Shaking on the downhills

[Lowly Swale]

I had such a problem with a BOB trailer on a suspension mountain bike, Marin/Whyte quad link back end. No problem with light loads but really un-nerving. The shimmy was so severe as to steer me across the road, luckily no oncoming traffic.
It occurred at about 20mph.
Dilemma, I used Bob trailer to allow a more comfortable full suspension bike.
Changing back to rigid cures the problem but then I could just have used panniers.
Lightening the load helps but then there might be times when I want to carry heavy stuff like water on dry sections of GDMBR.
Any advice/ observations?

[humunuku]

I had a shimmy caused by a jacket that I had strapped to my rack...when I got going fast, the slight flapping of the jacket then transferred to the bike...i now put everything that could blow around in panniers

[knolltop]

Guy's journal on CGOAB mentions this problem at length.  Semi permanent first aid treatment was to stretch bungee cord between head tube and seat post.  This significantly reduced shimmy.
Course would be MUCH better to solve problem permanently before ya leave.
Also, ya may find better gear weight distriution to be 60% front & 40 & rear.

[Tandem4Rider]

I'd second the suggestion to check the rack screws to ensure they are tight enough.  We had the kids in a trailer and hit a down hill segment that made the tandem sway in a most disconcerting manner.  Fearing something going seriously wrong my inspection revealed a slight flaw in the hitch attachment.  It was hard to believe such a small flaw could cause such a violent sway, but after easily replacing the offending part all was good.