The short answer is that a bumper auto watch works by using kinetic energy to bounce the rotor from one side of the watch to the other, thus winding it as you move your wrist. A conventional automatic lets the rotor slip back and forth using gravity, but the springs inside a bumper send the rotor back on its original path. It’s a watch technology you will find inside older 1940s and 1950s watches, usually from Jaeger le Coultre, Omega or this Mido Multifort.
The bumper auto evolved naturally from the first mass produced automatic or self winding watch, the Harwood – made by John Harwood who developed his prototype in the early 1920s. Harwood had served in WW1 and saw how damaging mud and dirt was to the first wristwatches that gained popularity back then. By the way, soldiers stopped using pocket watches because the flash of light as the watch was checked for time was all that a sniper needed to fix on a target.
Harwood’s original automatic had two plungers, bit like miniature shock absorbers on a motorcycle, to act as pushers inside the case. The rotor ran on a little track and bounced between the two plaungers. Harwood staked all he had on his own watch factory in 1928 and started making his watches, but the crash of `29 and the Depression put him out of business.
THE SWISS AND OTHERS RUN WITH HARWOOD’S IDEA
That left the field open for the Swiss and the Americans, who were very big on watchmaking in the 1930s. A Schild made a kind of bumper kit movement (or ebauche) for several Swiss brands. Companies like Omega-Tissot and others were busy forming partnerships and merging in order to survive the Depression, so buying in movements was commonplace.
I have seen Jaeger le Coultre bumper autos from the 50s sell for about £1000-£1500 at auction, or almost two grand in a watch shop. A nice Omega bumper auto might sell for £800-£1200, depending on codition. The Mido isn’t so valuable, but it does much the same job and with some regular maintenance will survive for 100 years I’d guess. Look at the solidity of that rotor, the chunky simplicity of the gears. For a small movement, the parts are kinda over-engineered.
What puts off collectors is the tiny size of most bumper automatics. Watches were small in the 30s, because many men worked outdoors in all weathers, and so hiding your expensive watch beneath the sleeves of a tweed jacket was a wise move. That meant 31-33mm case sizes were typical. Those would be ladies case sizes today of course.
Harwood was reportedly inspired by the actions of a wooden see-saw in a playground. He noted how the energy from the bump-stop sent the plank back into the air. Sometimes simple ideas are all around us. Harwood also invented the impact screwdriver and the automatic watch winder – he sold his own winding box design. A brilliant engineer, born in Bolton Lancashire, he is almost forgotten now. You are more likely to see a statue of Ash Sarkar being erected in Bolton today, than see any acknowledgement of John Harwood.