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Goldilocks Goodyear* And The Three Tyre Pressure Bears


Getting the tyre pressure right is a bit of a Goldilocks process – it can’t be too hard or too soft, but has to be just right.  If you don’t get it right, it could result in an accident that leaves you looking like you have indeed had an encounter with three grouchy, hungry grizzly bears. Or it could turn your vehicle into a beast with one heck of an appetite for fuel. (On a complete sidetrack, wouldn’t Ursus or the word for Bear in some other language make a great name for a 4×4?)

The most common scenario is that you end up with Mama Bear’s tyres: too soft.  This is because valves aren’t perfect and slow leaks happen over time, what with little air molecules being sneaky.  Ideally, we ought to check our tyre pressure monthly but not many of us actually do this (and that’s me at the front of the queue for the confessional!).

The problem with too-soft Mama Bear tyres is that they bulge out.  This leads to two problems.  Firstly, because the walls of the tyre weren’t designed to balloon out like that, you’re increasing the chance of the tyre going boom on you.  Yes – underinflation and being too soft is what increases the chance of getting a blowout, not being too hard.

The second problem of having too-soft Mama Bear tyres that bulge out is because this increases the area of tyre contacting the road.  A moment’s thought will tell you that this has to be better for grip, right?  Well, yes.  It does increase the amount of grip between the tyre and the road surface, and that’s just the problem.  This means increased friction, and this means that your car has to work harder to get up to the speed you want to.  Remember what it was like when you were a kid and your bike tyre started getting a leak so you had to pedal that much harder when the tyre was flat?  Well, the same thing happens when your car tyres are flat (or your trailer tyres for that matter).  What this adds up to is terrible, terrible fuel economy.  If you’ve wondered why you don’t get the same fuel economy as the stats in the car ads say you should, this is one of the reasons why (the other reasons are because the vehicles are tested minus any load at all and in the lab where there’s no crosswinds or headwinds).

OK, so having the tyres too soft is a bad thing.  However, is there such a thing as being too hard?

As Goldilocks would tell us, yes, tyres can be too hard.  Papa Bear tyres might not increase your chance of a blowout the same way that Mama Bear tyres do, in spite of what the cartoons tell us. Papa Bear tyres are dangerous in another way.  Because they make the bottom of the tyre narrower and more convex, there’s less of the tyre touching the road.  This means less friction.

Less friction, of course, means less grip around corners and greatly increased braking time.  If it’s wet, then proportionally less water can be channelled out of the way, so the friction decreases even further.  Let’s stop and think about the implications of that for a moment, but not for too long.  The results certainly won’t be pretty, especially if speed is involved.  It’s a wonder that the cops don’t have random tyre pressure checks the same way they do random breath testing and random speed checks.  Oops, maybe I shouldn’t have written that – I might give them new ideas and new ways to milk our wallets.

So how do you get those nice Baby Bear tyres that aren’t too hard or too soft but just right, where you’ve got enough friction to make the car handle well but not so much that your car guzzles petrol?

The answer, of course, is to check your tyre pressure regularly.  Some say that you should even check the pressure every time you fill up with fuel, but this may be going a bit too far.  Maybe.  Most modern vehicles are very, very nice to us and have tyre pressure monitors installed and provide us with an alert when the pressure strays out of the Goldilocks Zone.

OK, so how do you know what pressure you should inflate your tyres to?  The answer to that is usually provided very kindly by the car manufacturers, either in the owner’s manual or on the door pillars (either on the driver’s or the passenger’s side).  In my Volvo  S70, the info is in the manual.  In my Nissan  Terrano, the information is on a sticker on the door pillar on the driver’s side… unfortunately in Japanese where it hasn’t totally faded away.  Curses and naught words!  Fortunately in situations like this, you can use online tools and good old Google to help you out (here’s one possibility:

Tyre pressure, like porridge, can’t be taken too hot.  However, there is no such thing as too cold when it comes to measuring tyre pressure.   This is because heat makes the rubber a bit softer and the air inside take up a wee bit more space.

When you check the tyre pressure, you need to be sure that you use the right units.  Car tyre pressure is one of the few things that we still like to think about in Imperial units rather than metric (the others are height and the birth weight of babies).  The Imperial unit is pounds per square inch (psi) but the metric equivalent is kiloPascals (kPA).  The conversion formula is 1 psi = 6.8947 kPA, so if you use the wrong unit, you’ll either be underinflated or overinflated by sixfold.

Of course, getting Baby Bear tyres isn’t as simple as that.  If you’ve got a heavier than normal load in your vehicle, this will press down on the tyres so they bulge out and get a Mama Bear tyre profile and will therefore act like a Mama Bear tyre.  This really adds up to a beast with a big appetite, as the engine doesn’t just have to cope with the extra load, it also has to cope with the extra friction if you don’t increase the tyre pressure.  And don’t forget to make like Johnny Farnham and take the pressure down once you’ve dropped off the load!  Oh yes – and make sure that your tyres aren’t too worn or getting the pressure right won’t do diddly-squat.

To make things even more interesting, if you’re into off-roading, you need to adjust the tyre pressure according to the surface you’re driving on.  In sand, for example, you need the extra friction, so Mama Bear might be able to help you out if you get stuck.

Catch you later – I’m off to check the tyre pressure in both cars.

* This is not the name of a blonde model in the Goodyear equivalent of the Pirelli calendar.

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