For first time tandem riders it is often most difficult to fathom the extreme vibrations transmitted to the stoker via their seatpost. On a single seat bike the frame geometry allows for a balance between front and back end vibrations. Tandems on the other hand, with the long wheelbase, focus a majority of the vibrational and big hit forces to the rear rider.
As can be viewed in this photo, from the team TandemBicycling.com full suspension mountain bike tandem, even long travel suspension frames are susceptible to serious forces transmitted to the stoker.
To combat the inevitable fatigue and pains the stoker will feel most tandems will be spec’d with a suspension seatpost. The suspension seatpost, once adjusted properly, will help absorb the big hits and small vibrations resulting in a more smooth and comfortable ride for the stoker.
It is also important to work on communication between captain and stoker. In the case of the photo above, the captain failed to warn the stoker of a major bump on a fire road decent. The suspension performed its duty in softening the blow but the torsional force of the stoker rising and sinking was too much for this post to handle. The team would have been better off if the captain provided the warning and the stoker then transitioned her wait to the pedals and handlebar.
On road tandems clear signs of extreme vibrational problems can be found in saddle soreness and early muscle fatigue for the stoker. Simply replacing a standard seatpost with a short travel suspension post will alleviate the vibration induced pains.