Fatbike wheel in a 27.5+ fork (experience report)

Dmitri

Active Member
#1
Hi everyone! Just wanted to share the results of an experiment I did -- putting a fatbike wheel into a non-fatbike fork of my R&M Delite.

As you may or may not know, the choice of forks for fatbikes is really limited: there's basically Manitou Mastodon (the best option), RockShox Bluto (the OEM option, legs are a bit thinner) and a few exotic ones. Those forks typically have 150mm hub spacing.

Well, it turns out that a 3.8" wheel can happily fit into a 27.5+ fork such as Fox 34 27.5+. What this means is that you can put a 26/3.8 wheel in there (or, theoretically, 27.5/3.8 though I haven't tried this as it is an exotic size). Here's what it looks like from the outset (images are clickable):

20180810_140751.jpg

As you can see, the size is the same as if you had a, say, 27.5/3 wheel in there, so the angles are undisturbed. Now, in order to build this wheel, I basically took a 110mm SRAM X0 hub and laced a wheel with a 80mm DT Swiss fatbike rim. That's it. The tire I've put on is Bontrager Hodag (26/3.8).

The amount of clearance on the sides of the tire is about 4mm on each side as can be seen here:

20180810_140810.jpg

Naturally, this kind of wheel is incompatible with Pletscher R80 mudguards that I had on, so that had to be removed.

It's not all rosy though! The 80mm rims were never designed to be used with 110mm hubs, so the angle of the spokes can catch the brake caliper on a 180mm rotor. The contact is minimal, but it's extremely annoying, which is why I've filed away a bit of the caliper using some sandpaper:

20180810_140820.jpg

The solution to this problem is simple: install a 203mm brake rotor! Not only does this solve the problem of scratching spokes against the caliper, it also makes perfect sense because, with such a (comparably) heavy wheel, you need more stopping power.

So, what are the advantages of this approach?
  • Vastly improved cornering
  • Front wheel absorbs even large stones/objects with ease
  • Suspension is superlative... I can drive directly into a curb and the bike just floats over it
  • You get to use one of the best, most configurable forks out there (sorry, but it's just way better than Bluto/Mastodon)
And the disadvantages:
  • Spokes can scratch against your brake caliper (depends on brakes, obviously)
  • A 4mm clearance isn't sufficient for one of those plasticky mudguards... you'll need a fullsize fatbike mudguard with those cateye fork holders and everything
  • You need to build what's a fairly non-standard wheel that won't be reusable in other bikes
  • If combining with an 'ordinary' rear wheel, now need to pack two different tubes when traveling :)
My takeaway from this experiment is that, when building fatbikes, you don't necessarily have to go for a fatbike-specific fork unless you really want a 4.8" front tire. 3.8" is enough to drastically alter the riding experience, so if you were to buy, say, the Haibike e-fatbike (it uses 4.0" wheels), a 27.5+ fork is a sensible upgrade to the stock Bluto that comes with the bike.

(Ahh, I wish R&M would do a Delite with a 4.0" rear... that would be the ideal ebike. I can keep dreaming though, right?)
 
#2
Excellent info, @Dmitri! Thanks for the detail. I regularly read posts by members who don't like the stock fork, particularly on the fatter tires. Thanks for providing options and the very important hint of changing out the rotor size. Having a bit of rubbing or snagging of the brake calipers on spokes is problematic, not to mention an irritating noise :)
 

Dmitri

Active Member
#3
Actually, I now realize that a much simpler solution to the problem of scraping spokes is to simply choosing more narrow rims, e.g., 52mm ZTR Hugo rims.
 

RickyE

New Member
#4
A general question: What are the advantages and disadvantages of a suspension fork on a front drive Ebike
with a 250w motor and 36v source?
 

Ken M

Active Member
#5
A general question: What are the advantages and disadvantages of a suspension fork on a front drive Ebike
with a 250w motor and 36v source?
Personally I think suspension forks are of minor value on an urban / street ebike. If you run a tire width over 2" / 50mm on the front the ride quality is typically pretty good and the handling will be better without a suspension fork. Will typically drop the front end weight a couple lbs which can make handling feel a bit more lively and the lack of any bobbing makes riding more efficient.

I think the industry has done such an effective job of convincing bike riders that they need front forks even on none mtn bikes that it's not easy re-educating the consumers that wider tires typically provide effective suspension for urban riding.

I have ridden front street tires up to 2.8" and the ride quality only gets better but sometimes steering can feel a bit more sluggish. Regardless, I'm 57 and I don't think suspension forks on urban commuting bikes is justified. It's just marketing to the masses...
 
#7
Thank You. I'm trying to stuff a 3.0" Rocket Ron in the smallest non-step cast Fox fork I can. I've also read the 32 fox non step cast can do this, and for sure the 34 non step cast can do it, but had no idea could fit a 3.8". This is for a non-ebike solution. I mostly ride regular bikes.