Introduction: Homemade Fat Wheel Mini Bike
I've been looking to make a mini-bike for ages now and finally got round to it. For a while i was collecting ideas on my Pinterest board here:
In true OpenSource style, the design of my bike here is based on a bunch of ideas I've seen online remixed with my own twists which hopefully someone else can take and remix+share.
I was hesitating doing a write-up as I was sort-of learning/winging it as I went along since this my first build. It was also the first time bending tubing (which is something I've been meaning to figure out how to do for a long time!). Anyway the bike turned out great and is an absolute hoot to ride so I thought I'd go ahead and do an Instructables.
WARNING: Since a lot of this was learning on the job, the photos/steps are not perfect. That said I'll make every effort to include enough information in the text for you to be successful. I also reverse-engineered some rough plans and a CAD model of the frame which should help!
Failing that please comment on the thread and I will be more than happy to answer questions for all of you.
Anyway, enough preamble, lets build a bike!
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Step 1: The Stuff You'll Need
- 4M of 1"diameter 16swg mild steel tubing (for the frame and the exhaust)
- 1M of 3/8" 16swg mild steel tube for the foot pegs
- 30cm of 32mm mild steel round bar (for the drive sprocket hanger)100x125mm aluminium round bar300mmx300mm 6mm mild steel plate
- 300mm of 6mmx6mm metric key steel
- 100mm of 50x50 aluminium square bar to make the handlebar holder.
Wheels / hubs / axle etc
- 18x9.50-8 knobby tyre on four stud rim: https://www.ebay.co.uk/itm/18x9-50-8-knobby-tyre-...
- 1000x25mm 6mm full keyway axle like this one 25mm x 1020mm. https://www.ebay.co.uk/itm/25mm-x-1020mm-Solid-Re...
- 1" Bore Go-kart drive hubs 4"x 4 studs with 1/4" keyway M12 Studs with nuts https://www.ebay.co.uk/itm/112679862230
- 19mm Brake disk https://www.ebay.co.uk/itm/283426435555
- 429 pitch 102 link motorbike chain https://www.ebay.co.uk/itm/Pitbike-Motorcycle-Ste...
A brake calliper and lever(master cylinder) like this one. NOTE: I'm pretty sure this is identical but the listing for my one no longer exists as I took the brakes off my drift trike. Double-check the dimensions before drilling/cutting the mounting plate. https://www.ebay.co.uk/itm/Hydraulic-Rear-Disc-Bra...
- 1x Motorbike Flat Cushion Cafe Racer Seat https://www.ebay.co.uk/itm/333326505854
- Honda GX160/200 engine mount plate(could make your own out of a bit of 10MM mild steel but life's too short) https://www.ebay.co.uk/itm/Honda-Gx160-Engine-Mou...
- Monkey bike petrol tank. https://www.ebay.co.uk/itm/MONKEY-BIKE-TYPE-A-MIN...
- Rear Sprocket 60 Tooth Pitch 420 https://www.ebay.co.uk/itm/Rear-Sprocket-60-Tooth...
- Titanium exhaust heat wrap https://www.ebay.co.uk/itm/Titanium-Heat-WRAP-10M...
- Throttle grip https://www.ebay.co.uk/itm/7-8-Throttle-Accelerat...
- Some BMX handlebars (7/8" thickness)
- Throttle grip and cable https://www.ebay.co.uk/itm/7-8-Throttle-Accelerat...
- 300mm of M12 threaded rod.
- Plenty of M6, M8, M10 nuts and bolts.
- 4x UCFL205 bearings like these https://simplybearings.co.uk/shop/p21467/UCFL205-...
- Welder (Mig or big is fine)
- Tube bender
- Digital inclinometer https://www.amazon.co.uk/KKmoon-Inclinometer-Elect.
- Pillar drill
- 1" hole saw
- Lathe (to make the custom sprocket and brake disk hangers and the headstock bearings)
- M6 Tap
- Angle grinder
- Large hacksaw with a HSS blade
Engine and transmission
The engine I had lying around from a previous build was a clone of a Honda GX200 like this one:
Loncin G200F-EP Engine with 3/4" Shaft - Replaces Honda GX200 https://www.ebay.co.uk/itm/Loncin-G200F-EP-Engine...
This is a very popular engine design used in everything from compressors, generators, log splitters, go karts and even wake-board cable winches. Needless to say, there are loads of spares/mods out there and make them a common choice in kart and minibike builds. There is also a smaller sibling the GX160 which would be fine too. The GX200 is meant to be roughly 6.5HP stock but you can squeeze twice that out of them with the normal tuning hacks (i.e a big valve cylinder hear, higher compression piston, billet conrod + flywheel, larger carburettor, ignition advance, big exhaust etc...). What's also nice about these is that they are mechanically very simple and self-contained i.e. no external wiring loom to worry about. Pull start is also a plus.
Transmission-wise, I had a Comet Tav2 torque converters kicking about:
(10t 420Pitch final drive. 3/4" parallel shaft to match the engine): https://www.geminikarts.co.uk/product/218355a-tav...
A note on torque converters. These are the same transmission as you would get on a modern scooter like a Vespa GTS. It's basically an automatic transmission without discrete gear changes that uses a rubber drive belt (also known as a CVT for Constatnt Variable Transmission). With these, the engine reaches the optimum RPM range almost instantly and stays there until reving-out at the top end. This way you have maximum torque throughout the speed range.
This setup means my minibike shoots off from a standing start like absolute stink and will pull power wheelies from a walking pace with a firm fistful of throttle. Nice :-)
Pros: Really get the most out of the engine. Brilliant acceleration.
Cons: Expensive. Some maintenance to consider as the belt is consumable all will need replacing after a period. The unit is also quite wide giving the bike a fatter, and slightly lopsided look.
If the cons don't put you off go for it (remember to get the smallest drive sprocket you can and a pitch to suit your chain. In my case 10T 420 Pitch). If I find the time, I'll make a video comparing the two to help you choose.
If you don't fancy a torque converter, a traditional centrifugal clutch like this one will work fine.
Beware that you probably need a longer chain for this setup.
A note on gearing: I set the bike up to top out at about 30MPH. I calculated this using the wheel diameter (19"), the rear sprocket teeth (60), the front sprocket teeth (10) and the max RPM of the engine (3600) to give me 33MPH. If you are trying different ratios you can use a speed calculator like this if you can't be bothered dong the maths yourself:
The 6:1 ratio I have here is probably the highest you can achieve as the 60T rear sprocket is the largest I saw for sale anywhere online (at least in a 420 pitch). Some builds I have seen use a Jackshaft to further increase the gear ratio but I think this is a bit overt the top and makes the bile look messy.
Step 2: Make the Frame
So this bit was a little bit trial and error for me as I've only just managed to get my hands on a tube bender and this is my first time using it. I started getting up and running following the excellent tutorials from The Fabricator Series on YouTube.
After I finished the frame, I measured things and sketched up some rough plans (see the files section for the PDF).
- The first step was to figure out the parameters of my tube bender and you will have to fo the same for yours. In essence, this consists of doing a 90deg test bend (as explained in detail on the video series linked above), marking 1" gradations and marking the bend at 10 deg increments until you hit 90 deg. That gave me the Cost of Material (COM) =4.5" for my setup so I knew how much metal to use and where to mark it out. Following my marks too closely won't be exact for your setup so I really suggest understanding the principles before setting off.
- Bend the pieces shown in the plans using a 3" CLR die. The digital level comes in to ensure both sides of the symmetrical 'U' shape pieces are straight. As you can see from the pics, mine didn't turn out totally level because I didn't take enough care. That said, it was possible to fiddle things later on to make them line up better.
- I used a hole saw to notch the tubes (as shown) but those with patience can use a coping file too. I always try to get the best fit I can before welding to get a good strong joint. With all the angles a bit off from the tube bending, this was difficult to do! take your time here and it will pay off (as I should have done),
- Once the tubes look like they fit OK, tack the frame up and make sure everything is straight before welding up.
Step 3: Make 4x Bearing/Axle Hanger Plates
To make the bearing/axle carrier plates, drill out the 6mm steel plats per the template shown in the images. I later modified these to allow the axle to be removed more easily without needing to disassemble it (which is always a pain in the a....). Just cut from the edge of the cutout hole as shown.
Step 4: Mount the Engine and Rear Axle
So this bit was **major** trial and error but worked out OK. You can see some of the mistakes I made and how to avoid them when you come to build. In summary, the rear chain stay far too short and I had to mount the bearing carrier all the way off the back of the tube and then strengthen with some 6mm plate. This looks a bit messy and the whole build will be cleaner if that tube is longer and the carrier just welds on underneath. The plans I have included with this Instructable have a longer chain stay so this should be OK. Please check for yourself.
- Cut the 1M axle in half and remove burs at the end that would prevent it sliding into the bearing cleanly.
- Assemble a rear axle with Bearing-->hanger plate-->sprocket --> tyre-->hanger plate-->bearing
- Dry fit this onto the rear subframe and tack weld in place.
- Spend a decent amount of time ensuring that the wheel is running true with the frame. I didn't have a scientific way of doing this so I just eyeballed it + used a meter rule to help see angles vs. the frame. Bash true with a wooden mallet if necessary.
- Place the engine on the mounting plate and prop up with some bits of scrap wood etc as shown
- Get a chain around the drive sprocket and rear sprocket and move everything into alignment. Since we can't just the rear wheel on this setup, we'll have to make use of the mounting holes in the engine mount plate in order to adjust the chain tension. Make sure things are positioned so we are roughly in the middle of the adjustment holes to give us lots of room to play about later.
- Once things look ok, tack weld, straighten up and then blast it with a hot weld.
NOTE: not shown in the pictures/plans is a little cross-member tube i added to support the engine mount underneath. Simply just get an off-cut of the 1" tubing, not it both sides and slide into the gap + weld in place.
I had to move the engine really far forward to try and avoid chain slap against the chainstay given the angle of the chain. This meant relying on only 3 out of the 4 engine mount bolt holes (yuck!). See if you can do better when you mount the engine. Maybe a chain tensioner wheel? I'm not sure if the large size of the rear sprocket adds to the problem with the angle but with a bit of fiddling about you might be able to do better than I did.
Step 5: Make the Forks
To make the forks, I did the tightest 'S'-bend i was able to with a 3" CLR die in the tube bender. I would guess yours will be the same. Don't worry about the precision on the width too much. There is plenty of room to adjust things later/add spacers etc. Just make sure the fork legs are the same on both sides
The steering yokes (at least I think that's what they are called. The bits that the forks are mounted to) are cut from 6mm steel using an angle grinder / plasma cutter and drilled per the plans (1" hole for the forks. 16mm hole for the steering bolt. The distance between the yokes should be 160mm + enough space to add a couple of washers or bushing, In my case I added 8mm. (I think the plans show it a bit tighter).
The headstock is made out of a 160mm piece of 1" tubing mounted onto the frame at 114 degrees to the top tube. I got this angle scouring the web/discussion forum threads and came up with a rake angle that was about. average. Given the fat tyres, the bike is stable but doesn't turn very tightly. I think if building again I would try a little bit more aggressive/upright angle but the one shown here works fine.
I tacked the tube on roughly, adjusted the angle then welded on. Per the plans I added some reinforcement on either side with a bit of 6mm steel plate.
Lay everything flat and weld up nice and hot!
Note: I made a couple of brass bushes on the lathe for the headstock to make a nice tight fit for the steering bolt. Without access to a lathe, I'd recommend getting 2x 1" bits of mild steel round bar as tight a fit as you can get to the ID of the 1" headstock tube, drilling the 12mm hole through it, pressing in and then tack welding it in place while the bolt is through the holes just incase of any movement. That should work just fine.
Step 6: Make the Handlebar Clamps
I tried to figure out the quickest way of making this and this worked-out great.
- Take a 50mm square bar of aluminium and drill a 22mm hole
- Cut through the middle of the hole
- Cut off the other side as shown.
- Drill and countersink the two 8mm mounting holes in the bottom half to fit a cap-head bolt.
- Drill, tap and countersink the top half for the 4x 6mm fixing bolts. Try and tap a thread all the way through as these bolts will be screwed in TIGHT to hold the bars. I was quite scared of stripping the threads but it held-out fine.
- Use a transfer-punch to make marks on the top yoke then drill 8mm.
Step 7: Make the Custom Carriers for the Rear Brake and Sprocket
NOTE: making these from scratch proved to be quite time-consuming! Have a look so see if you can get a sprocket/carrier combo off the shelf that will just slot onto the 25mm axle with a 6mm keyway. There must be some of these on eBay and will save you a fair bit of grief.
Anyway, I wanted to have a go and is how I made the sprocket carrier.
- Start from an 8MM 125mm diameter disk (laser cut, from eBay) and mark out the centre and hole circle. In this case it was a (pitch circle diameter) PCD 100
- Use a hole saw to cut out 33MM hole
- Turn down some mild steel in the lathe down to 33mm (tightest for to the hole as possible.
- Drill out to 25mm.
- Broach a 6mm keyway.
- Weld together.
This worked O.K but there is a bit of run-out (wobble) in the disk as there is practically no way of keeping the pieces totally still while welding. The heat tends to move things about,
For the brake disk carrier, I decided to turn out of a solid piece of aluminium. This was simpler in many ways and guaranteed the disk would run true also. I think id use this method again next time (I might also remake the sprocket carrier at some point).
- Get a 50MMx125mm piece of aluminium billet.
- Drill the PCD100 holes 3/4 the way through.
- Drill a 25mm centre hole.
- Broach a 6mm keyway
- Turn down the billet fo leave only what's needed to give us enough strength.
Step 8: Make It Stop! Adding Brakes
So I first started off thinking the brakes would be on the front wheel as that's where the majority of stopping power comes from on a 2-wheeler but after playing around with the fit-up, I decided that it just looked too messy having the brake sticking out on the side like that. The back wheel setup gave me some more room to play with and, from riding the bike a few times now, this is plenty of 'anchors' for a bike that size anyway. Especially as there is just sooo much tyre making contact with the ground.
As you can see by the result this was a bit of a 'bodge-up' as they say in the U.K, but works great. I have included a template for a cutout for the brake calliper that would be a neater way to do it.
the ideal way to do it would be:
- Cut out a calliper mount from 6mm steel sheet to fit the calliper.
- Bolt the calliper to the calliper mount.
- Fit the brake disk to the rear axle.
- Slide the brake calliper over the disk
- Get someone (or some runner bands) to pull the brake lever
- Adjust so that the calliper mount is in a position that you can get a good weld against the rear stay.
- Tack weld and release the brake to ensure everything runs freely.
- Weld up.
- Add some washers if adjustment is needed.
Step 9: Make an Exhaust
This was way more fiddly than i thought it was going to be! I did a test ride with a straight through pipe (shown) but this was just ridiculously loud. The whole idea is that people can play on this all day without the neighbours getting the hump so a plan 'B' was needed. In the end I repurposed the original exhaust (muffler) and made a custom header out of the 1" pipe I had lying around. I started from a full 180deg bend and cut pieces with a hacksaw to get the shape I needed. I then welded together and wrapped in heat wrap to prevent melting stuff/ kids getting burnt.
It worked out pretty well and is quite enough. You get a noticeable drop in performance though as the bike definitely doesn't wheelie as aggressively with the new exhaust but its a decent compromise.
Step 10: Finishing Touches
Some other things that needed to be done but didn't warrant a step of their own.
- Fit some foot pegs. I was worried about the strength of welding two sections of tube to make the foot pegs. Not only was I worried about the pegs breaking off, i was worried about weakening the frame through the weld at a critical place. Instead I opted for the 3/8th in tube sitting on top of the chain stay and just welding a couple of scraps of 6mm sheet on in order to keep it in place. This is working v. well
- Fit the tank - I found some runner door stops that made the perfect petrol tank mount. All that was required was to weld on a metal rod for them to slot over. Not a great picture but you can see all this on the picture of the frame sprayed.
- Make some aluminium spacers to keep everything in place on the axle. I had some 28mm aluminium round bar kicking about that i drilled to 25mm and packed everything in. Despite there being a keyway and some grub screws, this just won't hold things in place.
- Drill some seats for the grub screws on the wheel bearings to grip onto. Without this, the axle will likely move about and also, the screws will mark the shaft meaning you'll most likely never be able to remove it again
- Grease up the axle good n' proper. If it tarts corroding you'll never be able to take it apart again.
- Fit the seat. V straight forward. Weld two lugs on the back of the frame and one small plate on the top-tube that the front of the saddle tucks under. Make sure to leave enough room for the petrol tank mount.
- Spray the frame. I used a bog-standard rattle can (Hammerite smooth / rust-oleum). Seems to look OK.
- Add some footpegs. I was in a hurry so i just 3D Printed some. Link to the Thingiverse page here https://www.thingiverse.com/thing:4078826
Step 11: Final Thoughts
Im really happy how this worked out and its been great fun to build. The project is quite forgiving for the beginner welder/tube bender so don't be put off by what looks like quite complex stuff.
couple things I'd perhaps consider doing differently:
- The setup with the wheels on live axles etc allowed me the flexibility to use those huge wheels which look great. It added a lot of additional work above sourcing some wheels that have disks already attached to the hub or even drum brakes inside. Here in the UK its hard to buy stuff like that but in U.S / Australia there seem to be many more things like that for sale.
- The 6.5HP and CVT transmission is great for an experienced rider who likes playing with the front wheel coming up into the air. For novices this thing is a bit frightening. I need to have a look at how to soften the power deliver a bit.
- I NEED to figure out how to make some fenders/mudguards now
The keyway hubs on those wheels rattle about a bit (although they can't move anywhere because they're wedged in nice and tightly with spacers.I might try a shim or two or just accept that this is a home-made rig and won't expect the ride quality of a BMW GS.
Anyway really hope you try this build. I'm more than happy to answer questions if you have any.