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Tire pressure monitoring systems put to the test: what do the mandatory systems bring?

Dino Eisele
Tire pressure monitoring systems put to the test
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D he opinions about the new TPMS obligation are divided. Sure, with directly measuring systems with radio sensors in the wheels, the additional costs of changing to winter tires hit the office hard. The equivalent, however, is convenience and greater safety: the annoying measurement, combined with dirty fingers, does not have to be the same every time you refuel.

Indirect measuring systems, such as those in many Volkswagen models, can do without additional sensors -Group are used. They measure the tire pressure indirectly via ABS sensors over the rolling circumference of the wheel. If the wheel rotates faster than the others, a smaller rolling circumference and thus a loss of pressure is assumed.

Are both systems really equivalent? Representing directly measuring systems, a Mercedes C-Class takes the test. Because your sensors actually measure air pressure and temperature in the tire, the tire dimension is not decisive here. The situation is different with indirect measuring systems: the stiff sidewalls of low-profile tires deflect less when there is a loss of pressure than the sidewalls of standard tires. This could result in differences in the system performance. With the VW Golf, we therefore check two tire sizes in 16 and 19 inches.

Tire pressure monitoring systems reliably display pressure loss

All systems reliably display the pressure loss on the urban and rural route chosen by us . The indirect system of the Golf does not show any nakedness, not even with the difficult to grasp low-profile tires. But while the Mercedes is able to reliably display pressure loss even when driving slowly in the city, frequent changes of direction and low driving speeds make it difficult for the Golf's indirect measuring system to detect low air pressure; in this situation, the message is clearly delayed.

At what pressure is a warning? The Mercedes system reports a pressure drop of 0.6 bar. The VW's indirect system warns a little earlier. Could the driver have felt the loss of pressure too? Very experienced and always attentive pilots sure, but in view of the decoupled steering and indirect chassis connection, this is not always easy on a flat track.

Hardly any pressure loss noticeable

We want to know: How is pressure loss noticeable - front or rear - while driving, and whenis it getting dangerous? First the braking: Here it does not matter whether you brake with front-wheel or rear-wheel drive vehicles, but not with what tire size. In addition to the standard size 205/55 R 16, the wide and above all flat size 235/35 R 19 is used here for comparison. Braking is carried out from 80 km /h to a standstill.

In a series of measurements, the pressure is lowered to 0.5 bar at the front and then at the rear. What is happening? First of all: the driver does not notice any pressure loss in the hindquarters, and the braking distances hardly change either. The driver does not notice for a long time if there is a lack of air at the front. The protocol only notes steering influences at 1.0 bar. The braking distances are surprisingly shorter at first! The reason: In addition to the tread, when braking, when the tire is compressed, its sidewall also comes into contact with the asphalt, thus widening the contact area enormously. But below 1 bar these positive effects are over, the tread buckles and the braking distances become dangerously long. The 235 tire paints a similar picture. It brakes at a slightly higher level, but buckles more quickly when the pressure drops. The good news: Even with only 0.5 bar, the car can be brought to a standstill safely and without rim breakdown.

Air pressures below 1 bar is dangerous

It looks different when cornering: It is clear that too little pressure on the front axle causes massive understeer. On the other hand, if there is a lack of air at the rear wheel on the outside of the bend, there is a spontaneous risk of skidding. Rear-wheel drive vehicles such as the Mercedes C 180 are particularly sensitive to this. In extreme cases, such as when quickly changing lanes on the motorway, even the sensitive and early-acting ESP of the Mercedes cannot save the situation. It gets really dangerous, and this is confirmed by our test, only at air pressures of around or below 1 bar. A pressure at which electronic tire pressure monitoring systems have long since sounded the alarm, but which the driver cannot always notice, at least when driving straight ahead.

At 1 bar pressure, the tire is almost flat, that should be visible ? In the case of standard tires with a high sidewall, reduced pressure is easy to recognize. If sporty low-profile tires are fitted, this will also be difficult. Safety is only achieved by regular air pressure measurement - or a comfortable TPMS system.

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