How could a cable car be made steerable?
July 12, 2021 4:30 PM   Subscribe

I'm brainstorming a sci-fi concept where a city has a complex network of aerial tramway cables suspended above it. These cables intersect at various junctions which allow an operator inside the gondola to change directions by switching from one cable to another. Basically steering. I assume this is wildly impossible, otherwise I'd be able to find some example of this already existing in real life. But can we conceive of some method that (with some handwaving) could make for plausible fiction?
posted by The Winsome Parker Lewis to Technology (23 answers total) 1 user marked this as a favorite
One possibility would be at the switch point, have the tram go "downhill" so it has some forward momentum, and that's when you direct it to one way or another. The momentum then carries it onto another engagement section. You can have mechanical "pushers" to make the process more predictable and safer too.

The OTHER way would be to work around it completely: make those the transfer points. The trams never switch "tracks".
posted by kschang at 4:38 PM on July 12, 2021

I have a rough picture of how you could do this for a cart that just rides gravity down the cables, which could probably be adapted to add motors and brakes and things, but is simpler to think about without those complications. On top of the gondola, it hangs from two rollers, each of which is supported on only one side. One roller has the bracket on the left side, and the other roller has the bracket on the right side. Most of the time, you'd have both rollers engaged, so the gondola couldn't be knocked off to either side. When you get to a branch (I'm assuming it's sort of Y-shaped) you'd have one roller lift off the cable and out of the way. If the left-bracket roller is on the cable, it would follow the left branch, and if the right-bracket roller is on the cable, it would follow the right branch. If you forgot to retract one of the rollers, I think you'd just get stuck at the intersection.

I can't say it's a good scheme, but it feels like the seed of an idea that might work.
posted by duien at 4:41 PM on July 12, 2021

Railroad tracks seem pretty analogous to this. A switch at each junction determines which cable the car would move to. Could be operated from inside the car or from other locations. With some hand waving I think it’s easy to imagine increasing the complexity from simple forks-in-the-cable to almost limitless directional choices.
posted by TurnKey at 4:42 PM on July 12, 2021 [4 favorites]

High speed ski lifts work by disconnecting cars from the cable at the boarding/deboarding parts, there's really no reason you couldn't move the car to a different cable given a bit of complexity.
posted by so fucking future at 4:42 PM on July 12, 2021 [15 favorites]

The problem isn't switching or clutching/declutching to a cable, because lots of gondola systems already do that at the stops, to slow right down. The problem is equalising weight on the system as a whole so that you don't have far more cars on the up or the down—overtensioning the cable or going beyond the limits of the drive motors. Since you're science-fictioning and you can do whatever the hell you want, why not remove those limits by having extremely high-tensile cable, or vast systems with so many gondolas that one or two here or there don't matter?
posted by Fiasco da Gama at 4:46 PM on July 12, 2021 [2 favorites]

My phone is currently charging wirelessly. There's nothing in my head to think that a possible future aerial tramway couldn't get its electricity by contactlessly connecting to the powering wires (and having enough battery reserves to deal with a short time of no power during transition), so moving between them should be relatively painless.
posted by General Malaise at 5:18 PM on July 12, 2021 [1 favorite]

Gondolas already often make tight turns in urban settings by switching cables. Either using turning "stations" or "towers."

You might want to read Bruno Latour's Aramis for an exploration of issues that arise trying to design a transit system that operates like a packet switched network.
posted by snuffleupagus at 5:22 PM on July 12, 2021 [5 favorites]

I think part of this is a question of whether the cars are moving relative to the cable, or if the cable itself is moving.

Alastair Reynolds does the first version in Chasm City; the car grabs cables and moves along them, grabbing another cable when it needs to.
posted by sagc at 5:24 PM on July 12, 2021

The first version existed IRL, known as a 'telpher' (electric motors and a base station electrifying the cable). Presumably the weight (and safety?) disadvantage was more significant than any advantages motorizing the car rather than the cable provided. Nature, 1885.
posted by snuffleupagus at 5:32 PM on July 12, 2021

Trolleybusses do this, here's an example of San Francisco's, which are still in service.
posted by jamaro at 5:37 PM on July 12, 2021

There's some interesting information here on how trolleybusses achieve this (although they are not the same as cable cars).
posted by alexei at 6:24 PM on July 12, 2021

Best answer: Roundabouts? Magnets?

Remember, even in science fiction (or hard science fiction, even) you don't have to explain how everything works - just explain the bits you actually need for your story. If you explain and it's plausible and you sound like you know what you're talking about on the things that are critical for your story, you can still get away with hand-wavium on the other things.

If either your viewpoint character, or your story's character in general, wouldn't know the details of how they work, especially if they just expect them to work in the background, it might not be relevant.

If you NEED one, though, as a plot point (does it break? does your character fix it? get stuck? need to escape along the cable? need to change its direction? etc) than use the parts that matter.
posted by stormyteal at 6:27 PM on July 12, 2021 [3 favorites]

I think part of this is a question of whether the cars are moving relative to the cable, or if the cable itself is moving.

Ski lifts switch from the fast-travel cable for getting you up the hill to the slower cable for loading and unloading, and this mechanism could be extended to involve a change of direction.
posted by mhoye at 6:33 PM on July 12, 2021

Autonomous ropeways are already here, just not the PRT version (yet, but concepts are coming). They top out at about 20 MPH though. I don't see any barrier to automating the transfer between lines, but it would have to take place on the ground or on a tower/building.

The higher the speed, the more gradual the change of direction has to be. I see a web of cables with a rigid ring (essentially a large roundabout) at each junction, with transfer cables at tangents to the ring. At junctions, either the left or right grip would engage depending on which link you wanted to travel. You could use unobtanium materials to make the web impossibly strong and light, but you'd still need support towers (maybe from orbiting satellites if your society likes to spend money)

But if you're going fast enough, can't you fly? Maybe the web could be used to distribute power on long straightaways (or uses point-to-point segments between towers) but the vehicle doesn't use the cable for support b/c it's a hybrid drone. To change direction, the vehicle detaches and uses its limited battery power to glide to another web segment.

The most implausible thing to me is that the passenger would be able to switch to new lines on-demand -- these systems would all be highly automated unless someone hacked the system or were in a maintenance mode or something.
posted by RobotVoodooPower at 7:26 PM on July 12, 2021

What if, instead of cables, there was a system of thin but rigid overhead rails? That would make some kind of maglev/linear induction setup workable.
posted by The Underpants Monster at 7:33 PM on July 12, 2021

I'm with snuffleupagus, the tricky part seems to not be the switching itself, but the change from a closed system with a constant demand on cars per cable, to something with more elastic demand due to arbitrary routing. Seems like you'd introduce sky-traffic jams unless the routing was centralized and the number of cars was kept to a low level. However - coming up with solutions to that problem could be interesting in itself imo - maybe the gondolas themselves are constant speed and stick to one section of track, and contain passenger pods that can auto transfer to new routes via junction pillars, or something (this is basically just buses with automated bus transferring). Impracticality aside, it's a cool image, so solving just enough of the obvious issues for suspension of disbelief definitely works for me!
posted by Jon Mitchell at 9:19 PM on July 12, 2021 [1 favorite]

Best answer: Monkey arms.

1. Cable cars/aerial gondolas are common because they are simple and efficient. They are simple and efficient because they are at their essence just systems that pull long-ass cables in a loop, and happen to have useful stuff like chair lifts or gondolas attached at fixed points to the cables.

2. Cable cars/gondolas/etc... almost always attach to the cable with a single, offset arm. This makes designing the stations where you enter/exit the cars and intermediate support towers a lot easier.

3. What if the cars instead had 2 articulated arms? With fancy computer controls, they should be able to seamlessly switch from one arm to the other without the passengers noticing - or at least feeling discomfort. This would also allow the car to transfer from one moving cable to another.

4. Assuming the arms are long enough, have enough 3-axis articulation, and good enough software, they could probably even make 45-degree changes in direction barely perceptible.

5. Like THIS but less cute.
posted by Anoplura at 9:39 PM on July 12, 2021 [3 favorites]

I came here to say multiple robotic monkey arms, so seconding Anoplura's comment. You could have a system that smooths put wind sway and pivots the cabin onto a change of cable with near-impercreptable grace -- until it doesn't.
posted by k3ninho at 10:46 PM on July 12, 2021

As others have noted, trolley buses do something like this. To me it's normal, many in Seattle have antennas going to cables overhead (and they detach frequently and everyone groans and the driver goes out back to maneuver them back onto the cables). There are bus routes that overlap and the buses have to go from the Broadway overhead cable to the Denny overhead cable or whatever. Sometimes there are sparks!

If you consider that the cables could deliver power, the cars or gondolas could hang onto them using the power delivered via electromagnetics or the like, or via a complex hanging-wheel mechanism. (Bonus plot point: the power goes out and all the cars drop/lock on/can't change direction).
posted by BlackLeotardFront at 11:06 PM on July 12, 2021

Cable cars? Tom Scott has five minutes posted yesterday on "The UK's last aerial ropeway uses no power, moves 300 tonnes a day . . ."
posted by BobTheScientist at 1:46 AM on July 13, 2021 [2 favorites]

Best answer: Suppose the part of the gondola that "grabs" the cable is a wheel, like a motorcycle or car wheel with 6-ish spokes. At the end of each spoke is a grabber (mechanical, magnetic, whatever), and the gondola moves by spinning the wheel. Two grabbers are always grabbing at any given moment, but as the wheel rolls, when the cable forks it's possible for the next spoke to grab either of the two cables; the rest will follow.

If the picture's not clear, memail me and I'll send you a sketch.
posted by dbx at 5:51 AM on July 13, 2021

I think we are at a point where, even if it doesn't widely exist in the present, people intuitively understand that there will eventually be autonomous machines that are so routine and commonplace as to be nearly invisible, and yet perform miraculous feats of balance, agility, and strength as a matter of course. If this is true in the world of your story I think thoughtful AI driven grabbers of some sort (monkey arms, or stalks of grasping wheels, or or or) are the way to go.
posted by dirtdirt at 6:17 AM on July 13, 2021

Response by poster: Thanks everyone! Tons of great food for thought, this should go a long way in my worldbuilding process.
posted by The Winsome Parker Lewis at 8:20 AM on July 13, 2021

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