49cc Mini moto engine build so far

Discussion in 'Photos & Bicycle Builds' started by Waincat, Sep 18, 2013.

  1. Waincat

    Waincat New Member

    Hello, this is my first build, it is a bit of a mess at the moment and obviously has some issues but I have high hopes for it's completion! Links to photos below as I'm having trouble with uploading to this site



    Schwalbe big apple tyres
    Big Dutch made handlebars
    The seat extension is a bar end from a mountain bike
    Brooks sofa saddle
    Cheap Chinese 49cc mini moto engine (huasheng?)
    Automatic gearbox
    Gokart exhaust, modified and fitted with 22mm brass pressure fitting (I actually quite like the way it looks, sort of steampunk)
    43 tooth rear sprocket bolted on with galvanized steel, plywood and motorcycle innertube
    Basic mini moto petrol tank, to be replaced with by a fancy board track style tank when I have the tools

    As you can probably see from the photos, I have had a few problems in my test runs!
    The engine shakes way too much on the mounting and is shaking everything to bits.

    The mounting is made from 1mm galvanized steel which I think is too thin at the moment. I have decided to either add another mounting bracket or two, or abandon it until I can make some thick stainless steel tabs and weld them onto the frame.

    As you can see the pull start is dangling because the recoil has popped out for the sixth time! Not sure what I can do to resolve this other than buy a new one unless anyone has a suggestion for fixing it?

    Also, I think there may an issue between the rear sprocket and the gear ratios in the gear box because I don't seem to be getting as much power as I should. Maybe the automatic gears are not changing because there isn't enough pull due to the high number of teeth on the rear sprocket?

    Last edited: Sep 18, 2013

  2. 5-7HEAVEN

    5-7HEAVEN Active Member

    Several issues can be seen in your picture.
    Firstly, keep your top engine mount and fab another one which will bolt onto the lower crankcase mounting holes.
    Secondly, your gear ratio is grossly undergeared. The engine gearbox might be 2.19:1.
    If the engine sprocket is 22 tooth and rear wheel sprocket is 43 tooth, then your final gearing is 4.28:1.
    By contrast, you might need 18:1 for your bike to run well.
    If you keep your 2.19 gearbox, you'll need another 8.22:1 to get 18:1
    I'll return with more info.
  3. IbedaYank

    IbedaYank Member

    what 5-7 has stated your going to need more motormounts top bottom and front the front will keep the motor from twisting in the frame. Power comes from the correct gearing to allower the motor to reach its RPM powerband. being your running a 2stroke it needs to turn faster then a 4stroke for this to happen
  4. 5-7HEAVEN

    5-7HEAVEN Active Member

    Looks like you have a 25 tooth gearbox sprocket chained to a 43 tooth wheel sprocket.
    That's 1.72:1 ratio. Multiply that to the 2.19:1 gearbox ratio to get 3.77:1 ratio. That's wayyyy undergeared.
    A #25 90-tooth rear sprocket is the largest pocket bike sprocket I found. A 15-tooth gearbox #25 sprocket is the smallest I came across.
    That makes for 6:1 ratio. Multiply that by 2.19 to get 13.14:1 ratio.
    It's not the greatest, but it's MUCH better than what you have now.
    If you change to T8F sprockets and chain, you could use an 11 tooth gear box sprocket and a 72 tooth wheel sprocket.
    That'd give you 14.33:1 final drive ratio.
    If you could order a custom 123 tooth #25 rear sprocket and use a 15 tooth front sprocket,
    that's give you 17.96:1 ratio. The sprocket's diameter would be about 9" tall.
    If you change to T8F sprockets, you'd need a custom rear sprocket with 90 teeth. It would be also be 9" diameter.
    Your bike would have excellent mid and top speed with this combination.
  5. zippinaround

    zippinaround Active Member

    Last edited: Sep 21, 2013
  6. 5-7HEAVEN

    5-7HEAVEN Active Member



    With this sprocket and a 15t gearbox sprocket, ratio is 8.33:1
    Matched with the 2.19 gearbox, final drive is 18.25:1

    With a 5:1 tranny, engine rotation would reverse.
    You'd have to turn the engine about face, then use a jackshaft to drive your left-side sprocket.
    Methinks this 125t and 15t sprockets would be simpler solution to get your bike running well.
    Last edited: Sep 21, 2013
  7. zippinaround

    zippinaround Active Member

    do all the 5:1's reverse rotation? and how did you figure out 18.25:1 would be perfect?
  8. 5-7HEAVEN

    5-7HEAVEN Active Member

    All 5:1 pocket bike transmissions reverse engine rotation.
    That's because they use only an input and an output gear.
    The 3:1 transmission uses an intermediate gear, input-intermediate-output gear. That transmission,
    as well as the Staton chain drive does not reverse engine rotation.
    A simple analogy would be friction drive. Engine rotates one way
    When it rubs against the tire, that wheel rotates in the opposite direction.
    I say 18.25 is perfect, because 18.75:1 is the ratio Staton-inc used for decades
    on their chain drive. Of course, it's a compromise for low-mid-high speed riding,
    because there is only one gear ratio. Actually, I consider my shift-kitted drive system
    to have perfect ratios for my street riding. First gear is a granny gear @ 46.16:1.
    That will allow my bike to climb very steep hills and ramps that I encounter. I use 38.18:1
    Second gear from a standing start, which is great. Eighth gear is final drive @ 15:1.
    That's kinda high for my engine, so I use Seventh gear (17.73:1) for 35mph cruising.
    If I had to, I COULD start out in Seventh gear, but Second gear @ 38.18:1 is much more fun.
  9. zippinaround

    zippinaround Active Member

    dang it! didn't know that i thought there'd be 3 gears in the box so it would return to CCW. also i think i may have found an 11t #25 drive sprocket the biggest driven sprocket i could find was only 78t where can you get the 90's? yeah a shifter kit would be cool! what if you turned the engine mount it over the front wheel on the right side? this site says that those gearbox's are 4:1 http://www.petrolscooter.co.uk/gearbox-47cc-49cc-air-cooled-mini-moto-dirtbike.html?
    Last edited: Sep 23, 2013
  10. 5-7HEAVEN

    5-7HEAVEN Active Member

    That is an EXCELLENT price for that gearbox.
    Wonder if shipping costs will kill the deal.
    An 11t sprocket with 8mm chain is the much stronger T8F drive system.
    I use that on my gear-shifted motorized bike.
    Let's work out the gear ratios with T8F drive for the OP.
    He will have to change gear box, sprockets and chain.
    Parts are reasonable, installation is easy bolt on.
    Strength of drive system will increase dramatically.
    Availability of components is good.
    Divide 18:1 by 4 = 4.5:1
    Divide 50 tooth rear sprocket by 11t gearbox to get 4.545:1
    4.545 x 4 = 18.181:1

    The closest to a 50t sprocket is this 54t gear:

    It has a large 54mm inside diameter to accept a lefthand freewheel.
    You might need freewheeling. I've heard that the 4:1 belt-driven gearbox
    creates heavy drag whe the engine is off.
    Orrr, if you can live with the drag, use this 54t rear sprocket w/35mm ID:


    54t divided by 11t = 4.909. Multiply that by 4:1 to get 19.64:1

    Something to start with. You'll have better low and midrange. At high end, engine will be screaming.
    With this gearing, your bike will be VERY enjoyable!
    Last edited: Sep 25, 2013
  11. zippinaround

    zippinaround Active Member

    do you have yours hooked up to the front chainring? i was thinking about using the 5:1 trans and run a 10t to the 48t on my chainring. just wondering what the cheapest way to get the cranks to freewheel?
  12. Waincat

    Waincat New Member

    Hey, thanks for the response. I was actually looking at the 4-1 gearbox the other day! I may have to get one. The setup I have is T8F so it won't be a problem changing. Are you sure the one I have is 2.19:1 though? I've found the gearbox I have in a few places but none of them give the ratio, the person I bought it from doesn't seem to know either, all they said was that it should work with the sprocket I have for top end power. I'll test it again today and see how it goes. The 4-1 seems like the best option though, thanks again!
  13. zippinaround

    zippinaround Active Member

    one easy way to find out is to remove the case and count how many teeth are on each sprocket inside. how many teeth are on your drive sprocket?
    Waincat likes this.
  14. Waincat

    Waincat New Member

    There seem to be 10 on the drive and 30 on the second sprocket in the gearbox, the teeth and chain are smaller though. The front sprocket, outside the gearbox, has 12 teeth.

    I don't know about freewheeling but I think you can get freewheel sprockets to fit onto the drive.
  15. zippinaround

    zippinaround Active Member

    that means you need a 74tooth sprocket on the back. the mini moto sprockets come in that size but only have 3 bolt holes so you would have to drill more.
  16. 5-7HEAVEN

    5-7HEAVEN Active Member

    I've never seen or heard of a T8F 12 tooth sprocket. There are tons of 11t sprockets available online for use with 6-spline shaft.
    Is your 12t sprocket 6-spline, round shaft or "D" shaft?

    Left-hand freewheel mechanism will screw onto a special hub from staton-inc.com or bicycleengines.com

    Your gearbox is 3:1, if you have 10t/30t gears. Smaller internal drive must be #25 gears and chain.

    A 63 tooth rear sprocket with your 12t gear finalizes to 15.75:1.

    A 63t w/11t gear = 17.18:1

    A 72t w/12t = 18:1.

    A 72t w/11t = 19.64:1.
    Waincat likes this.
  17. Waincat

    Waincat New Member

    Is the 5:1 box for mini motos? If so, do you have a link?
  18. Waincat

    Waincat New Member

    The front 12t sprocket is 6-spline. I just found a 10t on ebay!

    I tried riding my bike again yesterday (before the pullstart broke again!) and it is definitely underpowered so I think I will go for a 54t rear sprocket and the 4:1 gearbox you suggested, which should give me exactly 18:1 if my workings out are correct and if that still isn't good enough I'll get a 10t for the front.
  19. IbedaYank

    IbedaYank Member

    8mm chain and sprockets are used Why would they use ASE when everything else is metric???
    all these boxes are JUNK as there is no way to adjust chain tension as the chain stretches and sprockets wear.
  20. 5-7HEAVEN

    5-7HEAVEN Active Member

    What is ASE? I know for a fact that right-side 4-bolt freewheels will bolt onto T8F 63t and 72t sprockets.
    Those same sprockets can easily be drilled to accept 5-bolt freewheels.

    It should be possible to fabricate a simple chain tensioner to take up the slack in the gearbox.