You only really need one, unless you do lots of heavy offroading and your wheel takes lots of big hits. I have one and it rocks. The only problems I've had are that it sticks out too far (no big deal, just asthetic), and the little C-clip that holds the aluminum doowhacky fell off (and I need a lalf link, but that's hardly Surly's fault). Buy one (or two), you won't regret it.