Lightened flywheel

[speedfreek]

Hey guys. I have decided to lighten a spare flywheel for my 400ex to see if I like it on a 4 cycle. I have lightened the flywheels on my previous bikes which were a banshee and a 250R. I even lightened the smaller CR250 flywheel on my ESR 310 when I did the ignition "swap". I really like this mod on a 2 stroke but always wanted to know how it would be on a 4 stroke.

I love the torque on my 400ex. It has killer bottom end. I just don't know if lightening the flywheel will take away too much torque. When I put it on and drive it, I will let yall know the results.

I forgot to get the before weight but I will weigh the other one when I take it off.
http://i90.photobucket.com/albums/k264/pnewell44/250R/400EXflywheel.jpg
http://i90.photobucket.com/albums/k264/pnewell44/250R/400EXflywheel002.jpg

[honda400ex2003]

If you dont mind, i have a few questions for you. How much do you plan to take off and where. what do you plan to use to do it a mill or a lathe? how much did you take off also? I assume you are going to take some of the outer ring off but i am curious. i have read that some guys took off 9 oz but am still curious where they took it off and such, thanks steve

[Honda5]

I have one I got 11 ounces off and that was without cutting the bearing retainer. (make sure that you get it ballanced) I had it on a 13.5:1 426 motor and it made a big difference getting thru the rpms.

[speedfreek]

Originally posted by honda400ex2003
If you dont mind, i have a few questions for you. How much do you plan to take off and where. what do you plan to use to do it a mill or a lathe? how much did you take off also? I assume you are going to take some of the outer ring off but i am curious. i have read that some guys took off 9 oz but am still curious where they took it off and such, thanks steve I used a lathe. I did not take off as much as possible but I took off a pretty good bit. As you can see from the pics I took most off from the outer side. I also took some off from the front.

I cut the snout off of an old crankshaft and zeroed it in the lathe. Then I put on the flywheel and checked it for "out of roundness". This one was .003 out of round. Now it is less than .00005! I have turned MANY flywheels and this is an easy mod that I will do to all of my bikes.


Here is one I did for my ESR 310. This is the CR250 flywheel.
http://i90.photobucket.com/albums/k264/pnewell44/250R/000_0276.jpg
http://i90.photobucket.com/albums/k264/pnewell44/250R/000_0275.jpg

[speedfreek]

Originally posted by Honda5
I have one I got 11 ounces off and that was without cutting the bearing retainer. I cut this one today and got 13.3oz off and could have gotten more:
http://i90.photobucket.com/albums/k264/pnewell44/250R/400EXflywheel006.jpg

[sneaky11]

Do you have to rebalance ?

[riggsracin]

?

[zedicus00]

Originally posted by sneaky11
Do you have to rebalance ?

we did a bunch on a live center lathe and only cut on the outer drum and did not have them balanced. im still running one of then 10 years later and have not had any extra vibes or anything from not having them re-balanced.

[Logan #34's Dad]

How, Who would you ballance one?

Curious what does a lightened flywheel do for a 2stroke? Would a lightened one be good for a CR85 engine for motocross?

[zedicus00]

depends on your porting and riding style, i could ride with no flywheel and it wouldnt bother me. if you have ever bogged out of power band or killed it in a corner probably don't do it. some guys even opt to add weight, it smooths out the power band hit and makes it not stall as easy when you add even more weight.

on the 85 you should be in the top 1000 RPM pretty much all the time, if you dont have a problem with that then sure, cut the flywheel some. it will make the powerband hit harder and you will need microscopic throttle control if you have dreams of ever getting traction.

[wilkin250r]

Zed nailed it, but let's go into more detail.

A flywheel is a big rotating weight, and it is necessary to keep the engine running. If there was no weight at all in the flywheel or crank, then the engine would stall because there wouldn't be enough inertia to overcome the force of compression on the compression stroke.

Adding or removing weight will have an impact on the power delivery of the motor. Just as it takes energy and force to accelerate your car in a straight line (and a heavier car requires more force), it requires energy and force to accelerate the flywheel mass to rotate faster. The more mass, the more force required.

Removing some of that mass will allow the engine to accelerate faster. This could lead to increased acceleration of the bike as a whole, or it could lead to increased wheelspin, traction problems, and decreased lap times.

A really convenient way to think about it is to think about the difference between a 2-stroke and a 4-stroke and the power delivery of each. A heavier flywheel will make an engine feel more like a 4-stroke, with broad power and easy power delivery. A lighter flywheel will make an engine feel more like a 2-stroke with quick power delivery and a stronger "hit" on the powerband and faster acceleration (provided you get traction).

[zedicus00]

Originally posted by wilkin250r
If there was no weight at all in the flywheel or crank, then the engine would stall because there wouldn't be enough inertia to overcome the force of compression on the compression stroke.



the counter weight on the crank is enough to keep it running. have you ever set up a PVL ignition? there literally is no flywheel, just a small aluminum disk that weighs next to nothing, its there to control timing.

true if there was no weight at iether place it would not be able to complete a compression stroke. you need some amount of weight roughly equal to the piston and rod plus the force of compression. after that you are changing the effect of the power stroke.

but yeah, good description on the why of it.