[The Citrus Smoothie]

Summer is just starting to arrive here in the UK, getting up to a grand 21 celcius. Party.

In my new fuel conscious frame of mind I have been checking my tyres every two weeks, making sure they are kept close to 44psi and managed to achieve 62 MPG (UK) on a round trip to Devon last week (not bad at an average motorway speed of 85mph).

On my return the next day I checked my tyre pressures and was greeted by a blast of heat coming from the front wheels and brakes. Probably due to the extreme heat, the tyres (even after 500 miles) were showing 46psi and I deflated them slightly. I have felt heat come from front wheels on other cars in the past but not like the blast I had from the Insight. I would have burnt my hand if I was stupid enough to hold my hand against the wheel. Presumably the 'solid' style of the aerodynamic lightweight wheels trap more heat than usual 'spoke' style wheels. Anybody else had this kind of experience?

 

[Will M]

First of all, don't deflate the tires because the pressure goes up when they are hot. The tire pressure specs are always for cold tires. The pressure goes up and down all the time as the tires heat up and cool down, so the specs have to be given for one arbitrary temperature, so the standard has always been for cold tires, because that's the closest thing to a stable temperature/pressure. Don't even bother checking pressure if the car has been driven recently.

Second, I doubt it's the aerodynamics of the wheel so much as the material it is made out of. The wheels are made out of a magnesium/aluminum alloy that is extremely strong and light. A side benefit is that it also makes a remarkable heat sink. I don't know anything that conducts heat better than aluminum. If you look inside any personal computer these days, you'll see lots of pieces of finned aluminum attached to chips in order to cool them down by sucking the heat away from them.

Since brakes are actually energy converters, they basically use friction to convert kenetic energy to heat. Disk brakes are better than drum brakes mostly because they disipate heat better, allowing them to brake harder, more times without experiencing "heat fade". Drum brakes will stop working if they get too hot while you go down a mountain. Disk brakes don't usually do that.

So, on the Insight, the wheels actually help conduct heat away from the brakes, making them fade less and work better, longer. Hot wheels are good. Don't worry (but don't touch).