Well, Dan, there are old riders and there are bold riders, but not so many old, bold riders...
"It depends", surely -- on three clusters of variables: bike, rider and load, and terrain (the latter understood broadly to include not only road, surface, slopes and corners, but also wind, rain, june bugs, other traffic, pedestrians, etc.) (Much like the old dilemma of judging musical quality: singer-song-or-audience?) (Too many sidebars-itching-to-become-tangents here; sorry about that.)
So, for me, "fast riding" = "approaching the limits". But the notion of "limits" surely includes the bike's capabilities + terrain + how-much-do-I-want-to-terrify-myself-today, etc., etc. So I'm reluctant to put a (kph/mph) number on it. Out-riding your brakes or headlight is surely "too fast". 60 kph down a 14% grade is "plenty fast" -- but stupidly fast if there are unseen sharp turns (esp at the bottom), unyielding walls or barriers, sand-gravel-wet leaves, and roadside forest where lurk the twitchy deer; or, if you're in Africa, where always there lurks a clutch of goats in the high grass, goats which cross the road only in single file & only at the last possible instant, and which always include a youngster who didn't get the memo, looked up, saw Mum mid-way across the road, and panicking, dashed into the empty spot you'd just chosen so as to avoid ploughing into the line. So, "fast riding" = "approaching the limits" = "consciously diminishing one's margin of error".
I guess one could find out about shimmy, etc., if / if one could control the variables of "terrain" enough to be able to assess shimmy at speed, under load, etc. -- without finding out all-of-a-sudden that (dang!!) I've just crossed the limits of my bike or myself--funny they were there & not a bit higher as I'd assumed/hoped...
J.
