A friend of mine who is starting to get into biking (and who knows even LESS than me) needs to know the difference between road cassettes and mtb. He wants to know why he can't use a mtb cassette on his new TREK road bike. Someone told him he can't and he came to me with the question why? I know they're typically geared much differently, but technically they are interchangeable as long as he keeps the same number of cogs, yes? Or no? I think he wants to move up to a larger cog so he can climb easier, but I'm not sure.