Sunday, March 6, 2011
I want to express a few more thoughts on the subject that I raised in the last post, that being using something along the lines of the EN-V as a dual mode PRT vehicle. The first conclusion that I have come to is that the ability to balance on two wheels is not really that advantageous for PRT. This is especially true in countries that have the equivalent of the US’s “Americans with Disabilities Act.” ADA requirements for wheelchair accessibility mean vehicles need to be longer than the very short configuration that the self-balancing hardware was meant to enable.
I realize now that what enthused me most about the EN-V was not the self-balancing capability but rather the maneuverability afforded by the side mounted, independently engageable drive-wheels. Actually, steering in this way is not at all new. Inspired by aviation design, geodesic dome pioneer R Buckminster Fuller developed and prototyped what he called the “Dymaxion Car” back in the early 1930’s to address some of the same weight and efficiency issues that we are concerned with today. This mammoth eleven passenger vehicle got 22 mpg!
I remember reading anecdotes about amazed onlookers staring wide-eyed as the vehicle made a U-turn and parallel parked into an impossibly small spot in a single motion by using the full 90 degree pivoting ability of its single rear wheel. The same geometry is widely used today in the form of “piggyback” forklifts, primarily for the maneuverability it affords. This rear-wheel steering concept should not be confused with the many other “reverse trike” designs out there that have front wheel steering. Front wheel steering is undoubtedly better for roadworthiness at higher speeds but the space and position requirements tend to highjack a vehicle’s design more than Bucky’s layout.
Those following the comments section of the last post have already heard my opinions on the dangers of adding anything more than the most modest weight gains to a vehicle. I still believe that the concept of cheap light track should be the primary design consideration because the main advantages of PRT only really manifest themselves within a network. Configurations of simple loops or figure eights would be better served by GRT, shuttles, etc. If dual mode capabilities compromise this priority… Well, in Texas we call that “Lettin’ the tail wag the dog.” There needs to be some limit to how much weight that ground travel capability imposes on the system design, and that limit is a painfully small amount.
I think it is time to consider the matter though, because certain aspects of the whole system design may be contingent on the results. For example, having wheels on the sides strongly suggests having a front-loading door, and that influences station architecture. Any ground clearance creates an elevation that must be navigable by wheelchair. Seating changes could influence weight limits and distribution, possibly changing bogey design. So here are a few thoughts.
The case for 3 or 4 wheels is not completely clear. With a four legged table, if you remove a leg it may or may not balance on the remaining three. In any case it won’t immediately crash over. A four wheeled vehicle behaves similarly when going over a pothole. The momentary removal of support has little effect. This is not so with fewer wheels. At least with the two in-line wheels of a motorcycle you can steer around bumps. Not so true with trikes or Segways. This leads to the design choice (for 2-3 wheeled vehicles) of larger diameter wheels that can better span dips. This can also be accomplished by wider or double wheels but this adds weight quickly. For four wheelers, the wheels can be smaller, but only with all four wheels being highly steerable can you match the maneuverability of those two side mounted wheels.
None of these limitations bode well for the goal of speedy, comfortable, or long range dual mode. The question becomes one of how much hardware one is willing to haul around everywhere. Way back in Post 50, I brought up the idea of a drive-by-wire skateboard approach to address the problem of carrying this deadweight. Obviously this is a complicated solution, but one that completely addresses issues like larger batteries, robust suspension, etc. There is a tipping point where carrying around integrated dual-mode hardware becomes impractical weight-wise and the skateboard becomes the better choice. In my opinion this threshold is reached well before the vehicle is roadworthy. This is not to say that seldom-trafficked residential streets wouldn’t be drivable, just that busy streets are not safe for such vehicles just as they aren’t safe for golf carts.
If you preclude busy streets, with their crazy drivers and potholes, and assume that trips will be fairly short, then the PRT vehicle could carry around the needed hardware without too much extra weight. The emergency battery could be split between the bogie and the cab, and that cab portion could be sufficient for short trips. If travel is mostly on paths specially paved for the purpose, suspension requirements are minimal. If the trips are short, minimal tires will last an adequately long time. If trips are short, speed is not a concern, so motors can be small and light. If speed is not a concern, wheel and load balance geometries can be used that would be less than satisfactory for ordinary driving, such as the Dymaxion car design.
In conclusion, I think the best balance may be in the old Dymaxian car design, with a clamshell front door. Two large diameter (but thin) wheels (think dirt bike) with “in wheel” motors would fit into a pair of skinny wheel wells. In the rear would be an external (but shrouded) steering wheel. Some means would be needed to rock the vehicle forward to facilitate entry for wheelchairs. The wheels could be designed as modular, removable components and vehicles without wheels might coexist within the system. I would shoot for under 50 kg of total added weight. I was hoping to include some preliminary illustrations, but these things take a lot of time.