Crankset / ring options for mid-fat

Espresso_
Posts: 745
Joined: Tue May 25, 2010 3:22 pm

Crankset / ring options for mid-fat

Postby Espresso_ » Tue Jan 12, 2016 8:51 pm

Hi all

I've an old 26" rigid hard tail (On One Inbred) that I'm thinking of converting to a mid-fat bike with Surly Dirt Wizard 26x2.75 tyres or similar.

By my measurements the rear of the frame will accommodate a tyre that wide (front could go even wider as I run a Surly fork). The problem is likely to be that the fatter rear tyre will foul the small ring/big cog combo - I'm running an SLX triple on ten speed.

So is there a simple/cheap way to ditch the big ring and push the middle and small rings out? Or am I dreaming?

E

User avatar
Duck!
Expert
Posts: 7118
Joined: Tue May 21, 2013 8:21 pm
Location: On The Tools

Re: Crankset / ring options for mid-fat

Postby Duck! » Tue Jan 12, 2016 9:01 pm

You're dreaming. The crank is designed for a specific width bottom bracket assembly; adjusting spacing to push the rings outboard will reduce the available spindle to clamp the left arm onto, greatly increasing the likelihood of left crank departure.
I had a thought, but it got run over as it crossed my mind.

Espresso_
Posts: 745
Joined: Tue May 25, 2010 3:22 pm

Crankset / ring options for mid-fat

Postby Espresso_ » Tue Jan 12, 2016 9:04 pm

I wasn't thinking of that - more like a way of putting the small ring where the middle ring is now and the middle ring where the big ring is now ... And ditching the big ring (to make a double).

Still dreaming?

E

User avatar
Duck!
Expert
Posts: 7118
Joined: Tue May 21, 2013 8:21 pm
Location: On The Tools

Re: Crankset / ring options for mid-fat

Postby Duck! » Tue Jan 12, 2016 9:11 pm

Still dreaming. 'Cos of its small size, the small ring has its own set of mounts so can't be moved outward.

Only possibilities I can see are to source a fatbike-specific crank with a longer spindle & cobble some spacers to make it match the existing bottom bracket, or a "Boost" crankset, designed for the new "semi-fat" 27.5+ & 29+ bikes, which uses the same spindle length but moves the rings 3mm furter outboard. It's designed to pair to slightly wider rear hubs, so could have an adverse effect on the chainline, but probably really only if you do the not-advised-anyway Big-Big crosschaining thing.
I had a thought, but it got run over as it crossed my mind.

Espresso_
Posts: 745
Joined: Tue May 25, 2010 3:22 pm

Re: Crankset / ring options for mid-fat

Postby Espresso_ » Wed Jan 13, 2016 5:51 am

Right - thanks for putting me straight.

Looks like the Surly Mr Whirly crankset might also work for me? But not cheap once I get all the bits and pieces.

E

Espresso_
Posts: 745
Joined: Tue May 25, 2010 3:22 pm

Re: Crankset / ring options for mid-fat

Postby Espresso_ » Wed Jan 13, 2016 8:28 pm

Or a really wide square taper BB and pick up a cheap second hand eBay crankset to match?

E

User avatar
Duck!
Expert
Posts: 7118
Joined: Tue May 21, 2013 8:21 pm
Location: On The Tools

Re: Crankset / ring options for mid-fat

Postby Duck! » Thu Jan 14, 2016 12:13 am

That'd work, providing your frame is threaded & not press-fit.

Potentially the front derailleur reach might prevent shifting up to the big ring.
I had a thought, but it got run over as it crossed my mind.

Espresso_
Posts: 745
Joined: Tue May 25, 2010 3:22 pm

Re: Crankset / ring options for mid-fat

Postby Espresso_ » Thu Jan 14, 2016 4:32 am

How do these Boost systems handle that issue? Special FD?

E

User avatar
Duck!
Expert
Posts: 7118
Joined: Tue May 21, 2013 8:21 pm
Location: On The Tools

Re: Crankset / ring options for mid-fat

Postby Duck! » Thu Jan 14, 2016 9:05 am

It's only a 3mm offset from standard, so not beyond the reach of a normal derailleur.
I had a thought, but it got run over as it crossed my mind.

Espresso_
Posts: 745
Joined: Tue May 25, 2010 3:22 pm

Re: Crankset / ring options for mid-fat

Postby Espresso_ » Thu Jan 14, 2016 9:13 am

Duck! wrote:That'd work, providing your frame is threaded & not press-fit.


Frame is 68mm threaded.

How does the crankset - bb interface work for square taper bracket? Can you just put any square taper crankset on any width square taper bb? Just buy a wider bb and get a wider Q factor and different chain line?

Seems too easy ...

E

Espresso_
Posts: 745
Joined: Tue May 25, 2010 3:22 pm

Re: Crankset / ring options for mid-fat

Postby Espresso_ » Thu Jan 14, 2016 1:18 pm

Duck! wrote:Potentially the front derailleur reach might prevent shifting up to the big ring.


That's probably not the end of the world in my case as I only really want the middle and granny rings anyway. Might be some chainline issues if I go too wide though.

So my steps will be:
1. Get the new fat tyre in and assess degree of chain rub problem
2. (Assuming there is a problem) install the narrowest-splined square taper bb and crankset combo that will clear the rub on the lowest gear combination.
3. Bodge the front derailleur to make it shift.

E

User avatar
Duck!
Expert
Posts: 7118
Joined: Tue May 21, 2013 8:21 pm
Location: On The Tools

Re: Crankset / ring options for mid-fat

Postby Duck! » Thu Jan 14, 2016 1:43 pm

Espresso_ wrote:
Duck! wrote:That'd work, providing your frame is threaded & not press-fit.


Frame is 68mm threaded.

How does the crankset - bb interface work for square taper bracket? Can you just put any square taper crankset on any width square taper bb? Just buy a wider bb and get a wider Q factor and different chain line?

Seems too easy ...

E

There are a couple of different interface profiles, but one is far more common than the other, so you're unlikely to have a major issue there. The thing to watch is that different cranks align the chainrings differently against the butt of the crank, which is the primary reason for various lengths, so the rings don't bite the frame. A crank that has the butt deeply recessed from the small ring requires a longer spindle from the start, which limits how much further you can push it out. One that has the small ring virtually in line with the crank butt uses a shorter spindle, so you have much more scope to play with length to do what you want.
I had a thought, but it got run over as it crossed my mind.

Espresso_
Posts: 745
Joined: Tue May 25, 2010 3:22 pm

Re: Crankset / ring options for mid-fat

Postby Espresso_ » Thu Jan 14, 2016 8:03 pm

Duck! wrote:There are a couple of different interface profiles, but one is far more common than the other, so you're unlikely to have a major issue there. The thing to watch is that different cranks align the chainrings differently against the butt of the crank, which is the primary reason for various lengths, so the rings don't bite the frame. A crank that has the butt deeply recessed from the small ring requires a longer spindle from the start, which limits how much further you can push it out. One that has the small ring virtually in line with the crank butt uses a shorter spindle, so you have much more scope to play with length to do what you want.


Right - that's understandable.

Sheldon's database is a good resource for square taper cranksets, but I'm seeing another challenge. If I can find an old crankset that suits (e.g. designed for a 113 square taper, and put it on a 127mm square taper or whatever) there's a huge chance that crankset was designed for an 7/8 speed chain. Will these work on my 10 speed SLX? Or do I need narrower chainrings?

E

User avatar
Duck!
Expert
Posts: 7118
Joined: Tue May 21, 2013 8:21 pm
Location: On The Tools

Re: Crankset / ring options for mid-fat

Postby Duck! » Thu Jan 14, 2016 8:14 pm

Would probably still work. Although rings for 7/8-sp are a little bit thicker there's generally a reasonable bit of tolerance to allow running the narrower chain.
I had a thought, but it got run over as it crossed my mind.

Espresso_
Posts: 745
Joined: Tue May 25, 2010 3:22 pm

Re: Crankset / ring options for mid-fat

Postby Espresso_ » Sat Jul 09, 2016 11:07 am

It's only taken me six months, but I've actually done this.

Just for anyone doing a web search on this trying to find definitive proof, the Surly Dirt Wizard 26inch x 2.75 will fit on the rear of an On One Inbred 26 vertical dropout frame! There's not much clearance, but no rubbing anywhere.

The problem I thought would appear, did appear. That is the front derailleur fouls on the tyre and prevents shifting into the small ring on my triple.

E

eldavo
Posts: 1697
Joined: Tue Sep 01, 2009 12:21 am
Location: Perth, Western Australia
Contact:

Re: Crankset / ring options for mid-fat

Postby eldavo » Sat Jul 09, 2016 11:29 am

What width rims are you running?

Espresso_
Posts: 745
Joined: Tue May 25, 2010 3:22 pm

Re: Crankset / ring options for mid-fat

Postby Espresso_ » Sat Jul 09, 2016 11:46 am

eldavo wrote:What width rims are you running?


50mm Surly Rabbit Holes.

E

eldavo
Posts: 1697
Joined: Tue Sep 01, 2009 12:21 am
Location: Perth, Western Australia
Contact:

Re: Crankset / ring options for mid-fat

Postby eldavo » Sat Jul 09, 2016 12:03 pm

Time for some photos ;)

Espresso_
Posts: 745
Joined: Tue May 25, 2010 3:22 pm

Re: Crankset / ring options for mid-fat

Postby Espresso_ » Sat Jul 09, 2016 12:59 pm

Image

Drive side shot

Image

Clearance on the seat stay

Image

Clearance on the chain stay, in the middle chainring.

eldavo
Posts: 1697
Joined: Tue Sep 01, 2009 12:21 am
Location: Perth, Western Australia
Contact:

Re: Crankset / ring options for mid-fat

Postby eldavo » Sat Jul 09, 2016 1:44 pm

Great work, and how is that Kiwi made front seat going?

Espresso_
Posts: 745
Joined: Tue May 25, 2010 3:22 pm

Re: Crankset / ring options for mid-fat

Postby Espresso_ » Sat Jul 09, 2016 2:19 pm

It's great - were out riding on it now. Though his weight on the bike is part of the reason for this thread. I need my granny gear back!

E

Espresso_
Posts: 745
Joined: Tue May 25, 2010 3:22 pm

Re: Crankset / ring options for mid-fat

Postby Espresso_ » Tue Jul 12, 2016 8:11 pm

Ok so the update on this...

The front derailleur fouls on the rear tyre at the minute, meaning I can only use the middle and big rings on my current 22-32-44 triple.

However ... I can mount the chain on the granny ring without the dreailluer in place and there is a few mms clearance to the tyre when in the big cog. Boom!

That means I can go 1x10 with a narrow-wide chain ring, and ditch the front derailleur and at 64bcd granny size I should be able to get an acceptably low chainring to maintain low-end gearing without changing crankset, bottom brackets, spacers etc etc as discussed earlier in this thread.

E

eldavo
Posts: 1697
Joined: Tue Sep 01, 2009 12:21 am
Location: Perth, Western Australia
Contact:

Re: Crankset / ring options for mid-fat

Postby eldavo » Tue Jul 12, 2016 8:15 pm

Nice work. I didn't use FD on my 2x10 OnOne Fatty since the ride style was onroad large ring, offroad beach drop tyre pressure and drop to small ring manually shifting front chainring.
I liken it to my 4WD or motorcycle offroad experience. Onroad higher speed higher tyre pressure/taller gearing larger chainring. Offroad low speed low tyre pressure shorter gearing smaller chainring.

Espresso_
Posts: 745
Joined: Tue May 25, 2010 3:22 pm

Re: Crankset / ring options for mid-fat

Postby Espresso_ » Tue Jul 12, 2016 8:31 pm

I hadn't thought about manual shifting. It does make sense to give it a try as its a cost free option.

Did you have a clutched rear derailleur or any other mechanism for chain retention?

E

eldavo
Posts: 1697
Joined: Tue Sep 01, 2009 12:21 am
Location: Perth, Western Australia
Contact:

Re: Crankset / ring options for mid-fat

Postby eldavo » Tue Jul 12, 2016 8:50 pm

Nothing else, I've never thought of chain drop on it actually. It is an XT long cage RD and usually on the bigger rear half of cassette when offroad so takeup isn't run out like on the smaller sprockets.

Riding on the beach the speed and bumps aren't big like the sharper faster regular offroad with downhill speed potential so that may have saved me.

Who is online

Users browsing this forum: No registered users