Well they didn't call us "Baby Boomers" fer nuthin', and lower birthrates since then mean a smaller pool of potential riders.
A pretty good article here states that overall, motorcycle sales have been flat for about ten years....
https://www.bloomberg.com/news/articles/2017-07-05/can-millennials-save-the-motorcycle-industry...if Harley is only feeling the effects in recent years then they've been doing really well.
Hard for me to believe the Millennials are totally wussed out, heck seems like in that climate any guy going old-school macho would be deluged with more tail than they could imagine, access to women being a priority among young men after all (a major reason why I got into motorcycles in a big way, back then I couldn't afford transportation AND a social life, so had to combine the two).
I will say that it seems a bit off the mark to compare the old two-stroke "rrrring-dinnng" death machines to modern sport bikes. I don't recall that those twitchy triples with their narrow on-off powerbands, crappy frames, weak suspensions and inadequate brakes were copies of actual race bikes. Sport bikes generally are, at least incorporating racetrack-based technology, and have been simply excellent machines for decades now.
Birdwatcher
