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johnnyjazz

TY250A Flywheel question...

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Hello friends,

Having read on here about the modifications people do to the weight of the flywheel to affect performance, could someone comment if my flywheel has been adjusted or if this is standard? it appears to have some holes drilled in it and doesn't look exactly like the pic in the parts manual.  i know the previous owner was serious enough to swap the front 14 tooth sprocket to a 12 so I'm curious as to whether the flywheel was modified also or is standard.

And would someone be kind enough to briefly recap the difference between a heavier or lighter flywheel and the effect it has on the bikes performance?  (I'm still a little unsure)

with many thanks in advance and very best wishes from NYC, Johnny

flywheel.jpg

flywheel part.jpg

Edited by johnnyjazz

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The holes drilled in it are to balance it and were put there by the manufacturer.

The steel ring/band on the outside of the "cup" of the flywheel does look to have been machined slightly narrower but it is hard to tell for sure with your photo.

Trials two strokes generally have more flywheel effect built into the crankshaft than trail bikes and MX bikes. In the case of the TY250 motor the additional flywheel mass compared to the trailbike motor that the TY motor is based on is in the form of that steel ring fitted to the flywheel cup. The next and subsequent models of the TY250 twinshock have a bigger steel band there than the A model.

Trials two strokes benefit from the additional flywheel effect because it smooths out the response of the motor at low RPM to changes in throttle input. This smoothness is important when riding trials sections to help with controlling the bike. The ideal amount of flywheel effect depends on the skill level and personal taste and riding style of the rider. The B and later model TY250 motors have a very strong flywheel effect which helped them perform well in the 1970s but since then the performance of tyres and the way we ride trials has changed so much that reducing the flywheel effect on those models is nowadays a popular modification.

Back when the A model TY250 came out it was perceived as having a motor that was a bit too responsive which is why the steel band on the flywheel was made bigger. Nowadays the A model flywheel effect is seen as close to the ideal.

When riding a TY250 on trails and roads, the additional flywheel effect of the TY compared with a similar motor in a trailbike with less flywheel effect is of no practical benefit.

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Thanks David, really appreciate your succinct and as always incredibly knowledgeable advice!   not that i have any intention of messing with it, but glad to know it's as it should be.  Methinks i just got a wee bit confused looking at the parts pic where i couldn't see those holes that were drilled, and it seemed that the nuts holding the steel band on look recessed in the parts pic whereas on my mike they seem to protrude from the band hence making me wonder if something had been removed.  i was researching the threads on here about TY flywheels and they spoke about removing a steel band so good to know what that actually means now! 

on an aside, i see a lot of people talking about the modification of electronic ignitions... am I correct in thinking that the general consensus is that the TY250A isn't a bike that benefits greatly from this and the original ignition system is perfect (esp. if road legal with lights etc)?  Not that i am seeking to perform this upgrade as my bike runs great, more just curious as to whether it is beneficial from a performance perspective.  I'm also presuming the more upgrades one makes like that it lessens the value of the bike from an 'original' standpoint? 

many thanks always...i continue to learn so much! very best from NYC and hope all is well down there in the Antipodes :)  

Edited by johnnyjazz

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Those drawings in parts books often contain things that are misleading. I've seen a drawing in a parts book for the internals of the forks of one bike (maybe a TY) and they have drawn the springs unrealistically which can cause confusion.

Yes there is a performance advantage if you fit an ignition that changes the timing to suit the engine RPM compared with the standard TY ignition that has fixed spark timing. A fixed spark timing is only perfect at one particular RPM and choosing what timing you run it at is a compromise for the rest of the RPM range. In the case of your bike the advantages would include less tendency to kick back when starting and more power in the mid to high RPM range. My seat-of-the-pants estimate for the trials bikes I've fitted them to would be about an extra 10-20 % power available in the mid to high RPM ranges.

There are probably modern aftermarket ignition systems that also have stator coils for lighting.

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