Why do cars and motorcycles use such low tyre pressures?

Question

Cars and motorcycles typically use tyre pressures of 30-40 PSI.

By the standards of literally almost every non-car non-motorcycle type of vehicle that uses pneumatic tyres, this is ridiculously low:

• Truck tyres are typically inflated to 80-150 PSI (depending on the truck and the tyres).
• Aircraft tyres routinely go up to 200 PSI.

Now, both of the above could be justified as needing higher tyre pressure to compensate for a heavier vehicle, but...

• ...bicycle tyres are typically inflated to the 60-100-PSI range (with the exception of specialized fat-tyre bikes for use on loose snow, which use tyre pressures in the typical car/motorcycle range)!

Higher tyre pressures minimize tread deformation and sidewall deflection under load, which is advantageous in multiple ways:

• Since the tyre isn't squashed as far out of round as it would be at a lower pressure, less energy is wasted in continuously squashing it in new directions and unsquashing it in old directions as the tyre rotates; this decreases the tyre's rolling resistance, making it more efficient.
• The reduction in deformation under load reduces the amount of mechanical fatigue damage incurred by the tyre as it continuously bends and unbends, increasing the tyre's lifespan and decreasing the chance of failure in a given timespan.
• The reduction in deformation under load also reduces the amount of heat generated as the tyre flexes, dramatically reducing thermal degradation of the tyre material, and, again, increasing the tyre's time-between-failures.¹

So why do cars and motorcycles still use low-pressure tyres?

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¹: On the flop side, driving with underinflated tyres dramatically increases heat generation and thermal damage. (This is why it's not safe to drive with a flat for any length of time - such a tyre will produce so much heat when rotated that it can be counted on to disintegrate violently in a very short period of time.)

Answer 1

The pressure is chosen to keep the largest area of tread on the road. If the tyre had too much pressure, it will bulge in the middle of the tread and cause wear in the middle. If there is too little pressure the tyre will run on the side walls more and cause wear on the edges.

The uneven wear highlights the uneven use of the tread and hence the less grip you will be getting on a flat surface. The pressure is chosen to suit the tyre and weight of the vehicle.

Answer 2

I don't know that I'd consider it "ridiculously low", but the main reason for the tire pressures to be lower than what is typically used in commercial applications is to dramatically improve ride quality.

Secondarily, if you check the cost of commercial tires, they are hugely more expensive than passenger tires (in the neighborhood of 4 to 5 times as expensive). The reason for this is the construction materials which go into it. For instance, if you take a passenger tire, they probably have ~5 belted layers which go around the outside (circumference), while commercial tires have more than that. Commercial tires also have more side wall layers as well. All of this ads up.

Thirdly, passenger tires don't need to support the loads which commercial tires do. In the States, most 18-wheelers (typical over the road cargo transport vehicle) can carry up to 80,000 lbs gross weight (truck/trailer/cargo combined). By using a higher pressure, the tires can support more load per square inch of surface contact without deformation. A passenger car weighs between 2,500 to 6,000 lbs (large pick-ups and SUVs could be a bit more - some of which use "Light Truck" tires, which are somewhere between passenger and commercial tires as far as construction goes). The tire doesn't need to support as much weight. The same thing applies to a bicycle tire. It needs a higher pressure so it can carry a higher load. Their contact patch is very limited, which means there's a greater weight which must be carried there. The higher pressure offsets this. They also are much smaller in diameter, which means they can have the higher pressure without blowing out.

There may be other reasons I'm not thinking of at the moment, however, I believe this should give a fairly good idea of why we don't use higher pressure tires on passenger cars.

Answer 3

As Paulster's answer says ride quality is a significant driver - every bit of a bump that is absorbed by the tire deforming/compressing is that much less movement transferred to the suspension (it's an extreme case but on something like an F1 car the tire makes up the majority of the suspension travel!) so tire pressures being in that window between "high enough to support the weight of the vehicle" and "low enough to be compressible under more load than the weight" helps smooth the ride.

One of the reasons that cars/motorcycles use such (relatively) low pressure is to maximise and maintain the contact patch.

Having a certain amount of deflection/flex possible in the side walls ensures that the tire can absorb small bumps and lateral loads while maintaining as much contact of the tread surface and the road surface as possible. This is safer because that gives you more grip when doing things like braking and cornering, it allows the car to transmit the torque being applied from the drive train better. Reducing the amount the tire slips across the surface also helps tire tread life.

While it might not seem like much the effect can be very noticeable - I once had a tire shop incorrectly inflate my rear tires to 38psi as opposed to the 28psi specified and it completely ruined the handling, it felt like the rears were on ice!

Of course this extra grip comes at a price - increased rolling resistance. But in most vehicles of this nature the engine and drive train are more than capable of overcoming this - it's easy enough to generate wheel spin in most passenger cars and motor bikes if required.

Contrast this with a pedal cycle - on a road bike rolling resistance is the enemy, even the most powerful human is going to struggle to exceed the grip limit of bike tires, and they're typically intended to be ridden on (relatively) smooth surfaces with little need for malleability a lower pressure (and more flexing sidewall) affords. Mountain bikes on the other hand, while they aren't traction limited run on much rougher (and or softer) terrain and hence need the bigger contact patch (and the flex to maintain it) which is why they're wider and run lower pressures.