Dirt Rag Magazine

Reader’s Rides: Custom-fabricated full-suspension fatbike

Editor’s note: Chris saw our short video about the Salsa full-suspension fatbike prototype and wrote in with the story of his own build. Thanks Chris! Got an interesting or unusual bike you’d like to share? Send some details to web@dirtragmag.com

By Chris Willsey,

Ever since I started riding full-suspension bikes in 1994 I’ve been trying to mimic the experience of riding a motocross or enduro motorcycle, in that the suspension absorbs the terrain so you are in control all the time rather than trying to overcome a machine that only works really well when on smooth trails. I haven’t bought a hard tail since 1984.

About three years ago I started investigating the new “fat” or “snow bike” phenomena and got the idea that running fat tires with lots of volume and a bigger diameter might allow a bicycle to perform more like I wanted it to on rough terrain. However from off-roading with four wheels over the years I had seen that high volume fat tires are basically non-dampened springs that bounce uncontrollably if you hit something too fast or hard. I assumed that a rigid fat bike would work great on snow or sand but it might bounce on rocks and roots. So that is when I began designing a full suspension fat bike to get the benefits of the big tires but still keep them in control.

I had been building up longer and longer travel bikes over the years and each step was an improvement in performance so I wanted to have pretty long travel on this bike too. The most logical platform to start with was an old Intense Uzzi DH that I had built into a 7-inch all-mountain bike years ago when 4-inch was the longest travel any XC bike could put out and before "all-mountain" was even a category.

After thinking for a while about how to do it, then waiting some six months gathering up the parts, which included several Intense rear ends, I started the fabrication. The main part of the transformation entailed cutting up the rear end parts, machining, gusseting, welding them back together then heat treating the finished product, all to make room for wider and taller tires.

I built the wheels using 47mm trials rims. I wanted a more round tire profile for cornering and less width because I didn’t need the flotation provided by an 80mm or 100mm rim that works well for snow. Plus I wanted to save weight where I could. To avoid tire chain clearance issues I built around a 170mm rear Hadley rear hub provided by Fatback out of Alaska. Then I had to get the chainrings in the right place, but I only had a 68mm BB shell rather than the 100mm width that has become the norm on current Fatbikes. After some experimenting I ended up with the widest Ti spindle that Profile Racing makes, which allowed for the chainline I needed. With chromoly cranks it probably adds at least a pound to the overall weight but for a prototype it works fine.

As far as gearing, I had been riding a 26-inch downhill frame as an all-mountain bike for a while and since that had no provision for a front derailleur I was running a 24 front ring only which provided me with low gears for really steep technical climbs and the 24/11 is high enough that I don’t spin out around here with our short downhills.

But when I tried that on the new FS Fat bike I found that since the bike rolled over things so much easier I was up at least 3 gears basically everywhere compared to the 26-inch. Unfortunately with the chainrings hanging out it space so far from the seat tube, any normal front derailleur setup was not going to work. After several other attempts I ended up fabricating an E-type hanger that lowered and spaced out the derailleur to just the right spot and it’s been working ever since with 22-32 front rings.

My theory was right about the fat tires and long travel suspension combination. I roll over rough terrain so much easier and faster that I can keep up with guys who would usually be much faster than me. They only catch up when it gets smooth. I am pushing a 37lb bike after all. I run 9psi front and 14psi rear. That seems to be a good compromise offering compliance but good rolling resistance and rim protection. The rigid Fat guys run considerably lower pressures.

There is one really rocky trail that I always ride when I build up a new bike or modification to test for bump compliance. There are several technical rocky climbs where I have to be pretty on to get up and sometimes I don’t. There is also one section that I have never gotten up, even once, despite trying since the mid ‘80s. Yeah the mid ‘80s!

When I took this bike on that trail two things happened. One was that I cleaned the "can’t be ridden" section and after getting to the end of the trail I was thinking "What happened to those hard climbs?" Turns out in my enthusiasm I had just ridden right up them like they were hardly there. To prove that it wasn’t a fluke I’ve been back a couple of times to that trail and had the same results.

I just built up a long travel DH/AM 29er bike out of a newer Uzzi with VPP to see if it was just the big tire diameter that was giving me all the advantage sine I had yet to own a 29er prior to this one. I’ve been riding it for about three weeks now and while it is better than normal 26ers it’s still not on par with the full-suspension fat bike, even though travel is about the same and tire diameter is slightly larger, plus the bike weighs about 5lbs. less in all-mountain trim. I have several cool 26ers and I really don’t want to ride them any more, except for novelty sake. I know I’ll go slower and use more energy to get there and probably go shorter too.

I’m sure it’s not for everyone. The rigid SS guys will probably put up the greatest resistance. Many will think "Yeah whatever. It can’t be that much different." and "Aren’t those big heavy fat tires slow?" Well, since there are almost no full-suspension fat bikes in the world for them to try, I guess they will just have to wait until someone starts producing frames so they can experience the revolution for themselves. Hopefully I can get the motivation to make more myself to help push that along.

 

Print

Back to Top