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Picture of EXTREME FAT TIRE BICYCLE
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
 http://www.ridingthespine.com/Journey/category/chupacabra
 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.

IF YOU LIKE, PLEASE GIVE THIS INSTRUCTABLE A GOOD RATING. THANKS!!

 
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Step 1: Getting started - what you will need

Picture of 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

Step 2: Bike layout and design

Picture of Bike layout and design
This took quite a bit of time and experimentation.
I started with a large piece of cardboard. You can get a bike box or two from your local bike shop. Mine was a weed trimmer box from my local lawn tractor dealer.
I cut up and used various pieces from several bikes to get a sense of how the bike might be configured. I started with some small frames I got for free from (my local freecycle.org). I ended up buying a larger framed bike for $40 from Craiglist to cut up.
In any event, do the layout and figure out what frame piece you can reuse and assemble into your new frame and what pieces might need to be fabricated from conduit or other steel tubing you can find.
I used the primary donor bike frame along with (oval) pieces from a bmx bike to form the horizontal bottom tube and 1 1/2" rigid galvanized electrical conduit for the new extended top tube. I ended up cutting the seat tube and inserting it through a hole in the conduit then welding the seat tube back together. I welded it together with an outer strap in order not to reduce the inside dimension required for the seatpost.
Note the drawing showing the wheel locations, head tube/fork angle, bottom bracket height, etc. Also critical here was the measurement for the swing of the crank and the rearmost dimension of my (shoed) heel in order to clear the rear (stationary) swingarm frame.
I returned to this layout drawing many, many times. Take you time and work things out here where changes are easy to make.

Step 3: The wheels

Picture of The wheels
As I mentioned earlier, the wheels are from a Yamaha Blaster atv.
The rims have a 4-bolt pattern that is close to one of the go-kart industry standards of 4on4 (4 bolts on a 4" diameter circle).
As I said, they are close.
I ended up drilling out (enlarging) the (3/8"?) holes for the larger (1/2") bolt diameters of the go-kart hubs and enlongating the holes a bit due to the not-quite 4on4 pattern of the rims.
But it was close enough and I did it all with a battery powered drill with a 1/2" bit.

Step 4: Rear swingarm and front fork

Picture of Rear swingarm and front fork
I had a little help on this part of the build.

While fishing through the dumpster of my local bike shop for the cardboard to do my layout, I came across a bunch of car-mount bike rack parts (from display models) that they were discarding.

It just so happens that one of the u-shaped armatures was the perfect width to serve as the rear swingarm. It provided enough clearance for the chain drive and a disc brake.

Another was slightly narrower, which perfectly suited the width of the front tire without chain drive or brake and therefore served perfectly as a front fork.

I was really lucky. I had some ideas for building these components, but didn't have to.
I wold recommend visiting the atomiczombie.com website for suggestions on this if you can't work it out yourself.

Step 5: Rear swingarm, axle and hub

Picture of Rear swingarm, axle and hub
The rear wheel used a live axle, that is the axle was secured to the hub and turned the wheel.

The axle is a 1" solid steel. It has threaded ends and I bought 1" lock nuts for it. It has a keyway to keep the hub fixed from rotation around the axle. I think I used 16" length.
1" Live Axle with 1/4" Keyway. Lengths include 14", 16", 18", 36", 40", 45" or custom size.
Part #4000**, SKU Number:4000**
1" Axle Nut
Part #400050, SKU Number:400050

The axles rotate in a set of 1" bearings supported on steel bearing hangers
1" Bearing Kit (2 hole)
Kit Includes:
  • (2) 1" Bearing Hangers (have very minor surface rust)
  • (2) 1" Axle Bearings
  • (2 pair) 1" 2 hole Bearing Flangettes
  • Nuts & Bolts
Part #400405, SKU Number:400405

The swingarm was slotted to accept the bearing hangers (steel plates) which were then welded in place. they also could have bee just welded to the tube, but this swingarm had such thick tubing and my welding skills were so bad, I thought this was safer for this project. My frame layout and geometry (on the cardboard) was based on the slotting method, otherwise I would have had to shift something somewhere about 3/4".

The rear hub is a (Wheel Hub 1" Bore Wheel Hub with 1" Bore 1/4" keyway, 1/2" 4 bolt pattern on a 4" bolt hole circle SKU Number:260050

Also shown in picture is the sprocket hub I later cut down to size and mounted a chainring to in order to turn the rear axle.
Unihub - Sprocket Hub
1" Bore with 1/4" keyway, 6 Bolt Holes on a 5 1/4" Bolt Hole Circle
Part #AZ2556, SKU Number:AZ2556

Step 6: Front fork

Picture of Front fork
I cut the legs from the fork of the donor bike, and grinded the legs to suit the new fork, then welded the two together.

I cut two flat plates from another rack part and slotted the new fork legs to accept the plates.

The plates were predrilled with 3/4" holes to accept the front axle (my brother-in-law drilled tehm at work since I didn't have a 3/4" bit), before welding the in place.

Front axle was solid 3/4" steel

The hub and axle are not live like the rear. The hub is free spinning on the axle, with integral bearings.
Heavy Duty 4 on 4 Wheel Hub
Has a 4 on 4 bolt pattern and accepts a 1 3/8" OD bearing
Comes with 3/4" bearings but 1/2" and 5/8" are available.
SKU Number:260056

The axle is held in place on the fork and the hub held in place on the axle by several locking collars
1 Piece Steel Lock Collar
High quality steel lock collar. 1 piece design with grade 8 set screw.
Part # 600034, SKU Number: 600034

Step 7: Front wheel hub

Picture of Front wheel hub
A view of the front wheel hub with integral bearings

Step 8: Locating the jack shaft

Picture of Locating the jack shaft
I had figured out that the jack shaft would be located up high, behind the sweep of my heel on the arc of the pedal motion.

I laid it out on cardboard based on the heel arc, the size of the freewheel cassette ( I opted for a Shimano 7 speed with the 34 tooth mega-range cog).

Step 9: Basic frame assembly

Picture of Basic frame assembly
So now I was pretty much ready to assembly the basic frame.

It's starting to look like this thing might actually happen.

Euphoria sets in, my wife still thinks I'm crazy, but others start to see the vision as well.
This is going to work!

Step 10: Constructing the jack shaft/transfer hub

Picture of Constructing the jack shaft/transfer hub
I started with a steel rear bicycle hub with freewheel off one of the donor bikes (I think it was a Magna).

I cut the hub in half and inserted a length of steel tube cut from one of the bike rack parts that formed the front fork or rear swingarm.

I had done some basic calculating and prealignment to figure out roughly how wide the overall width had to be.

I welded a 17 tooth cog from a donor freewheel onto the side opposite the freewheel.

I also ground flats on the freewheel side of the hub flange in order to grip the hub in a vice to remove the freewheel, since it is nearly impossible to remove the freewheel from the hub after you remove it from the spoked wheel (I found that out too late, but this was my solution)

I made a jig for keeping things aligned (as much as possible with my crude techniques. Things are not perfect, but good enough for a prototype.

Step 11: Aligning jack shaft

Picture of Aligning jack shaft
I used one of the fork legs I had cut off earlier as the left side support for the jack shaft, having aligned it with the rear tire chain drive and tacked it in place.

I bolted the jackshaft in place and used the fork to hold it in alignment while I went to the other side and cut and fit pieces of a rear triangle from a donor bike to support the other side.

Additional conduit framing sections added to complete frame.

Step 12: Jackshaft alignment and installation

Picture of Jackshaft alignment and installation
This photo show the right side support of the jackshaft using the rear triangle of a donor bike.

I had to tack this side in place as well and then tweak things around on both side to complete the alignment and weld it all home.

I also added a piece of seat tube to mount a front derailleur. Since the angle of the rear (right side) drivetrain was changed by locating the jackshaft high, I had to provide a front derailleur mount that matched the revise drivetrain angle.

I later had to add side and rear struts to the left side fork support because the torque on the chain shifted the fork mount and misaligned the whole left side drive train.

Now I'm set for the 21 speed drivetrain.

Step 13: Rear drive

Picture of Rear drive
As mentioned earlier, I used a sprocket adapter to mount a 30 tooth chainring.

I used spacers to keep the chainring clear of the sprocket adapter.

This step was a mess and a bit of a problem. The sprocket adapter was a 6-hole setup. The chanring was 5-hole. Needless to say, the chainring holes could not be aligned with the spaces between the sprocket adapter holes. I had to add extra shims and back plates to get the mounting straight and rigid.

Step 14: Disc brake

Picture of Disc brake
I installed the right side disc brake similarly to the method I used for the left side drive chainring.

Instead of using a sprocket adapter, however, I used a go-kart disc brake. It was a solid steel disc for mounting on the live axle.
Disc Brake Rotor with 1" Hub
7.87" OD, 1" Bore with 1/4" Keyway
Part # 400285
SKU Number:400285

All I had to do was drill and thread 6 bolt holes to match the bicycle 8" disc rotor. The difficulty here was the rotor bolt holes were on a circle diameter that did not coincide well with the disc brake. The holes were half in and half out of the main hub portion of the disc brake. I also had to find the right size tap for the rotor bolts. Anyway, after much aggravation, I was able to make it work.

The mechanical disc brake caliper and 8" rotor were Gator brakes purchased from Pricepoint.com. They were the most economical and they work great.

I tightened the caliper to the rotor, then aligned a steel plate and marked the bolt hole locations. I then drilled the holes and welded the plate to the frame.

Step 15: Chain tensioner

Picture of Chain tensioner
I had to add a chain tensioner (Sette chain tensioner also from Pricepoint) to the left side drivetrain, since there was no axle adjustement or derailleur to take up the slack.

Another mounting plate, complemented with a spring, and there yuo go.

Step 16: New tires

Picture of New tires
As I mentioned before, I found the largest tires I could for the 8x8 rims. These were the blue knobby 11x22 tires you've seen in most of the pictures.

The problem is they were so square in profile that you had to lean 2 feet over the side of teh bike to steer at speed.

I ended up buying a used pair of tires/wheels. These are much rounder in profile and handle much better.

Step 17: Transport

Picture of Transport
Don't forget that you'll need a way to carry this baby around if you don't have a truck.

I opted for the custom hitch-mount rack, courtesy of a Harbor Freight hitch-mount cargo carrier and some wood and tie-down straps.

Step 18: Visit the beach

Picture of Visit the beach
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You also might want to construct a surf fishing trailer to get all your gear to and around the beach.

Bottom frame is made from aluminum angle screwed together at corners.
Bottom is heavy duty aluminum mesh.
Sides are made from 1 1/2" PVC conduit posts and 3/4" PVC conduit rails.
Axle is 1/2" steel rod.
Tires are from lawn tractor
Hitch arm is electrical metal tubing (EMT)
Articulating mount is made from a 2"x2" steel tube welded to a swivel caster. Caster is cut square to fit into bike hitch.


Step 19: Ongoing Tweaks and Project Development

Picture of Ongoing Tweaks and Project Development
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After 6 months of use, I decided I needed to make a few tweaks.

The tires were Carlisle Turf Tamers, which had regularly spaced square knobbies. I had purchased the tires, on the rims, on Craigslist for cheap and threw them on, after the initial blue, square tires turned out to be, well, too square. The tires ended up being too tame for the soft sand at the beach and the tires would spin at times and always when pulling the cart (in the soft sand).
With winter approaching, I decided it was time to install a new tire that might work better in the sand and possibly snow. I just purchased and installed a Maxxis 4-Snow tire and, while I haven't tried it at the beach, I did try it on a nearby sand volleyball court and it works well, although the sand was not super soft at the time. The reviews on the tire performance were excellent (for an ATV) and it ended up being super-light compared to other ATV tires. Hopefully snow performance will be OK.

I also ended up changing the sprocket on the jack shaft from a 17 tooth to a 28 tooth. The 17 tooth reduced the gearing too low. I was never getting into the granny gear (smallest on the triple crank). I just welded the 28 tooth onto the 17 tooth (this was a sprocket from a freewheel cluster). Now I will also have a higher top end when cruising around town.

One issue that arose with the new sprocket was that the chain length was too long for the chain tensioner, so I had to add a leg onto the chain tensioner to extend it and take up the extra slack. No big problem, but things were getting pretty close to scraping on the frame. Oh well, it's all just a big experiment anyway.

link to Video of test ride on sand volleyball court
http://s226.photobucket.com/albums/dd52/mscarani/FAT%20BASTARD%20BIKE/?action=view&current=004.flv

Step 20: Ever evolving uses for the Fat Bastard

Picture of Ever evolving uses for the Fat Bastard
BionicBlue5 years ago
Kudos on the rad bike.
mankysteve5 years ago
Why did you have to remove the freewheel surely having a free wheel here would make a huge difference.?