This is bugging the hell out of me!
I can induce front wheel wobble/oscillation at any speed so far (still experimenting) between 12.5mph and 21mph by simple taking my hands off the bars and almost sitting upright. The further forward I lean, the less the effect, also the further back/upright, the wobble diminishes. The oscillation never gets uncontrollable or scary like a tank slapper at very high speed - I can ride and watch or feel the bars wobble without losing absolute control. It can also be produced by taking only one hand off the bars - which is how i discovered it - hand signalling - I don't make a habit of riding no hands!
It appears to start sooner with more of a load in the saddlebag or when using rear panniers. I went on a jaunt last week with heavily loaded (small Super Cs) on the rear and a lightly loaded barbag. The "shimmy" was way more pronounced, even at lower speeds and sometimes with both hands on the bars. It was still just about controllable.
At speeds in excess of 20mph it's rock solid, loaded or not - which took a lot of confidence building on my part, having had a tank slapper on another frame and could only see death, fortunately avoided with buttocks and knees automatically clenching the saddle nose and top tube respectively!
The frame is a Longstaff tourer which I bought new from George in 1993 - heavily ridden and toured with solid as a rock predictable steering for many years. I had it refurbished in 2007 (by Longstaff's), when I requested the 1" steerer tube to be replaced to make it 1" Ahead compatible. It looked so naff (traditional lugged frame with UGLY chunky Ahead stem = FAIL on my part ) I reverted to the more elegant quill stem and cut the steerer tube down and threaded it. A new Ultegra threaded headset was fitted.
This is when I suspected/imagined? a different feel to the bike? I kept checking the adjustment of the headset - I still keep checking and have had it checked - it is properly adjusted.
The front wheel has been changed to a Shimano 3N71 dyno hub model. I've checked the frame over for cracks. I've checked the alignment of frame front to back.
Suggestions? PLEASE??
Further thought - what could be the outcome of not threading the steerer "square"?
Now then Slope
Sorry about the delay in replying I have been stupidly busy the last couple of weeks combined with a nasty attack of Dutch Lurgie courtesy of one of my framebuilding course pupils.
Over the past 37 years that I have been building frames I have come across this problem fairly regularly. If the bike rode OK before then clearly something has changed.
It sounds as if the trail has altered. This could happen if the new column was put in at a slightly different angle to the original thus altering the offset. First check that the fork blades are in line with the steering axis. Do you know what the original fork offset was.
Is the dynohub wheel the original? if not have you tried it with the original, or near offer, wheel
The effect of theading the column off square would be a bit of a tight spot somewere as it turns.
The weight distibution thing is consistent with a lot of examples of this problem that I have seen most of which have been cured by altering the fork offset however there are too many variables to give a firm reply. Sorry to be a bit vague but I normally solve this sort of thing "hands on" rather than by electronic means.
My money would be on the fork offset.
Cheers
Dave Yates