Pros and cons of Adding flywheel weights.

Discussion in 'Trials' started by MATTY, Nov 13, 2017 at 1:29 PM.

  1. MATTY

    MATTY BORDER RAIDER

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    In Trying to run my latest trials bike project a 203cc i am noticing despite lower gearing than original. i am getting labouring bogging and virtual stalling in very tight turns at low speeds, i richened the mixture slightly this did improve things but i still feel its lacking what the engine is capable of..
    Bike is based around a 125cc engine with longer stroke slightly bigger bore but i think it is same flywheel.
    Thinking if i increase flywheel mass it might held in its running low down, but have no experience of modifying engines in such a way, any advice please on increasing flywheel mass, what can i reasonably expect to gain if anything.
    Any additional advice of what sort of extra weight to add and how to go about this practically, i have a couple of lathes and a mill and reasonably well equipped workshop, so advice on how to go about it in a diy sort of way rather than looking to buy aftermarket add ons off other bikes.
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  2. lineaway

    lineaway Long timer

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    Go for it. Remember an outside the diameter would have more effect and need less weight.
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  3. MATTY

    MATTY BORDER RAIDER

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    I am limited on available space inside the cover thats the only problem with going outside by much, Might get some on the face about 5mm clearance more on the edge so that might work.
    Any idea what sort of weight in say grains or grams will start to make noticeable effects lineaway. ? I really have no idea where to start here.
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  4. mung

    mung Been here awhile

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    It usually takes about 4 to 5 ounces on the outside of the flywheel for it to be noticeable. Just have to experiment to see what fits [might have to make a spacer for the cover] and what feels right.
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  5. MATTY

    MATTY BORDER RAIDER

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    5 oz or so i should be able to make that work even in mild steel, i thought about rolling a ring and wilding it on the flywheel or would welding it up so slightly undersized, then turn it down inside so it is a close fit then heat it up and shrink it on to the flywheel perhaps rivet or bolt it between the magnets some where thoughts here please. .?
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  6. Huzband

    Huzband Team Dirt

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    Don't forget it needs to be balanced.
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  7. MATTY

    MATTY BORDER RAIDER

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    Yes thanks for the reminder i did actually think about this, was thinking an old tyre balancer the vertical bubble type spin it up on there, and drill out the steel ring to balance it up.
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  8. Nodabs

    Nodabs Been here awhile

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    +1 on increasing the diameter as much as possible. I did a bunch of machining a few years ago for a top rung AMA flat track team, and did a lot of work on taking weight away from the center of flywheels while adding diameter. The engine builder was adamant about the diameter helping to "find " traction.
    I'd also think about rebalancing the entire rotating assembly, changing the bore and the stroke might really make it into a shaker.
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  9. MATTY

    MATTY BORDER RAIDER

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    It is a stock engine capacity increase is from the factory, T212 Minsk engine they are 203cc and based on a 125 minsk engine, but use different crank and piston.
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  10. D2W

    D2W Adventurer

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    With the mechanical skills of my brother, I did a similar thing with my son's TXT 70. I went to a metal shop and purchased a piece of brass or bronze "pipe" [as the brass or bronze has a greater density than common steel]. We then machined the OD to clear the cases/side cover. The ID was machined to be slightly smaller than the flywheel. We then put the flywheel in the freezer and heated the weight. The heated weight then slipped right over the cooled flywheel and when it was back to normal temps there was no movement at all (so we did not use any mechanical fastners). To remove the flywheel I did have to purchase a puller (which I believe had a left-hand thread). As for a balancer, I used one made for the heads of model helicopters. :)
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  11. MATTY

    MATTY BORDER RAIDER

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    Nice one D2w Brass a good idea thanks.
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  12. D2W

    D2W Adventurer

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    One trick I forgot to mention ... when trying to "measure" the clearance between the flywheel and the engine cases/covers, I used a blob of Playdough/putty from my son's art-supplies box and formed a representative flywheel weight. I then attached the "weight", reassembled the engine, and kicked it over (by hand after taking out the spark plug). Any areas of interference between the engine/case and weight were now easy to see and I could determine the maximum size of the weight. I believe I allowed for a wee bit of extra clearance when I sized the final weight.
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  13. motobene

    motobene Motoing for 46 years

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    How much weight to add or not too is a matter of total inertia and engine power and torque.

    Inertia isn't only rotational, it is also from the mass of the piston assembly moving linearly, with the the rod serving to add to to inertia with motion in between rotation and linear motion. A bike of the same weight but more displacement will thus feel heavier.

    Calculating total moment of inertia involves lots of measurements and many hours, so it is usually done by 'seat if the pants.'

    Total inertia will affect the timing of response to throttle input and shut off, and it also affects starting, with too little total inertia contributing to lumpier compression response. This is why adding additional flywheel weight can make bikes easier to start one kick.

    Too little total inertia can lead to stalling when interruptions in forward progress happen in time with the compression stroke, leading to too little piston velocity to carry adequately through compression.


    Too much total inertia leads to a delays in engine response during quick opening and closing of the throttle. Example, the stock '80s TY350s with significant delays in coming on and shutting off.

    Ideal for trials depends on rider preference, but it is usually high enough for no stalling problems, but low enough for throttle responsiveness.

    Oh and if space is limited, machine weights from brass or bronze as the density is higher than that of steel. And more weight farther out from the rotational axis helps as rotational inertia is a function of the radius of the centroid of the revolving mass.
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  14. MATTY

    MATTY BORDER RAIDER

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    Thanks all for replies, I can relate to what bene says on adding weight, this bike i am as sure as i can be is bored and stroked yet i feel its a 125 stock minsk flywheel assembly.
    Bike feels good its smooth yet its some how lacking engine momentum at the bottom end rolling along on tickover or just above if you back of on any kind of resistance a tree root a small incline it gets verging on a stall and if you dip the clutch save the day its bogged then wakes up but its exasperating the lack of grunt too much and it is very stressfull concentrating on keeping the engine with us its detracting from the ride and makes it getting close to a chore rather than fun.
    Put a fair bit of time into this bike because i like soviet stuff and trials but its just lacking the edge right at the bottom i know it can give hence the flywheel mod being contemplated on to be included in the mix now. .
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  15. motobene

    motobene Motoing for 46 years

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    Wow, riding a 125 that is not, say a grunt monster like the latest generation Sherco-Scorpa 125 is hard to contemplate.

    Sounds like work to stoke the fires at tickover when rolling over a root!
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  16. MATTY

    MATTY BORDER RAIDER

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    We have a sherco 125 no comparison on anything, but the Minsk motor is smooth if not good at low end and give it the clutch its surprisingly powerful once it is on it, but it is stressful verging on unpleasant low down a constant chore if you understand.
    To try and explain in words its nature it is rather like a half way house between the crude basic yet capable BSA Bantam Harley hummer engine, and a MZ etz 250 which i suppose is pretty close to what it is.
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  17. motobene

    motobene Motoing for 46 years

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    Very interesting. Minsk, as in Russian? Exotic stuff.
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  18. MATTY

    MATTY BORDER RAIDER

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    NO! Minsk as in Russian not so exotic stuff.
    Ours is one we had given about 18 months ago now much breathed on and with mods and different cycle parts suspension but same basic frame and pannelwork, its been a fun project for me and my son he is 14 now.
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  19. Buschog

    Buschog Been here awhile

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    I like the general lines.
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  20. Brewtus

    Brewtus Buffoonery, Inc.

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    Da. :D
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