I have been running 13/50 on this bike. I bought it this way. In the dessert it works well for me, but in the tight forrest near my house that I ride in the summer it seems a bit too tall. I just bought the primary drive brand 13/51 with their gold x-ring chain 114 links and the chain is too short. I have a couple questions. 1st I read that someone said that the 51 tooth was hard on the chain guide. Is this true? 2nd How many links in the chain should I use with 13/51? 3rd I am wondering if anyone has some extra links in this style chain that I can buy from them? If I could get some links I could also get a extra master link and and easily change the chain length for the 50t or 51t sprocket. Comments please.