I've wanted to build a custom bicycle for many years. I started wondering how fat of a tire I could accommodate on a bicycle. I had seen pictures of the Hanebrink Technologies extreme terrain bicycle, but wanted something still more extreme.
The problem I considered was one of drive train alignment. With the fat tire, the chain had to shift too far outside of the normal bottom bracket to rear wheel alignment, in order to clear the tire width. I also wanted to make this work with gears, as I knew the large tire would weigh a lot.
Originally I wanted to spoke up a fat tire rim with a standard bicycle hub, but this proved impossible due the the small rim size with the large tire.
After seeing one of the most creative bike ever (see Ode to the Chupacabra on
 I got the idea of using a standard geared rear hub as a jack shaft or transfer hub between the front (bottom bracket) alignment and the rear tire alignment. This also solved the problem of gearing the bike, since I didn't have to mount the gears on the wheel itself. Thanks to The Atomic Zombie for instruction on one of his bike for the widened hub concept and for much other inspiration.
It weighs a lot, but it is a blast to ride and you never saw a bike get so much attention.
It took me a few months of noodling this around in my head and on paper before I started in earnest to create the "Fat Bastard" extreme fat tire sand bike. Hope you enjoy it as much as I do.

Let me also just say, for the record:
I did not set out to make the most efficient, or lightweight, or highest performance bike I could. It was all about maximizing tire size and working out the drivetrain issue. This was for fun! And also to learn how to weld and to see where this project might lead.


Step 1: Getting Started - What You Will Need

What you will need are:
1. The ability to weld. I never welded before this, but wanted to learn. I bought a $99 120V stick welder from Sears and taught myself on this project. It was painful, since the stick welding was pretty dirty so I had to do a lot of chipping (away the slag) and grinding and rewelding. Eventually, body putty covered all the rough (but strong enough) welds.
2. Some steel donor bikes and rigid electrical conduit. I cut up quite a few bikes to get the parts I needed. What parts you use and exactly how you configure the overall frame design are not critical. What is critical is placement and alignment of critical components. I
3. Wheels - I ended up using rear wheels from a Yamaha Blaster atv, because that is what I could find on Craigslist locally. It turns out they were a good choice because of the 4 bolt pattern which I will explain in later steps. Also, you can finda decent selection of used ones on ebay or other places online with tire sizes up to 11x22 (yeah baby!). I started with the wheels and designed the bike out around those. You need to know how wide to make the frame. I actually started with giant blue knobby tires and ended with rounder black knobbies (the first ones were a bit too square and messed with the handling)
4. A bunch of purchased (motorized) go-kart components from BMI Karts. More about those later. And some purchased bike components.
5. Basic knowledge of bicycles and power tools. I bought a 4" angle grinder and used a ton of cut-off wheels, flap wheels and grinding stone to cut and shape almost every component on the bike in some manner. This was a critical piece of equipment. I used my welding gloves and a face shield. I used flap wheels to remove galvanizing from conduits prior to welding. Do not weld galvanized steel. Also used conduit benders for curved framing members.
6. A big piece of cardboard for layout
7. Patience and some creativity
8. Time
Nice job on getting it all together. I have built a fat tire bike for kinetic sculpture racing. Here are some solutions that builders may find interesting. Dealing with the chain line and gearing are really a puzzle. Here is the drive system I built. It uses an 8 speed hub as a jackshaft that runs cogs on the disc brake mounts of the hub and rear wheel. It has a second gear set up as a one speed on the other side for road riding.  The 8 speed is for sand, mud and hills. I rode this at Arcata and Corvallis last year and managed to ace the sand at Arcata, and ace the course at Corvallis. I will put up an instructable about this eventually. The basic bike weighs about 50 lbs. add the water gear and it gets up to 75.  What really makes this challenging is that for effective traction in the sand, you need the rider's weight on top of the rear wheel, which makes all the chain issues even more extreme. 
You are correct. I've thought of all those issues and have been contemplating a new version with Nexus hub. The problem with a 10" wide tire is the pedalling clearance at the wheel, necessitating either pushing the rear tire back quite far, as in my case, or pushing the pedlas forward to get the seat (weight) over the rear tire. Also, with the extar wide tire, the hub gets pushed quite a ways outward as well. All things I have been noodling over since I built this. Nice ride. Would like to see close up photos.
<p>NICE!,But I would use a solar panel connected to a electric motor,with 12 v batteries.I seen a motor powered and it was used for a dessert machine.With mini tractor grip type tires.front rim held water extra fuel.While the rear held water or fuel also.It to was a wild to ride in muskeg .</p>
<p>I was thinking one way to reduce weight would be to use an aluminum front portion of a frame. I have a wrecked 21 speed mountain bike with a mono shock that was given to be. All that was damaged is the rear rim and swing arm when the guy came down wrong off a jump. That part would be trashed anyway except the cassette, hub and a part of the swing arm, the lower pivot. The swing arm is steel and bent, but the rest of the bike is fine and aluminum cutting that part of the frame weight in half. A steel swing arm made for the fat tire could bolt rigidly to the aluminum frame, no need for the shock with a tire like in this instructable. </p>
That is so baddass! You need to sell those
Amazing Great :-) Sent to Nephew so he can make it for my Brother :-)
<p>Just a thought..... relocating the back gears to be on the same line as the front gears...</p>
The rear cassette is in line with the front crankset.
<p>cant wait too build me a fattie</p>
<p>I love it! Going to show my son who would love it</p>
<p>Hi, great job. I restore sortlike ( Motor- ) bikes from Yamaha with 350 and 200ccm.</p><p>Therfore a hint for the tires: </p><p>in the EU you can buy those semitrailer tires in DIY stores like Biltema etc. </p><p>Otherwise seems pretty difficult because Dirtbike tires are very rare.</p>
Do you only make custom bikes or could make a racing lawnmower
<p>amazing job there . i'm planning on doing something similar as I want to fabricate my own fatbike for beaches . for mine will be trying to make my own double width wheels by welding 2 together iditabike style . I hadnt thought of the issue of wheel rubbing against the chain but what you did with the extra chain is a clever solution . </p>
<p>I like it! How is the conduit frame holding up?</p>
simple beautiful
make it gas powered
<p>Thanks for sharing.</p><p> I have a Rokon, this is the eco version of it !!</p>
I need to build this right now
That looks awesome, but it looks kind of hard to pedal, so I've got a question for you: is it hard to pedal?
Easy to pedal because of the 21 speeds, however, it is very heavy.
Easy to pedal because of the 21 speeds, however, it is very heavy.
So cool and I just moved to NC too. I have my winter project!!! Thanx. Shhh my wife thinks its over time
i have a moror cycle that has idenical tires to that and requires no pelataling baja mini bike
Several people have commented to me that all this needs is a motor. Since the purpose of the project was to make a bicycle (the pelataling kind) , that kind of defeats the objective. Some places do not allow motorized vehicles.
&nbsp;Putting a motor on this would essentially amount to a Heald Super Bronc....a VERY cool MAXI (not mini!) Bike.
...or a Rokon. Crazy little things.
Rokons are badA$$ AND 2 wheel drive!! Healds were one wheel drive but still killer.
true. one with both would be relay cool. I have tryed to ride a bike on a beach and it is hard so I like the fat tires
Agreed ;) Love finding the perfect object like that! Its one of the best feelings in DIY.
That is a sweet looking ride!
I Seee <br>
Your Ride the spine link is no good it keeps redirecting me. other than that this bike is a great imaginative project
i looked, it's riding the spine, not ride.
Holy crap!<br>I've had the wrong link out there for 4 years.<br>Thanks for picking that up.<br>I have corrected it now.
last triumph, turning it into a trike or 4 wheeler, 49cc bike motor added, it could be rather fun.
YOU my dear friend, are a friggin genius !!!!!!!!!!!!!!!!!!!!!!!!!! <br>Thanks for all the tips and tricks and steps and thinking you went through with this bike!!! <br>I made one just like it only started with a Yamaha 7 inch fat tire dirt bike frame and disappeared the motor and transmission and welded in a Schwinn 12 speed bike frame and then proceeded to play with it until it worked. Current rendition has a 3 speed front derailer and a one speed rear. Kept the shocks and the front brake and it rides a little funny as your knees have to be out at a 45 degree angle to avoid hitting the rear shock brackets and it's one really looooooong chain with no jack-shaft like yours.
Also, I beat you, mine weighs 110 pounds! And needed two 4 inch steel threaded extensions on the crank arms to keep from hitting the rear shock brackets. Those dirt bike frames are really sturdy and seriously heavy !!!
it appears the bottom bracket is lower to the ground than on a regular 26&quot; mtn bike since the rims are so much smaller and you didn't modify the basic frame design of the donar bike. Does this matter when cornering? I would imagine if you're not bunny hopping logs it wont matter.
Actually, the bike was constructed to obtain a normal mtn. bike BB height of around 12 1/2 - 13&quot;.<br>Handling in deep snow is not great due to weight of bike and geometry. Better in sand.
how well does it work on snow? <br>
I had trouble connecting with the Ride the Spine website, but I did find a direct link to the Chupacabra bike mentioned in the text body here http://www.ridingthespine.com/Journey/category/chupacabra. There are some amazing and innovative resolutions to design problems in both bikes. <br>
hey can u send me the measurements?
Can you be more specific on what dimensions you mean?
srry bout the way i put it but is there any specific mesurments for the bike?
You will be pleased to know that I thought (from your trailer vid) that you had bought this bike. Excellent work and idea.
thanks for sharing your amazing bicycle with us. can we put an engine in this bicycle?
i want to build one of these for around town use i just need to make is hard to steal what would you suggest?<br>i cant think of a nicer bike thanks
Just put it next to a really nice car :D
2 things <br>1 a lock<br>and 2 a removable chain setup

About This Instructable




Bio: Architect
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