KB’s uber-custom fatbike – The Swan

This frame was really cool to see go from computer screen to reality. A few people said it looked Swan-like so…there you go.
Thanks to Walt and Brad for their help with this. I probably couldn’t have made it like this had they not given me great advice and in Brad’s case in-house gusset lessons.

KB wanted a 5″ fatbike with a Bluto and 2×10 gearing allowing a 20 x 42t for her lowest gear. She requested the lowest standover possible with a 80mm Bluto, BB height no less than 12.5″, and 4.8″ tires. I was able to get the standover (unsagged) to 695mm and lower as you get closer to the seat tube (~630mm).  I bent the top tube to get it as low as possible but also to give a nice long coped interesection with the True Temper 38mm Supertherm downtube.  The top tube comes into the downtube in the thicker butted section (1mm wall) and a big long gusset was added to strengthen the headtube junction made out 2″ diameter x 0.049″ wall 4130 straight gauge tubing.  She also wanted her bars to be 4″ higher than her saddle.  So this build was very different than anything I’ve attempted before.

Geometry:  68.5 degree head tube angle, 73 effective seat tube angle, 430mm long chainstays, 612mm front center, 41.4″ wheelbase.

Components: SRAM 10-spd twist shifters, Hayes Prime Expert disc brakes, Truvativ Noir T20 carbon riser bar, Thompson 70mm stem and seatpost, Surly Mr. Whirly Offset Double 165mm crank is on there with a 20x33t ring combo paired to a 10spd SRAM 12-36 cassette.  But the cassette is a wide range 10, with a Wolf Tooth 42t ring and a 16t to replace the 15 and 17t rings. I used their longer B-tension bolt too to get it to shift right.

The front derailleur is a direct mount X7 mounted to the Wolf Tooth clamp for Moonlanders/190 rear hubs.   Because of the weird crank when set up as spec’d (2mm from bottom of cage to top of big ring) the cage rubs the crankarm. It’s really easy to adjust the direct mount derailleur upwards and out of the way and it still shifts pretty good.

Wheels were built by Lacemine29.com and are Bontrager tubeless Jackalope rims laced to a 190 Fatback 10mm through-axle on the rear and a Borealis 150x15mm hub on front, Vee Rubber Snowshoe XL’s run tubeless! Very easy to set up without any tape on these rims. Super psyched about this. It didn’t work well (at all) for me using Lou/Buds.   Headed to the Colorado high country!

 

10 Responses

  1. Wow, this is one superb bike, and I know KB will be waiting for it to arrive. What a frame, details galore for all of us who have anyone in our family into any kind of biking. And I guess the only thing to make it better would be personal delivery!!! Christmas greetings to you both, from Jean in NZ.

  2. Very nice.
    Are you using the Nova 29er stays? Did you need to further manipulate them to get to 430mm?
    I built my first fatbike with fairly standard angles 73/71. I’m pondering fatbike 2.0 and am considering changes. Are you sold on the short as possible CS with slack HT? I have noticed that I want to get my weight back on soft stuff. What’s the idea with the really slack HT and long front center, kind of let the front tire ride up more like a ski?
    Thanks!

  3. Thank you Jean in NZ, that’s really nice of you to comment

  4. Jbyrne, I did dimple the Nova chainstays a bit in my normal way. I pretty much have to do all my frames that way, fatbike or not since everybody wants short chainstays. The bend on the Nova’s is probably right on a 4.8″ tire fatbike with 445mm stays or longer. But as you can see from this photo on my Flickr they could use a bigger bend to get down to 430.
    Rave Face Turbine Cinch is really a cinch for clearance. Huh huh

    For geometry, I’m sold on short chainstays because of exactly what you say – weighting the rear wheel to get traction. That also pairs well with a slack HTA because unweighting the front wheel with a long front center (not too long though) and lots of trail to more easily track and push through soft snow instead of taking your front wheel where you don’t want it to go (off the trail). I’ve found with steeper angles you have to be so much more on it (concentrating where you don’t want your wheel to go) than with slacker angles/more trail. It’s just like riding in sand, you can stop your progress with too much forward weight. Lean back and you get through it. I feel it’s a similar feeling to riding in soft snow. Hardpack conditions are a different story, and if the trail is wide and groomed you can pretty much ride whatever geometry you want.

  5. Thanks. The one weird thing is that slack/lots of trail traditionally means high speed stability but poor slow speed handling. The wheel wants to flop and it’s hard to hold a line going slow. But this rule of thumb seems turned on its head when riding in soft conditions. Is there some logical explanation for this?

    • What you say is true and…all I can say is that you should give it a try. I’ve written about this before on the blog and what holds true theoretically for ‘regular’ bikes has to be thrown out the window with Fatbikes. They are a different bike with such high volume, low pressure tires, and with such potentially soft conditions vs. dirt and pavement. If you think about it, riding on snow is similar to riding on a constantly changing, uneven and inconsistent terrain…so making the geometry slacker like a freeride bike makes sense. The front wheel and your steering becomes less sensitive to every little change in the “trail” and allows the bike to stay balanced while you plow through the varying snow conditions. But if you just ride well packed groomed ski runs it doesn’t matter as much.

      What I’ve noticed is that when you first get on one of these bikes, whether it be a slack all-mountain rig or a slack fatbike, they do feel less stable on steep uphills and under around 5mph on flats, because of wheel flop. But once you actually start moving at even a pretty slow speed it stays more stable than a steeper-angled (twitchier) bike. You NEVER are climbing steep hills on a fatbike, if you are you’re walking or have studs and are on ice. I’ve done back to back testing on slack vs. steeper and there’s a noticeable difference.

      By way of example, the new owner of the bike in this post went out on her first ride today. She has owned a Ti Fatback for about 5 years. She had this to say,

      “Wow, I had an awesome ride. I rode the same trails as I did with my Fatback yesterday. Yesterday, it just about 50/50 walk/ride due to 8-12” of loose deep snow and no one but me packing trails. Your bike churns through the snow with incredible stability – I barely walked at all today! There’s so much less “losing control of the front wheel, falling off the track, and then flailing while I try to restart”. Seriously, I didn’t imagine that the new bike would make SUCH a huge difference. But it does. I seem to have better control over what the front wheel does. You nailed the geometry.”

  6. Thanks so much for all the detail. Can I ask what kind of trail numbers you try to hit, >100mm?

    • I havent gone over 100 yet. I added more offset to that fork on the E-stay bike to keep it lower. There are a few new bikes coming out with this style geometry- the ice cream truck and beargrease i think are two.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: