Why are bike tires so narrow and large diameter compared to car tires? What tradeoffs are here exactly? Motorcycle and some ebike tires are more similar to car tires than to bike tires, so i guess it has something to do with braking length at maximum expected speed, and probably also with weight of vehicle, as to not exceed some specified pressure on road. There has to be so many more reasons (weight? air resistance? some other things affecting efficiency or safety? ???)
The TL;DR is that at one point in history, automobile wheels did in-fact use the same construction as bicycles. But the needs of automobiles diverged somewhere in the first half of the 20th Century. And since tires are mounted onto wheels, we need to discuss those first.
I’ve written prior comments here about bicycle wheel/tire fitment and wooden carriage wheel design.
Basically, early horseless carriages used the same wooden wheels that horse-drawn carriages has used for centuries, which have a squared off profile that contacts the ground, sometimes with a steel band – a tyre – to both hold the wheel together and reduce wear on the wheel itself. The only requirements for carriage wheels were to: 1) roll, and 2) bear weight. And using thick wooden spokes, a wagon wheel could achieve those objectives just fine but were really heavy.
When the bicycle was invented in the 1820s, the first iterations used slender variants of wagon wheels, but since 100% of the moving power came from the human rider, this is still unnecessary dead weight to haul around. So bicycle wheels evolved to use very thin spokes, which by the late 19th Century were made of steel in tension, rather than the compressive loads through wood that wagon wheels used. Although steel is heavier than wood, a thin steel spoke has more tensile strength than the same weight of wood has in compression. So overall, it’s a weight savings. Specifically, we say that a bicycle wheel must: 1) roll, 2) bear some weight, and 3) allow for leaning.
The last requirement is crucial for bicycles: they cannot use squared-off carriage wheels, or else leaning the bike will start riding on the edge of the wheel/tire. The solution is simple: round off the contact point so that leaning doesn’t change the profile.
As it turns out, by the 1910s or so, automobiles also realized that wooden wheels were too heavy, and so they also adopted the steel spoked wheel. But they kept the squares off rubber tire, precisely because an automobile does not (normally!) lean during a curve, and instead should be firmly planted on all four wheels. So at this point in history, both automobiles and bicycles are using spoked wheels but just have different shapes for their rubber tires. Great!
But this wouldn’t last: the spoked wheel – which already is a phenomenal structure, essentially being a suspension bridge wrapping upon itself – has one small quirk which bicycles tolerated but automobiles do not. When a spoked wheel is subjected to a straight downward force, the structure distributes the force essentially evenly. But if the force is sideways from the left (ie along the axle), the spokes on the left are heavily stretched but the spokes on the right aren’t. This is uneven loading, that then reverberates from side to side.
This is no issue for bicycles, because they usually lean and so the sideways force is often zero. Sure, a BMX rider can intentionally ride the bike askew, but it’s workable. For an automobile, sideways forces are a regular occurrence, such as during a sharp turn. But also during motorsports where the car is sliding. Spoked wheels can disintegrate when subjected to enough sideways force, which is why cars switched to solid wheels.
Also around this time, cars got very heavy – some would say “land yachts” – and this required making the tire wider to deal with the weight. Since the tire and wheel are the same width in cars, this means wider rims as well. Bicycles have no such issue, because most bicycle tires are “balloon shaped”, and so already are wider than the rim, sometimes almost comically.
In the end, the closest that bicycles and automobiles got was in the early 1900s, and have diverged ever since. Fatbike bicycles and now ebikes pushed the width of tires to some 4+ inches (100+ mm) while touring cars are luxury vehicles meant for long distance, high speed cruising on the Autobahn, and so need wide, high aspect ratio tires.
As for wheel diameter, that’s much simpler to answer: as Jeremy Clarkson noted in the Vietnam Top Gear Special, smaller wheels fall into potholes easier. Bigger wheels roll over them. Automobiles for paved roads use modest diameters, capable of slowly rolling over a 4-6 curb to access a driveway. The same diameter on a bicycle would be the 27-inch (aka 700c) or 29-inch class used for road cycling or mountain biking. Whereas smaller folding bikes used exclusively for last-mile commuting can tolerate smaller wheels, because the benefit doesn’t outweigh the diameter penalty when folding it down.
For overlanding or bouldering, 4x4 automobiles have some enormous tire diameters and even then, they sometimes have to intentionally reduce the air pressure, so the tire can conform to rock surfaces and thus get more traction. But such tires are wholly inappropriate on a roadway at freeway speeds.
grue@lemmy.world 4 weeks ago
The pressure the tire exerts on the road is always equal to the pressure it’s inflated to. When the vehicle weight increases while tire PSI stays the same, the contact patch (area squished flat against the pavement) increases in size.
Bike tires are narrower than car tires because bikes are much lighter (so the contact patch doesn’t need to be as wide), and also because they lean into turns (so the contact patch can’t be wide). Bike tires are often larger diameter than car tires because they have more gyroscopic effect and thus make the bike easier to balance. They also make it easier to ride over bumps, but on a road bike (as opposed to a mountain bike) that’s probably a relatively minor reason.
I think motorcycle and ebike tires are a little wider (but still round in cross-section, so not like a car tire) for durability reasons because all the forces they’re subjected to are larger.
litchralee@sh.itjust.works 4 weeks ago
This is merely a convenient approximation for properly-inflated tires carrying a load, not a hard rule rooted borne out during empirical examination. After all, removing a wheel from an automobile and rolling it along clean concrete leaves tire tracks that are full width, yet the tire will not substantially deform at the contact point because 20-30 pounds is not much of a burden. If there’s no deformation, then the contact patch is a line with a tiny area, which would wrongly suggest a ludicrously high tire pressure.
While bike wheels do act as gyroscopes – as do all rotating masses without a contra-rotating mass – this is not substantial to bicycle stability. If it were, kick scooters or e-scooters which have substantially smaller wheels but with the same physics as bicycles would be unrideable.
The bicycle has existed for about 200 years, and for most of that time, how it remains stable was an open question in physics until roughly the late 20th Century, when researchers built enough intentionally-bad bicycles to prove what was minimum and sufficient to have a functioning bicycle. This empirically ruled out trail, caster, and gyroscopes as necessary factors. But the most prominent factor that remained necessarily is centrifugal balancing, aka leaning/banking. Turns out, bicycles lean into curves just like airplanes so.
grue@lemmy.world 3 weeks ago
Sure, the tire itself has a certain amount of strength, but (unless it’s a run-flat tire, I suppose) it’s negligible compared to the load carrying provided by the tire pressure.
No, you’re overstating your case. First of all, I didn’t say that gyroscope forces were the only factor. Second, they are a “substantial” contributing factor. Your own wiki link agrees with me:
The important part is the “gyroscopic effect… contribute” part, not the “solely responsible… discredited” part.
Remember, OP’s question was “why are the wheels big,” so the effect that’s relevant to discuss is the one that’s different between wheels of different diameter. And that’s the gyroscopic effect, not any of the other things that contribute to bicycle stability but don’t depend on wheel size. There’s a reason people generally don’t prefer things like Bromptons unless they really need the packaging advantages, and it’s because bikes with small wheels are (relatively) weird and twitchy to ride.
NeatNit@discuss.tchncs.de 4 weeks ago
If the vehicle gets heavier, doesn’t the tire pressure increase?
Elting@piefed.social 4 weeks ago
Yeah I think in that case they had meant to say that the amount of air in the tire stays the same and the PSI increases when the tire deforms. They are right that the PSI in the tire has to match the pressure on the road, that can only happen if the PSI increases when you increase the load.
marcos@lemmy.world 4 weeks ago
No. It often decreases.
Tire pressure is the one main variable that determines pavement wear. It is highly regulated for heavy vehicles. Those have to compensate by using wider tires or more wheels.