It is the sugar/protein ratio. Turns out chocolate milk has the right ratio.

For 90% of our longer rides, it is chai. Made from scratch chai in the winter and a packaged concetrate (Rishi) in summer. Always with soy. It seems to work well as we notice when we don't have access to protein after the ride.

Of course, if we're on the road we've also been known to stop for a smoothie or a chocolate malt at a custard stand. They aren't too rich and bad for use--they're recovery drinks.