Depends on where you live. I used to live in So CA Mojave desert area and used 50 sunblock and still get some red action going by the time a 3 to 5 hour ride was over. But now I live in the Midwest and haven't used any sunblock all summer, even on 5 hour rides and barely got a little red. I think where I live now there are a lot more clouds in the sky whereas in the Mojave desert rarely did I see a cloud. And maybe the sun angle might have something to do with it also.

But if you burn easily or live in a desert area you should use nothing less then 50. Sunblock will make you feel hotter, but radation burns to radiate skin cancer is even hotter, or dying of skin cancer and the possiblity of going to hell could be a WHOLE lot hotter!!!