Off the Beaten Path
Join Date: Aug 2007
Location: ALBANY, OR, USA.
Interest: SEA KAYAKING
If your bikes are in good maintenance condition, probably just flats and maybe a tire blowout. I had a rear pannier rack lose a bolt one time, rattled out on a gravel road. Fortunately, I had a replacement bolt and noticed the problem before I suffered any wheel damage from the support dropping down into the spokes. I ripped the sidewall out of a tire by accident another time, got to close to a curb with my bike fully loaded. Non-repairable and was 55-60km from the nearest bike shop and I didn't have a spare. The kindness of strangers helped me out there.
Try to have some contingency plans just in case. Loading up a bike with a lot of weight can put quite a bit of stress on areas of a bike that under normal circumstance would hold up just fine. Riding several days in a row for several hours at a time can place enough stress on weak areas to sometimes cause failure. The main thing is to not be surprised if you do have a breakdown. That's one of the reasons for doing the shake-down ride.
Consider a few pair of latex gloves in your repair kit in case you have to mess with your chain at all or change a flat on the rear tire. Avoids the greasy hand problem. If you are going to be doing a bunch of downhill make sure to start out with new brake pads or take replacement pads along.
Quite a bit of the risk depends on how much distance you plan to cover and the road surface so if the island roads are in good shape and the bikes are in tune, you should be good.