automotive functionality! In the case of prior PRT designs, there was always an inherent conflict between the holy grail of minimal track size and the obvious merits larger, automotive-sized wheels for heavy loads, higher speeds, and less maintenance. While simply splitting the difference is one solution, this essentially means that future designers must “round up” both the track and the bogie size to accommodate the largest and fastest anticipated vehicle and impose that same design on neighborhoods where such a system could prove unpopular or too costly. But the only other alternative is to limit the potential speed or size, which does not seem like a good starting point either. A high speed, 6-10 person, scheduled shuttle between an airport or a park&ride and downtown seems like a practical arrangement, and would be better yet if the track is PRT compatible. But let’s keep such “muscular” track out of peoples’ front yards! Simply eliminating mechanical reliance on a track’s “ceiling” height allows this flexibility, so that the smallest vehicles, designed for using equally small track, can still larger track wherever needed. Smaller track, we should remember, is much cheaper and so has inherent advantages in addressing the “last mile” problem, (or simply providing more coverage for a given cost) and we should not dismiss places like campuses, that don’t need a family-sized vehicles in the first place.
In conclusion, I would just like to point out how mechanically simple this whole thing is. There are two moving assemblies that go up and down on a cam, mounted on a frame (yellow) that holds four motor powered wheels. Other than that, what is left is basically an exercise in cooperative robotics, a technological environment that is becoming increasingly commonplace. I will be addressing aspects of that next time.
Posted by Dan at 6:25 PM
Tuesday, March 10, 2015
As I wrote in my last post, to pay its own way and generate additional profits to reward stakeholders, a system must achieve a certain threshold of passengers, and these are, by definition, pedestrians – a relative rarity in many (especially younger) US cities. I illustrated the problems of gaining passengers with many low volume stations with a heat map. What I left out, however, were the special cases where there are natural congregations of people for some reason. There is an obvious disadvantage in only catering to such hot spots, even if almost all cities have them. To do so is like planting daffodils too far apart. They may fill in someday, but it could take decades. Still, it is worth listing some of these “special cases” as many are actually quite common.
>A bridge – The cost of building a PRT bridge or putting track on an existing bridge is almost trivial compared to adding a lane for ordinary traffic and getting through such a bottleneck quickly can potentially create high volumes of passengers.
>A large topographical obstruction – a related issue is when a city is located on a lake, ocean or river or even against a mountain, where straight-line travel is obstructed. A good example would be a city on a bay, where the only routes are C-shaped and through highly developed areas.
>An existing, pedestrian-based transportation infrastructure. There is obvious advantage to sharing stations with other networks where possible. This includes bus stations, airports, subway stations, etc.
>Park & Ride locations - Get passengers where they are leaving their cars anyway!
>Tourist destinations and parks – It should be noted that many times parks front rivers and streams, which may even provide a ready-made route for PRT. Parks may be advantageous from the city’s point of view as well, involving less red tape and/or alternative funding possibilities.
>Privately owned commercial real estate – this could include office buildings, strip centers, big box stores and so forth.
If I have left out any categories, post ‘em in the comments section! For the moment, though, I would like to concentrate on this last one, because it is both universal and well-distributed, and so should influence a PRT system’s design. I will skip going through multistory buildings for the moment because there is already a lot to cover.
In the US, it is common to have “big box” stores and strip centers running along freeways, accessible by the freeway’s one-way feeder roads. This tends to create elongated commercial zones … little “Main Streets.” What used to occur in the town center now often happens here. Moreover, highways reserve overpasses for only the major cross streets. This makes these overpasses the only portals between adjacent neighborhoods and stimulates commercial activity on these crossing routes as well. The corners between highways and these cross streets tend to have a lot of traffic and are eventually prime sights for multi-story buildings.
I regard these sites as extremely important to PRT design since they often have terrible traffic, both on and off of the highway, and it is a particularly difficult situation to remedy by conventional means such as road expansion or widening the roads a block over. These sites have added significance to the commuter, as they tend to occur “on the way,” and so would seem to be one way to extend PRT out from downtown areas to allow suburbanites a way to access a downtown PRT system without driving all of the way there first. They are naturally distributed in key areas. If PRT is to have a real impact it must be able to spread, to join the various hubs and trafficked areas, and so such areas are essential footholds.
One requirement that PRT should have, to service such areas, is the ability to cross the highway. Here suspended designs have a huge advantage over supported ones as they can, in many cases, simply run under the overpasses, using their ability to quickly change elevations and tilt into turns to comfortably navigate what is likely a very serpentine route. These advantages come into play even if the only route is over the highway, as this would be extremely high (supported designs would need very long ramps) and would ideally use some sort of cable-stayed or suspension design, which is also very well suited for suspended systems.
The next requirement PRT must have to service such areas takes some consideration regarding ownership of these properties. (I am not including using public easements in this discussion, for brevity.) First of all, the retail establishments probably do not own the land. Such land parcels are often owned by developers and whoever has their store there at the time is simply babysitting it… paying the bills and generating some return while the land appreciates. Therefore putting PRT stations on such land is contingent on what serves those developers’ needs. For this reason, it is worth exploring what strategies could be employed to entice these land developers to allow stations on their property and under what terms.
One thing that needs mentioning is that these “developers” are probably not individuals but rather partnerships or corporations, often set up expressly for a particular property. As such, they may have certain bylaws, have silent and managing partners, and so forth, so any kind of agreement regarding adding a PRT “station” should be as simple as possible, as they have their own internal politics. This would involve some kind of monetary benefit for them and provisions for eliminating risk and complications. On the cash side, the solution seems easy enough. One way that a city can incentivize corporations is with a tax break, something that is increasingly done to attract new businesses. This way cities can knock down the cost of the project, at least on paper. Never underestimate a city’s willingness to “rob Peter to pay Paul!” For the landowners, they see their tax bill reduced, their tenants see higher foot traffic, so leasing values go up. Win win.
The next criteria is to make it risk and hassle free. This is where the physical PRT design comes in, particularly in regards to the matter of permanence. No landowner wants to permanently relinquish sovernty over any section of his property, nor does he want to be tasked its upkeep, utility costs, security and so forth. He also won’t want to be responsible for its removal and repair of the site if he decides to repurpose or sell the property. This means that ideally the “station” would be powered from the track and require minimal anchoring or other foundation work. For example, holes could be augered into the parking lot for subterranean concrete anchors, where removing bolt-on components leaves only small holes to fill and patch. The idea is to make the entire spur and station easy to install or remove.
Also note that many of these properties have super-sized parking lots, the owners being understandably reluctant to subdivide the land. This raises the possibility of creating a mini “park and ride” lot. It also raises the matter of ensuring convenient parking for store patrons is not sacrificed for commuter parking. This will take some creative problem solving! Luckily the system will probably know who the passengers are.
Another related instance of private ownership is the strip center. These tend to be smaller lots, and usually have somewhat less parking. They may also have vacancies, raising the possibility of using indoor space for a station. Again, this would take a PRT design capable of tight, serpentine track configurations.
Then there is great American mall…Many of these are dying and have parking galore, indoor space and so forth. These folks should jump at the chance to have PRT, and have “Park & Ride” written all over them.
The original creators of PRT envisioned a citywide transit solution, not a system designed for just a few niche markets. They also lived in a time when people believed in big government funded projects, and cities where not so spread out. These days, if PRT is to demonstrate its worth, we will have to find a formula where it can spread, station by station, by the benefits each new station brings to everyone involved, from cities to investors to passengers, to merchants along the way. One overlooked source of revenue is the immersive experience of the trip itself. After all, the pod knows who you are and where you are going, and potentially much more… It can talk to you, play games with you, even tell you about the attractions along your route.
Imagine, for instance, the pod itself offering special sales, issuing coupons, etc. for upcoming stops. “Battery sale? Next stop?” “Half-priced fries for the next 30 minutes at McDonalds? – Honey, better hit that button for station 22!” For the merchants, it’s like home delivery in reverse. It is a big machine to automatically attract and deliver customers to the front door, and at an exact time of their choosing! That item you looked up on Amazon last night might just “happen” to be on sale just a few stations down. A personalized, temporally-tuned billboard… You could hunt for (or potentially even negotiate) custom tailored deals along your way!
One thing that has, so far, characterized SMART PRT designs is that they are not cheap. The track, yes. The stations, yes – but the vehicles have capabilities that far exceed other designs. I have always considered that the vehicles themselves are best left to a deeper pocketed partner – that they should be, in the words of Steve Jobs, “Insanely great.” This business model – the intelligent delivery of customers directly to merchants – is not well supported by slower systems that need big loops, large stations, long ramps, etc. If you are going to stop at the store on the way home, it needs to be a fast, red carpet experience…which, by the way, is the same experience we need for PRT to become the long-lasting paradigm that it deserves to be. To do this right, we really need an Apple, a Google, an Amazon - and the right business model to get their attention. Maybe this is it. You listening, guys?.. Tim? Elon? Sir Richard?... Call me!!
Posted by Dan at 6:14 PM
Wednesday, February 4, 2015
Recently I have been corresponding with a new PRT developer, and I was struck by how similar his logic is to the many who have come before. He initiated his communications by saying that the first step to a successful solution should be a fresh look at the problem. He then stated his list of design criteria, tying back each decision to the problems that it would solve. Before very long, he had narrowed his design to something very similar to the SMART platform, yet quite different in many important ways, and I found myself defending the specifics of my approach. I thought I would share.
Design differences don’t just come from differing views on the practicality of one mechanical solution over another, but also from the original assessment of the problem itself. PRT is the would-be solution to a host of different grievances. To some, it is primarily an energy/climate saver. To others the main emphasis is downtown mobility, while some would try to solve the problem of gridlock on our highways. The various PRT solutions that are out there, it seems to me, are all pretty well crafted for their intended purposes. What worries me is they stick to their respective missions a little too tightly.
Current PRT offerings are not versatile enough to harvest sufficient passengers from the varied, real-world cityscapes to be economically viable until they have reached a critical mass of coverage. The cost to achieve this break-even coverage is simply larger than what cities can afford or, at least, are willing to risk their careers over. Here is a brief synopsis of the problem:
For PRT (or any other public transportation system) to be a serious contender, it must pass two tests. First, it must not lose money. Second, it must perform its function better than alternative choices. When PRT can clearly meet these two criteria, it will be adopted, pure and simple. Starting with the first test, let’s imagine the simplest, cheapest system, a simple loop between two stations. PRT costs money, and that only gets returned through fares. The separate costs of track, stations and vehicles must each be amortized though their respective lives and these numbers (along with operating costs) tell us how many passengers need to use those assets daily to make the system break even. Here’s a hint. It takes lots and lots of passengers to make the payments! With two stations, the only people who will use the system are those who need to go to one of those two destinations. While you can buy fewer vehicles, and you can make smaller stations, you can’t reduce the track costs to where only the occasional pod will pay the bills, so that component (the track) bleeds money from the stations and vehicles, leading to its logical elimination. Now we have another shuttle bus on the streets.
As for the second criteria, such a simple route lends itself better to a larger, scheduled vehicle anyway. Since everyone is going to the same place, why not ride together? Even a few stations between would not represent much added inconvenience.
Above is a “heat map” of 5 layouts, representing increasing numbers of loops and stations. The warmer colors represent greater traffic. These are not representations of city blocks nor would it ever be possible to have such evenly spaced stations. Passengers are only where you find them, and then only some of the time. What is shown, though, is how stations are interdependent for passenger traffic. Each station is both source of new passengers and a destination for others who would not otherwise ride. In a full-sized system, (not shown) the vast majority would be hot yellows and oranges, and only the outer perimeter would be the unprofitable purple or blue. PRT clearly can’t start too small.
Needing more closely associated network nodes to be effective is not unique to PRT. The same heat map and lack of profitability at the outer edges can be seen with a bus system or parcel delivery, for example. Yet networks are not necessarily all equal when it comes to the ability to grow “from seed.” The key is making each part of the network viable in its own right. No doubt the builders of the very first stretch of railroad already had a pretty good idea that it would be profitable on its own, and each subsequent branch surely met this test as well.
When it comes to priorities for designing the perfect PRT system, this business of needing a whole network from the start is a whopper. It is the elephant in the room. Yet with proper design I believe a formula for growth can be found. There’s more. Next time you are somewhere that you think would benefit from a PRT system, try identifying the next closest station sites, and estimate how many passengers each such station would attract. If your experience is anything like mine, you will soon see that good sites are not at all easy to come by, and those with lots of foot traffic are usually on private land or are squeezed by roads that have been widened as far as limited easements will allow. At best they are in patches, separated by areas that don’t need PRT at all. I pity the poor city planner tasked with finding station sites every half-mile that have enough pedestrian traffic to warrant an elevator equipped station.
The mobility problems faced by the urban and suburban populations differ greatly, and often PRT designers are biased by their own experience. There are differences due to geography (such as climate) and even city-to-city topographical differences, such as water frontage or hills. Political differences are important because they effect funding and attitudes toward public works in general. There are even historical differences. One that I am often reminded of is (then) President Eisenhower’s “cold war” strategy of initiating of the US interstate highway system, resulting in urban neighborhood segmentation and the suburbanization of an entire nation. Now, 1 out of 4 miles driven in the US is on an interstate, often at maddeningly slow speeds! The point is that these are among the factors that individualize a city’s needs, even as the economics of starting a PRT company push toward a more “cookie cutter” approach, and therein lies the rub.
This is where “SMART” comes in. “Suspended Multi-axis Automated Rail Transport” is not a PRT system per se, but a development platform. Any system that can move vehicles in 3D can also work in 2D. Any system that can travel at 90 mph can also travel at 30. If it can turn in a 6’ radius, it can certainly manage any street corner. The idea to avoid “baking in” needless limitations. The SMART vehicles are very capable and full featured because someday they might be expected to be that way. After all, people will eventually want, from PRT, the same qualities they would expect from a manually driven taxi service, and investors, equally, will prefer a fast, efficient and comfortable fleet to maximize returns. In the meantime, there are tough choices in store. SMART, with all of its “bells and whistles” is, admittedly, an extremely (if not prohibitively) expensive way to start. Yet its basic architecture, especially the track, addresses the above problems squarely by being uniquely versatile.
With PRT requiring a whole network to operate profitably, and with the vast differences between different cityscapes, the problems that predicate a PRT designer’s job are immense and varied. Some examples include how to put dirt-cheap stations everywhere and anywhere; how to take the shortest route between lucrative sites instead of being detoured by worried landowners; how to piggy-back other services, like carrying utilities or street lighting; How to give local planners more and better options; How to get property owners to allow stations on their parking lots and in their lease spaces; How to attract the most riders without cutting fares, how to encourage night traffic and freight, how to minimize operating costs, and so forth.
Why PRT has not entered the mainstream is not simply a matter of suspended vs. supported design, linear vs. rotary motors, or third rail vs. battery power. It is the big structural issues regarding network size, versatility, and the business model, and translating these overarching issues that into physical form should be “job one” for the PRT designer.
Saturday, January 17, 2015
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.