On Lock-down Tyres

Why my car tyres are flat?

I left the house this morning and found that my car tyres had gone soft and one was completely flat. This was a rather curious state of affairs, because I had not driven anywhere to acquire a puncture, due to the covid-19 lockdown. I live in a quiet village and have good neighbours with whom I am friendly; so it seemed unlikely that anyone would have tampered with them. Nevertheless, I did inspect the tyres to make sure that there had been no foul play. There was no visible damage to the tyres and layers of grime on the valve cap indicated that it had not been loosened recently.

I like a mystery and this got me to start thinking about whether there was an engineering reason why my car tyres had become soft. The first potential culprit was the weather. The differing seasons bring with them settled and unsettled weather, which is of course linked to air pressure. It would be rational for the internal pressure in the tyres to be affected by changes in external air pressure. While this may have been a factor it did not seem to be a good explanation, because though the weather has been inclement recently it has not been unremarkable. Scotland regularly experiences inclement weather, often far worse than it is now. This begged the question that if weather was responsible for my tyres’ loss of pressure why had it not happened before?

The only thing that was really different to normal was that the car had not been used, as there was nowhere to go during lock-down. Cars normally deteriorate due to wear and tear; inactivity would therefore seem like and odd, and somewhat ironic, causation. That got me to thinking about what the mechanism for such an outcome could be. I soon found myself reverting to type thinking about materials and load paths.

The weight of my car is split between four wheels, but not evenly. With an empty boot [trunk for American readers] the car engine is responsible for there being more load on the front axle than the rear. This was consistent with the front tyres being flatter than those at the rear, so perhaps I was on to something.

Weight is transferred from the rotor to the wheel hub by four bolts and then from the hub to the tyre by air pressure. When I was growing up tyres had a pressurised inner tube, but today there would seem to be reliance on the joint between the hub and tyre being sealed tight by pressure.

The hub must be exerting a downward force onto the pressurised air at the base of the tyre, which is resisted by an equal and opposite upward reaction from my driveway via the tyre. The air inside the tyre does not like being squeezed and will try to escape out of the way causing the sides and top of the tyre to experience an outward thrust that would cause it to be stretched. Thus, the bottom of the tyre would be experiencing compression while the top and sides experience tension. 

With inactivity seemingly the critical factor I concluded that my tyres did not like this state of affairs for an extended period. I reasoned that this might be because they are intended to be spinning, such that each part of their circumference takes its turn at bearing compressive and tensile load.

Conversely if my car remained stationary then the load experienced would become quasi-permanent rather than temporal. This opened up several possibilities. Perhaps constant tension resulted in the structure of the rubber becoming elongated in such a way that it was more permeable to air, or perhaps constant compression made the rubber at the base less flexible and therefore more permeable. 

Alternatively, perhaps the seal between the tyre wall and the hub starts to slip with the constant application of load. It would seem reasonable to postulate that such an effect would become more pronounced as pressure is lost from the tyre, because the seal is reliant on their being a positive pressure.

I am not sure if any of these potential explanations are correct or if they all play a role, however irrespective of this I shall in future be making sure that my car wheels turn regularly; even during covid lock-down. 

I shall view my theory as proven if my tyre problem does not return....we shall see.