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weighted flywheels

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Hey fellow thumpers, looking for some advise on weighted flywheels for a 2003 crf 450. I mostly ride trails and a few orv parks. I never ride the tracks. I am being told to go with the lightest by some people and go with the heavyest buy others. I really do not want to lose any wheel spin and i enjoy doing wheelies. Any advise?

Sloneze

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why do you need a flywheel weight? because people told you? is it really affecting your riding that you need a flywheel weight? if you cant keep the front wheel on the ground...i would go with a heavier one...mabe 12 ounces...but if you do some moto or just want to slow the bike down...get a smaller FWW, maybe 7 or 8 ounces

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Hi I have a 400 yzf. It is noted for haveing a lite flywheel. It is so lite you would probly kill it the first time you went to take off. If you crack it open it will stand rite up for you, like a two stroke. When you let off you don't slow down. A heavyer fly wheel on my bike whuld be fine with me. Any how the liter you go the faster it will wind up and the less backpreshure to slow down. If one of your buddies has one "400 yzf " try it out . They go !!

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It matters. If you are often banging trees up in the woods, a heavier one like Dwight suggested would suit you well. But if you still want some hit and slightly reduced stall power, a lighter one would be best. It also just matters what kind of a rider you are. If you are agressive, go lighter. But if you like to lug, go heavier.

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The general principle to flywheel weight is rotational mass.. The flywheel is designed to absorb some of the impact of the explosive power of the combustion in your cylinder and then give it back when it's time to put energy into compressing the next charge of fuel and air.

With a multi-cylinder engine, the other cylinders can help with compressing the next fuel charge in the other cylinders. With our single cylinder bikes, the flywheel is a little more important. The 4-Stroke needs a little more weight to carry that piston around a second time and for the higher compression that is typical in a 4-stroke engine.. That is where the "Thump" comes from.

Remember, the primary function of the flywheel is to store energy. This means it will absorb some of the engine power and then give it back. This can mean less wheel spin and better traction and staying alive and smooth at slower speeds under load.

So, if you want your bike to idle smoothly and not die easily at low engine RPMs, And you want the engine to come back to idle speed more slowly for that "glide affect", then add weight. At lower RPMs, the extra flywheel weight will allow the engine to go slower and smoother.

If you want a more rapid or instant increase AND decrease in Revs with each twitch (or twist) of the throttle... and idle speed and low end smoothness is not important to you, then lighten the flywheel. Keep in mind that quicker revs also means a faster return to low revs

If you are a trail rider and you need to go slow at times as in tight switch backs and difficult sections of a mountain trail, then you will benefit from a heavier flywheel. You will also be less likely to kill the engine with rear braking too.

But the flywheel is NOT the only thing that affects the low and high end power.. Your Carb plays a big part too. If you are thinking of a heavier flywheel for reasons mentioned above, then you might also look into a smaller carb to go with it. A small carb is off of the idle circuit and on the main jet sooner.. The large carb needs more air passing through it (Higher Revs) before the venturi affect can get the main jet circuit working.

My 300cc 2-stroke Trials bike uses a tiny 26mm carb and a very heavy flywheel.. It will pull stumps out of the ground and crawl slower than you walk with the clutch fully home without dying at idle throttle.. And this is a 2-STROKE !!! If I increase the carb size, I increase the upper power but I hurt the lower end power.

And actually, there are 3 things involved here.. Flywheel, Carb and Exhaust. They all combine to give you a different power curve. It doesn't really matter if it's 2-stroke or 4-stroke. Compare the 2 extremes in 2 or 4-stroke engines. Compare the Moto-Cross Bike and the Trials Bike. Both in 2 and 4-stroke versions.

The Trials style engine uses a small carb, heavy flywheel and a restricted exhaust.. All three of these factors are for Super Tractor low end torque and smooth power.

The Moto-Cross engine uses a super large carb, very small flywheel and a very open exhaust.... for Explosive power at the extreme upper end of the power band for that Crotch Rocket Experience!

Other than that and a few other minor items like ignition and port timing, they are the same engines doing a COMPLETELY different job

So, using that as the far extremes, consider what you want in the way of power for your favorite riding style. It's Not Rocket Science, but it's amazing how so many people have so many twisted notions of what does what... :cry:

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It's personal, but I like the heavier ones. I have a 14oz on my yz250f and wish I could get more. I thought it would slow me down on the track, but it actually allows me to clear obsticles out of turns easier, because I can carry more cornering speed.

In the woods it's no comparison to a stocker. Now that I have a 262 big bore kit, I wish I could get a custon flywheel weight of about 20 or so oz.

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Yeah! What 2-Ply said :cry:

I ride a YZ426 in tight trails, and combined with slightly lower gearing, it was the best mod I've done to the bike. The flywheel really helps avoid the dreaded "cough and die" while trying to creep thru rockpiles or snotty tight trails and really reduces wheelspin in slipery conditions (clay).

I'm not sure I would want one for moto, but it's all about where and how you ride. If you don't like it, you can always take it off.

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Thanks guys, I hope it helped.... There's more to it, but I'd have to call my publisher before adding any more.. :cry:

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so what do i have on my 05 yz250f right now? i kind want it a little snappyer. how much would i go down in flywheel weight to make it snappyer? thanks a lot.

-Taylor

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I've heard of TTR riders milling weight off their flywheels to give their bikes more snap, but these are trailbikes and have kind of a heavy flywheel. Do a search in the YZ section of the forum and see what turns up. :cry:

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so what do i have on my 05 yz250f right now? i kind want it a little snappyer. how much would i go down in flywheel weight to make it snappyer? thanks a lot.

-Taylor

I don't recommend this at all, in fact I recommend adding some weight.

You will bog badly in sandy corners or on hills.

Dwight

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The Trials style engine uses a small carb, heavy flywheel and a restricted exhaust.. All three of these factors are for Super Tractor low end torque and smooth power.
And, due to gearing also, a 4th factor, they will still wheelie like there is no tomarrow.

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I don't recommend this at all, in fact I recommend adding some weight.

You will bog badly in sandy corners or on hills.

Dwight

ok, thanks for the insight. :cry:

-Taylor

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And, due to gearing also, a 4th factor, they will still wheelie like there is no tomarrow.

Except that what I'm talking about has nothing to do with gearing. But you are right, these bikes have 4 super low gears for the actual competition sections and then 2 VERY tall transportation gears for quick travel and saving fuel.

I'm talking about RPMs just above idle no matter what gear. In fact, I can leave the clutch home in 6th gear and slow down to less then 10 MPH and then roll on the throttle all the way to 75 MPH. On regular single track trails, I leave it in 5th gear, no shifting, from 5 MPH all the way to 50 MPH

See the large round black side cover below? That is ALL flywheel

280pro.jpg

One demonstration on dirt is to let the bike idle. Then with the front tire against a tree or car. slowly let the clutch out until fully engaged, increasing the power WITHOUT increasing the Revs above idle with all of my weight (225 pounds) planted on the bike and watch the tire slowly dig a hole without stalling the engine and without shooting any dirt past the rear of the tire. The front tire becomes completely smashed flat againt the tree... And remember, this is a 2-Stroke!!

Of course, I move the bike, fill the dirt back in and tamp it down with my boot.

I once drowned out my van in deep water with three of us, all of our gear and three bikes on a trailer. I drained the battery trying to get the engine started again, so we took one of the bikes and strapped it to the front bumper of the van (including the trailer and the other 2 bikes). We attached the strap to the rear axle of the bike so that the force would be low and help prevent the bike from looping. We towed the van for 1/2 mile, flat ground and no help from any hills with the Van in second gear, clutch engaged all the way until it pumped out all of the water and fired up!! The Trials bike was in third gear! To be honest, I was not sure it would do it.

A word of caution for those that are thinking of messing with the flywheel weight. It's not always a simple mod. Balance is critical! Making your own weights is risky. And most bikes can only be slightly changed. You are not likely going to make a TTR 125 into a Moto-Crosser and for the Moto-Crosser, the cases are probably too small to fit much in there. Diameter of the flywheel has a big affect. If your flywheel and case is small in diameter, it will take more weight to get the same affect as a larger diameter wheel. Look again at the size of my flywheel cover in the photo above.

Get professional advice and/or support from someone that has already done the mod for YOUR particular bike. :cry:

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MXrider, if you want more snap, I would recomend leaving the flywheel alone, and try adding a couple of teeth to the rear sprocket. After that you might try some standard hop-up mods like a James Dean jet needle or a power-now.

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