3244: Pullback Drive
| Pullback Drive |
 Title text: "How does the spring not run out almost immediately?" "We pull it back REALLY far." |
Explanation[edit]
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This comic is a reference to toy cars with pullback motors. Normally used for small toy cars, a spring motor stores potential energy when the car is pulled backwards, and the potential energy is suddenly released as kinetic energy when the car is released.
The very simple version of pull-back car will only go as far forward as it is drawn backwards in the first place. More complicated versions can use a change in effective gearing (through the use of a 'flappy' gear that meshes differently depending upon the relative direction of movement of the cogs it is meshed with) between 'charging' the spring by back-pulling, and then letting it 'expend' in the forward direction. This can allow it to store a lot of torque from a little pre-pulled distance and then expend it to give far more effective speed/distance to the very light toy. Through a free-wheel gearing at the end of its 'powered' phase, the car may run on 'unpowered' for a significant further distance. However, since there is a finite amount of energy that can be stored in the spring, they may hit a hard limit where the spring cannot be wound any further, or commonly a slip-gear will simply click as the mechanism no longer tries to convert pull-back movement into sprung potential (letting the child know that their toy is at maximum readiness).
In this comic, Hairy suggests a full-scale version of a pullback car. Making this type of energy store work on the scale of a full-size car would be extremely impractical, due to the low power and the requirement to pull it back far enough to then go anywhere meaningful, even assuming a multiplying effect on forward travel compared to the initial backwards travel. Such a car would also have the significant disadvantage of not being able to provide a reverse gear.
Hairy tries to sell the car by saying that Cueball won't need to worry about gas or electricity prices. This is technically true, but cars that run on petroleum or electricity have the advantage of their energy supply being refillable, while this pullback car seemingly does not (without another factory-style 'pull back' facility). The worries about electricity and gas prices may be a reference to the closure of the Strait of Hormuz due to the 2026 Iran war. The strait was a very common waterway used for the international trade of natural gas and petroleum from the Middle East, but the Iranian government is currently not allowing any foreign ships to pass through it. If this type of propulsion works, this would negate the need to fuel the car, making it a good energy-efficient alternative if it could be practically implemented. But hopefully they don’t give the car too much energy.
It would be possible to "recharge" such a car by repeating the process of pulling it backwards, or perhaps by placing it on a treadmill-style arrangement and running this forward relative to the car, while holding the vehicle stationary. However, the energy being stored in the spring motor would have to come from somewhere. No clue is given to what form of mechanical device is used to pull the car back at the factory and if/when it needs to be retensioned again, but the means used to power that might entirely defeat the main purpose of the pullback car (that that it doesn't rely on various fuels to keep it going) if it relies on such fuels itself.
A flywheel connected to the motor could occasionally be lowered onto the road when the car is under braking and then automatically raised, which might work as a form of charging if the released kinetic energy could be diverted to the main wheels. However, the automatic lowering and raising of the flywheel would require an external energy source, and it would be much simpler to just use an electric car at that point. Plus, this would produce diminishing returns and the car would still need to be "recharged" every so often.
In the title text, Hairy tries to ally Cueball's range anxiety about the low capacity for energy storage meaning that it would run out almost immediately compared to a traditional combustion or electric engine, by stating that they "pull it back REALLY far". Due to the inbuilt 'clicking-limit' that already is implied to have been reached, this wouldn't really help, since any further 'pulling back' would simply be wasted energy. If the mechanism is large and powerful enough to store the energy needed to make the car go any appreciable distance, the acceleration that results when it is released is likely to be a deeply unpleasant and dangerous experience. Furthermore, more pulling back would not address any of the other problems noted above.
To some extent, the spring-powered car is a direct analogue to electric cars (whether pre-charged at the factory or not), where forms of externally-generated power are transfered to a 'potential' held within the vehicle to be re-expended (with acceptable losses in conversion efficiencies) as movement. By contrast, fuel-powered cars provide the energy in the form of potential-holding material (LPG, fuel-oils, or even solid fuel like coal or wood, depending upon the vehicle), which is expelled after use and refilled with new supplies. The advantages of electrical power are that it can be relatively easily generated by means other than burning fossil fuels, and (while not currently at energy densities comparable to common engine fuels), the weight of batteries required to power a car over a given distance isn't anything like as problematic as the equivalent spring-based system would be.
Fun Fact: Paolo Bacigalupi's 2009 dystopic SF novel "The Windup Girl" is set in a post-petroleum world where this kind of propulsion is normal. Spring technology is incredibly advanced compared to our present. Cars are powered by springs that are wound-up in factories on treadmills with genetically engineered mastodons. These springs can then be placed into cars and other machines. They are exchangeable, so if one spring runs out, you replace it with another. Fuel-powered cars still exist, but are only used by the military, and the motor sounds they produce have a terrifying effect on the general population, because they are not used to those sounds any more.
Transcript[edit]
- [Hairy and Cueball are standing to the right of a medium-size car. Hairy has raised one hand slightly to point to the car.]
- Hairy: You won't have to worry about gas prices or electricity prices with our new pullback drive model.
- Hairy: At the factory, we put the car on the ground and tow it all the way backward until it starts clicking.
- Hairy: To drive forward, you just release the brake and it goes.
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Is that supposed to be Elon Musk? 185.114.120.233 (talk) 09:24, 12 May 2026 (UTC) (please sign your comments with ~~~~)
There are toy cars that work like this, or that use a flywheel to store energy in a similar way, for example the Fisher-Price Rev 'N Go Stunt Vehicles. The salesman is offering a full sized car on the same principle. The toys don't go very far. Neither will the full sized car, though he implies it will go "far" before stopping. He doesn't offer any practical way to rewind the spring. 2A12:F43:1462:CC00:583C:B3A7:2A0B:2140 09:56, 12 May 2026 (UTC) dww
- Pull-back and Rev-up toys work significantly differently (though a pull-back toy may have a flywheel effect, it doesn't usually rely on this). Both are, of course, ways in which a child puts their effort into a much smaller vehicle that then expends that effort into forward motion (from my childhood, there were 'friction-toys' or even the Evel Knievel Stunt Bike, on one hand, and things like the Penny Racers, on the other).
- The Rev'N'Go type perhaps can be far more indefnitely charged with energy, upon being released (barring mechanical failures and stripping/melting the plastic cogs), but would not give you the described Clicking that the comic says is happening, which is a sign of a sprung-toy with basic overwind-protection designed in.
- Also, riding a (for example) full-sized Evel Knieval flywheel bike and applying the brakes (assuming they could stop it in time!) would probably just stop it, and leave no more energy to move off again (unlike the spring-powered car, halted before too unwound). I suppose you could make the brakes disengage the drive from the still-running flywheel, then 'clutch back on' when you released them again, but still not something that mofe pulling back at the factory is going to help reduce your range-anxiety with. 82.132.221.157 16:00, 12 May 2026 (UTC)
Energy storage. The devil is in the detail, and the magnitudes. It turns out a rubber spring (aeroplane elastic) stores rather more energy weight for weight than a steel 'clockwork' spring. However batteries, and hydrocarbons, store orders of magnitude more. This subjectwould be good for one of Randall's 2D graphs, plotting use case against energy stored. For instance, a diver's harpoon gun uses a pull-back mechanism quite effectively. 82.19.218.32 (talk) 10:22, 12 May 2026 (UTC) (please sign your comments with ~~~~)
I italicized the "or" in the first section of the transcript SomebodyElse (talk) 11:09, 12 May 2026 (UTC) SomebodyElse 12:08, 12 May 2026 (UTC)
- Isn’t it also bolded?Commercialegg (talk) 12:51, 12 May 2026 (UTC)
- Looks like it is. :) SomebodyElse (talk) 18:09, 12 May 2026 (UTC)SomebodyElse 17:08, 12 May 2026 (UTC)
Is the mention of worries about gas prices a reference to Trump's war on Iran? SectorCorruptor (talk) 14:52, 12 May 2026 (UTC)
- Almost certainly. I've added it in. --DollarStoreBa'alconverse 14:38, 12 May 2026 (UTC)
Does this count as a Tuesday comic? Not sure when it went live but it hit explainxkcd at 0900 UTC, which was 2AM Tuesday morning California time. It was still Monday in Hawaii (UTC-1000) though. 64.201.132.210 15:18, 12 May 2026 (UTC)
Probably the real vehicle getting most close to the pullback drive is the Gyrobus: https://en.wikipedia.org/wiki/Gyrobus 2a02:3100:8bac:fe00:1e1b:dff:fe9f:401d (talk) 20:40, 12 May 2026 (please sign your comments with ~~~~)
I'll let another editor handle the writeup and citations, but here are some order-of-magnitude comparisons: The springs actually in your car's engine store a few joules. A garage door spring might store a few kilojoules. An eight-ton chunk of rubber can absorb 5 megajoules, and a single gallon of gasoline stores >120 megajoules. Even if you have some incredibly machinery to get that energy out, there's several orders of magnitude between (the spring energy you can store inside a car) and (the spring energy needed to make a car go even one mile). 206.209.15.112 (talk) 21:02, 12 May 2026 (please sign your comments with ~~~~)
- You can make springs out of petrol? 2001:569:FBCA:1700:986:CE3B:3243:855 00:47, 13 May 2026 (UTC)
Just reading this article and seeing this sentence:
> This is technically true, but cats that run on petroleum or atomicity have the advantage of their energy supply being refillable, while this pullback cat seemingly does not (without another factory-style 'pull back' facility).
82.196.111.48 19:06, 13 May 2026 (UTC)
- I think you needed to paws and read those clawses again. 82.13.184.33 08:30, 14 May 2026 (UTC)
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