Sunday, March 6, 2011

119> Further Thoughts on Dual Mode

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.


Juho said...

I agree that "the concept of cheap light track" is important. It is particularly important in the beginning of the PRT era since the high cost of the track network may well be the largest single obstacle to getting PRT systems implemented. And an extensive network instead of just a short trial circuit makes the PRT system so much more useful to the end users.

People may build whatever kind of dual mode vehicles they like. Someone maybe likes a one person dual mode vehicle with good off-road capabilities. Or one could use one bogey to carry a regular two or four person light weight gondola and another bogey to carry the skateboard. These could be two separate packages to carry. The skateboard is anyway not needed in the gondola while traveling in the PRT network.

(In addition a third bogey could carry all the extra luggage, and a fourth bogey could carry a trailer to carry all the luggage while traveling on the old fashioned road system. Well, maybe not very often, but in principle.)

This proposed arrangement requires some time and effort to connect the gondola to the skateboard. In many destinations skateboards could however be available - at least at some skateboard rental spots.

As long as you can break the cargo in modules PRT can handle it (e.g. building materials of a house). Maybe the key design criterion is to agree the weight category of the track (e.g. two or four regular size persons with no heavy luggage + gondola). I wonder what the weight of the skateboard is. It would make sense to be able to support also this kind of skateboard carrying scenarios.

Dan said...

Dan the Blogger Responds -

Welcome aboard Juho,… Always nice to see a new face….

You know, your comment, along with various comments in the last post, has gotten me to thinking. What is missing from this discussion is the question of a timeline. In a world where extensive networks of raised track exist, the possibilities and probabilities are quite different than for a world where no such network exists. The size of the track, for example, is essentially contingent on what the public considers in its best interest. Right now anything dangling above someone’s street is fair game for intense criticism. Anything that costs any money that has no track record is suspect.

This could all entirely change with a single successful PRT implementation. It strikes me that a good design must acknowledge the probabilities that come out of this timeline. For example, private vehicle ownership seems inevitable in a world of PRT style track networks, but totally unlikely as a driver to build those networks in the first place.

I think I had better do a post on this matter… Come on, guys! Give me some food for thought!

Juho said...

I tend to see PRT as a very useful system that people would not be willing to give up if they just had first gotten used to it. The problem is to get over the huge threshold of high cost of the first implementations and expanding those to wide coverage networks. (I don't particularly like references to PRT as an "urban network" since this is a very limiting approach.)

I'd like to have track specifications that allow tracks with different kind of requirements (but connected together as one network). For example I'd like to build a private track to my house from the nearest public track. That track could be for low speed and light weight traffic only (one vehicle at a time, using only one two-way track), but it would be cheap. And I'd be happy to start a trip from home to work or to some holiday location (24h ride away), using also tracks that allow high speed, heavy loads and many vehicles (I'd attach the skateboard to my vehicle only after leaving the home track).

Dan said...

Dan the Blogger has had a slow day...Yeah!
I completely agree Juho.
Private “spurs” are a big part of the usefulness of railroads and I think that they would be a good idea for PRT as well. Allowing differing track capacities seems like a good way forward as well. That is one reason that I made the “smart” spec as large as I did. (posts 53,54, 83) It won’t preclude some of these heavier payloads if suddenly everyone becomes a believer and wants roadworthy dual mode, the occasional GRT, and high speed capabilities. I wish I could make it still smaller, though, without giving that up. It can, at least, be made of thinner gauge steel with more frequent supports to trim costs. Conceivably a small gauge bogey from a smaller gauge track could go to some destination on the “big boy” network, but the idea really needs some careful thought.
With regards to a timeline, I am toying with the idea of a track that is designed to be stiffened at a later date. We get economy going in, robustness if and when traffic or higher weight loads demand it.
Also, on a different subject, as I consider Bucky’s wheel layout, I am struck by how good such an aviation style design would be for “landing” a suspended PRT vehicle. I could see the active wheels being replaced by feather-weight passive ones in vehicles not intended for any dual mode use, or even something in-between that would just be used for maneuvering within a station, garage or parking lot.

cmf-seattle said...

As soon as you introduce freight or private vehicles, the "T" no longer stands for Transit. It's a general transportation network, which has to compete successfully with current modes.

I think ULTra guideways could accommodate pallets with built-in wheel motors (although, you'd likely need linear motors to boost/slow them on inclines).

Two exhibits to serve as food-for-thought:
-- Garden railways often have most or all of the track elevated, while easily supporting several hundred pounds per vehicle. Is there a simple, semi-portable and low-cost pylon design? Would our military find it useful (like the Internet)?

-- The ore will be trucked to the Australian port of Fremantle and transported by container ship from there. Unlike the Chinese coal-hauling application of magtube. So, maybe rail doesn't often pencil out as well as we'd hope it would.

qt said...

Well, you asked for it!

re: the timeline and private/dualmode vehicles:

Private ownership might not drive the very first networks, but it might be an inducement in the early expansion.

One set of scenarios I imagine is kind of a tree-branch of alternatives:

Phase 1A
A series of small networks, that establish the utility of the "direct from anywhere to anywhere" aspect of PRT.

I call this "1A" because it's the likeliest way to begin. In fact, it's what ULTra has tried to start up in Masdar and Heathrow.

The ultralight PRT you mentioned in the last post could be a contender in this environment, by the way. In a campus system, or a people-mover replacement in an airport, for example, the ADA requirements might be more rational. After all, the "pod seats" wouldn't be the only way to get from one point to another. It could be argued that they're more like escalators than a bus line--the wheelchairs could move using existing infrastructure.

Phase 1B
One or more of your trunk lines, connecting two or more major nodes, with not much network at the endpoints.

This is "1B" because it seems less likely early on. Most of the people considering such a system will gravitate to HRT or LRT (or gondolas or cable cars, etc.). Established technology in an environment that plays to their strengths.

But it could happen.

Further down the timeline, you could have:

Phase 2A
An extended network, covering a fair area with closely spaced stops, but without much in the way of high-speed links. If there are links, they use more established tech (HRT, etc.)

This could easily happen, though I hope not. ULTra seems to be in some danger of painting themselves into this corner.

Phase 2B
A set of high-speed trunk lines, with limited networks near the major nodes.

You could get this from either 1A (linking networks that have developed a following) or 1B (feeder lines developing as the trunk lines develop a following).

In either case, your long term goal seems to be:

Phase 3
An integrated network, with massively networked feeder lines converging into high-speed trunk lines. Useful both for local trips (entirely within a local network) or long commutes, etc. (local to trunk to another local).


qt said...


I agree that dual-mode wouldn't be much of an inducement when trying to sell a Phase 1A system. But what about a Phase 1B? Or when you're trying to sell an upgrade from 1A or 1B to a 2B?

Until you reach Phase 3, the last-mile problem is going to be a big stumbling block to popular (and political) acceptance. But if you can offer a way for people to buy something that acts like a second car--for the price and gas mileage of a motorbike--you might have something. And your trunk lines would allow that.

The crosstown golf cart.

To my mind, the necessary performance is going to be only slightly scaled down from the EN-V. Even in a Phase 3 system, some trips won't be worth the trouble of hooking up for the rail jump. Especially if you're talking multiple short trips--the "honey-do" list I keep harping about, for instance.

The vehicle should probably have a top speed in the 35-45 mph range, with a range of 20-50 miles. Such a vehicle can take to the streets--Atlanta is full of motorscooters with about that kind of speed. And to sell the "cart," you should be able to sell it as a "second car," not an "attachment to the PRT system." Which means it needs to appear useful in its own right.

As to weight--as I said in the last post, I'm thinking of something that would hold one or (better) two people, plus a few grocery bags or briefcases, and would weigh (loaded) about what an ADA-compliant "pod" (loaded) would. You pay for the privilege of hauling the wheels and motor around, but for some trips it would be worth it.

It could be any size or shape, as long as it would fit into the space taken up by an ADA-compliant pod--and hung properly once hooked up. Let the makers and the buyers sort out what features it would need.

Hopefully, you could get the things legally roadworthy under the motorcycle rules in most U. S. states. It's not as if you're going to put them on the freeways.

For a PRT system, neither more performance nor more size is really needed. Dual-mode minivans and off-road vehicles are properly matched with intercity systems--like the MegaRail people are pushing, for instance. You only need that capability if you're heading into the country.

qt said...

As to Mr. Fuller's car:

If I recall, the Dymaxion was said to drive fairly well. Fuller was engineer enough to design the steering system to compensate for the high-speed instabilities. Or so I've heard.

For the "pod-carts," you could probably get away with something like a forklift's layout, though. The kind of cheap computing power that makes the Segway and EN-V possible would likely be up to a smart fly-by-wire system. That could make such a vehicle street-drivable by the ordinary commuter. And you might get a lot of the EN-V's advantages (tight-quarters maneuvering, etc.) in a less expensive package.

(I've seen several shopping malls that have reached similar conclusions. They may or may not use Segways to get their mall cops around inside the building--but out in the parking lot they've gone to a kind of electric stand-up tricycle. About as fast, fully half as maneuverable, every bit as handy when you're not dodging pedestrians on a sidewalk or an aisle floor--and probably a lot less expensive.)

The "landing gear" is a nifty idea, too. One of the things I've wondered about in your system is how you'd reassure the passengers who get nervous when the pod rocks and bounces when they're boarding. But you could lock the landing wheels after touchdown, and the pod would be solid as a rock.

It would also solve a problem with floor-height tolerances in your stations. No worries about the pod scraping the concrete, or hanging free an inch or two and adding another "axis of bounce" to unnerve the passengers above. You wouldn't need a fancy suspension--the bogey arm would still be handling most of the weight--but a little spring travel would simplify the design of your minimalist suburban stops.)

I do like having something like this to comment on when I get home. It's such a nice change from backing trailers into docks...

qt said...

Now, as to making your dual-mode system ADA-compliant...

Would that actually be necessary? It's not as if all cars on the highway have to be wheelchair-friendly. Granted the pods have to be set up for it, but the "pod-carts" would be privately owned. We have not yet reached the level of insanity that would require (for example) an airline to be responsible for getting a loaded wheelchair from the customer's front door to the airport. (Though I'm sure someone's thought about it...)

And that makes the wheelchair boarding problem something the cart-makers would worry about (assuming one of them wanted to build a wheelchair-friendly podcart).

It wouldn't need to influence station design, because the cart would not be loading and unloading at the station. As long as it could drive into the pod space, and the attach-point on the roof was in the right place to mate with the carrier-arm of the bogie that's come to take it where it's going, the system wouldn't really care. As far as the system was concerned, it could be a normal pod or a freight pallet.

For that matter, it wouldn't have to be as big as an ADA-compliant pod. It's a private vehicle. And quite possibly it's driven by the disabled person himself. Why should it need an extra seat?

Dan said...

Thanks for the links cmfseattle… It’s interesting that that pipeline is essentially a hanging, PRT like system, although it doesn’t look like it supports intersections. It is also interesting to see just how skinny rail can be if you support it every few feet. It makes me appreciate how much benefit cable stayed or suspension bridge designs can bring to PRT design.

BTW, my son just landed a job at Microsoft. Any advice for someone who doesn’t know the area?

Dan said...

I tend to be a little more optimistic about longer ranges for PRT than you are QT, at least absent a cable based gondola system that is specifically designed to compete for this type application. The other alternatives are simply much more expensive, and shrinking infrastructure budgets and space will tend to favor PRT over time. I see routing from a “park-and-ride” to a business center loop (with a few stations) as something that might happen fairly early. Also between multiple business centers that aren’t too far apart, like a hospital district and a downtown. This would favor relatively high speeds, and semi-stout track, fairly early on. Very light, slow vehicles (such as might be used for campuses) would tend to get in the way off campus, but compatibility can still be a plus.
I think PRT loses some advantage in very short distance networks, because the shorter the distance, the less inconvenient intermittent stops become. Consider the elevator. If you have less than 10 floors, it’s really no problem to make a few stops to let others on and off. Tall buildings, however, almost always employ express elevators. Similarly, buses are impractically slow for long trips, but for a few blocks they work OK, so long as they come along frequently. This factor favors higher speeds, fewer stations, more track. It’s still a network, but one that tends to pick out destinations more selectively than just trying to fill out a grid of coverage. This kind of layout might favor some integrated, limited distance dual mode capability. I do not mean private ownership. I just mean pavement capable. (at least shorter term) Such an embodiment could freely roam campuses or airports.
It occurs to me that to many, dual mode automatically means privately owned. I tend to think more in terms of how something is physically engineered, not who owns or leases or houses the vehicles, which is more a matter of business modeling. I probably have confused a lot of people. I guess we all need to be careful in this regard. In the old days, being road-capable meant being user-driven, which implied private ownership. Now PRT can presumably switch from track mode to road mode to extend the network under multiple, even concurrent, business models.
I see three weight classes, one for economy, slower speeds and shorter distances, one for general purpose, and one for very high speeds, private dual mode, or GRT. (Group Rapid Transit) Some of the track requirements for the third could be addressed by increasing the vehicle spacing on type 2 track where practical. As I mentioned, it would also be good if track can be designed to be stiffened at a later date to fit the third category. It would probably be impractical to try to make track for the first category carry vehicles designed for the second or third, but vehicles of any weight class would able to travel on the heavier track, but only when and where there is vacant space for slower traffic.
The ADA compliance issue is strictly limited to public transportation. Anything like privately owned pods or skateboards are exempt. Can you imagine otherwise? ADA compliant canoes or motorcycles?
Thanks for the links cmfseattle… It’s interesting that that pipeline is essentially a hanging, PRT like system, although it doesn’t look like it supports intersections. It is also interesting to see just how skinny rail can be if you support it every few feet. It makes me appreciate how much benefit cable stayed or suspension bridge designs can bring to PRT design.

qt said...


Actually, I'm not that pessimistic about longer ranges. I just think the shorter range systems are likelier in the short term. Politics and salesmanship don't always follow logic, reason, or economics.

You'll notice, I said my "Phase 2B" (trunk lines with separated but growing networks) was the preferable next step whether you started with "1A" or "1B," and that my concern was that the "1A" companies like ULTra were painting themselves into a "2A" corner. That's one reason I follow this blog the way I do--I like your attitude toward the development process.

As for connector applications--I do worry about something like gondola systems doing the linkup work Myself, I think it's a good substitute for "Light Rail," but not really desirable for an integrated system like you're designing. But I can see government types thinking differently, eventually.. After all, it is an established technology with more than a century's track record.

Not a bad point on the "express elevator" analogy. But I might point out that the elevator's extra stops are tolerated because they don't slow you down much. If you had to stay in one for more than five minutes, the "public transit" aspects would start costing ridership, I think. Unless you didn't give them other choices...

Buses are much the same way. They're not just impractically slow, they're crowded, noisy, unpleasant, and (sometimes) fear-inducing. The privacy and short wait times are two major selling points for PRT, whether short distance or long. That's one reason I'm not a big fan of GRT. It doesn't really offer anything that's not out there already. And what's out there already isn't selling to anyone not stuck with it.

qt said...

As to dual-mode and private ownership, multiple business models may be a good thing. It's here already with autos--Taxi's and Zipcars(tm) and Hertz(tm), oh my! But the business models need to be considered--they directly impact the design side. A marvelous piece of technology that no one wants is not much of a success.

And one or more of those business models will almost certainly involve private ownership, unless communism makes a MAJOR comeback.

I'm not sure you need to restrict private dualmode to the Type 3 track. You seem to be assuming that "dualmode" means "Interstate capable," if not "SUV lite." But there are a few towns that have integrated golf carts into the city infrastructure (Peachtree City, just outside Atlanta, is one of them). And scooters have made a comeback in the last decade or three--small scooters, with a 45mph or less top speed, not allowed on the big highways but welcome on city streets. A small, light vehicle, with limited but useful range and limited but useful speed, might let the network grow a little faster than it could otherwise.

In your scenario (which seems to resemble my "1B to 2B to 3" scenario), I suspect there will be at least some transitional period where the "last mile problem" will be more like a "last 3 to 5 miles problem." A light dualmode vehicle that could double as a family errand-runner might be attractive, both as a Zipcar(tm) analog and as a cost-effective substitute for a second car. And if it were done right, you might could build a 2-seater the (loaded) size and weight of a (loaded) ADA-compliant 4-seater pod. Which could put it on your Type 2 track. For an extra charge, of course, to cover the extra weight if nothing else.

Agree on the ADA issue. That's what I thought I was saying. Must not have been clear enough. That, and the fact that variations in the dualmode vehicles' layout wouldn't impact station design much--nobody would be using the station's boarding facilities to get in and out of their vehicles.

(This is SUCH a nice change from truck-stop conversation!...)

Dan said...

qt, you will be glad to see me bringing some of these ideas to the forefront in my upcoming post. Be glad, meanwhile, for your occupation. Few people these days get to think more or less freely on subjects of their choosing while they earn a living.