Do flat tires make your speedo lie?

There are some engineering questions that may not be very important in the world, but which have become the subject of intense passive speculation. A good example is the question of whether the lower tire pressure of a motor vehicle will differ in the indicated speed. There are several contradictory theories about what should happen [mechatronicsguy] It took time for some experimentation to find out what was really happening.

The effective radius change from a flat part of the tire, the so-called tank track effect so that the entire circumference of the tire is still exceeded and the possibility of circumference change due to different pressures was at risk. The test wheels were made of foam and were seen to give a different texture when compressed. It can solve toe problems, but of course the car has radial wires to make them stiff. When these were simulated on foam wheels with packing tape, the difference evaporated. This was later confirmed by GPS-measurement of an original car with deflated wheels.

All of this makes for an interesting read, because after all, sometimes there is no substitute for real-world testing.

Header image: Gerlach, public domain.

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