Why do car tires not need a tube or liquid sealant but bicycle tires do?
In: Engineering
Bike tires are incredibly thin and have microscopic holes in them that let air out if not filled with a sealant.
Vehicle tires are much thicker, comprised of multiple layers of material including rubber and steel bands, and are completely sealed.
There are no holes in them, and if there are holes in one of the layers, there’s several other layers present to create a seal.
bicycle tires are thin and lite whereas car tires are thick and robust…if we had thick heavy tires are a bicycle then that would make the bike more difficult to pedal due to the extra weight….sealant ‘might’ work on a bike tire, however, the PSI is often greater, the sealant in the tire would throw off the balance of the wheel thus causing vibration….
Most expensive bikes do not use tubes or liquid, You will only find tubes in Walmart quality bikes and others equal to them.
Bicycle wheels have spokes, which make it really hard to be airtight without some sort of sealant. A tubeless car wheel, the defacto standard for years, is smooth and reasonably easy to seal against the ‘lip’ of the wheel when it is inflated.
People are missing the 2 important aspects: spokes for bicycle tires are hard to seal and if you get a flat, you don’t want to keep a spare tire on you.
Bike tyres are much thinner then car tyres. This is done to make them lighter making them easier to bike. The problem with the thin tyres is that they tend to puncture much easier. The tyres are expensive and also somewhat large and heavy to carry. So cyclists carry spare tubes instead. These are much cheaper and fold up to fit in a small tool big that you can mount under the seat or on the frame. It is also much quicker to swap an inner tube then swapping the tyre.
For road racing you do often see tubeless bike tyres as these are lighter then the tubed tyres most bicyclists use. This may make a difference in a tight race. In old photos of bike races you can see the riders carry spare tyres around their torso but this is done away with in modern cycling. You can also get tubed tyres for cars and trucks. If you drive on rough roads where punctures are common this may save you a lot of money and even allow you to carry multiple spare tubes. I have also seen innertubes be installed as a way to fix a leak in an otherwise tubeless tyre. Care should be taken though as not all tyres and rims can have a tube as they might be abrasive on the inside.
Bicycle tires aren’t rigid enough to maintain their shape under load. If you did away with the tube, you wouldn’t be able to maintain a good enough seal on the rim.
The main reason histrically (and still valid) is: wire spokes. Way back in the day before the days of single-piece wheels, the only realy practical/affordable way to make a tire was to have a separate hub (center part where axle goes) and the rim (outer part where the rubber/tire mounts, and they were connected by a bunch of wire spokes. Each spoke had a little hook on one end that would hook onto the hub, and the other end would have a little threaded portion and a tensioning nut that would secure to the rim. While these individual wires were not very strong in compression, when you installed them you would tighten them up verrrrrrry tight like a guitar string, so the hub and rim were under concentric tension, which made everything strong and stiff while still very lightweight. Also very easy to assemble. Problem though: it left a whole bunch of holes in the rim (outer part) where the tire isntalls. If you tried to inflate a tire, the air would just leak out the rim holes. So they used a tire to prvide structure, tread/traction and wear and tear protection, and use a thinner rubber tube inside THAT that seals the air and keeps it pressurized. This also has the benefit that if you have a puncture, you dont neccesarily need to repalce the tire, but jsut the tube (and a spare tube was small enough to carry in a pocket).
Cars rapidly went away from wire spoke wheels, though they remained popular on bicycles and motorcycles through the present day. There are a few companies like BMW that make tubeless wire wheels (instead of penetrating through the rim, the spokes interface with some flange on the interior of the rim). And also here are modifications you can do to a standard wire rim that will seal the spoke holes well enough that you can run without a tube. and most weird, there are even “mousse” options. Where instead of a pneumatic tube (i/e/ inflated with pressurized air) its basically like a big donut made of soft foam rubber. Still rpvides the cushioining effect that a pneumatic tire will, but is immune to puncture. They are msotly used on off-road motorcycles where punctures may be more common, and where top speeds are limited (a mousse tube will tend to heat up a lot faster and can be damaged by longer rides at higher speeds).
Your premise is wrong.
The main way to lose air is spokes, the rest of the rim is basically equivalent between cars and bicycles.
Tubeless has been available on MTB and gravel rims for about 20 years now, and solidly the default for at least 10. You need rim tape, sealant, and tyres with the right kind of hooked bead, but it’s pretty trivial to set up.
On road bikes tubeless doesn’t work nearly as well because of the much higher pressures and smaller tyres. While MTB runs in the 20-40 psi ranges depending on specific chonker on the bike, road bikes are more in the 70-120 range. MTB tyres are like 2″ thick in all directions, road bike ones about ½”.
The combo doesn’t work that well, at 100 psi air **will** find a way to leak, and due to the much lower volume, any lost air will have more of an impact. If you blew your car tyres up to 100 psi (**DON’T**, thought r/justrolledintotheshop has had those examples), and the tyres didn’t fail catastrophically, odds are they’d be down to 80 in a day due to air escaping.
Cars have engines, so they can be big and heavy, therefore built to be very durable. Bikes are powered by your legs, so every part of them must be as thin and light as possible, at the cost of durability. Thus the tires are not as puncture resistant and other helpers like sealants are needed.
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