# ­xkcd thread **URL:** \ **Category:** Cafe Society\ **Tags:** comic-strips\ **Created:** [May 28, 2020, 12:57am UTC](https://boards.straightdope.com/t/xkcd-thread/854574 "2020-05-28T00:57:03Z")\ **Posts on this page:** 20\ **Page:** 294
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [May 12, 2026, 2:43pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5863 "2026-05-12T14:43:44Z")
Interesting idea. If the spring was wound at the factory to provide life-of-the-car energy, how much is that? Feynmanning forth … Of course the energy needed to move the car is roughly the same for an ICE or an EV or a pullback. But the conversion efficiency of each of those varies. Starting with ICE as the base case … If a typical car lasts 300K miles and goes 300 miles on a tank of gas, we can say 1000 tankfuls is a lifetime fuel supply. For a basic transportation small sedan like in the comic that gets 20-30MPG, those are probably 12-15 gallons per tankful. Being pessimistic, 15,000 gallons of gasoline. One gallon of gasoline is ~120E6 Joules of energy. Times 15K gallons is 1.8E12J. An EV would be more efficient in converting stored energy into motion. So might get away with 1/3rd to 1/10th as much stored energy. A mechanical spring will be nearly 100% efficient at energy out versus energy put in. But the mechanical transmission to manage the RPMs and get them to the wheels won’t be so efficient. Probably less efficient than an EV, but better than an ICE; that chemical-to-mechanical energy conversion step is a real efficiency killer. In any case, for any energy source & power conversion system that is life of the vehicle we’re talking ballpark 100E9 to 1E12J. If that much energy gets out all at once it’s going to make a very impressive mess. SPROINNNNGGG!!!11!1!!! 🤪 ---
**Author:** ![puzzlegal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/puzzlegal/32/3827_2.png) [@puzzlegal](https://boards.straightdope.com/u/puzzlegal)\ **Post date:** [May 12, 2026, 2:50pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5864 "2026-05-12T14:50:57Z")
How large is this spring? ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [May 12, 2026, 3:07pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5865 "2026-05-12T15:07:17Z")
More important, how tight is it? ---
**Author:** ![Ponderoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponderoid/32/19221_2.png) [@Ponderoid](https://boards.straightdope.com/u/Ponderoid)\ **Post date:** [May 12, 2026, 3:15pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5866 "2026-05-12T15:15:32Z")
I kind of figured the spring would have no more than a single gas tank equivalent in energy, thus the hefty tow truck fees when you need a rewind. ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [May 12, 2026, 3:27pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5867 "2026-05-12T15:27:59Z")
You can also do it with a crane: roll it back, hold the wheels, pick it up, move it forward, put it down, roll it back again, and so on and so on until it starts clicking. ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [May 12, 2026, 3:33pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5868 "2026-05-12T15:33:48Z")
Recharge stations would be reverse dynamometer systems, driving the power wheels in reverse until fully wound. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [May 12, 2026, 3:52pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5869 "2026-05-12T15:52:12Z")
Which plays havoc with the odometer. 😉 ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [May 12, 2026, 3:55pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5870 "2026-05-12T15:55:08Z")
Interesting. And I took it the other way; rewinding in the field would be soo all-fired difficult that the car had to be delivered from the factory with a lifetime energy supply. The number of different ways to inject sense into the nonsense is one of my main measures of merit for an xkcd comic. This one scores pretty high. 😁 ---
**Author:** ![Q.Q.Switcheroo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/q.q.switcheroo/32/5713_2.png) [@Q.Q.Switcheroo](https://boards.straightdope.com/u/Q.Q.Switcheroo)\ **Post date:** [May 12, 2026, 6:54pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5871 "2026-05-12T18:54:30Z")
As Matilda Wormwood’s father knew. ---
**Author:** ![smithsb](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/smithsb/32/12107_2.png) [@smithsb](https://boards.straightdope.com/u/smithsb)\ **Post date:** [May 12, 2026, 7:26pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5872 "2026-05-12T19:26:09Z")
We need to do this with a factory spun up flywheel. Should be smaller than this spring assuming the density of a neutron star for compactness. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [May 12, 2026, 8:20pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5873 "2026-05-12T20:20:07Z")
Mechanical methods of energy storage are always going to be less efficient than fuels that burn oxygen; the energy density of oxygen burning fuels are increased by the fact that you don’t have to carry oxygen around. Unless you are in space or on the moon, etcetera. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [May 12, 2026, 8:26pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5874 "2026-05-12T20:26:15Z")
> [@puzzlegal](#): > > How large is this spring? I haven’t been ignoring this most excellent question. But it’s a bit beyond my mechanical engineering chops. An hour spent digging around for background knowledge, for spring designs, and materials parameters was getting me nowhere. So I cheated and asked Google in AI mode to design one. As expected, Google sez it’s somewhere between ginourmous and impossible with current known materials. Provided herewith for your entertainment with no editing by me and no guarantees by me either. > **Google AI's Opinion of a Wind-up Spring Sufficient to Power a Car for its Design Lifetime.** > > My prompt: > > > Give me a design for a torsion spring able to store 1E12 Joules of energy. > > Response: > > > [@Google AI](#): > > > > Storing 1 Terajoule (10¹² Joules) of energy in a single torsion spring is physically impractical with current materials and engineering. To put this in perspective, 1 TJ is equivalent to the energy released by about 239 tons of TNT. > > > > ## Engineering and Material Constraints > > > > Using high-strength spring steel, a torsion spring of this capacity would require a volume of roughly 400,000 cubic meters. > > > > - Mass: Approximately 3.14 million metric tons. This is roughly the weight of 9 Empire State Buildings. > > - Dimensions: If designed as a solid steel cylinder, it would be roughly 80 meters in diameter and 80 meters tall. > > - Structural Integrity: At these scales, internal defects in the material would make the structure extremely prone to catastrophic failure under the immense mechanical stress required to hold the energy. > > > > ## Theoretical Design Parameters > > > > If one were to attempt this using the best available spring steel (Tensile Strength \approx 2 GPa): > > > > - Material: Chrome Silicon or ASTM A228 (Music Wire) grade steel, though manufacturing it at this scale is beyond current industrial capabilities. > > - Energy Density: Approximately 2.5 kJ/kg. For comparison, a Lithium-ion battery stores about 900 kJ/kg, which is about 360 times more efficient by mass. > > - Spring Rate (k): To store 1 TJ with a 90-degree (1.57 rad) twist, the spring constant would need to be roughly 8.1 \times 10^{11} \text{ Nm/rad}. > > > > ## More Efficient Alternatives > > > > For storing 1 Terajoule, mechanical springs are one of the least efficient methods available. More practical methods for large-scale energy storage include: > > > > 1. Pumped Hydro Storage: Moving water between reservoirs at different elevations. > > 2. Flywheels: High-speed rotors, often made of carbon fiber, which offer significantly higher energy density than steel springs. > > 3. Chemical Storage: Large-scale battery arrays or hydrogen storage systems. [1, 2] > > > > Is the inquiry regarding a smaller energy target for a specific mechanical application, or is this related to a theoretical physics concept? > > > > [1] [https://www.fsp-group.com](https://www.fsp-group.com/en/knowledge-prd-44.html) > > [2] [https://www.govinfo.gov](https://www.govinfo.gov/content/pkg/GOVPUB-D101-PURL-gpo3023/pdf/GOVPUB-D101-PURL-gpo3023.pdf) ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [May 12, 2026, 8:27pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5875 "2026-05-12T20:27:49Z")
> [@eburacum45](#): > > Mechanical methods of energy storage are always going to be less efficient than fuels that burn oxygen Maybe you mean better energy to mass ratio? In this context, efficiency is the fraction of stored energy that gets converted into kinetic energy. Heat engines are not particularly efficient at converting chemical energy to motion. While devices that convert stored mechanical energy to motion tend to be very efficient. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [May 12, 2026, 8:38pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5876 "2026-05-12T20:38:12Z")
Yeah. Two very different notions of efficiency. Both matter. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [May 12, 2026, 9:50pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5877 "2026-05-12T21:50:30Z")
> [@Pleonast](#): > > Maybe you mean better energy to mass ratio? Yes, that’s right. A machine with stored mechanical energy needs to be heavy, in order to store the energy. A kilogram of spring or a kilogram of flywheel will store less energy than a kilogram of kerosine, assuming an internal combustion engine that burns atmospheric oxygen. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [May 12, 2026, 10:01pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5878 "2026-05-12T22:01:54Z")
Wait 'til Randall describes a car powered by the awesome energy of popsicle sticks [![](https://img.youtube.com/vi/l2f_EW2mzZU/maxresdefault.jpg "Stick Bomb 23,600 Sticks!") ](https://www.youtube.com/watch?v=l2f_EW2mzZU) ---
**Author:** ![DesertDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/desertdog/32/11503_2.png) [@DesertDog](https://boards.straightdope.com/u/DesertDog)\ **Post date:** [May 13, 2026, 5:08am UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5879 "2026-05-13T05:08:49Z")
Boy, talk about maximizing entropy. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [May 13, 2026, 8:02pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5880 "2026-05-13T20:02:22Z")
I’m sort of wondering how you set something like that off. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 13, 2026, 8:52pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5881 "2026-05-13T20:52:02Z")
> **[Results Age](https://xkcd.com/3245/)** [![](https://imgs.xkcd.com/comics/results_age.png) ](https://imgs.xkcd.com/comics/results_age.png) > [@](#): > > Please, we need your help. Our research suggests you're the last living descendant of the person who knew how to format this config file. Oh, and going back a few days, how are zircon crystals used in geochronology? Do they have inclusions of uranium, or something? ---
**Author:** ![Mollie\_Magill](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mollie_magill/32/20882_2.png) [@Mollie\_Magill](https://boards.straightdope.com/u/Mollie_Magill)\ **Post date:** [May 13, 2026, 9:10pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/5882 "2026-05-13T21:10:20Z")
> [@Chronos](#): > > Oh, and going back a few days, how are zircon crystals used in geochronology? Do they have inclusions of uranium, or something? Yes - You’re spot on. [Previous page](https://boards.straightdope.com/t/xkcd-thread/854574.md?page=293) [Next page](https://boards.straightdope.com/t/xkcd-thread/854574.md?page=295)