Heavier HT flywheel for more tractability

Discussion in '2-Stroke Engines' started by AussieSteve, Nov 26, 2009.

  1. AussieSteve

    AussieSteve Active Member

    I've been thinking a lot about which mods would best suit these engines. (66cc HT)
    Most of us have a fixed single gearing, so the engine needs to have good torque/power throughout the entire rev range. Fast acceleration is not an issue. (We'll never achieve that.)
    A couple of the biggest faults are the bucking and surging at very low RPM and the difficulty of slipping the clutch to take off smoothly when the engine is running, especially at higher gearings with 40T or 36T rear sprockets. Lower gearing would help, but only at a cost of top-end speed.
    A heavier flywheel would minimise both problems. This is what makes a 2-stroke trials bike work so well. (Better than 4-strokes)

    I found this:-

    On the downside, the engine wouldn't rev out as quickly and would be a little harder to get spinning when starting.
    I'm wondering how much extra weight might be needed to make a noticeable difference.
    Any thoughts?
    ... Steve

  2. spad4me

    spad4me Member

    Easy solution .
    Make your single speed setup into a multi speed.

    A brass flywheel would probably run for hours after you killed the engine.
  3. AussieSteve

    AussieSteve Active Member

    That's not the solution. I didn't ask about adding gears, I asked about flywheels.
    ... Steve
  4. Scotchmo

    Scotchmo Member

    If you could add enough flywheel mass, it should help with the bucking and surging at low speeds. It won't improve your torque or power at low speeds. It might seem like it since it should be more drivable at low speeds (like a trials bike). Where would your put the extra weight? There is not much room inside the crankcase.
    Last edited by a moderator: Dec 15, 2015
  5. 210061741

    210061741 Guest

    I think the surging is caused by the inability of the carb to provide the proper mix at all rpm's.

    The flywheel allready ways a ton and if anything should be lightened.
    The flywheel is what make these motors soo heavy.

    A heavier flwwheel would actually rob some power.
    And the only way to do it would be to use heavier material like Carbide to make them.
    There isn't space in the CrankCase to make it any larger.
    If you try let me know how it works out.
    I think i'll be removing some weight.
  6. AussieSteve

    AussieSteve Active Member

    Excerpts from the article I referred to. (It's worth a read):-
    I'm interested in 6. and 8. above. (More driveable at low speeds and smoother clutch takeoffs.)
    I'm already tuning towards more low-end power which has already improved torque at the lower speeds.

    I'm still working out a way to squeeze it in for testing. I'll probably try to get a second magneto cover, chop the top off so that I only have the sides, then fit that with the normal cover and longer screws. That should give me plenty of room to play with. My engine is mounted higher than my pedal cranks, so nothing will hit there.

    I rang a friend yesterday who said he'd make a brass flywheel for me, when I've got the dimensions. I'll try to get a couple with differing weights. There isn't much spare thread on my crank, but there is some. I'll use a thinner washer and nut and should have enough room for attachment if the thickness of the flywheel in the centre where the nut tightens is about 2.5mm. That's fine - I want the weight towards the outer edges anyway. If we can't get the brass thin enough at that point, we discussed using a thin steel disk, (large washer), riveted/screwed to the bottom of the flywheel.
    Can't locate using the original woodruff key, though, so I need to find a way of attaching the flywheel to the magnet using a locator pin or similar to stop it spinning off.

    ... Steve
    Last edited by a moderator: Dec 15, 2015
  7. 210061741

    210061741 Guest

    If you split the crankcase.
    And pull the crank.
    There is a metal flywheel on each side of the crank attached by 3 screws.

    It's about 30min to 1 hr to tear down the motor.

    Why not replace the original steel flwheels with brass.

    I'm sure it would be quite a bit heavier.

    And you wouldn't need to mod anything.
  8. AussieSteve

    AussieSteve Active Member

    Not a bad thought. A bit tricky for me - I don't have the resources to do that here and it's too much to ask of my friend. He's 82 and 2 hours away. I wish I knew someone locally with the equipment.
    I just checked on the relative densities of brass and steel. Brass isn't much higher, so replacing the original flywheels wouldn't make too much difference on it's own. I'd like to do both.

    From Wiki:-
    ... Steve
  9. 210061741

    210061741 Guest

    If you google Flywheel weights you can find some specialty high desnity materials.
    But i'm sure they would be more difficult to machine.

    To take apart the motor you only need a few tools.

    Good impact driver. $6.00 at harbor freight.
    Good Screwdriver.
    The Sprocket / Gear puller tool that comes in the KITS with the motor.
    A 10mm, 13mm , 14mm , 15mm and 19mm sockets.
    Maybe not all of the sockets but if you have those your good.
    Some ViseGrip Plyres and a pair of channellock plyers.

    Pretty sure thats about all i have needed to take these motors apart.

    They really are very simple. And with all the Pics and info around here I'm sure you could do it.

    I use an old microwave stand for a workbench.

    Anyway if you ever get a new motor or the current one breaks take it apart.
    I'ts lots of fun and on an old motor it's easy to see where to improve things.
  10. AussieSteve

    AussieSteve Active Member

    Hello 210061741, the high-density materials sound interesting. I'll do a search later.

    You misunderstood me. When I said that I didn't have the resources to do that here, I meant the resources to machine up a new crankshaft flywheel.
    I have all of the tools you mention and then some and as an ex dirt-bike and road-bike rider, I'd never be without an impact driver.
    I've also split heaps of crankcases, 2-stroke and 4-stroke bikes and VWs, years ago. More importantly, they all went back together properly. No problem with a workbench. The kitchen table works fine, but I do also have a bench in the workshop.
    ... Steve
  11. 210061741

    210061741 Guest

    Figured you had that covered.
  12. AussieSteve

    AussieSteve Active Member

    I only asked Jack to turn the simple disk design on his lathe. He doesn't have the gear to make a new crankshaft flywheel pair, especially from high-density materials.
    ... Steve
    Last edited: Nov 30, 2009
  13. 210061741

    210061741 Guest

    I bet Manic Mechanic has the equipment.
    If we sent him a simple drawing i bet he'd do it.
    I'll be tearing apart my Grubee as soon as i get the PK done.
    I'll pull the flwwheels off the crank and make a drawing.
    Maybe someone can make them cheap.
  14. AussieSteve

    AussieSteve Active Member

    Apparently Manic Mechanic is gone. I heard last night that he was going out of business and was to close down on November 15, 2 weeks ago. He stopped taking orders as of November 2.
    His business had no work for a year.
    Sad if true. The website is still there.
    I just emailed him for confirmation.
    ... Steve
  15. 210061741

    210061741 Guest

    I read that he got some orders and is doing ok.
    Last i heard he was taking orders for a billet Engine.

    Hopefully he's ok.
  16. AussieSteve

    AussieSteve Active Member

    Now I'm happier. Besides this flywheel idea, I want to buy one of his sprocket kits a little further down the track, unless I decide to go with a shift-kit.
    Another billet engine manufacturer. That could only be a good thing, especially for everyone in the US.
    I should hear back from Jim over the next couple of days.

    ... Steve
  17. 210061741

    210061741 Guest

    Yes lets hope that enough people help to keep him in buisness.
    Loosing people like Jim is bad for everyone.