Ash makes a pretty stiff backing and can overpower a softer wood like maple that does not handle compression well leading to compression fissures. Hickory is not as stiff and is very durable, but tends to take more set in humid climates (watch out for that as I just leaned this week), but is an excellent backing material. Maple is the softest of them and doesn't add much weight to the bow or overpower the belly wood, so it is better for softer belly woods. It is also incredibly bendable assuming it doesn't have any blemishes. I prefer using ash because a slightly thicker backing will actually add stiffness to the bow, but I only use it with mid-density belly wood. I just did a black-walnut/ash bow and a myrtle/ash bow. For those two woods, the ash worked much better than hickory or bamboo in comparisons I tried before. Took less set, had more zip, but again, I am in a very humid environment so I am not seeing hickory as it could be in a dry climate.
