Chapter 11- Bishop, and What Heat Does

Friday started at fifty one degrees in a car park in Bishop and reached a hundred and five in the Owens Valley by two in the afternoon, which is a fifty four degree swing inside seven hours and which almost everything on both vehicles noticed.

The warning that was not a warning

The van's tyre pressure warning came on at nine forty in the morning, thirty miles south of Bishop, and Bea saw it first and said so calmly, which she had got good at by day five.

Sarah pulled into a wide gravel turnout, which is available in the Owens Valley in a way it is not on Tioga Road, and read the individual pressures off the phone rather than looking at the dashboard symbol, which tells you only that something is low and not which thing.

Front left thirty two. Everything else thirty four or thirty five.

And this is where Wednesday's note earned its place. On the Wednesday morning in Mariposa she had put two pounds into every tyre because they had read low in the cold, and she had written down what she did and where and at what temperature. The front left had been the one reading lowest that morning too.

Two readings, four days apart, both showing the same wheel a pound or two below the others, is a pattern rather than a temperature effect, because temperature affects all four tyres equally.

The rule for telling those apart

This is worth stating as a general principle because it applies far beyond tyres.

A condition that affects everything equally is environmental. A condition that affects one thing while its neighbours stay put is mechanical.

All four tyres dropping two pounds on a cold morning is the weather. One tyre dropping two pounds while the other three hold is a slow leak, and the fact that both things look identical on a dashboard symbol is precisely why the individual readings matter.

They found it at a tyre place in Independence forty minutes later: a small screw in the tread, sealed enough to lose a pound or so a day, which is the most common thing in the world and which had been quietly happening since somewhere before Wednesday.

Repaired properly with a plug and patch from the inside, in thirty five minutes, for a figure Sarah described afterwards as embarrassingly small.

The part the vehicle does not know

Something worth recording for anybody who relies on a monitoring system: after the repair, the vehicle had no idea anything had happened.

A pressure sensor reports pressure. It does not know whether that pressure is being held by a tyre in perfect condition or by a plug fitted in a workshop in Independence. It cannot report a repair, a patch, a tyre's age, tread depth, or a sidewall that has been kerbed.

So the monitoring system said everything was fine before they arrived and everything was fine after they left, and it was correct both times, and it had nothing to do with the decision to have it repaired properly rather than topped up and ignored.

Heat, and the things that cope with it

By early afternoon the outside temperature was over a hundred and the van was working harder than it had all week, and none of it was about the engine.

Air conditioning is a load. The compressor takes real power from the engine, the cooling fans run more, and the whole system is fighting a hundred and five degrees on one side of a piece of glass and four people on the other.

What Sarah watched was engine load at a steady cruise, which sat noticeably higher than it had at the same speed in cooler air, and coolant temperature, which rose a few degrees and then held, because a cooling system that holds a temperature in a hundred and five degrees is a cooling system doing its job.

The bike had the opposite experience and it is worth recording. A motorcycle in a hundred and five degrees at highway speed has enormous airflow and cools beautifully. The problem is entirely the people on it, who are being convection heated by moving air that is hotter than they are, and who dehydrate without noticing because sweat evaporates instantly and never feels like sweat.

Mia filmed a thermometer reading in a fuel station forecourt at Lone Pine and it went into the video and got more comments than anything else that week. Nobody commented on the tyre repair.

What Emma noticed

From the middle row, somewhere near Big Pine: "The tyres went up when it got hot but the engine didn't."

She had been reading the numbers off the phone for two days by then and had spotted, without being told, the thing this chapter is about. A tyre is a sealed volume of gas with no thermostat and no fan, so it simply follows the temperature. An engine has a cooling system whose entire purpose is to hold a temperature regardless of what is happening outside.

Sarah told her that was a genuinely good observation, and Emma said she knew, which is fair.

What They Learned

Read the individual pressures rather than the dashboard symbol. The symbol says something is low. The numbers say which one, and that difference decides everything.

Something affecting all four wheels equally is environmental. Something affecting one while the others hold is mechanical, and dated notes are what let you tell them apart four days later.

A pressure sensor knows pressure and nothing else. It cannot report a repair, a plug, a tyre's age, tread depth, or sidewall damage.

A regulated system holds its value against the weather and an unregulated one follows it. Coolant temperature holding in extreme heat is success. Tyre pressure rising in the same heat is physics.

In extreme heat the vehicles usually cope better than the people do, and on a motorcycle at speed the rider dehydrates without ever feeling as though they are sweating.

This curated preview is provided for personal reading. Copyright remains with Nobo Astoria.