Ok…so I’m not sure who all’s reading this here blog but you have an obligation to call out my crap when you see it. I’m surprised nobody cried foul on my “Combo Bikes Suck” post. I mean, look at my bikes. No, not THESE types of Combo bikes (although those would be pretty cool too) but the ones I build. Cyclocross-ish frames that fit 40+ tires? I have an All-rounder that looks like a cross bike but fits 2.2″ tires. My favorite bike is a “29+” that is basically the genius offspring of a fatbike and 29er. What could be more “Combo” than a 29er half-caff fatty!?
But I’m not a total hypocrite, the post was mostly about making a fatbike also fit a 29+ wheelset just because they can fit. I still do not recommend doing something just because you can do it, but hey, it’s a personal thing. However, I have been toying with the idea of making my next MTB a 29er that fits the Surly Dirt Wizard 2.75″ tires. So…a combo bike. That way it has the same 135mm rear axle and 73mm bottom bracket shell width as a normal 29er but can have a 2nd wheelset that would have either Knards (3″) or Dirt Wizards (2.75″).
For the small # of races I plan on doing, I’d use the lighter and faster 29 x 2.25″ wheelset, then have the 29+ wheelset for most of my other riding. In reality all 29+ (mid-fat is the new moniker?) bikes fit regular 29″ tires, there’s just that issue of wheel diameter and any change in geometry from going from a rigid to a suspension fork. The bottom bracket height difference isn’t trivial, it’s a difference of about 3/4″. So say the regular 29er has a BB height of 12″ (as low as i’d like to go), the 29+ would have approx. 12.75″ BB height. Maybe 12.7″ with the lower tire pressure. I guess you could make it with an EBB to account for some of that change in height. Or figure out what travel sus fork would even it out as much as possible? Hmm…
There’s also a small difference in front center, wheelbase, and trail but let’s forget about those. For other parts of the geometry, you can realistically only get 420mm chainstay length using Knards (I’d be surprised without a very skimpy yoke and/or an offset rear axle if you could get that short and have any semblance of a normal chainline). So for my hypothetical bike, I could set the chainstay length to about 430mm and use Sliders or Rockers. Wolf Tooth makes a lot of different rings to fit their splined GXP cranksets including a “BB30″ model that can be flipped for increasing the chainline by 5mm or so. For touring gearing it’ll be difficult (impossible?) to fit a double or triple ring on front so it may have to be a 1×11. Not ideal for bikepacking but putting a 26t up front would help. I’m not a big-ringer but being spun-out isn’t fun either. This is the stuff that I put my brain through…it’s incessant I tell you.
What do you think?