Smaller lighter and quieter MB engine

Discussion in '2-Stroke Engines' started by Steve Best, May 18, 2016.

  1. Steve Best

    Steve Best Well-Known Member

    A thread came up in the other forum with the idea of smaller lighter and quieter MB engine. It proposed something about 30cc-35cc. I feel it will be harder to sell the idea of less displacement to the US market where most jurisdictions specify less than 50cc and 20mph, most of us want to take full advantage of these limits. So how to make it smaller lighter and quieter?

    A large weight reduction is in the flywheel. Aluminum cheeks with steel weight rings on the outer diameter could reduce weight 3-4 lbs in a standard China Girl. Thinner precision casings and plastic covers would take 2-3 lbs off and reduce all dimensions 1" in every direction. BMX chain and aluminum sprockets. These things would double the price of production.

    The plastic gear and chain covers would dampen a lot of their noises. A new air filter cover and fine foam filter would quiet a lot of the intake noise. Rubber pegs between the cylinder and head fins would quiet those noises.

    The exhaust would be a pipe tuned for max torque at 20mph and +50% over-rev so extra pedaling would be rewarded with up to 30mph speeds but still meet the legal 20mph condition on flat and up hills. Ideally the torque tuned pipe would be thinwall construction with a surround type muffler to quiet it to a whisper.

    The beauty of this approach is that each component would still interchange with all the 50cc and 66cc motors that are still out there. Development could start as aftermarket sales of individual components as a complete package is developed.

    Think about it;
    meets all legal requirements, 50cc, 20mph flat speed
    will haul touring weights up hills
    can still pedal or down hill to 30mph
    whisper quiet
    half the weight due to precision thinwall castings and aluminum/steel hybrid crank
    1" smaller in all dimensions
    Complete interchangeability

    Wouldn't this be great? Any manufacturers listening?


  2. bluegoatwoods

    bluegoatwoods Well-Known Member

    Well, I like your ideas. Expense would be a problem, though. You're right on that as well.

    And if the flywheel were lightened, then wouldn't vibration (not to mention poor warm-ups) be trouble too?
  3. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    Quieter and lighter and less vibration sounds great to me. But no one is even bothering to sell a drilled out wrist pin for the 66 and how easy would that be for anyone with a big drill? :-/
    Also 30-35cc does sound small. What RPM would it need to do and how would it then be geared to a shift kit so it is well matched to a human's 80-90 RPM?
    You can already pedal to 30mph down hill without the engine running of course. 56:11@80rpm on 26" is 31.5mph, 90rpm is 35.4mph
  4. butre

    butre Well-Known Member

  5. 45u

    45u Active Member

    Take too much off the flywheel and you will have very poor low end torque. I know this from trying this on a Harley crank and trying lighter flywheels on radio control cars. One of the reasons that Harley s have such good low end torque is there heavy flywheels. 2 storks do not have very good low end torque to begin with so I need all I can get.
  6. Steve Best

    Steve Best Well-Known Member

    The idea with the crank is to lighten the center with aluminum cheeks but to put a steel ring for weight at the outer diameter, which can be even larger than stock because there is more room in the case. This would drop pounds off the crank but keep the same inertial mass as the present crank. It is all about polar moment of inertia. Less weight but moved our further. The weight at the center of the crank does nothing for momentum. And I agree Jeff, these motors need all the inertial mass they can get.
    libranskeptic likes this.
  7. jaguar

    jaguar Well-Known Member

    aluminum isn't strong enough. and it would only save about 1 pound anyways.
    taking a few pounds off the engine won't make it any better at climbing hills.
    a plastic side cover may allow more gear noise to exit. maybe not.
    20mph limit sucks! I had a 24 volt electric bike that had that limit so I upped the voltage to 36 volts, which worked well.
    I love speed. maybe it's just me.

    If you want your ride to be lighter then stop eating so many potato chips and french fries! Most humans can easily lose 10 pounds.
  8. KCvale

    KCvale Motorized Bicycle Vendor

    If you want to use a tiny engine best consider bike drive gearing, it would be pretty useless without mechanical advantage.
  9. AutOtaku

    AutOtaku Member

    I would think a BMX chain and sprocket wouldnt be strong enough for these motors.....
  10. libranskeptic

    libranskeptic Member

    dunno respective weights vs chinese kits, but fyi:

    as an example of whats doable:

    their trimmer motor 25cc is 2.8kg & despite what the weird power number they give here (see footnotes in link), is 900 watts sustained i read elsewhere

    thats a whomper by ebike power standards & needs a big heavy battery

    my; 24 speed, 350 watt, 30 kg inc battery, alloy mid drive is, as fast as i need to get me to the wind barrier, and climb anything it can grip on. It would weigh about 25kg if i swapped in the gx25 (commonly used in various sizes in their trimmers, the "upside down ok" type carbys are convenient for bikes spillage wise btw)

    torque issues are a transmission problem, not a power problem.

    900 watts pushing a 25kg vehicle is too much if anything.

    my ideal would be a hybrid with a 100-300w tiny generator, 2 stroke if need be (model aircraft/boat engines?), powering the ebike and charging the power boosting battery, and maybe a stealthy 450 watt version of my above bike, or maybe just add 2WD, using a 2 speed front hub motor as well.
  11. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    Wow I did not know there were any-way-up four strokes. It sounds great if it can be made to mount to a bike, output the right sort of rpm, pedal start, look less like garden equipment and be narrow enough to fit between crank arms.
  12. libranskeptic

    libranskeptic Member

    furry, getting a sprocket on the motor shaft shouldnt be hard. then get a sprocket on existing pedal crank for the chain to the motor. with 24 gears existing usually, some of them are bound to suit :) as long as the initial reduction produces similar torque to pedaling.

    this motor may be used in some honda chain drive products which you could intelligently mimic for your purposes - half the reduction of a garden tiller e.g.