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Building a small transmission

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  • Building a small transmission

    Alright so i'm bogged down by homework right now so i can't go outside and really work on anything - so instead i do the next best thing and piddle away my time thinking about what i could be doing instead of homework I've been throwing around this idea of a funky mini-bike type vehicle. I'd really like to have a small transmission to choose a couple of different speeds and it needs to be like a transaxle on a lawnmower - i'd like to use a 6 hp lawnmower engine on it. The rear tire is an ultra wide tire from a riding mower and the front will be a 13" wheelbarrow tire because thats what i have lying around .

    Anyway, i've built a homemade manual transmission before but it was set up like the trans on a mower - i.e. no synchros. Shifting can be pretty sloppy like this and i would often break the shift keys. On this transmission i'd really like some kind of synchronizer and i'd like it to be well thought out. I was thinking about using a centrifugal clutch because the engine i'd be using has a head that i milled down so it has ultra high compression. When you let off the throttle it drops to idle in almost no time flat and i figured everything is small and lightweight in the trans so is should be easy to snychronize with the driver taking it out of gear and letting off the throttle. Not the best system i realize but advantageous for me because i've got all the parts lying around.

    I wish i could be outside fooling around with it and designing it, but since i can't i figured i'd see what you guys had to say. Maybe i can save myself some material and time in mistakes.

  • #2
    I would look at non running motorcycle engines.

    They have a constant mesh transmission, and a clever person could use a lot of the gears and shafts that are already made?


    • #3
      Yes you could have a large selection of readily available transmissions at a motorcycle boneyard that could be salvaged cheaply from a motor that was toast.Like Gene said all the expensive parts and enginerring have already been done.
      Or you could go with a continuously variable transmission and automatic clutch like I'm using and save yourself some work if not money.
      Here's a link to the type of unit I'm using (
      Home, down in the valley behind the Red Angus
      Bad Decisions Make Good Stories​

      Location: British Columbia


      • #4
        I have thought about looking at old motorcycle drive trains but i don't really want to spend money on this thing - after all i've got my truck to work on and its only going to be 6hp. I figured i'd ride it around for a while and then give it to my niece and nephew to have some fun on. More importantly i want to build it for the expierence - i'm hoping it will be a good lathe project. I don't have a whole lot of expierence there and every project on it is a learning expierence. I've got quite a number of old gears from a transaxle lying around and some plans in my head of how it should all be made but i'm sure that will go under many revisions as i find problems with it. Thanks for the suggestions though!


        • #5
          No sense reinventing the wheel. The others have made good points. Although I'm a little old for mini bikes, I've thought about building a 4-wheeled kart to traverse the property. My plan is to use hydraulics. The engine would drive the pump and the pump would drive a wheel motor(s). Through engine speed and flow control, the speed and power would be infinitely adjustable throughout it's limits. If you want "gear" changes, the flow control could be set up with fixed detentes on a shift lever setup.

          With a similar setup on a mini bike, a high speed hydraulic motor (about the size of your fist) could be used to drive the chain to the rear wheel. Set up your engine throttle in the usual way and have a foot "shifter" to control speeds.


          • #6
            Speaking of a continuously variable transmission BELOW is a basic drawing showing how to assemble one, but I will leave it to you to figure out how it works.

            A unique feature of this transmission is that with the input running at a constant speed the output shaft can be slowed from a 1:1 ratio down to zero (output shaft stopped, input still turning) and then back up to a 1:1 ratio in the reverse direction. This all takes place WITHOUT the use of any clutches or the physical shifting or changing of gears.


            • #7
              I like the hydraulic drive. Torro(sp) lawnmower uses a hyd motor to the drive wheels. Seems to work a long time mowing parks. Speed changer/ throttle is a reliable set up that is an old tested system.
              my two cents worth
              Herm Williams


              • #8
                Mad S. i think your diagram is missing a braking system is it not?, in order to change direction or speeds a friction device is needed --- this equates to loss of power, I say forget reverse (its a mini bike right?) and just go with your standard variable pulley torqe converter, dirt cheap (find one on an old junk snowmobile) and very practical and pretty efficient also easy to adapt...


                • #9
                  Never mind, i looked again and it must use a lockout but thats not diagramed either, still to many components to run power though for the H.P. conscience performance oriented mini bike owner,,, Could be a real reliable maintenence free lawnmower drive system though..........


                  • #10
                    oops, re-read your post and if this is taking place gradually than i go back to my first post of a braking mechanism, in wich case its a horsepower robbing SOW.... Very interesting to try and follow though, somebody was doing some thinking...


                    • #11
                      Thanks guys -

                      Like i said earlier though i've got the material and gears sitting around and i'd like to build a transmission just for the heck of it. At this point, i'm not building this because i want a mini-bike. I've built plenty of go-karts w/ torque converters or just chain drive and they work pretty well, in fact one will hit 45mph on a straight shot. I've built small transmissions for go-karts but they were from small gears used in a self-propelled walk behind mower. They were never designed to take the stress that driving a go-kart at 20+mph put on them. If i wasn't breaking a part i made then i was breaking gears. Now i have some heavy duty ones from an old heavy riding mower. I'd like to do something with those-build a more sophisticated transmission just for fun. I though a mini-bike would be an interesting medium for the new trans since i've never made one before. I really appreciate all of the advice though. I guess i should have made it clear that the main purpose of this mini-bike was to have something that used a new, "sophisticated" homemade transmission. I'll keep thinking and playing around until i settle on something. Hopefully i can post some pictures. It should be a fun project.


                      • #12
                        Sorry we got off track but what an undertaking if your talking a real syncromesh transmission, unbelievable amount of design work with tapered gear hubs and syncro rings and engagement recesses --- your talking boocoo time and lots of work for a mini bike! If i had to id do what Topct said, go with shifting dogs like in a motorcycle trans, lot less work and if you build it tough enough you can shift it without a clutch of any kind, expect a little hash in the oil after awhile, still tons of work for a mini bike, do something cool like a diesel powered pogo stick, or put a little YZ 80 engine and trans on one of those 6.2 volt rascal scooters for the handicapped, thats what i want to do.... ooops - thing i got off track again.


                        • #13
                          A.K. Boomer:
                          Sorry no brake or extra parts are required to make this work. All that is needed is what you see. Well maybe some bearings and a case to mount all the parts in. But the only thing needed to cause a speed change, is to change the "ratio of the variable pulleys".

                          Lets see if this helps?
                          To achieve an overall one to one ratio, forward rotation, set the variable pulleys to a one to one ratio.

                          To achieve a neutral, set the variable pulleys to a two to one ratio. At this point everything will be spinning around joyfully EXCEPT the output shaft.

                          To achieve an overall one to one ratio, reverse rotation, set the variable pulleys to a three to one ratio.

                          Yep this can be a real mind bender, however when you are dealing with differential or planetary gearing it is typical to have your brain flip over a few times and go bouncing across the floor.

                          As far as its maximum power handling ability it is like any belt driven device and is limited to how much torque the belt can transmit before it starts to slip.


                          • #14
                            I've built plenty of go-karts w/ torque converters or just chain drive and they work pretty well, in fact one will hit 45mph on a straight shot.
                            Those were the good ole days. I had one that was "tagged" at 116mph at Mid-Ohio Race Track (Summer of 1989). ... On a slightly more than stock Briggs 5hp. And I didn't win. Some of the upper class karts were hitting 130+. The ground moves by mighty fast when your rear-end is only 1" off the pavement.


                            • #15
                              Wow! What did you use as your drive train? The only ones i've ridden that would hit near 100mph was a shifter kart. I'm not sure what a well designed one would hit with a torque converter. The kart that had a torque converter on it was a heavy beast - good for towing the others but that was about it. I don't think i ever broke tripple digits the course was too small for me to feel comfortable to push it very hard. Besides it wasn't my kart - i'd feel pretty bad if i busted it up because the guy who owned the track was letting me and some other kids drive for free! :O