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The principal steering oscillation instabilities of motorcycles are now well documented: low-speed ‘flutter’ and high-speed ‘weave’. This paper describes a third wobble instability which occurs on periodically undulating road surfaces. Although its incidence is much less common - indeed it has only been reported recently - it is potentially more dangerous than either flutter or weave, being highly resonant, and with fierce growth rates. A physical mechanism is set out and tested experimentally, on both test track and in laboratory simulation. A long corrugations test track is used; in the laboratory both vertical and lateral responses are recorded. This then leads to structural modifications to the machine, which are evaluated and produce a configuration with the instability eliminated. One important aspect of the changes is a small increase in front wheel diameter. Reservation is expressed about current trends in reducing wheel size.