The Blowerdrive is a one-piece chainring spider and trials-style bashguard that mounts directly onto Shimano’s popular Deore LX M569 and Deore XT M739 splined crankarms. This extremely durable and uniquely styled unit’s coolest feature is its ability to fit 74mm, 94mm, or 110mm BCD chainrings - no more gear limitations due to poor freewheel selections, and no need to buy a bunch of new chainrings to experiment with strange gear combinations. We bet there’s stuff hanging on your bench that will bolt right on.
The Blowerdrive’s integrated bashguard is "gnarled" for extra grip when bushwhacker mode is on, and will protect chainrings up to 36 teeth. Blowerdrives include chainring hardware and look pretty tough, as well. Available in silver.
Strengths: Durable, simple, easy to install, fits just about any 5 bolt chairing you can find.
Weaknesses: After mistakenly blaming an offcenter chairing, I discovered that the spline on my blowerdrive was "precision CNC machined" approx. 1/32" off center from the chainring bolt circles. 1/32" might not sound like a lot, but with horizontal dropouts and no chain tensioner it is enough to make it nearly impossible to get your chain tension adjusted anywhere near half decent. I ended up taking the assembly apart and trying the spline in all four orientations to find the one that worked out best. It is better than where I started, but I'm still too slack on one side of the crank rotation and too tight on the other. I double checked the chainring and it is dead nuts.
The Blowerdrive will protect up to 36 tooth chairings, which inherently makes it a bit large and reduces your ground clearance. If you are doing any riding where you really need to protect your chairing with something this beefy, then you are probably not using a 36t up front.
Good way to hold on to your XT or LX splined cranks if you like them and want to go single speed. The large diameter means it takes more abuse than it really has to.
If you have a chain tensioner, then I don't think you need to worry about the off center machining. I bought mine second hand already assembled to a crank, so I didn't bother contacting Surly, though I have heard their cust. service is good.