Going to the Next Level

Discussion in 'General Questions' started by Timbone, Dec 12, 2015.

  1. Timbone

    Timbone Active Member

    I am very happy with my one gear HT motorbike but I can't help but wonder about how you can make an even better product. I'd welcome any and all feedback because I need all the info I can get to make good decisions.

    First, it would be good to source a good engine. I've been looking at those RTM clones that Dax offers. $500. Right now, usuing a 41T rear cog, I get a cruise of about 29 at about 7K RPM. This little engine (49cc), even if you ran it at a meager 9K, would provide a cruise of at least 35mph. It would be really nice to run in the low thirties just to filter better into traffic.

    Any and all feedback on this motor would be very appreciated by me.

    Second, bicycle frames are relatively small and yield short wheelbases. My Trek carbon fiber roadbike is at 39". My Skyhawk frame is about 44". It would be better to have a bit longer wheelbase.

    Instinctively, we think about welding frames in a jig, a very difficult task that requires real skill and expertise. Has anyone ever considered a bolt up frame? I think very affordable 12 gauge Unistrut, properly designed and gussetted, could make a superior frame. Clamp/ weld the headtube and bottom bracket tube. Maybe even fashion in a tube for a swing arm for rear suspension.

    Anyone ever done anything like that?

    Then there's that idea I have to work in a simple 2 gear dog clutch...


  2. mikedabomb

    mikedabomb Member

    When I took apart my 66cc I found the cylinder gasket to have horrible matching to the intake transfer case ports. I had a really big amount of gasket leaning into the port, it covered a fourth of it easily, on both sides. I haven't ran my engine yet but I probably did it a great justice.
  3. sbest

    sbest Active Member

    Timbone, with very little work my 66cc is into the high 30's (mph).
    Cylinder head squish shape, matching the exhaust and jetting are the main improvements.
    Transfer gasket matching was not noticeable and matching the intake made things worse.
    Widening the intake and exhaust brought more torque but no extra rpm.
    A fancy drink bottle carb filter helped a bit too, if you get the right one.
    I am running at a calculated 8700rpm with 44t sprocket.

    I think you are getting too complicated. Start small, find what works and what doesn't.
    You can buy your way there, or built it. In the end, which do you think will bring inner satisfaction?

  4. KCvale

    KCvale Motorized Bicycle Vendor

    The 'next level' for a 48cc 2-stroke is just an expansion chamber exhaust and gears.
    Gears change everything.
    All these engine mods are a joke compared to just those 2 things.
    That's next level.

    Heck this 48cc would do ~40mph.


    Light bike, legal engine, and the brakes for the speed.
    That is 'next level' for a small engine ;-}
    Timbone likes this.
  5. Timbone

    Timbone Active Member

    I am reading your posts and trying to understand what you are doing and why you are doing it. Your 8700 rpm will take you places in a hurry. Very cool. I appreciate your input and idea, really enjoying your experiments. Thanks!
  6. sbest

    sbest Active Member

    What are you are doing and why you are doing it?
    Over the years, most of the engines and vehicles I have worked on have been complicated and expensive, and especially complicated and expensive to do engine modifications. Most of what I have done has consisted of reading and learning, tearing into an engine, doing a dozen mods, putting it back together and reaping or ruing the results. When I could I tried to follow the experimental method, but usually did not have the opportunity due to time and money. Well, here is a golden opportunity in these beautiful little engines. Like a Heathkit or breadboard or Lego blocks, these engines are a simple learning tool.

    From Wikipedia
    These steps can vary slightly, such as:
    Ask a Question
    Do Background Research
    Construct a Hypothesis
    Test Your Hypothesis by Doing an Experiment
    Analyze Your Data and Draw a Conclusion
    Communicate Your Results

    You guys are my final step by-the-way, my peer review!

    KC Vale, I know you are right, just taking my time getting there. Starting simple and cheap first.
    The straight pipe and single gear does make jetting and engine removal a bit easier too, and pushes me to look for broad power, a big fat curve. If I could gear down to make it up the hills the urgency for more power and torque would be gone wouldn't it? :)

  7. KCvale

    KCvale Motorized Bicycle Vendor

    By 'straight pipe' you mean the stock 2-stroke exhaust and not some no back pressure pipe correct?

    2-stroke engines have no valves and try to do intake and exhaust at the same time, this results in some incoming fuel going rite back out with the exhaust.
    Just follow the green fresh gas mix in this animation for what an Expansion chamber exhaust does.


    A tuned expansion chamber exhaust pushes the otherwise wasted fuel back in and makes a huge difference, like 1/2 a HP, the single best performance enhancement possible.

    Second, every engine has 2 curves, torque and HP, and they are at different RPMs, in short you can't get one big fat curve.
    Torque is in lower rpm range, power at the higher rpms.

    When accelerating you start with all the torque and run it up to full power, and shift up to drop rpms back down to the torque side and run it up to the power rpms again.
    Downshifting on a hill for example drops you down to the most power range of the lower gear you shifted up from.

    I find a 3-speed to be all you need and the difference is staggering.
    I have put one of my stock 66cc direct drive builds head to head against a stock 48cc 3-speed shifter build and they ran neck and neck.

    You wanted 'next level' advice, that is mine as I have been building shifters for that reason for a long time, 44 of them listed here.

    Hope that helps you in your quest ;-}