Homemade Fat Wheel Mini Bike

Step 1: The Stuff You’ll Need

Some Metal

  • 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 50×50 aluminium square bar to make the handlebar holder.

Wheels / hubs / axle etc

  • 18×9.50-8 knobby tyre on four stud rim

1000x25mm 6mm full keyway axle like this one 25mm x 1020mm.

1″ Bore Go-kart drive hubs 4″x 4 studs with 1/4″ keyway M12 Studs with nuts

19mm Brake disk

429 pitch 102 link motorbike chain

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.

Other stuff

  • 1x Motorbike Flat Cushion Cafe Racer Seat

Honda GX160/200 engine mount plate(could make your own out of a bit of 10MM mild steel but life’s too short)

Monkey bike petrol tank.

Rear Sprocket 60 Tooth Pitch 420

Titanium exhaust heat wrap 

Throttle grip

Some BMX handlebars (7/8″ thickness)

Throttle grip and cable

  • 300mm of M12 threaded rod.
  • Plenty of M6, M8, M10 nuts and bolts.

4x UCFL205 bearings like these

Tools

  • Welder (Mig or big is fine)
  • Tube bender
  • Digital inclinometer
  • 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

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):

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 ūüôā

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:

Beware that you probably need a longer chain for this setup.

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

Once all the sealer was dry it had been time to maneuver all my pieces in and begin assembling. Since the top and therefore the foot of my bed was glued up into one piece, there was a complete of only 8 pieces to the present bed that needed to be assembled (the head, the foot, 4-2″x6″ bed rails and 2-2″x3″ top bunk rails). Assembly was pretty simple , put the square peg into the square hole.

I wanted the joints where the rails attached to the foot and head of the bed to be durable but not permanent. After all, I imagine the beds won’t stay in there forever. due to this, they were still joined using mortise and tenon joints, I just skipped the glue, and rather than dowels, I countersunk some 3″ lag screws to carry them in situ . Because I wanted a cohesive look with the opposite glued up joints, I countersunk the lag screws in about 3/8″ into the 4×4’s with a 1/2″ bit that might allow me to use my 1/2″ dowels as plugs to hide up the heads of the screws. The dowel plugs stay pretty much without rupture , while are also not too hard to tug out if you would like to urge back to the screws again. This ended up being the right thanks to hide the lag screws.

After the frames were built, I needed¬†to get¬†within the¬†slats. Now, all I had for the the slats were a a support piece¬†that might¬†hold them up, but not¬†in situ¬†. My solution for this was to drill a hole through the slats at¬†the top¬†and foot of the bed into the supports. I glue the dowel into the support piece but¬†didn’t¬†glue the slat¬†in situ¬†(again,¬†i would like¬†to be¬†ready to¬†disassemble this¬†within the¬†future). I then evenly spaced all the slats down the length of the bed and ran two strips of vinyl webbing across the tops of the slats and stapled the webbing¬†to every¬†piece using 5/8″ small crown staples. This was¬†an excellent¬†way of holding all the slats¬†in situ¬†, but if¬†they have¬†to be disassembled, the slats can all be rolled up together.

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

You can see a number of the mistakes I made and the way to avoid them once you come to create . In summary, the rear chain stay far too short and that i had to mount the bearing carrier all the way off the rear of the tube then strengthen with some 6mm plate. This looks a touch messy and therefore the refore the whole build are going to be cleaner if that tube is longer and the carrier just welds on underneath. The plans I even have included with this Instructable have a extended chain stay so this could be OK. Please check for yourself.

Cut the 1M axle in half and take away burs at the top that might 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 situ .
Spend an honest amount of your time ensuring that the wheel is running true with the frame. I did not have a scientific way of doing this so I just eyeballed it + used a meter rule to assist see angles vs. the frame. Bash true with a wooden mallet if necessary.
Place the engine on the mounting plate and prop with some bits of scrap wood etc as shown
Get¬†a sequence¬†round the¬†drive sprocket and rear sprocket and move everything into alignment. Since¬†we will not¬†just the rear wheel on this setup, we’ll¬†need to¬†make use of the mounting holes¬†within the¬†engine mount plate¬†so as¬†to regulate¬†the chain tension.¬†confirm¬†things are positioned so we are roughly¬†within the¬†middle of the adjustment holes¬†to offer¬†us¬†many¬†room to play about later.
Once things look ok, tack weld, draw up then blast it with a hot weld.
NOTE: not shown¬†within the¬†pictures/plans¬†may be 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¬†each side¬†and slide into the gap + weld¬†in situ¬†

Step 5: Make the Forks

To make the forks, I did the tightest ‘S’-bend i used to be ready to with a 3″ CLR die within the tube bender. i might guess yours are going to be an equivalent don’t be concerned about the precision on the width an excessive amount of there’s many room to regulate things later/add spacers etc. Just confirm the fork legs are an equivalent on each side

The steering yokes (at least i feel that is what they’re 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 space between the yokes should be 160mm + enough space to feature a few of washers or bushing, In my case I added 8mm. (I think the plans show it a touch tighter).

The headstock is formed out of a 160mm piece of 1″ tubing mounted onto the frame at 114 degrees to the highest 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 feel if building again i might try a touch 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 touch of 6mm plate .

Step 6: Make the Handlebar Clamps

  1. Take a 50mm square bar of aluminium and drill a 22mm hole
  2. Cut through the middle of the hole
  3. Cut off the other side as shown.
  4. Drill and countersink the two 8mm mounting holes in the bottom half to fit a cap-head bolt.
  5. 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.
  6. 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

  • 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.
  • Face.
  • 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

  1. Cut out a calliper mount from 6mm steel sheet to fit the calliper.
  2. Bolt the calliper to the calliper mount.
  3. Fit the brake disk to the rear axle.
  4. Slide the brake calliper over the disk
  5. Get someone (or some runner bands) to pull the brake lever
  6. Adjust so that the calliper mount is in a position that you can get a good weld against the rear stay.
  7. Tack weld and release the brake to ensure everything runs freely.
  8. Weld up.
  9. Add some washers if adjustment is needed.

Step 9: Make an Exhaust

This was far more fiddly than i assumed it had been getting to be! I did a test ride with a straight through pipe (shown) but this was just ridiculously loud. the entire idea is that folks can play on this all day without the neighbours getting the hump so an idea ‘B’ was needed. within the end I repurposed the first 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 urge the form I needed. I then welded together and wrapped in heat wrap to stop melting stuff/ kids getting burnt.

It¬†figured out¬†pretty much¬†and¬†is sort of¬†enough. You get¬†a clear¬†drop by¬†performance though¬†because the¬†bike definitely doesn’t wheelie as aggressively with the new exhaust but its¬†an honest¬†compromise.

Step 10: Finishing Touches

  1. 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
  2. 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.
  3. 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.
  4. 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
  5. Grease up the axle good n’ proper. If it tarts corroding you’ll never be able to take it apart again.
  6. 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.
  7. Spray the frame. I used a bog-standard rattle can (Hammerite smooth / rust-oleum). Seems to look OK.
  8. Add some footpegs. I was in a hurry so i just 3D Printed some

SOURCE —————— www.instructables.com

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