Heavier flywheel for more tractability

AussieSteve · #1 11-26-2009, 08:48 AM

See 2 posts down for thread starter
... Steve

**Bikeguy Joe** · #2 11-26-2009, 09:32 AM

You'll need a few post before you can post a link...welcome to the site! You have a good idea, more details?

AussieSteve · #3 11-26-2009, 05:04 PM

Quote:

Originally Posted by AussieSteve

No link no posts

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

**Bikeguy Joe** · #4 11-26-2009, 06:03 PM

I like your thinking....pioneer anyone?

I would start by getting some weights of existing flywheels.

AussieSteve · #5 11-26-2009, 06:56 PM

Quote:

Originally Posted by Bikeguy Joe

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

RecycleBill · #6 11-26-2009, 07:30 PM

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.

AussieSteve · #7 11-26-2009, 08:46 PM

Quote:

Originally Posted by RecycleBill

It might be that you could add weight to the clutch assembly and get the same effect as a heavier flywheel.

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

**2door** · #8 11-26-2009, 09:30 PM

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

RecycleBill · #9 11-26-2009, 09:59 PM

Quote:

Originally Posted by AussieSteve

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

You're right. Could the weight be added to the ends of the crank, perhaps to the starter assembly?

AussieSteve · #10 11-26-2009, 10:12 PM

Quote:

Originally Posted by 2door

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

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