Fenix has several other comparable lights that use a variety of cells. I chose the TK41 because I own 30 or more cells already and the TK41 is one of the few lights that provides many hours of run time in medium setting. >300 lumen
Clearly for anyone who cares - the best power-format for a strap-on flashlight for bicycling is a 2x18650 tube. Of course I can safely ride with my LD20 as well 2xAA.
I still think 200 lumen with "throw" is safe up to 20mph in most scenarios. But everyone is different.
I road tested the TK41 last night and am pretty disappointed. I had always understood how broad the MS 900 beam is, along with the great "flood" the hotspot is nearly non-existent. I had hoped the TK41 would make a great all purpose light, while running on medium. However, the hotspot never diffuses very much and the surrounding beam is weaker than the MS. This results in needing to use "high power" on the TK41 to get enough "flood light" for good road lighting. (and a wasted hot spot)
In other words, in my opinion the MS has too wide of a beam, and the TK41 has a little "too much throw" and "too little flood" to make a great bike light.
I should have expected this, yet I am going to have some fun with that TK41 - I can't wait for a big organized night ride - I figure most riders will think a motorcycle or car is coming -
if not a freight train! 