I also use an old Hero3 black. I found on my Brute that the best location was on the helmet. Everywhere else has vibration issues...even my front plastics which I took the time and added vibration mounts to had issues. Yeah tried the bars too. Too much swinging back and forth for me and too much jolting. As for settings, after all these years for most all rides I settled for 1080/30 for anything under 10mph and if in darker woods and 1080/48 over 10mph but have to take the light into consideration as faster shutter speeds need more light and if you don't have it.. it will blur and loose definition. I also like the Med width because the wide fish eyes/distorts things too much. I render at 16:9 anyway and at 22000kbps.
On my last video I did speed-up the clips a little to get more in but that does effect the smoothness. This is a version of that same video with music that doesn't have the speed adjusted..just some sections removed. Shot in 1080/30.