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So I did my 'bike instuctors course last weekend. Spent a bit of time reading the Mountain Leader book by Eric Langmuir.
One thing he mentioned in the navigation section was how compasses get affected by large steel objects, which made me think about our collection of assorted steel frames. For a giggle I waved the compass around near the nearest available Cotic.
Cor, blimey, but it makes a hell of a difference. Even worse if I stick it in the bike cupboard which has 4 steel frames in close proximity - it was going round like a yoyo. Interestingly my Dedacciai framed steel Condor didn't affect it in the slightest.
Anyway - you have been warned. Don't expect to get a sensible compass reading if you're less than a couple of feet away from a steel bike...
Ships have little magnets arround the base to calibrate the compass.
But I wouldn't have thought that a bike frame was making that much of a difference? Most leisure boats manage fine with the compass installed a couple of feet form the engine which weights a darn sight more than a 5lb hardtail.
Are you sure it was definately the frame, not a wire, lightbulb or something else nearby?
Also be aware that in some locations the rock has magnetic properties. Can put a compass out by a fair enough chunk to send you in the wrong direction.....
Definitely, it's very noticeable. If you have a steel frame and want to take a bearing, get off and walk 10m or so away from the bike. Even in an alu frame, there can be enough steel bit (bolts, chain, etc.) to make a noticeable difference.
The Langmuir book (or maybe the new Mountain Leader one - Hill Walking by Steve Long) also warns about those wee slidy counter things that all the DofE kids put on their compass lanyards. The steel springs in them can be enough to make the compass innaccurate. I had a play with this while I was going through the ML & IML schemes and it did seem to have a small effect. I put a knot half-way down the lanyard to keep them away for this reason.
Are you sure it was definately the frame
Well with my SOs Roadrat, which ever way the bike was pointing, the needle aligned itself to the top tube....
I work a lot on rooftops looking at telecoms sites...often you have to walk to the edge/centre of the roof to get a decent reading...the smallest bit of steel can affect a compass....