Lightening the flywheel:)
 

Ok, the other day I was lightening a flywheel for a customer. Now...I've done about 12 of these over the past few years, and when I set them up on my lathe I always indicated the center bore to make sure it's at zero run out. Here's what i have found on a couple of KLX flywheels(including my own) they are NOT concentric. Meaning the center bore and the outside diameter are a bit out of wack....about .007 to .009 thousandsths of an inch. Now this would explain why my KLX had a slight vibration at 5k rpm. So, not only did the 1lb. of material that I took off the flywheel help the motor rev up quicker but the annoying vibration is now gone as well.:)

1lbof weight - that's a good bit! How does it feel after that?

I like the tractor-like feel - but at times I wonder what it'd be like with a much lighter flywheel.

Is that bad manufacturing...or what?
How much weight would be removed just to true it up and take out the vibration?
Interesting...

How much does it weigh overall to begin with?

Lighter flywheel sounds like something I'd be interested in.

Pics please. How much do you charge to do it?

I charge $55 to lighten the flywheel.

Yes it's a manufacturing flaw on some of the flywheels. The flywheel backing is riveted onto the magneto rotor, and if everything is not lined up 100%perfect...then you have what we got here. To just true it up, it would only take less than half an ounce of material to be removed.

You don't lose any of the tractor-like feel, but what you do gain is a little bit more throttle response.....since there's less weight to spin quickly.

Hmmm.. I may have to try this. I'm going to try to source an extra flywheel so I can have both.. I'll be in touch :)

EDIT - what am I talking about? I forgot I have an extra entire bottom end... I am going to pull that flywheel. I will shoot you a message when I have it off to see where to send it.

Thanks!

Cool.

What is the down side to a lighter flywheel?
Dan

less bottom end faster revving but not necessarily a bad thing. Could be interesting.

Sounds like it may be a worthwhile mod for these bikes.

Just to note; just because the assembly ID and OD are concentric, doesn't necessarily mean the assembly is perfectly balanced. I don't doubt an improvement over OEM though, since removing eccentric weight from the OD will and reducing OD will have a big effect on imbalance.

Easier to stall at very low engine speeds (not really less low end power, just more likely to stall). Less idle stability, or need for increased idle speed. Increased torque fluctuations through the drivetrain (may equate to increased wear). ...But there might be enough rotating weight in the crankshaft that it won't matter much anyway. However, for those with high-compression pistons (such as the BB351), I'd be cautious. I'm not saying don't try it, just know what you're getting into.

Found a little on-line. Lightening the flywheel can cause more engine vibration. Flywheel and clutch mass can mask other vibrations. You may need to adjust carb to keep engine smooth. Clutch could slip. May need to up idle speed, and keep the revs a little higher when running low rpm. Don't know how much of this applies to our KLX.

Here's what I found about lightening the flywheel, and this is from my own REAL experiences.....everytime I've done one for a customer the results were always positive. Less vibration, better throttle response. I've NEVER had any issues with the idle...or stalling....or increased vibration...engine reliability or anything of the sorts.
When the job is done right the outcome of the product/service shows when my costumers come back to me with a big smile on their face......need I say more?

Have a nice day.;)

This is neat, i'd deffinetely just want the flywheel trued up a bit though.

I dont think there is much negative to lightening it though either, just less energy stored in the rotating mass... i mean for all intensive purposes the flywheel is similiar to a capacitor for electrical purposes... it floats out power inputs and outputs.

Kinda like pouring water outa a bucket, if u fill and dump a bucket repeatedly; ull see at times there is no water flowing across the ground (like when ur filling the bucket before u dump it again lol, this would be the equivalent of no flywheel at all) but if you were to put another bucket on the ground and dump ur bucket into that one (now we have a flywheel), as the bucket stationary on the ground began to fill and overflow... you would notice there is still flow out of the bucket on the ground, even when the bucket being dumped over and over again is not always dumping water into it...

The point is i believe reducing flywheel weight is similiar to just using a smaller stationary bucket on the ground (now we have a smaller flywheel)... which is wat leads to faster response

So yea u gain faster throttle....
But the downside would be less THUMP when u pop the clutch at the same RPM's for a trimmed flywheel vs. a non-lightened flywheel...

and there is ur physics for the day lol, enjoy ;-)

You may want to reconsider if you FI, unless you can remap your idle speed to higher. If you take significant material off, the engine will have to idle faster.
If the flywheel was not installed concentrically, just making the outside round does not necessarily balance the thing, it may make it less unbalanced. Round & balanced are two different things. If the outside is not concentric, the inside isn't either.

I've lightened flywheels for many FI street bikes as well, and it had NO effect on the idle at all. And yes, a less skilled person can install a flywheel incorrerctly and throw the balance off....but remember this...the flywheel is on a tapered shaft end and it also has a key way. Kind of hard to mess up the install, but certainly not impossible.

why would the mass of the flywheel determine idle speed... that doesn't make sense to me really... once u get the speed in it, its not like its more or less efficient; its just stored energy

Well said.:)

The stored energy keeps the motor turning into the next piston firing. Without the mass keeping everything moving, the compression can be high enough to stop the motor. Kind of like turning your idle speed down low enough that the engine dies. You may need to turn the idle up with a lighter flywheel. Perhaps not on our KLX, but reportedly so on other bikes.

^ yea that makes sense now, probly a threshold point we haven't crossed yet on the klx... someone turn their flywheel down to nothin and let us know the resulst lol....

My bike's Prev Owner (guy in western PA) had the flywheel lightened... So I've been riding with a lighter flywheel all along.

I want to know what it's like with a heavier flywheel! ! ! haha..

I believe there's some misunderstanding between the above quoted posts. durielk is saying that if the flywheel was riveted together with the OD eccentric to the ID of the hub, then even by machining the hub ID and flywheel OD to be perfectly concentric, there's still eccentricity left within the assembled part. That's part of what I was getting at in my earlier post, too.

Other than during the firing stroke, the engine needs to make those one and a half revolutions to get to the next firing stroke; a lot of energy is absorbed through friction, compression, and accelerating the reciprocating parts, etc. If the rotating mass doesn't have enough stored energy, the motor can stall before it gets a chance to fire again. Energy is a product of mass and speed; more mass and/or more speed equate to more energy; less mass and/or less speed equate to less energy.

In a lot of cases a lighter flywheel won't affect idle speed, but it certainly can. It depends on how much energy the engine needs to make it though a cycle in relation to how much energy the flywheel must contribute to the total energy required. There may be an excess of rotating mass (aka energy storage) in an engine (such as a smooth street bike), or it may be marginal (such as a max effort drag race bike).

In a lot of cases (most cases), the idle speed of an engine is set much higher than the threshold where it may or may not stall out. This means there is an excess of energy potential to keep the engine turning between power cycles. From this point, some mass can be removed from the rotating parts and may not have an effect of the apparent idle quality, as long as there's enough mass still spinning in the engine.

Let's look at it another way. Every engine has a minimum idle speed, below which it will start to run rough and eventually stall. Yet that minimum idle speed is significantly higher than the speed we spin the engine at to get it started in the first place. Why is this? It's not because we lose spark, or air, or fuel below the minimum idle speed...after all, those are there when we kick or crank the engine over. It is because below that minimum idle speed, there is no longer enough rotating energy to consistently get the engine through the exhaust, intake, and compression cycles... And that minimum rotating energy is directly related to flywheel mass (to a more or lesser degree depending on application). So, if you take 'X' engine and turn it down to the lowest possible idle at which it will remain running, and do nothing else but remove 'flywheel' weight, it will now require a higher idle speed to stay running.

That's a long way of getting at...exactly what IDRIDR said, only he did so with much less verbage. Anyway, our bikes may, as per cpr1's experience, do nothing but benefit from a lighter flywheel. The only way to know for sure is to try it! But still, a number of 351 owners have reported needing to increase idle speed from stock; a lighter flywheel could exacerbate that issue. Like I said before, just know what you're getting into, and have at it.

Hi there,

here is the real deal, got my flywheel lightened and took a ride today.

WOW. it idle good like before, it is like having 13 front gears instead of 14.
they took out 1.4pounds of the flywheel . very trottle happy revs very fast.:)

Did you have cycle pro do the work ? I'm thinking about having it done on my bike once i get my 331 kit back from them and installed.

I had cycle pro do the job very nice and professional..

Thanks Robert.:)

Anybody knows the thread size on flywheel puller?
Possibly somebody can measure it?

Thanks
Mikk

I used the rear wheel shaft.worked like a charm.:)

Wow that is nice :) Thanks my friend

Info on tool path for for lighting the fly wheel.
 

[QUOTE=cpr1;456969]Here's what I found about lightening the flywheel, and this is from my own REAL experiences.....everytime I've done one for a customer the results were always positive. Less vibration, better throttle response. I've NEVER had any issues with the idle...or stalling....or increased vibration...engine reliability or anything of the sorts.
When the job is done right the outcome of the product/service shows when my costumers come back to me with a big smile on their face......need I say more?

Have a nice day.;)[/QUOTE
i have a few buddies that are machinist and was curious as to where and how much metal you guys are removing when turning down a flywheel.

Info on tool path for for lighting the fly wheel.
 

[QUOTE=cpr1;456969]Here's what I found about lightening the flywheel, and this is from my own REAL experiences.....everytime I've done one for a customer the results were always positive. Less vibration, better throttle response. I've NEVER had any issues with the idle...or stalling....or increased vibration...engine reliability or anything of the sorts.
When the job is done right the outcome of the product/service shows when my costumers come back to me with a big smile on their face......need I say more?

Have a nice day.;)[/QUOTE
i have a few buddies that are machinist and was curious as to where and how much metal you guys are removing when turning down a flywheel.