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Adaptive suspensions with adjustment rates on the order of milliseconds are going mainstream in consumer vehicles. Air suspensions have been common on commercial trucks and some SUVs for some time.

Why are pneumatic tires necessary or helpful on vehicles equipped with such suspensions?

I understand why a wearable tire is necessary: the friction coating on the wheels does need to be replaced. Recent years have seen a proliferation of very low-profile tires for road vehicles, but to my knowledge they are still always pneumatic. Why? The downsides to inflatable tires are that they perform poorly when improperly inflated and are subject to failure, including catastrophic decompression. What are their upsides? For example, do they provide damping responses that can't be replicated with an air or adaptive suspension?

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  • $\begingroup$ DaveTweeds link to 'other tire designs' is broken but searching for 'airless tyres' (or tires' in US) brings up some interesting alternatives. These are most commonly seen on construction plant but might soon be viable for other road vehicles. $\endgroup$
    – rdt2
    Commented Sep 3, 2015 at 14:03

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A pneumatic tire provides the mechanical "decoupling" of the tiniest variations in a road surface (the highest freqeuncies), involving a small "unsprung mass" (the rubber of the tread) and a spring (the air pressure) working against the "sprung mass" of the wheel and the axle.

The vehicle's suspension (springs, shock absorbers, etc.), working between the axle and the frame of the vehicle, decouple the larger, slower (lower frequency) variations in the road surface. In this case, the entire tire + wheel + axle assembly is considered the "unsprung mass".

While there are other tire designs that have similar qualities, they are generally heavier and much more complex to manufacture. At least for now, the pneumatic tire remains the most cost-effective way to get the desired overall performance. (Not to mention the fact that the entire vehicle service infrastructure is currently set up to deal with pneumatic tires.)

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  • $\begingroup$ It sounds like you're saying that it's more demanding to decouple the highest-frequency impacts at the suspension rather than where the rubber meets the road? Is it beyond the capacity of current active or air suspensions? Or is the problem that the unsprung components can't readily absorb those vibrations? Or are vehicles with adaptive suspensions still shod in pneumatic tires just because of the supply and service infrastructure? $\endgroup$
    – feetwet
    Commented Jan 27, 2015 at 22:41
  • $\begingroup$ All of the above. Ultimately, it's an economic (business) decision. Only when an "active" suspension can deliver the same performance and reliability as the pneumatic tire at a lower cost will we see a shift in mass-market vehicles. $\endgroup$
    – Dave Tweed
    Commented Jan 27, 2015 at 23:52
  • $\begingroup$ My observation is that the "reliability" component of the pneumatic tire seems pretty bad in comparison to having a solid tire. But even on supercars where money is no object, and where catastrophic failure seems most problematic, we're still seeing pneumatics. I wish I could see the engineering analysis that results in that decision. $\endgroup$
    – feetwet
    Commented Jan 28, 2015 at 0:02
  • $\begingroup$ So tires damp high frequency road variations - imagine then if you asked the suspension to try to account for frequency content one order of magnitude higher. This means that all of the suspension parts are moving that much faster. Assuming that suspension life is roughly proportional to number of cycles, this means that it would cut suspension life by a factor of 10 - from about 50k miles to about 5k miles. Imagine the cost of replacing your suspension every 5k miles instead of replacing your tires every 30k miles. $\endgroup$
    – Chuck
    Commented Sep 3, 2015 at 14:32
  • $\begingroup$ Active suspension might last a little longer, but again, assuming life is proportional to cycles, an active suspension system that might last 100k miles would be cut to 10k miles if you cycle the suspension at the higher frequency. Again, replacing the suspension system 3-6x more frequently than you currently replace tires is a big cost. $\endgroup$
    – Chuck
    Commented Sep 3, 2015 at 14:36
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While I don't see them going obsolete any time soon, you make some great points that really made me think. I thought of some upsides to pneumatic tyres that overcome some edge-cases, but actually they all boil down to the same thing:

Any protrusion will cause a focus point for the weight which is usually spread over the entire contact surface of the tyre.

A pneumatic tyre will distort around such protrusions up to a point. If the entire weight usually supported by the tyre were focused onto a small area, this could cause damage to the road surface or the tyre. Here are some examples:

  • Small rock in the road: The rock may safely crush; or it may ping out at a high speed; or it could embed itself in the tyre causing damage to the tyre and road surface at multiple points further down the road; or it could crack the road surface and embed itself where water could get in and freeze causing more damage.
  • Exposed metalwork: The suspension may ensure that the dynamic impact is not too severe, but the weight focused on a single sharp corner could significantly cut or dent the hard tyre.
  • Driving up a curb: Repeated driving up a curb would wear the corner of the curb away much more quickly than a pneumatic tyre, which would distort and start to take weight on the high part of the curb before the weight was fully removed from the lower side.
  • Accidents: A human leg (for example) being literally run over by a pneumatic tyre, while not pleasant at all, would suffer much less damage than a hard tyre. The hard tyre would probably crush/shatter bones into very small pieces and completely destroy muscle and blood vessels by flattening them into the bone fragments.

There is one other upside that I can think of which doesn't fit into the above category:

  • Pneumatic tyres are adjustable: You can lower the air pressure to get more contact surface (and therefore grip) in slippery conditions and raise the pressure to get lower friction if grip is not an issue.
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  • $\begingroup$ re the last point: Pneumatic tyres with lower pressure are less damaging to the ground, relevant for farming applications. $\endgroup$
    – mart
    Commented Apr 14, 2015 at 11:51
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Part of the answer is that the dynamics of pneumatic tires are well understood. In basic suspension geometry design, your caster and kingpin angles contribute to the creation of pneumatic trail, which provides a return to center force when cornering.

The same deformation of the tire contact patch when presented with aggressive steering input, or acceleration or braking, allows the tire to grip more than if it was solid, and increase the response time of the whole vehicle to a particular input. There is a sweet spot there, as a flat tire gives no response, but a rigid tire will break traction too easily.

This is not to say that these things couldn't be overcome by an active suspension, but the suspension would need to actively deform in directions that are not currently considered. State of the art computer controlled suspensions very quickly change the response profile of the shock absorber, but they don't actively adjust the steering or camber angles of the wheel.

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jhabbott lists some terrific reasons for continuing to use pneumatic tires, here are some additional reasons:

  1. Pneumatic tires are cheap to manufacture
  2. Pneumatic tires get the job done with little maintenance.
  3. Pneumatic tires' failures are easily repaired in the field.
  4. Similar tires can be used across a wide variety of vehicles and even vehicle types.

In contrast, when active suspensions fail they are expensive, not field repairable, and require specialized parts.

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To add to other answers, pneumatic tire absorbs most of the minor impacts smoothing the load variations on studs, control arm and bearings. Once fitted with hard tire, you will have to redesign the suspension to cope with much more instantaneous stress from small bumps.

Also, when traveling over gravel, pneumatic tires have the most contact patch due to absorption of high spots(gravel).

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