What's this red cylinder thing below the front axle?

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Criggie: What’s the red cylindrical thing on the drive side of the front fork? Is it part of the timing system?

Carel: They used ‘simple’ speed sensors in those days, no GPS! It is the sensor with a magnet glued to one of the spokes.

Criggie: I know about those (I have basic cateyes on all my bikes) but the big red "cowbell" looking thing in the picture above is way larger than any bike computer magnet sensor. Was it a transmitter to trip sensor wires laid over the roadway for timing segments/climbs ?

Carel: My guess is an aerodynamic housing for a wired reed-contact. Electronics weren’t that sophisticated and transponders had the size of car-radios in those days.

Criggie: That’s about a hundred times too large to a reed switch. Even in 1999, electronics were smaller than that.
And its not at all aero hanging down when it could have been in-line with a fork leg.

What’s that red dangly thing below the front axle?

Cross-loaded from: Jan Ullrich's TT bike in la Vuelta 1999: why the bigger rear wheel?