As far as I understand, the difference is in the strings' core. The Hi-Beams have a round core while the Lo-Riders' core is hexagonal. The guys at DR say that the round core results in a more flexible and brighter string, but they require an extra care: If you're going to cut a Hi-Beam string before inserting it in the tuning post, you must make a 45 degree bend before cutting since there are anchor spots in the strings that may be compromised if you don't do that.
Here's a picture of an hex core string:
The core doesn't follow the wraps' shape, while in the round core string there's the possibility of the wraps to unravel if you don't bend the string before cutting it.
Anyway, I bought once a set of Hi-Beams through juststrings.com and must say that those are the best strings I've tried.
Hope this helps.
the difference DR hibeam and DR lo-rider? 
