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Virgin "It's literally cheaper to keep repairing it" vs. Chad "I don't care how 'overengineered' you say this is, the budget is coming from the Emperor himself"
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this post was submitted on 13 Sep 2025
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I will admit that my pre-1800s wheel knowledge is quite limited, but one thing which is clear from wood as a building material is that it's mostly used for its compressive strength. That is, the spokes of a wooden wheel are there to only bear weight when directly below the axle. Otherwise, they're not doing too much.
Some wood wheels do have massively thick spokes which can singularly bear the entire weight from the hub, as the wheel rotates. This makes the rim less important, apart from providing a smooth surface to meet the road.
But other wood wheel designs used a massively thick rim which would hold the circular shape. These designs meant the downward force was spread across multiple spokes (not just the immediate bottom one) and so each spoke could be slightly narrower.
The big innovation with bicycle wheels circa later 1800s is that the spokes are in tension, not compression. And that simultaneously makes the rim into a natural circle while saving rim weight, plus it distributes the load forces across all the spokes, making them all thinner. Add the fact that steel has incredible tensile strength and the result is pencil-thin wires supporting over 100 kg easily. Since material science has blessed us with materials with more tensile strength than compressive stength, this is almost as optimal as it gets, for a wheel with purely vertical loads (near-perfect for bicycles; not as good for automobiles).
The simplest intuitive understanding is to imagine what happens if a bicycle wheel tried to deform at the bottom. If so, three or four spokes will be relieved of tension, but some 30 spokes other would undergo extra tension. And that would work to resist the deformation, returning back to the intended circular shape. Wood wheels have no such restorative feedback loop. Nor can wood wheels spread out an overstress to multiple spokes, and instead would probably collapse the wheel with attendant injuries.
The drawback of the wire spoke wheel is that as the diameter gets larger, the transverse strength will diminish, because the spoke angle from the rim approaches 90 degrees. That is to say, if a bicycle were doing a power-slide, a really tall wheel would run the risk of "dishing" out, where the rim basically slides off-plane from the hub, turning into something like a cone, potentially catastrophically. Old timey penny-farthing bicycles are at the diameter where this is a problem, but fortunately those bikes can't practically be slid sideways. This is also why cars moved away from spoked wheels, because of this lack of lateral strength.
The other drawback is that the structure is the most efficient (read: saves the most material/weight) when the rim is narrow. But if being paired with a wide tire, then the rim has to be wider and that costs material. Bicycles use "balloon tires" that expand wider than the rim, and those have worked great for even MTB bikes that need the extra tire width. But today's fatbikes and e-bikes with >4 inch (100 mm) tire widths necessarily require wide rims, and that's taking efficiency off the table. It's another reason cars (which need tire width bc they're so heavy) moved away from spoked wheels.
As for the parallel with barrel making, I think the cooper uses steel/iron bands to bind the barrel together, right? I think I've seen steel along the tread of wood wheels, but I don't know if that's structural to the wheel or if it's more like tread to prevent the road from destroying the rim surface.
Well, the wood of the spokes, which is usually chosen to be something like oak for that reason. That's a nitpick, though, because you clearly understand the rim and hub experience more complex forces.
The fascinating thing is that if you use really basic materials, that's not exactly true! With a few ultra-modern exceptions, ceramics can't tolerate much tension. Wood can a bit, and so was even used by the Inuit for tools, despite their total lack of access directly to it. But, it has has a few other weaknesses, like anisotropic strength, anisotropic shrinkage that doesn't even follow sane, straight lines, softness, a grain of appreciable size and lack of heat resistance. Natural cordage has great tensile strength, but ~zero compressive strength and will stretch to laxity over time. There's similar stories for things like hide, sinew and cloth. Finally, bone lands somewhere in between ceramics and wood, and can be hard to find large pieces of depending on available fauna.
It had huge impact on the history of how machines developed. And it was the whole reason metal found great use even in the days when it was extremely, extremely precious. Tin is a critical ingredient in historical bronze, and it's actually about as abundant as a precious metal. Arsenic was also available for that, but it was so toxic they didn't even want to expose their slaves, and so the Near East moved over time to tin. In other metalworking societies they rapidly moved to much more difficult metals and/or made do with the limitations of stone-age tools, maybe due to even less abundant tin.
It's structural. I suppose you could turn a wood ring out of one big log, and if you did it along the tree's axis it wouldn't shrink or grow into an oval, but it would be tricky to get the spokes in, and tricky to get (and keep! seasoned wood can still swell or shrink as humidity changes) it all tightly in contact. In reality, the rim in in pieces. IIRC they're steam-bent, with the grain running along the axial direction, and made of something like yew that responds well to flexing.
The origin of the word tyre is that metal band. It's put on hot, and the compression as it quenches is what holds the entire assembly together. Earlier wheels weren't necessarily the same. At least some ancient Egyptian chariot wheels used sinew as the compressing element, and it sounds like it may have been in a different place.
You're absolutely right, and I was more-or-less generalizing, given that modern structures tend to focus on maximizing the tensile strength, such as carbon fibre for fuselage sections. There's even the combination where tensile strength is used to augment compressive strength, like with tensioned concrete slabs. Honestly, the fact that some steel wires pulling within a piece of concrete makes it stronger is kinda nuts to me.
We're at a unique juncture where wood is being revisited as a sustainable material, but also because it's quite interesting if its natural drawbacks can be tamed. The fact that wood bends well before it snaps gives it some very forgiving characteristics, unlike steel which will hold until catastrophically failing.
I suppose if we look more closely at wood, it does in-fact have some tensile strength, as the bottom-side of a heavily-laden beam does stretch. But I'm not material engineer.
I have to mention that this is precisely why the phrase "you can't push a rope" exists, for folks not familiar with this expression for an impossible assignment.
TIL!
I once considered this as a thought experiment, just to see if it would work. And my conclusion is that it would be a phenomenal waste of a log to make a solid disc, when a spoked wheel made from conventional, longitudinal boards cut from the same log would yield a satisfactory wheel at greatly reduced material consumption. That said, it may very well be viable for smaller-diameter wheels, where rendering a slender log into boards would result in too much lost material as sawdust.
If I'm not mistaken, this is how steel tires for steel railway wheels are attached. Plus ça change!
TIL! I'm kind of surprised that significant compressive forces don't inevitably show up in something big and highly mobile like that.
In a way it's too good to be true; a fibre composite that literally grows on trees, and that produces several useful compounds and enormous amounts of energy when burned. The Inuit weren't wrong to scrounge for it.
It still exists because trees end up solving a lot of the same problems as human inventors.
The grain is strands of mostly cellulose, so it's almost rope-strong. Even across the grain, lignin is a decent glue (and with high strain at failure, resulting in the flexibility you mentioned), although the fact you can split wood obviously implies that that is a relatively weak pair of axes, under tension.
I actually have seen wooden wagons careening around pretty good. Modern wheels are going to be better, but most of the time when you read about a historical wheel failure it involves, like, a panicked animal and/or some truly rough off-road terrain. Which TBF definitely came up.
There's actually another factor here! The exact shrinkage of a green log as it seasons is something like 15% radially, 5% axially and not at all along the grain. If you think about the geometry of that, it's going to build up quite a lot of stress and spontaneously fail, unless it has been cut and can warp instead. And that's exactly what you see in old, unsplit logs; they get one big, longitudinal crack that spreads open almost to the center.
IIRC your wheel will have to be wrist size or smaller, by the conventional wisdom. Or, I guess, stay permanently soaked and soft, either by way of sealant or having a job underwater.
TIL! I had always assumed they were bolted.
In the case of aircraft fuselage -- using the Boeing 787 as the prime example -- the pressurized cabin means that the airplane is basically a balloon, as the pressure inside is typically something akin to being at 6000-8000 ft MSL (~2-3 km) but the outside air pressure is something like 32,000-38,000 ft MSL (~10-12 km).
Expressed another way, the delta between 6000 ft pressure and 38,000 ft pressure is about 600 hPa, or roughly 60% of sea level pressure. Since 1 Pascal is 1 Newton per square meter, and if we say an aircraft door is 2 sq meters, the force trying to push the door open at cruise is 120,000 Newtons, or the equivalent gravitational force expressed an object measuring 6 tonnes!
As a balloon-like tube, the 787 fuselage has a lot of similarities with how its fibres are arranged, not unlike the plies that stretch across the tread of a pneumatic bicycle tire.
Ok, I have to know: what sort of situation would this have been at? Medieval chariot racing?
I've never thought about the shrinkage rate before, but it makes perfect sense that wood would shrink at different rates in different dimensions. Would this also suggest that steam bending of wood is less effective in certain directions?
Hmm. Is that why passenger jets have stuck with aluminum for the wings, then? They do have to actually bend a visible amount, as opposed to just being an inflated balloon.
I can't remember now how much chariot racing I've seen. I don't think ever in person, although wooden carts have been around at re-enactor things.
I was thinking about this local specialty, which I'm pretty sure still uses traditional wooden wheels.
Maybe, but not directly for that reason. The relevant effect there is the lignin reaching a heat and moisture-induced glass transition and plasticly deforming. I'm not actually sure if the relatively large shrinkage along the growth rings is due to capillary and cell shape, arrangement of the polymers, or both.
I want to say the 787 also has composites in the wings, but I'm not certain. It does, however, bend quite a lot during takeoff. But I think most wings do too, but controlled so it doesn't cause fatigue cracking at the wing roots. A composite wing wouldn't have the same fatigue failure mechanism.