148 Boost Dropouts with a 68mm BB?

Education
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Hey all - great resource for researching my first frame build!

Wondering if I could get some second opinions on building my first frame using 148 rear dropouts, with a road (68mm) BB on a gravel bike.

Trying to achieve 700x45/50c clearance, with a 2x10 groupset. Frame’s intended use is for Drop bar, loaded bikepacking.

Obviously I understand this approach is unconventional and mixes MTB with road standards; which I have a few reasons for doing;

1- Ideally keeping a lower Q factor for the crankset (thus 68mm shell).

2- future proofing by avoiding 142 hub sizing and going straight to 148. This is based on a number of 2nd opinions from LBS head mechanics I am friends with; with murmours of an industry move to road/gravel boost (and 142 slowly becoming less adopted?)

3: Having some mix-and-matchablilty with my Surly Krampus (currently w/ boost wheel set). Not so much expecting to cram 29x3 wheels into my new frame, but the reverse. (and fantasizing about some utopic future where all the bikes in my house share at least the rear spacing as a standard for wheel swapping and tinkering etc).

I was planning on building up a bit of a mix and match groupset (I’m particular fond of 3x8 vintage mtb, with friction bar end shifters - but looking to maybe go 2x10 for this). So not so worried about a being able to slap on a off-the-shelf groupset, and willing to do a bit of mix and matching to get what I need.

What are some of the caveats of this approach? Assuming at the very least my crank’s chainline will be 3mm inboard of where it should be (68vs73mm BB width)?

Any thing else I should be aware of (or talk me out of this approach hahah).

Cheers!!
Lester

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Welcome @lesterlh!

Sounds like a fun bike.

Some thoughts:

  • Boost 148 spacing does not effect tire clearance. Tire clearance is driven by the chainring size and chainlines.
  • I disagree that the industry is moving to boost for road/gravel. There is no reason for it. The only thing I have seen is Focus, but I think they did that to maintain parity with their E-gravel bikes (which need boost because the motor Q-factor is wider)
  • Swapping wheels is an OK reason. But you would need to source or build a boost fork, which is harder than picking up a new wheelset.

One last thing you should be wary of is the chain and derailleur interference with the tire. The max official tire clearance of 2x gravel is 45mm. Otherwise, the tire can hit the derailleur or battery (in the case of SRAM), or is too close to the chain.

My suggestion: just go 1x for true offroad riding. It is way more reliable.

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I was hoping you’d chime in on the move to 148. I can imagine from a MFR standpoint, there is some benefit in standardizing on 148 so they have fewer SKUs to manage from everything from hubs, axles, shipping boxes, dummy axles etc etc. I agree 148 is overkill for Gravel.

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Thanks for your input Daniel!
Got me thinking maybe 142 is a better bet - especially if I ever do want to be able to just put on a GRX or similar group.

I understand boost will not help with chain clearance; was mostly based on future-proofing info interpreted from others (but nice to know this gravel move to boost is not wholesale/happening soon).

When you note “just go 1x for true offroad riding” - are you suggesting I go 1x with my planned setup as mentioned above? (148 Boost-68mm BB)?

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That was just a general suggestion. 148 will work. Pushing the cassette further outboard is usually not a problem. The only issue I have seen is if you have a 10t or 9t cog, the chain might get hung up on the cog next to it.

Make sure you figure out the fork situation before you decide to go with Boost wheels. Most people don’t realize you design a drop bar bike around a fork.

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