Hello everyone. My name is Robert and I'm located in FL. Before I begin a seemingly never ending post, let me just say thanks for letting me join this fantastic website/forum regarding this mini-motorbike bug we all share.
I know my creation does not fit the "old minibike" title, but nonetheless I thought you all would enjoy it.
I grew up in the shop, my father getting my brother and I started at an early age with mechanical devices. First it was speeding up lawn mowers, then building go karts, then hot rods and mud trucks with 49" tires (remember FL is basically a swamp haha). To satisfy mom, we never built anything with less than four wheels...until now. However I got the bug from riding my cousin's Honda XR-80 at the age of 10 or so. After that I grafted an old Lauson cast iron 2hp engine to a bicycle frame. It was slow to take off but sped up after a while. Mom wasn't happy, but dad assured her it was slower than riding a bicycle down a hill (for the most part was true). A year or so later (with the help of good grades) I put a 5hp B&S engine in place of the Lauson. It worked for about a week until the torque twisted the hub in the spokes (what a sight)! So a phone call to what was back then Northern Hydraulics, some 6" rims, 14" tires, hubs, chain, and needle-bearing Comet clutch was ordered. To fit this to a bicycle frame was impossible, so the only remaining pieces from the bicycle were the steering neck and bearings. Without a governor, the bike was so much fun it never got painted and remained natural rusty steel!
I forgot to mention that I lived on dirt roads. On more than one occasion I would take a corner too fast after a rain storm and from the ruts running across the street, the front tire would bounce out, sending me flying over the handlebars! A few pulls on the B&S engine and away I went (with a few scrapes and a pound of sand in my pants!). Sometime in high school a friend was riding it on the sand and he fell off and got skinned on the jackshaft chain (I think he was just being stupid). So that put a dent in the fun factor. The engine was also becoming tired as it served a hard life on a go kart before I got a hold of it. Eventually the bike was taken apart and the frame junked to save space. I didn't miss it at first, but as I went through college, I started to miss the fun I had on it, but had too many other projects to begin building another minibike (brother's mud truck, my 4x4 van chassis, tandem axle trailer and keeping mom happy with kitchen/living room renovations).
Sometime in 2001 I got a Dixie Chopper lawnmower tire and bought a rim with a centered mounting flange to fit it. Not knowing what to do with it at first, it sat around for a year until my dad suggested using it on a mini-chopper project. I thought about it somewhat and then began collecting parts on eBay (Sportster gas tank, Comet needle-bearing centrifugal clutch, Bendix ATV drum brake, etc.). With college and my girlfriend requiring more time, the mini-chopper project was set on the back burner (more like it fell off the stove and landed in the dust on the floor behind the oven haha). I always knew I wanted more hp than what a 5hp or even modified 5hp B&S could offer. I even wondered if an 8hp B&S I/C engine with Kehin carb and no governor would work. Then Northern had a price drop on the Honda GX-390 13hp engine and I thought this would be perfect. Another year went by and I realized people were putting Vanguard engines in Cushman motor scooters and doing in excess of 70mph! So it was in that aspect I knew I needed a V-Twin! And luckily I had one sitting on my go kart from a few years earlier.
Fast forward to January of 2007. I was graduating college and needed a mechanical type project to design/build in order to pass the senior design project requirement. I really did not like the projects other students were contemplating, so I decided I was going to go solo on my project. This was allowed and I realized if I ever wanted to build another minibike this would be the way and how to do it.
So the months of January and February were spent researching and designing a chopper bike. I realized there were several goals I wanted accomplished that I have yet to see another individual create or use in their mini-project. These include:
1.) A huge engine! No 5hp or even 15hp but a 25hp Kohler CH25S 725cc V-Twin. Vanguards were the norm so I wanted something different
2.) Hydraulic disc brake with a remote reservoir for master cylinder
3.) Cog belt drive system. No chain to oil or make noise doing 60mph
4.) A dead rear axle. The wheel/hub will rotate over the axle on sealed bearings
5.) Internal throttle handlebar system for no unsightly wires or cables like real chopper builders use
6.) Headlight and brake light system
With such a large engine, right away several problems were presented:
1.) Transmission: a centrifugal clutch was out of the question. Comet's 20 and 30 series TAV's were also too small. We had some success from the Comet 40 series torque converter on the go kart application, but Comet only recommends the 40 series for up to 16hp and now I see why. With 25hp and a heavy go kart, the roller weights are not heavy enough to engage the belt to keep from toasting it. So the clutch "slips" for a longer than normal period. I won't go into detail about the go kart's specs, but the 40 series did work well for what it was, but a bigger clutch could have made things work even better. So with that now known, I was now in snowmobile territory looking for the 94C driver. Snowmobile clutches are not purchased in "kits" rather everything is separate. Once I got the driver and driven clutch pulleys, a belt length had to be determined. This proved way harder than just flipping through a parts catalog. Long story short, Don Jackson of Comet Industries personally sold me a "proper" belt according to the specs I gave him. Rarely does a manufacture sell and ship a product directly. Bear in mind this belt dilemma was very stressful as it was one of the last items purchased insuring the engine to rear axle distance was as short as possible. The belt finally showed up after the frame was just tack welded and pulleys were in place.
2.) Battery: the engine actually has electronic ignition, so it will not fire without a minimum of 10 volts to the circuitry. This means a battery will have to be on-board at all times. This really was not a problem as no engine above 18hp can be started by rope/recoil. But an almost fully charged battery is a must.
3.) Foot peg position: with the engine being so wide with the clutch pulley and exhaust mounted, it created a minor problem for rider position. This was later remedied by moving the foot pegs rearward.
Before any construction began, part of my project was to simulate the stress being developed on several items: the pulley drive hub, the axle shaft and the front fork. I used two methods for determining the stress in each component; (1) analytical (mathematical/textbook) and (2) FEA (finite elemental analysis/computer software simulation). Both methods gave very similar results (a very good outcome when practicing engineering!). A safety factor is a number assigned to a part which tells how many more times stronger it is until failure will result. So 2 means it is twice as strong, 3=3 times as strong, etc. As it turns out, the pulley hub has a safety factor of 8.7, so it is fairly strong considering the weight it will carry and the torque being applied to it from the engine/transmission.
At the start of February, this is what I had: an engine, front and rear tires/rims and a Comet 40 series torque converter along with a dead battery. Within a week I upgraded to a better front rim with many more spokes to carry the substantial weight of the Kohler V-Twin.
View attachment 651
View attachment 652
Starting with the rear, I needed to make my own hubs to bolt to the rim since nothing like them exists anywhere else (adapting a lawnmower rim to Harley Davidson belt sprockets and brake rotors). It was just easier to make my own parts rather than modify go kart parts. There will be two hubs, one for the belt pulley and the other for the brake rotor. Below is a picture of what needs to be created:
View attachment 655
Now with the addition of both hubs and the rim in between, below shows the rear wheel assembly in a cross section view:
View attachment 654
Here is a graphical representation of the stress developed within the pulley hub:
View attachment 653
I know my creation does not fit the "old minibike" title, but nonetheless I thought you all would enjoy it.
I grew up in the shop, my father getting my brother and I started at an early age with mechanical devices. First it was speeding up lawn mowers, then building go karts, then hot rods and mud trucks with 49" tires (remember FL is basically a swamp haha). To satisfy mom, we never built anything with less than four wheels...until now. However I got the bug from riding my cousin's Honda XR-80 at the age of 10 or so. After that I grafted an old Lauson cast iron 2hp engine to a bicycle frame. It was slow to take off but sped up after a while. Mom wasn't happy, but dad assured her it was slower than riding a bicycle down a hill (for the most part was true). A year or so later (with the help of good grades) I put a 5hp B&S engine in place of the Lauson. It worked for about a week until the torque twisted the hub in the spokes (what a sight)! So a phone call to what was back then Northern Hydraulics, some 6" rims, 14" tires, hubs, chain, and needle-bearing Comet clutch was ordered. To fit this to a bicycle frame was impossible, so the only remaining pieces from the bicycle were the steering neck and bearings. Without a governor, the bike was so much fun it never got painted and remained natural rusty steel!
I forgot to mention that I lived on dirt roads. On more than one occasion I would take a corner too fast after a rain storm and from the ruts running across the street, the front tire would bounce out, sending me flying over the handlebars! A few pulls on the B&S engine and away I went (with a few scrapes and a pound of sand in my pants!). Sometime in high school a friend was riding it on the sand and he fell off and got skinned on the jackshaft chain (I think he was just being stupid). So that put a dent in the fun factor. The engine was also becoming tired as it served a hard life on a go kart before I got a hold of it. Eventually the bike was taken apart and the frame junked to save space. I didn't miss it at first, but as I went through college, I started to miss the fun I had on it, but had too many other projects to begin building another minibike (brother's mud truck, my 4x4 van chassis, tandem axle trailer and keeping mom happy with kitchen/living room renovations).
Sometime in 2001 I got a Dixie Chopper lawnmower tire and bought a rim with a centered mounting flange to fit it. Not knowing what to do with it at first, it sat around for a year until my dad suggested using it on a mini-chopper project. I thought about it somewhat and then began collecting parts on eBay (Sportster gas tank, Comet needle-bearing centrifugal clutch, Bendix ATV drum brake, etc.). With college and my girlfriend requiring more time, the mini-chopper project was set on the back burner (more like it fell off the stove and landed in the dust on the floor behind the oven haha). I always knew I wanted more hp than what a 5hp or even modified 5hp B&S could offer. I even wondered if an 8hp B&S I/C engine with Kehin carb and no governor would work. Then Northern had a price drop on the Honda GX-390 13hp engine and I thought this would be perfect. Another year went by and I realized people were putting Vanguard engines in Cushman motor scooters and doing in excess of 70mph! So it was in that aspect I knew I needed a V-Twin! And luckily I had one sitting on my go kart from a few years earlier.
Fast forward to January of 2007. I was graduating college and needed a mechanical type project to design/build in order to pass the senior design project requirement. I really did not like the projects other students were contemplating, so I decided I was going to go solo on my project. This was allowed and I realized if I ever wanted to build another minibike this would be the way and how to do it.
So the months of January and February were spent researching and designing a chopper bike. I realized there were several goals I wanted accomplished that I have yet to see another individual create or use in their mini-project. These include:
1.) A huge engine! No 5hp or even 15hp but a 25hp Kohler CH25S 725cc V-Twin. Vanguards were the norm so I wanted something different
2.) Hydraulic disc brake with a remote reservoir for master cylinder
3.) Cog belt drive system. No chain to oil or make noise doing 60mph
4.) A dead rear axle. The wheel/hub will rotate over the axle on sealed bearings
5.) Internal throttle handlebar system for no unsightly wires or cables like real chopper builders use
6.) Headlight and brake light system
With such a large engine, right away several problems were presented:
1.) Transmission: a centrifugal clutch was out of the question. Comet's 20 and 30 series TAV's were also too small. We had some success from the Comet 40 series torque converter on the go kart application, but Comet only recommends the 40 series for up to 16hp and now I see why. With 25hp and a heavy go kart, the roller weights are not heavy enough to engage the belt to keep from toasting it. So the clutch "slips" for a longer than normal period. I won't go into detail about the go kart's specs, but the 40 series did work well for what it was, but a bigger clutch could have made things work even better. So with that now known, I was now in snowmobile territory looking for the 94C driver. Snowmobile clutches are not purchased in "kits" rather everything is separate. Once I got the driver and driven clutch pulleys, a belt length had to be determined. This proved way harder than just flipping through a parts catalog. Long story short, Don Jackson of Comet Industries personally sold me a "proper" belt according to the specs I gave him. Rarely does a manufacture sell and ship a product directly. Bear in mind this belt dilemma was very stressful as it was one of the last items purchased insuring the engine to rear axle distance was as short as possible. The belt finally showed up after the frame was just tack welded and pulleys were in place.
2.) Battery: the engine actually has electronic ignition, so it will not fire without a minimum of 10 volts to the circuitry. This means a battery will have to be on-board at all times. This really was not a problem as no engine above 18hp can be started by rope/recoil. But an almost fully charged battery is a must.
3.) Foot peg position: with the engine being so wide with the clutch pulley and exhaust mounted, it created a minor problem for rider position. This was later remedied by moving the foot pegs rearward.
Before any construction began, part of my project was to simulate the stress being developed on several items: the pulley drive hub, the axle shaft and the front fork. I used two methods for determining the stress in each component; (1) analytical (mathematical/textbook) and (2) FEA (finite elemental analysis/computer software simulation). Both methods gave very similar results (a very good outcome when practicing engineering!). A safety factor is a number assigned to a part which tells how many more times stronger it is until failure will result. So 2 means it is twice as strong, 3=3 times as strong, etc. As it turns out, the pulley hub has a safety factor of 8.7, so it is fairly strong considering the weight it will carry and the torque being applied to it from the engine/transmission.
At the start of February, this is what I had: an engine, front and rear tires/rims and a Comet 40 series torque converter along with a dead battery. Within a week I upgraded to a better front rim with many more spokes to carry the substantial weight of the Kohler V-Twin.
View attachment 651
View attachment 652
Starting with the rear, I needed to make my own hubs to bolt to the rim since nothing like them exists anywhere else (adapting a lawnmower rim to Harley Davidson belt sprockets and brake rotors). It was just easier to make my own parts rather than modify go kart parts. There will be two hubs, one for the belt pulley and the other for the brake rotor. Below is a picture of what needs to be created:
View attachment 655
Now with the addition of both hubs and the rim in between, below shows the rear wheel assembly in a cross section view:
View attachment 654
Here is a graphical representation of the stress developed within the pulley hub:
View attachment 653