Monday, November 10, 2014

Kevin's Low-Q fatbike EDIT: added frame geometry discussion

This was a cool project. Kevin did a bunch of the hard work figuring out chainline and researching appropriate cranks, and we kicked around ideas a bunch and ended up with something I think will be really fun: a fatbike without a ridiculously wide pedaling stance/Q factor.

Now, the tradeoff here is that a 4" tire on a ~80mm rim is about the limit for tire clearance, but that's ok if the alternative is horrible knee pain and no fun on your fatbike. 4" tires are damn fun, kids, even if the cool guys all have 5" ones now and look like babies waddling with full diapers on their ICTs.

I kid, I kid. Few pictures, geometry update in a minute.

Hello, Bean
Now, geometry:
-69 degree head tube angle, 45mm offset fork, and around 90mm of trail
-695mm front center
-310mm BB height (obviously depends on tire/inflation, though)
-435mm chainstay length
-83mm BB shell for happy knees and low Q
-170mm rear spacing
-150x15 spaced/Bluto swappable 480mm rigid fork.
-About 115mm of tire clearance at 345mm from the rear axle - this means basically any 3.8/4" tire will fit on 65-80mm rims. Bigger tires (or rims) are probably a no-go, though. 29+ and (more logically for BB height reasons) 27.5" will both fit with gobs of room.

Lots of bending. Damn, the only straight tube is the head tube...and fork blades, I guess.

9 comments:

Anonymous said...

So 84mm shell? I have a fragile knee-ed friend who's looking for something of this sort.

DG

Anonymous said...

Photo credits to Walt. Chalk art by Andy

Chris said...

If your knees hurt with a wide q, push your knees out more and keep your feet pointed forward on the pedals. This will help keep the cruciate ligaments crossed and stabilize the knee. Yes, your hips and ankles will have to get used to the new range of motion, but they're mobile joints that, unlike the knee, move in more than one plane and will benefit from the adaptation.

Anonymous said...

People thought (and still think) I am crazy when I say that I sold my Pugs because of knee pain.

Anonymous said...

I'm glad that you and Kevin figured this out. Please give an update on the build, so we can see the cranks and wheel/tire combination that was chosen.

Kyrke Gaudreau said...

Hi. I'm pretty new to these different BB widths. What type of crankset would work with this lower q-factor fat bike?

cheers
kyke

Walt said...

Hi Kyrke -

Shimano Saint, SRAM XO1-DH, and Raceface SixC are probably the most common, but you can get cranks from loads of other companies (Truvativ, E13, Gamut, etc).

Kyrke Gaudreau said...

Hi Walt,

Thank you for the quick reply. When will you be selling these frame sets?

Cheers
Kyrke

Walt said...

Erm... now? I mean, I build bikes constantly, it's my job. If you want one like this (or anything else), get in touch with me via email.