Heavier flywheel for more tractability
- Thread starter AussieSteve
- Start date
You'll need a few post before you can post a link...welcome to the site! You have a good idea, more details?
My post relied heavily on a link, but anyway-
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)
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
I like your thinking....pioneer anyone?
I would start by getting some weights of existing flywheels.
I would start by getting some weights of existing flywheels.
The problem is that there isn't a whole lot of space under the magneto cover to add much weight. Still, look how small and light the rotor is compared to those of typical small 2-stroke bike engines. It shouldn't take much extra weight to make some difference.
If it's not heavy enough, it might be possible to make a deeper side-cover by buying a second, chopping off the top, then attaching the remaining sides of the second one to the bottom of the first, with longer screws.
(Can you picture what I mean?)
My other concern is that the original rotor is stepped at the sides, probably for waveform shaping for the CDI ignition timing. Any additions might upset the field and therefore the ignition system.
I'm thinking about making up a metal disk that just fits under the mag cover, then see what happens. Brass might work OK.
Here's that link again, (fingers crossed):-
Flywheel Weight and Inertia Comments
... Steve
It might be that you could add weight to the clutch assembly and get the same effect as a heavier flywheel. Of course you'd then risk having main bearing concerns but that might also be true with anything that adds rotating weight to the crank assembly.
Thanks for the input, RecycleBill. The same thought crossed my mind. The problem, however, is that then the flywheel would need to be much heavier if it came after the 4.1:1 engine-clutch reduction. The ideal place is on the crankshaft itself, for best effect.
... Steve
I like the concept but I can't visualize how you will mount a larger rotating mass and still incorporate the magneto coil and core into the equation. I recall older 4 stroke motors with the flywheel being hollow on the inside with the magnets embedded into the inner circumference of the wheel and the mag coil surrounded by it. Is this what you have in mind? Brass would probably be good its weight to mass is high. Keep in mind also that the crankshaft diameter at the rotor is very small, about 5/16". Any unbalanced condition could seriously effect things there. Keep us informed.
Tom
Tom
You're right. Could the weight be added to the ends of the crank, perhaps to the starter assembly?
Hello Tom. I'm not sure just how I'll do it yet, either, but I'm sure it's possible. I have a crude sketch for a first attempt attached. (Not scale or even in proportion.)
Once I get measurements, I still need to talk my friend (with a lathe) into making it for me.
Balance is critical, but that shouldn't be a problem with a carefully lathe-turned disk, I hope.
... Steve
Attachments
I already have fast acceleration with my bike and i have no bucking or surging and my bike runs smoothly. but i weigh only 120lbs.
LOL, I weigh about 140 and I don't have those problems either.
Are you thinking of attaching this flywheel under the nut that holds the magneto rotor in place? That is definitely doable, that is the way that pull start assemblies attach. Now that I think about it, the cover from a pull start assembly might be just what you need to give you extra room. Here is the only site I know of to get one, and they seem to be out of stock again. But you can see what I mean about extra room.
Are you thinking of attaching this flywheel under the nut that holds the magneto rotor in place? That is definitely doable, that is the way that pull start assemblies attach. Now that I think about it, the cover from a pull start assembly might be just what you need to give you extra room. Here is the only site I know of to get one, and they seem to be out of stock again. But you can see what I mean about extra room.
That does look interesting. For test purposes, it's just what I need. (Beats buying a second std cover and modifying it, from a practical poit of view.) Not a bad price, either, at $20US. Trouble is, postage to Australia will add about $35AU to the cost - total about $58AU.
Pity they're not available here in Australia, or are they? Anyone?
I figure if I use a diameter that fills the side-cover, I won't actually need too much weight to make a good difference.
You say you don't have that problem. I guess it's not so much a problem as a limitation. Surely you'd like to be able to idle along even more slowly than you currently do, if need be. Also, this will improve clutch take-offs.
It won't rob noticeable power from the engine. On a fast-revving/accelerating two-stroke, it will slow the rate that the engine can rev out. On these little things, it won't make a real difference.
They don't rev out very quickly anyway.
... Steve
Yes, it would be nice. I remember when my engine was first breaking in and I was using the standard carb (I use a CNS now) I couldn't run smoothly below 15mph. Now I can do as low as 6 or 8. With an absolute top-end on flat ground of 28mph (48cc, 48T sprocket) I'd say that isn't bad, but having more speed range would be great. What interests me most is the clutch take-offs. I can do them now, but with a lot of clutch squealing and engine lugging to get up to running speed. I usually just peddle to get moving and save the clutch pads.
Actually, mine's about the same for bottom speed, 12kph, despite higher gearing, but it is a 66cc. (44T, stock NT carb, jetted down to #76 when speeds were last measured.)
Even slower would be better.
And it would be nice to only have to pedal to start the engine.
Just worked it out, I get 27.5mph max speed.
... Steve
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