I've lost all my links to show you but when you are hunting larger game the heavier arrow is always better to a point. It starts losing its benefits at about 15 gr per lb. However the Black Bear is not exactly the thoughest skinned animal on the planet so I would make a moderate weight arrow that still has decent speed to it. At 70 lbs I'd recommend a setup in the 500-550 weight range. Not the heaviest in the world but not the lighest. I've taken an American Bison with a 55 lb High Country bow shooting 407 grain Arrows. Complete pass through at 30 yards. I like to hover in the 7-8 gr per lb area for pretty much anything in North America. It will give you the weight you need to penetrate and the speed you need so you don't have to aim 4 feet high at 40 yards. Some of my heavier arrows I shoot right now I end up aiming with my 30 yard pin for 20 yard shots (they are up in the 1000 gr area). And the best thing you can do is practice practice practice with your setup get a good feeling of how the arrows react. One thing I would worry more about then my arrows though are my broadheads. Do you know what broadheads you are planning on using?