The first bow I ever made was from ERC; basically knotless, 70lbs at 29 inches, unbacked, 2/3rds heartwood to 1/3rd sap wood ratio, two inches wide at the handle, six feet long, ELB. I knew nothing about preserving the back of a bow, so this one had cuts, dings, and just about every type of violation across the back you could think of. Yes, it broke. At about six inches down from the upper nock and whacked me on the head, but that was after probably 400 or 500 shots. Soon after, I switched to hickory and have had fewer headaches, but, I think, with a bit of luck, you can get away with it unbacked. Just my humble opinion. Good luck. And you might want to invest in a hard hat, just in case.