Combo bikes…maybe they are OK

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?

7 Responses

  1. I’d say 73mm BB square taper, spindles from 107mm to 122mm or something like this, no?
    But I’ve heard that bearings should be shown to everybody these days and I might be the only shy MTB’er left keeping them hidden, non-external 🙂

    For BB height, just get a pair of short cranks and a pair of long ones…j/k

    Anyway, I hope for you that more technical-savvy people than me will chime in ’cause I don’t think I helped you much here… but I wanted to let you know that I do read and enjoy your blog.

  2. Raoul, thanks for the comment!
    No, you’re not the only one, I still have a square taper BB on one bike! They just never die. But i do like the ease of removing the cranks and bearings on external bearing BB’s and feel that they are stiffer. But i don’t feel they last as long as a good square taper bottom bracket (Phil Wood). Stiffness aside, it’s a good recommendation for chainline adjustment alone. I go back and forth on caring about real and perceived BB axle stiffness and weight on a non-racing MTB. We’ll see. I think the chainline is more important than the BB height so maybe I’ll just put a Rohloff on the bike and solve that problem. Wait, then i’d need TWO Speedhubs! Damn.

  3. Whit,

    I have been lurking for a while but I like to keep up with what you are doing because you speak a language (frame design details) that I just don’t hear about elsewhere. That and when I read your stuff it is like your know my thoughts sometimes.

    Ok “someday soon” I want to come to you knowing I will be asking for a disc all-rounder 29er/700c wheels with tires typically 38-60 wide, set up for dirt drops, with as low a BB as possible (using 180 cranks) and wanting a tall headtube.

    So I am clearly on the urban side of your extreme rides for some reason finding these things is like pulling teeth elsewhere. Oh and using a decent HT tubing, not the lightest and not the heaviest either, still trying to really learn the effects of trail (and the different ways to get there) are in real life.

    So yes multiuse, but I also see that the more narrowly defined the criterion the better performing the expectation can be.

    Kevin

    • Kevin,
      Thanks for the comment, and i’d be happy to make you a frame and fork for that type of riding! (It’s actually something I’ve been wanting to build myself as a v2 for my current all-rounder.)

      The Trail effect is a hard one to get a good handle on unless you really exaggerate the trail on the same frame by varying the fork offset. I made an adjustable rake fork to do just that when I first started building bikes a few years ago.

      I’m sure you’ve read all about it online but just in case Wikipedia has a good writeup and motorcycle geometry books go into it well beyond my mathematical prowess. I tend to like more offset on my forks since that “allows” a slacker head tube angle to be used while preserving a relatively quick-steering feel. More offset shortens the trail figure (and lengthens the front center a bit which can be compensated) for a bike that’ll roll over stuff smoother and be more stable at speed. For something that’ll be loaded and going fast on pavement this could be a nice addition to a low BB. On dirt, it’s been researched a lot with suspension and more fork manufacturers are starting to offer higher offset forks than before (not just 44 or 46 but 51mm).

      • Whit,

        I had a hard time posting a reply, and maybe it was best to take it off line anyway. So I sent you a note btw good to hear you are busy building.

        Kevin

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