Manual trans minibike project
- Thread starter sparkwizard
- Start date
That is a BSA A7/A10 transmission, the lever on top actuates a rod through the center of the the input shaft to operate the clutch, which would be attached to the other side of the case. the input shaft goes through the center of the output shaft so the clutch and output sprocket would both be on the other side of the transmission.
there's a cheaper way to do this. you can get honda clone CT70 motors with a 4 speed and clutch for just a few hundred. they even support a headlight. and are available in 80 or 100 or even 125cc, in the same form factor. i was thinking of rigging one of these into a Speedway mini bike frame, but never followed through with it. the biggest issues is the rear brake on a mini bike. your left side handlebar becomes the clutch, so now the rear brake has to be converted to a foot brake. for some reason that seems like a bigger hurtle than mounting the motor...
Also available in 140, 155, 190 and 212cc (maybe others). The Zongshen/ Piranha/Daytona 190-212cc engines produce around 18-20 HP at 9,500 RPM and have a 5-speed manual transmission. As you may imagine, they aren't what many would consider cheap, running around $1,000-1,500 (there are several models that vary in price). However, a Lifan 125cc 4-speed engine that produces 11 HP at 9,500 RPM can be had for $400-500. You need to be careful buying a 125 because there are so many variations. Due to various bore/stroke and compression ratio combinations, output varies from around 7-8 HP at 7,500 RPM (max) to around 11 HP at 9,500 RPM (max). There are also clutch/transmission variations. Some have a manual clutch (lever) and some have an automatic clutch (no lever). Some have 4 forward speeds, some have 3 forward speeds plus 1 reverse (for ATVs, etc.). Some are "fully automatic"; I don't know for sure but I think they have a centrifugal clutch and a single gear ratio. I've never seen an explanation of how they work but I've watched a few videos of go karts with the "fully automatic" 125 and they sounded as if they had a single ratio. So, you need to read all of the fine print in the engine description before spending your money. As far as the brake issue when putting a manual clutch lever on the left handlebar, if you are running a rear only brake (like most minibikes), just move the brake to the right handlebar, simple. If you use one of these Honda clone (different type than what we usually discuss) engines, mount it using the engine mounting bosses at the top and rear of the case; you will need to fabricate brackets. Do not bolt the engine to a plate using the four footpeg mounting bosses on the bottom of the case. The sump (bottom of case) is relatively thin and bolting the engine to a plate in this manor will likely result in the case breaking. It may crack and oil will begin leaking or the whole bottom of the case may break away. I do not speak from personal experience but I did a lot of research years ago and saw plenty of pictures and videos of this happening. It was a project I spent a lot of time researching and planning but never executed, it happens...
You can get some idea of engine variations here: T Bolt USA
Search Amazon and eBay for engines (due to the many variations, read descriptions thoroughly).
Engine mounting bosses
I can't find a picture of the bottom of a case to show the footpeg mounting bosses that I warned against using to mount the engine but here's a picture of a pit bike footpeg bracket and you can see the four mounting holes used to mount it to the bottom of the engine case.
One may think that, if the bosses are good enough to mount the footpegs, they should be good enough to mount the engine. However, consider that the footpeg assembly is low mass, it will shake/vibrate with the engine with very little resistance. The bikes frame is much higher in mass and will resist the engines shaking/vibration and that is what will stress/fatigue the relatively thin cast aluminum sump if the engine is mounted using the footpeg mounting bosses. Notice that the footpeg bolt holes are very close together which would stress the sump much more than if they were farther apart. Fabricate mounts and mount the engine properly.
There is a very good 125 manual build thread on here; I'll post a link if I can find it... (the search feature isn't doing me any favors today)
Wow, this is a long post...
You can get some idea of engine variations here: T Bolt USA
Search Amazon and eBay for engines (due to the many variations, read descriptions thoroughly).
Engine mounting bosses
I can't find a picture of the bottom of a case to show the footpeg mounting bosses that I warned against using to mount the engine but here's a picture of a pit bike footpeg bracket and you can see the four mounting holes used to mount it to the bottom of the engine case.
One may think that, if the bosses are good enough to mount the footpegs, they should be good enough to mount the engine. However, consider that the footpeg assembly is low mass, it will shake/vibrate with the engine with very little resistance. The bikes frame is much higher in mass and will resist the engines shaking/vibration and that is what will stress/fatigue the relatively thin cast aluminum sump if the engine is mounted using the footpeg mounting bosses. Notice that the footpeg bolt holes are very close together which would stress the sump much more than if they were farther apart. Fabricate mounts and mount the engine properly.
There is a very good 125 manual build thread on here; I'll post a link if I can find it... (the search feature isn't doing me any favors today)
Wow, this is a long post...
I found the build thread. I had searched "shifter" and a few other words I thought might work. Searching "Lotus" is what did the trick...
https://oldminibikes.com/forum/index.php?threads/shifter-minibike-build.133618/
https://oldminibikes.com/forum/index.php?threads/shifter-minibike-build.133618/
i've had a couple speedway 10" wheel frames (shrike) where someone made the 20 degree slant engine mount flat. that would be a perfect candidate for doing a motor swap. as the guy in the green manual trans thread said, i would keep it at 70 or 80 or 100cc. i think more than that would be out of control. and like the green frame guy, i would want a clutch and manual 4 speed.
I have a honda ct70 clone i bought a long time ago for $400. i took out the original china 70cc motor and replaced it with a 125cc version (which was less than $300 new, including shipping.) the conversion was a pain in the butt, mostly due to wiring harness issues. but in the end had a manual trans four speed 125cc bike. unfortunately those honda ct70s (and clones) really are crap to ride, they handle badly, with small wheels and too much junk (to make it heavy), and non-adjustable handlebars. i would much prefer putting a manual trans motor in a speedway 10" wheel frame. but hey that's me...
i do still have the china 70cc motor from the clone switch, but it's automatic. and it's kind of crappy anyway. the newer china motors are built better and bigger. still thinking about do this, but have to find a Speedway scorpion or scarab frame where someone mangled the motor plate to a flat mount.
https://www.amazon.com/125cc-Stroke-Engine-Complete-Motorcycle/dp/B0BLGKHKRX - 125cc 4 speed manual horizontal ct70 style $150
https://www.amazon.com/Vertical-Cylinder-Air-Cooled-5-Speed-Motorcycles/dp/B0DHL4ZB3N - 200cc 5 speed manual vertical xr75 style $285
i can't believe how inexpensive these motor sell. not sure if the vertical style would fit, but that would be ideal, oppose to the ct70 style. but 200cc would be a freaking death trap! unfortunately the vertical 200cc motor is electric start only, no kick start. that kills it for me...
I have a honda ct70 clone i bought a long time ago for $400. i took out the original china 70cc motor and replaced it with a 125cc version (which was less than $300 new, including shipping.) the conversion was a pain in the butt, mostly due to wiring harness issues. but in the end had a manual trans four speed 125cc bike. unfortunately those honda ct70s (and clones) really are crap to ride, they handle badly, with small wheels and too much junk (to make it heavy), and non-adjustable handlebars. i would much prefer putting a manual trans motor in a speedway 10" wheel frame. but hey that's me...
i do still have the china 70cc motor from the clone switch, but it's automatic. and it's kind of crappy anyway. the newer china motors are built better and bigger. still thinking about do this, but have to find a Speedway scorpion or scarab frame where someone mangled the motor plate to a flat mount.
https://www.amazon.com/125cc-Stroke-Engine-Complete-Motorcycle/dp/B0BLGKHKRX - 125cc 4 speed manual horizontal ct70 style $150
https://www.amazon.com/Vertical-Cylinder-Air-Cooled-5-Speed-Motorcycles/dp/B0DHL4ZB3N - 200cc 5 speed manual vertical xr75 style $285
i can't believe how inexpensive these motor sell. not sure if the vertical style would fit, but that would be ideal, oppose to the ct70 style. but 200cc would be a freaking death trap! unfortunately the vertical 200cc motor is electric start only, no kick start. that kills it for me...
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