Why do we need air-filled tires?

Question

Why do we need air in bike tires?

Having changed hundreds of annoying flats, carried a change kit for thousands of miles, and pumped up tires before pretty much every ride, I got to wondering: why do we need to fill tires with air anyway?

Why can't we just have a thin strip of solid rubber meet the ground?

I've been told that air-filled tires give a smoother ride, but I think that the same ride can be achieved with springs and shocks between the rider, frame, and wheels.

Is this a conspiracy of bike and wheel manufacturers, to maintain a steady demand for tubes and tires?

Answer 1

Is this a conspiracy of bike and wheel manufacturers, to maintain a steady demand for tubes and tires?

No, because if it worked as well as you suspect, someone would have made it happen and taken all the money tube and tire companies are getting.

The reason pneumatic tires continue to be used is because air itself is a great spring. Additionally, being a fluid, it has the benefit that all of the air in a tire can work as a spring all the time. The rubber in the tire casing also works great as a shock absorber, and just like the air, the entire tire flexes at once, not just the contact patch, no matter which way the tire is oriented. These two effects mutualistically complement each other, and make the tire what it is.

If you had a solid strip of rubber, only the rubber in the immediate area can do the job, so you'll either have a harsher ride, or a heavier tire. The same problem exists with other air-free alternatives as well.

Now sure, you could make a full-suspension bike with a suspension tuned to make a smooth ride despite solid rubber wheels. But you'll quickly find out that such a suspension, both the changes to the frame, and the components, will be considerably heavier than a regular tire. And bikes are an application that is highly weight sensitive.

Other applications, such as Tanks, have a different set of constraints and trade-offs, and so there tracks are preferred, and they have some pretty sophisticated suspension systems allowing them to drive over bumps and stuff at high speeds, because when hauling around 50 tons of armor, another extra ton for a good suspension is worth it.

Sometimes, things stay the same way forever and ever because they just work.

Answer 2

There are two good answers here already, I'll add the relevant physics.

Any suspension has three key variables:

• Its spring constant, i.e. how quickly the force rises as the suspension is compressed.

• Its dampening, i.e. how much energy is lost in every compression/decompression cycle.

• Its unsuspended mass, i.e. the amount of material that needs to move around due to the uneveness of the road.

Bicycles are a type of vehicle that requires very low energy losses (simply because typical energy input is limited to about 250 Watts). As such, bicycles absolutely need to keep the dampening low, because all the energy that you dampen away needs to come from your kinetic energy (conservation of energy!), which the rider worked hard for to produce. Now, the more unsuspended mass you have, the more dampening is required to keep continuous contact to the road. You don't want your wheels to bump up and down after riding through a pothole, that would be extremely dangerous. That's the reason why any suspension fork or car suspension includes shock absorbers: Their whole purpose is to drain energy, but they are required for safety.

Pneumatic tires have the great advantage, that only the rubber within the contact patch to the road is unsuspended mass. The entire rest of the tire, the rim, the spokes, the hub, the fork, they are all suspended, and thus do not require dampening. The tire itself also has enough dampening within its rubber to ensure a good contact. As such, you simply cannot find a suspension system that's more efficient than a pneumatic tire. In addition, pneumatic tires also allow easy adjustment of the spring constant by pumping them to different pressures, but that's just a bonus.

Now, alternatives have been tried and are even used by some. Most importantly by the use of airless tires. These replace the tube with a core of rubber foam. Unfortunately, the added rubber already increases the dampening significantly, and thus reduces the tires efficiency. Nevertheless, it's the next best thing to do after the pneumatic tire. All other alternatives that would move the suspension further away from the contact patch, and thereby massively increase the unsuspended mass, would lose too much energy to dampening, and leave the cyclist thoroughly frustrated.

Answer 3

We don't, and alternatives exist

What we need is some level of compliance in the tyre. Rolling resistance at this level is governed by the tyre's ability to roll over small surface deviations. Air (in fact any gas) is highly compressible and gives a great way of soaking up these small changes in surface height, allowing the tyre to roll well.

They aren't the only game in town though. Solid tyres with some kind of internal foam rubber structure have been around for a while - I used them for a little while in the early 90s, but there were versions around in the 70s. Cycling Weekly has this review of a couple of more modern versions.

More exotically, a new company has developed metal spring tyres made from "memory metal". They claim they'll be hitting the market next year; whether it happens or not, we'll see.

But they have never worked very well

The issue with solid tyres historically has always been that whatever the internal structure of the tyre is, they simply aren't as smoothly compliant as a balloon full of high-pressure air. Rolling resistance has always been worse, so that even if someone today does come up with a high quality solid tyre, they have 50 years of bad press to compete with.

On the plus side of course, they don't get punctures. So if you're cycling in winter when the roads are likely to be full of bits of debris and you really don't want to be stuck fixing a tyre in the pissing rain, you might trade off rolling resistance against reliability. That is still a trade-off you'd have to choose though.

And there are options if you really can't use air

Historically, penny-farthing bicycles used solid tyres and relied on spokes to provide all their compliance. They actually worked very well, to the extent that "safety bicycles" (the standard triangular frame we know today) were called "boneshakers" because the smaller wheels had shorter spokes which did not give the same quality of suspension. Pneumatic tyres (or some solid equivalent) were required before our modern bicycles became a practical method of transport at reasonable speed.

Amusingingly, this directly feeds into the design of wheels in space. Wheels on the Lunar Rovers were a spring structure, similar to the memory-metal bike tyres mentioned earlier. The Mars Rovers have taken the penny-farthing approach instead, relying on springy spokes.

Answer 4

A wheel is essentially round, a road is essentially flat. For reasonable contact with the road that does not damage either wheel or road, the tire needs to be deformed at the contact point to make a contact patch rather than a point. As the wheel rolls on, the deformation needs to move on the wheel. If the deformation energy is expended locally, you need perfectly elastic material.

A constant pressure tire, however, effortlessly transfers the deformation energy round the wheel without loss. The local deformation travelling around the wheel turns into a semi-constant deformation at the contact patch with the tire itself not exchanging significant energy while following the deformation.

Railroads do it differently, but are rather limited in acceleration and inclination and require trickery for curves that results in the typical screeching sounds. They also require working with much smaller tolerances than the usual wheel/road coupling.

Answer 5

This is one of those ideas that comes around every couple of years, over and over.

And it is quite plausibly a good idea in certain circumstances, like your martian rover (AA doesn't service there), or in a war-zone (where a flat would be bad for your health), or a wheelchair (where you're literally stranded)

However every time, the idea never takes off because pneumatic tyres just work better.

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The best way for you to make your own judgement is to ride some. Note that some bike shops refuse to fit airless tyres, so you might have to fit them yourself.

If you can't find any to buy, its possible to make your own solid tyres. Watch this for an example:

You will need a mold, the correct rubber, and a pressure pot big enough for the mold.

Another solution is to get the tyre used on a penny farthing or a light wagon tyre. They come in a roll and you secure a loop with twine/string.

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Another way to trial this is to inflate your tube with water. If you got your tube filled 1/6 with water it would essentially be solid, for the purposes of slow speed riding. Slow speed and consistent velocity allows the water time to flow down, while at high speed the water would line the outer edge of the tube exactly like a sealant in tubeless.

To do this, you use a track pump and stand it in a tub of water, it will inhale and pump water down the hose into the tube. It is immensely harder work than pumping air, and your pump will need a disassembly, clean, and re-grease after ward.

The air also in the tube should be inflated to whatever "normal" pressure is for your wheel. The water will not compress, and will sit down the bottom to provide the solid feeling.

Naturally this wheel will be just as vulnerable to punctures as a normal pneumatic tyre, the whole point of this is to experience a "solid tyre" feel without the cost.

Answer 6

I had a Specialized ALibi with Nimbus solid rubber core tires... It was the best set of tires i have ridden on... Zero worry about street glass/ nails/ debris near the curb causing a deflation... Great power transfer to the surface... the rigid tires made pedalling much nicer... over all i think solid core tires are great... bring back Nimbus tires...

AAAAHHH .... but they won't ... They learned thier lesson with Nimbus... They had no repeat customers... no need to replace the tires so the profits from solid core tires is not as good as tires that need replacing on a regular basis... Nimbus was a lifetime tire , and the company couldnt afford that.. sucks but that is our world....