Lightned Flywheel Opinions

[ducpainter]

But not at the crank, correct?

Correct...

but you ain't riding a crankshaft bro.

The increase is real world...

not sales hype.

[Jarvicious]

Not having fiddled with my engine extensively (yet), could anyone recommend which parts of the stock flywheel to mill?  That sounds like a fairly cheap winter project to me 

[Ducnial]

One more opinion on this over opinionated topic.    Think of the flywheel or any mass that has momentum (rotating or linear) like a bucket that holds energy, call it potential energy.   A lighter flywheel spinning at the same speed has less capacity for potential energy,  for this reason it spins up faster (accelerates) because it take less energy to accelerate it. Therefore for a given energy input (motor Hp) more energy is available from the motor to accelerate the bike faster.   The thing to remember here is a flywheel only resist acceleration and deceleration,  once its rotational velocity is constant mass does not matter for anything other than its gyroscopic effects.
 
If engine breaking is done through down shifting  the bike decelerates as the momentum is dissipated through the engine as friction (heat) PLUS is re-stored as potential energy within the engine's rotating mass as it accelerates due to energy transfer from the bike momentum into the engine's rotating momentum.    A more massive flywheel will add to engine's rotating mass causing it to resist acceleration even more.  This will lead to increased engine breaking but only as long as the engine is actually accelerating from braking.  He Man's first hypothesis.  But once it reaches a constant velocity during engine breaking the mass of the flywheel acts to decrease engine breaking because the additional stored energy must be dissipated through friction in the engine, Spidey's hypothesis .  In the end the mass of the flywheel within the range we are talking about has little effect on engine breaking because these factors tend to cancel each  other out.

[Dirteagle]

 
Bravo Ducnial! A very composed expose` of the dynamics involved with mass and energy. I tip my hat to your erudition on the subject matter.

[Spidey]

If engine breaking is done through down shifting  the bike decelerates as the momentum is dissipated through the engine as friction (heat) PLUS is re-stored as potential energy within the engine's rotating mass as it accelerates due to energy transfer from the bike momentum into the engine's rotating momentum.    A more massive flywheel will add to engine's rotating mass causing it to resist acceleration even more.  This will lead to increased engine breaking but only as long as the engine is actually accelerating from braking.  He Man's first hypothesis.  But once it reaches a constant velocity during engine breaking the mass of the flywheel acts to decrease engine breaking because the additional stored energy must be dissipated through friction in the engine, Spidey's hypothesis .  In the end the mass of the flywheel within the range we are talking about has little effect on engine breaking because these factors tend to cancel each  other out.

You're assuming a downshift.  And you're basically saying the same thing I did.  A lightened flywheel will reduce wheel hop (or call it engine braking if you want) because it more easily matches the revs of the engine.  In practical terms, if you downshift, the bike won't slow down as fast (again, call it engine braking if you like), but the revs will skyrocket.  Thus, as I said, it acts like a poor man's slipper.  But if you just roll off the throttle without downshifting (real engine braking), the lightened flywheel is more responsive.  It will allow the engine to slow down faster.  In practical terms, the bike will slow down faster.

So, what do we know about lightened flywheels?  In practical terms, that is. 

1)  You can bang a downshift with less fear of locking up the rear.  The bike will slow less than bike with a stock flywheel until the engine catches up with the wheel speed.  Then it will slow down faster than a bike with a stock flywheel.

2)  If you roll off the throttle with a bike with a lightened flywheel, it will slow down faster than a bike without a lightened flywheel.

They don't cancel each other out.  In (1), you have two separate things happening at different times.  In (2), there is only one thing happening.

[Howie]

You're assuming a downshift.  And you're basically saying the same thing I did.  A lightened flywheel will reduce wheel hop (or call it engine braking if you want) because it more easily matches the revs of the engine.  In practical terms, if you downshift, the bike won't slow down as fast (again, call it engine braking if you like), but the revs will skyrocket.  Thus, as I said, it acts like a poor man's slipper.  But if you just roll off the throttle without downshifting (real engine braking), the lightened flywheel is more responsive.  It will allow the engine to slow down faster.  In practical terms, the bike will slow down faster.

So, what do we know about lightened flywheels?  In practical terms, that is. 

1)  You can bang a downshift with less fear of locking up the rear.  The bike will slow less than bike with a stock flywheel until the engine catches up with the wheel speed.  Then it will slow down faster than a bike with a stock flywheel.

2)  If you roll off the throttle with a bike with a lightened flywheel, it will slow down faster than a bike without a lightened flywheel.

They don't cancel each other out.  In (1), you have two separate things happening at different times.  In (2), there is only one thing happening.

Spidey gets an A+

[Ducnial]

You're assuming a downshift.  And you're basically saying the same thing I did.  A lightened flywheel will reduce wheel hop (or call it engine braking if you want) because it more easily matches the revs of the engine.  In practical terms, if you downshift, the bike won't slow down as fast (again, call it engine braking if you like), but the revs will skyrocket.  Thus, as I said, it acts like a poor man's slipper.  But if you just roll off the throttle without downshifting (real engine braking), the lightened flywheel is more responsive.  It will allow the engine to slow down faster.  In practical terms, the bike will slow down faster.

So, what do we know about lightened flywheels?  In practical terms, that is. 

1)  You can bang a downshift with less fear of locking up the rear.  The bike will slow less than bike with a stock flywheel until the engine catches up with the wheel speed.  Then it will slow down faster than a bike with a stock flywheel.

2)  If you roll off the throttle with a bike with a lightened flywheel, it will slow down faster than a bike without a lightened flywheel.

They don't cancel each other out.  In (1), you have two separate things happening at different times.  In (2), there is only one thing happening.

Spidey,

We are both saying the same thing.    If restating it a differtent way helps others [and me] understand the physics of this topic,  that's OK.     You are right I was assuming only down shifting vs rolling off the throttle.  However we'll just have to agree-to-disagree on the issue of total breaking effect of  light vs heavy flywheels.   As you said, a stock flywheel will slow less than a light one during engine spin-up from a down shift  and slow more than a light flywheel as the engine catches up and breaks the bike from friction. If you treat these two phases together as total engine breaking  then there is no difference between light vs stock. E.g. comparing a light flywheel to a heavier one will flip-flop these relationships netting identical braking.  [assuming the contribution to braking from each of these two phases are the same, which they are not.]  I agree a light flywheel will reduce wheel hop, that's good, but probably wont make much of a difference total engine breaking within the range of weight we are talking about.

My Brain Hurts.. 

[rule62]

boobies

[Dents]

My uncle, who's a farmer, pulls a plow with an tractor that's too small for the # of spades on the plow and the amount of clay in the soil.  He had too many instances where the tractor would bog down in certain areas of the field.  His fix was to place a HEAVIER flywheel on the tractor.  It definitely takes longer to get the tractor's rpms up to speed but once he's there the tractor pulls like a uh well a bigger tractor.  He ended up doing the same mod to one of his trucks he uses primarily for pulling a gooseneck trailer.

Of course he's trying to accomplish something different but its interesting to see that in some applications heavier better than lighter.