Hey Paul,"Summer Side..." did reach #98 on the Billboard Hot 100,so I guees you could say it was a very "minor" hit.
The folks at radio(then,like now) like to play god with people's career's. Radio giveth & then radio taketh away.
If you'd like to see what it may have been like in America if GL were a "superstar"...check out my topic,"If Lightfoot Were Bigger Than Elvis".
