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I recently got a 27.5+ Fox fork only to discover that the distance between the legs of the fork is 4.3". This made me think that one could fit a 4" tire (obviously, using a 110 mm front hub instead of 150 mm) in there, and sure enough, after googling I found people doing exactly that.

So my question: can we consider 27.5+ forks a good alternative to 26 fat-specific forks such as Bluto/Mastodon?

Update: I did it, installed a 110mm Boost hub into an 80mm rim. It's not ideal because spokes are almost vertical. To make it work, you need:

  • Either a 65mm or 52mm rim; or
  • A 203mm brake rotor

Either of these two things fixes the problem. Apart from that, the experience is amazing. A 26/3.8 Hodag fits into a Fox Factory 34 27.5+ (plus!) wheel perfectly, affording about 4mm clearance each side (lots of space on top, maybe 2cm or so). Cornering, grip, much softer suspension — this is all vastly superior to what you get from RS Bluto or Mastodon Pro.

So if you're OK with a 3.8" front wheel, you can totally do Fox. For example, many electric fatbikes (e.g., Haibike FullFatSix) are limited to 4" on the rear, so having 3.8" on the front is not such a big deal. Here, I made a video about this.

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Here are potential issues that may or may not arise from such solution that I see.

  1. Geometry changes. As it has always been, control that the head tube angle of the bike changes in a way that does not worsen its handling. I, for one, use 27.5+ wheels and tires with a fat bike fork made by Surly originally shipped for 26" wheels with 4.8" tires. However, the vendor in this particular case states compatibility with 26", 27.5" and 29" wheels, and so far it was so good.

  2. Clearance issues. Yes, a 27.5+ tire alone would fit; but would all the mud and snow sticking to the tire fit? I've had situations in my life on some bikes when either rear or front wheel got clogged with a mixture of clay, grass, sand and dirt, or sticky snow, to the point the wheel stopped rotating until cleaned.

  3. Tire width variation outside of vendor specs. The actual installed width of a tire depends on the width of the rim used, and it could be wider or narrower than specified. So your 4" can become 3.8" or 4.2" in reality.

For the rest, a trail fork should be as good as a fatbike fork, or even better. For example, RockShox Bluto is considered by some to be "noodly" because its stanchions are just 32 mm and seem to be derived from a cross country fork. So a narrower fork with 34 mm stanchions could be stiffer.

  • I'm really considering this set-up in the context of an ebike so weight doesn't matter as much as riding comfort. I purchased a 26" rim and am getting it laced to try with a 27.5+ fork; I doubt it will give me much benefit, though, because the rear wheel is still 2.6" but it should be fun to try. – Dmitri Nesteruk Aug 1 '18 at 20:24
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Note that a 4" tyre would have less than 4mm of clearance on each side. If your wheel goes slightly out of true or starts picking up debris, it may rub or even jam.

  • True, 4" is pushing it. 3.8" should be fine though. – Dmitri Nesteruk Jul 13 '18 at 12:44
  • Haha, turns out a 3.8", not a 4", has about 4mm of clearance on each side. – Dmitri Nesteruk Sep 2 '18 at 14:20

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