Shimmy is the potentially dangerous problem of uncontrolled steering oscillations in bicycles, at a frequency too high for pilot reaction. It typically occurs at high speeds and causes cyclists to be afraid of descending hills at full speed. It is possible that shimmy can be prevented by allowing a controlled side-motion of the bicycle seat. We have designed and fabricated a user-friendly shimmy preventer, based on a lockable sliding carriage, that fits just under a normal bicycle seat. The prototype is manufactured completely from machined aluminium parts, and will be ready for evaluation in late spring. If the solution prevents shimmy as expected, we will work on automatic activation in response to vibration. As bicycle travel is a key method of sustainable transportation, improving its safety and efficiency will contribute to furthering transportation sustainability.