Introduction: Fat Tire Trike

About: I'm a mechanical engineer and an avid creator of things; often ridiculous, frivolous things, and sometimes just plain old practical things.

A tricycle is eminently practical for medium distance travel - where you could walk but it would take a while, and you could bike but you've got stuff to carry, and maybe sights to see along the way. Maybe you're going to have a drink or two at your destination and the added stability of three wheels eases your worry that you'll scrape your knee!

Before I started working on this trike, I had another tricycle (pictured above deep in a desert dust storm) that I managed to rescue from an industrial scrap dumpster. It was a sun atlas industrial cargo trike, and it served me very well for 5 years. It had some shortcomings though - if anyone sat on the rear "trunk" it would lift the front wheel up, making it impractical to carry a passenger. Over soft ground it would also bog down, and even with a 5 speed gear hub, sometimes it would sink in and stall out completely.

In the last few years we've seen the advent of sort of silly looking, but sort of awesome looking fat-tire mountain bikes, even from economy manufacturers, and I've started to see these trike adaptations, so I set out to make my own! This turned out to be a bit of an undertaking, as it's not a simple bolt up conversion. I had to weld, I had to learn how to build wheels, I had to learn all about the different kinds of fat wheels and how they work. PERFECT.

Step 1: Things You'll Need

I don't like putting a bill of materials with these projects, as I'm sure anyone else building this will put their own spin on it. So I'll list the basic elements you need, and give you some options for how to figure them out.

Donor Bike: Discussed in detail in the next step. Starting with a fat tire bike gets you a lot of expensive parts for relatively cheap right off the bat, but does require some extra work to attach the trike rear end.

Three Fat Wheel Assemblies: Trikes use special hubs. This means you will need to either have custom wheels built, or build them yourself. I'll discuss this in more detail in its own step later on.

Trike Rear Axle Housing Assembly: I actually had one already, but they are available online for $70-$100. I will discuss this in detail in its own step later on.

Some steel plate and some welding skills: You could just convert an existing trike (like my sun atlas) to a fat tire model, in which case you won't need to weld anything but in general you will probably need to do at least a little bit of fab work to make the trike rear end work with a bicycle frame.

Other Bikey Stuff: You may have all of this already from your donor bike; stuff like a seat and handlebars and grips and shifters (if using a gear hub) and brakes and so on.

SAFETY NOTE: Tricycles do not have rear brakes, unless you use a gear hub with a coaster brake. It's very important to have a good functioning front disc brake or you will have a very hard time stopping. This is a heavy machine compared to a bicycle and it takes some stopping power.

Step 2: Procure Donor Bike Frame

I used a Huffy special edition "Darth Vader" bike. I dunno if this will anger any memorabilia collectors - I hope not. I like to think I took a limited edition cool thing and made a one of a kind awesome thing, so there.

The things I wanted from the donor were:

  • 2 fat tires (these are around $40-50 to buy separately)
  • 2 fat tire tubes - also about 20 bucks apiece
  • 2 100mm fat rims (these are $50-$70 each for cheapies and up to hundreds for nice lightweight ones)
  • Frame - a regular bike frame would actually be easier to work with due to the rear stay spacing. More on this later when we start trying to attach the rear axle housing. I really liked the look of this frame though, even though it's very cheaply made, and quite heavy - I'm not planning on doing any acrobatics with this beast.
  • Fat Tire Fork - These are around $100 to buy separately - you need a special fork to fit the wider tire in. If you're using a regular frame you'll need to make sure you get a fork that fits your frame's head tube. In my case the two came together so I didn't have to worry about it.

So at this point I had parts that would cost around $400 to buy individually just by starting with a fat tire bike.

Step 3: Build Your Wheels

I tried to get around building my own wheels. Really, I did. Then I toyed with making outlandish laser cut and welded aluminum 5-spoke wheels for a while... then I decided I wanted this to happen in reality and I bit the bullet.

The tricky part is that tricycles use special hubs for the back wheels, so either you need to buy custom made tricycle wheels from a wheel builder, or you need to buy the hubs and the spokes and build them yourself. The good news is, fat tire wheels are rather easy to build. The hard part is figuring out what spokes to use.

Now there are some resources that go into great depth on the subject of wheel building. Since I'm definitely not an expert after 3 wheels, I'll just assure you that it's really not that bad, and point you to a few sites I read before I started:

An Excellent Assembly Guide:

Fat bike Spoke Length Calculator

An Instructable just for wheel building!

The parts I used were:

  • 2x 26x4" 36 hole rims from the donor bike.
  • 2x trike hubs from husky bicycles.
  • 10" 12GA spokes from husky bicycles that I painted black to match the wheels.
  • 12GA spoke nipples that were included with the spokes - you may need to buy seperately depending where you get your spokes.
  • 1x 26"x4" 36 hole fatbike rim I bought on eBay for around $70 from seller "junkyrustybikes".
  • 1x front hub recycled from the donor bike.

I ended up taking apart and reassembling all the wheels so I could have the one slightly different wheel on the front, with the original bike hub, and the two matching ones on the back with the new trike hubs.

Step 4: Rear Axle Housing Assembly

Husky Bicycles sells several variants of trike rear axle housings. I used the Sun Atlas 17mm version.

You will also need:

  • Axle Bearings (Included with the axle housing, I mention this only for reference)
  • 17mm axle assembly - Comes with key, nuts, washers, and spacer.
  • 17mm axle sprocket
  • 17mm hubcaps (Optional)

Note 1: I needed to modify this housing to work with the wide rear stay spacing on my fat tire bike frame. In the photos above you can see first how I mocked up the unmodified housing, then welded new brackets to it that line up with the wider stays, then cut off the original brackets. I also included a couple of reference photos from my atlas trike so you can see how a factory produced connection looks.

Note 2: You'll notice the bike frame has already been modified to accept the trike axle housing brackets in this step. This is because I had an existing trike to reference, and I modified the bike frame to match my trike frame before I got to this step. Assuming you do not have this luxury, I'm leaving that step until later in this instructable.

Step 5: Modify Bike Frame to Accept Trike Axle

For this step, I had a leg up on anyone doing this without an existing trike to reference, but I'll lay out the steps I would take if I hadn't had this luxury.

Basically, the idea is to mock up the rear axle housing to the bicycle frame and scratch your chin. Then make a cardboard tracing of the axle housing brackets, and hold it up to the bike frame and do a bit more chin scratching. Then make a couple more cardboard pieces, with the holes marked in the right place, and the outline of the rear axle housing brackets traced on them. Hold these un-trimmed tracings up to your frame and decide where to weld, then trim away excess material until you have something that will both fit to your bicycle frame, and accommodate your rear axle housing.

A note on how these brackets are supposed to work:

The axle side brackets have one hole towards the rear - maybe 1.5" forward of the axle tube. They also have a slot, about 4" forward of the hole. On the bike frame side, this is reversed, with the hole at the front and the slot at the back. This is very important - do not skip the slot. The purpose of this is to allow you ~1" of forward and backward adjustment on the distance between your crank axle, and your rear axle. The reason this is so important, is this is how you tension the chain. When you finally put your drive train on, the way you go about it is to push the rear axle all the way forward in its mounting slots, put the chain on, then pull it back until the chain is tight, and tighten the bolts. I will try to add a picture of this later on.

Photo Note 1: There are two holes shown on each bracket - the forwardmost hole is there so I could bolt the bracket to the existing hole in the frame designed for the bike wheel. Because I made the two brackets identical, I knew if I bolted them both through the axle holes on the frame before welding, they would be in the right place relative to one another. The holes serve no further functional purpose.

Step 6: Modify Bike Frame to Accept Gear Hub

You can skip this step if you're not using a gear hub. I had a Sturmey Archer 5 speed hub on my other trike and it was really a leg saver, so I decided to salvage it and put it on the new trike. The problems are:

  1. Bicycle frames have no brackets for gear hubs
  2. The rear chain stays on my trike are considerably wider than the mounting width for the standard gear hub.

I dealt with these issues both at the same time by bending some steel brackets out of 1/8" mild steel plate and welding them to the chain stays where they would not interfere with anything. This bike frame is already not usable as a bike frame anymore, so it doesn't matter that these brackets are now right where the wheel should go.

Note: The slot here is important for the same reason it is important on the rear axle brackets. With a gear hub you install the chain on the front sprocket and the gear hub, and pull the gear hub until the chain is tight. Then you put the chain from the gear hub to the rear sprocket, and pull the rear axle housing until BOTH chains are tight, tighten the rear axle brackets, then tighten the gear hub.

Step 7: Slap It All Together

I'll assume that if you've got this far, you know a thing or two about how to set up a bike, so this should all be pretty self explanatory. Get your tubes and tires on your wheels and pump them up. Get your wheels on the trike, hook up the brakes and your gear hub if applicable, put the chains on and tighten everything down. Once again make sure your front brake is in solid working order, and/or that your coaster brake (if equipped) is working properly, then take'r out for a rip. I guarantee you will get some unusual looks of either confusion or admiration or both.

Enjoy; and please send me your questions; I'll endeavor to update and clarify to answer any points of confusion.