These Beru tyre pressure sensors are sealed units. When the battery is flat, you replace the whole thing.
These Beru tyre pressure sensors are sealed units. When the battery is flat, you replace the whole thing.

The European Parliament, in its usual high-handed fashion, decreed in 2009 that all new passenger cars must be equipped with tyre pressure monitoring systems from November 2012, and new vehicles from November 2014.

This is not only aimed at improving road safety (underinflated tyres are often regarded as at least partly responsible for a high proportion of accidents), but also at protecting the planet, because the increased rolling resistance of too-soft tyres uses more fuel.

About 15 percent of the new cars now on the market have tyre pressure monitoring systems but few drivers know how they work.

There are actually two different types - direct and indirect - each with it own strengths and weaknesses.


Direct systems have pressure sensors attached to the rims inside the tyres; they measure the pressure (and often the temperature as well) and send the information by radio to a receiver in the car. One of the most obvious benefits is that it is possible to have an individual pressure readout for each tyre.

The downside is that they need batteries, which inevitably need replacing after a while. With a little luck, that coincides with tyres changes; otherwise the tyres have to come off, and be re-balanced when they go back on.

Not only that, most sensors are sealed units, and have to replaced when their batteries go flat. At well over €50 (R515) apiece that becomes a costly inconvenience.

There has to be another way.

Antilock braking systems - and there are very few new cars today that don't have them - work by measuring the difference in the rotational speeds of the car's four wheels. If one wheel is turning significantly slower than the other three, it's safe to assume that it is sliding and thus release the brakes on that wheel to get it turning again.

Simple, no?

It wasn't long after antilock braking was introduced that people began using comparative wheel speed to measure other parameters, such as over or understeer.

Now it gets interesting:

As a tyre goes flat, the axle drops closer to the ground, effectively reducing the rolling diameter of the wheel - which then rotates faster in order to keep up with the other three.

It's the exact opposite of a wheel locking up under braking, although (barring a catastrophic blowout) it usually happens much more slowly, and any anti-lock braking system can measure it. The rest is merely a matter of writing suitable computer algorithms and installing a warning light (or four!) on the dashboard.

It's much cheaper than the direct method because it doesn't require any hardware at all.

There are drawbacks, however. It can't tell you the pressure in an individual tyre, and it needs to be reset (usually by pushing a button on the dashboard) every time you pump the tyres, change a tyre or rotate the wheels.

There is no such thing as a tyre that doesn't leak.

And it won't warn you if all four tyres gradually lose pressure at the same rate, but it will work - once you've used the reset button to give it a baseline setting - even if the four tyres (or rims) on your car are not identical.

From this it seems obvious that direct tyre pressure monitoring will remain the purview of so-called premium models, the indirect systems will become the norm, particularly after antilock braking becomes a global requirement.

Maybe in this case the Eurocrats weren't so high-handed after all?