Has anyone added flywheel weights to their YZ450 to smooth out the throttle?


14 replies to this topic
  • Wikki

Posted October 26, 2008 - 07:40 AM

#1

Has anyone added flywheel weights to their YZ450 to smooth out the throttle? I have an 05' and was wondering what weight I should try. I mainly ride on tracks.

  • sacfelix

Posted October 26, 2008 - 09:01 AM

#2

I added a GYTR 9.21 fly wheel weight to my 06 450 and love it.

  • rexbond007

Posted October 26, 2008 - 09:57 AM

#3

yes, best money spent, i put on the gytr yamaha off road,

cuts down in stalling, keeps the front end down when hill climbing.

http://www.yamaha-mo...5/0/detail.aspx

  • KAS

Posted October 26, 2008 - 12:27 PM

#4

Yes, they work great.

Do a search in this forum (and just this YZ450 forum only) for "flywheel".

  • gtxkid

Posted October 26, 2008 - 02:26 PM

#5

I use a rekluse.
I would like to see you ride someone's bike with one.
No stalling

  • FLjoyride

Posted October 26, 2008 - 02:53 PM

#6

I have a zipTY flywheel weight, just bolt it to your stock flywheel! its an 8 oz one and I really liked it, but my stock flywheel magnets started sliding out so I got the GYTR 10oz, which I think is a bit too much weight for my riding terrain.

  • grayracer513

Posted October 26, 2008 - 04:06 PM

#7

I use the GYT-R off road model (9+ ounces over stock). Love it.

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  • tnl

Posted October 26, 2008 - 06:30 PM

#8

I started with the Dr. D 6oz one and increased it to the 8oz one for trail riding (stalling issues). Get the larger one!

  • pobit

Posted October 27, 2008 - 07:10 AM

#9

I bought the 4.9 GYT-R Flywheel. I think it is plenty heavy for me. I ride A class off road in very tight rocky rooty stuff. Too much flywheel and you can't torque down to as low an RPM to feel your way up or through slipery conditions. I do use a flywheel but if you looking for a smoother throttle response, a programable ignition is what you need. The YZ 450 has a very aggressive advance curve at off idle for racing motocross. If you get an aftermarket ignition that allows you to retard the spark at off idle it will make it smoother. This what an "enduro" ignition does. If you have a chance to ride a newer GasGas EC250-300 or KTM XCW 250-300 with the dual ignition you can see for yourself the differance. In a nutshell, a lighter flywheel with a differant ignition will give the smoothest and longest power curve. A heavy flywheel does smooth things out but it also shortens the power band and hurts acceleration. Dave

  • grayracer513

Posted October 27, 2008 - 08:35 AM

#10

...Too much flywheel and you can't torque down to as low an RPM to feel your way up or through slipery conditions. ... In a nutshell, a lighter flywheel with a differant ignition will give the smoothest and longest power curve. A heavy flywheel does smooth things out but it also shortens the power band and hurts acceleration. Dave

These statements are categorically untrue. The last statement is theoretically correct in that an increase in rotating mass will, at some point, begin to hamper acceleration, but from a practical standpoint, there isn't room in a YZ450 ignition case for a flywheel big enough to accomplish that. No matter how heavy you make it, it's still smaller than your gas cap, and side by side on asphalt, there's no difference you're going to be able to see.

Since the flywheel weight simply adds or subtracts from the work the engine has to do, it has absolutely no effect on the power band, other than to make the engine pull more smoothly at low speeds, any more than changing from second to third gear does.

And your first statement is the precise opposite of fact. The reason weight is added to the rotating assembly of the crankshaft is to increase its inertia. This added inertia is basically stored energy form the preceding power stroke that helps keep the engine turning until the next one hits, carrying the engine through the next compression stroke, and against whatever load is being placed on it. Far from making it more difficult to lug an engine down to lower RPM's, additional flywheel inertia facilitates this by keeping the engine from fighting itself to stay running, and smoothing out the operating speed.

Your point regarding the ignition is well taken, but a light flywheel to the extent that they are used on a YZ450 will not help at low speeds.

  • Wikki

Posted October 27, 2008 - 08:43 AM

#11

Is it ok for track (MX) riding?

  • grayracer513

Posted October 27, 2008 - 09:25 AM

#12

Yes, they are fine for MX. You may notice an improvement in traction leaving corners.

  • pobit

Posted October 27, 2008 - 08:00 PM

#13

These statements are categorically untrue. The last statement is theoretically correct in that an increase in rotating mass will, at some point, begin to hamper acceleration, but from a practical standpoint, there isn't room in a YZ450 ignition case for a flywheel big enough to accomplish that. No matter how heavy you make it, it's still smaller than your gas cap, and side by side on asphalt, there's no difference you're going to be able to see.

Since the flywheel weight simply adds or subtracts from the work the engine has to do, it has absolutely no effect on the power band, other than to make the engine pull more smoothly at low speeds, any more than changing from second to third gear does.

And your first statement is the precise opposite of fact. The reason weight is added to the rotating assembly of the crankshaft is to increase its inertia. This added inertia is basically stored energy form the preceding power stroke that helps keep the engine turning until the next one hits, carrying the engine through the next compression stroke, and against whatever load is being placed on it. Far from making it more difficult to lug an engine down to lower RPM's, additional flywheel inertia facilitates this by keeping the engine from fighting itself to stay running, and smoothing out the operating speed.

Your point regarding the ignition is well taken, but a light flywheel to the extent that they are used on a YZ450 will not help at low speeds.


What I stated is true. A lighter flywheel does indeed pull at a lower RPM than a heavier one does. Maybe you’re not understanding what I am saying so I will put it another way. A motor with a lighter flywheel can accelerate from a lower rpm than a motor with a heavier one. The RPM at which a motor can still accelerate is lower with a lighter flywheel because it has less weight to turn. You do understand I am talking about changing RPM's. I am not talking about a flywheels ability to prevent stalling. I was simply stating that flywheel weight is a compromise between acceleration and the width of the power band verses stalling. I also stated I race A class. I do not need a heavy flywheel as I have enough talent to prevent stalling with a lighter one. I use the lightest flywheel I can use without stalling so that I can up shift sooner and accelerate from lower RPM's This gives the most traction and helps the suspension work better.
Your statement that the heavier flywheels for the YZ450 make no noticable difference in acceleration tells me you have no idea what you are talking about. The heavier flywheels have a very noticeable effect on acceleration that makes the bike slower. A YZ450 with a heavier flywheel will lose a drag race to one with a lighter flywheel. If you look at the specifications for the Yamaha GYT-R flywheels you will see they give a mass inertia number. You will also see that a little extra weight spinning fast makes a very large differance. If your a trail rider and don't care about the loss of acceleration, a big heavy flywheel will help smooth things out but the correct way to smooth out the power on a YZ is with a new ignition. Dave
__________________

  • tnl

Posted October 27, 2008 - 09:19 PM

#14

:lol: . Ummm......? No idea huh?

  • grayracer513

Posted October 27, 2008 - 09:22 PM

#15

If you look at the specifications for the Yamaha GYT-R flywheels you will see they give a mass inertia number. You will also see that a little extra weight spinning fast makes a very large differance. If your a trail rider and don't care about the loss of acceleration, a big heavy flywheel will help smooth things out but the correct way to smooth out the power on a YZ is with a new ignition. Dave
__________________

I already conceded the value of remapping the ignition, so let's set that aside. When you look at the inertia masses of the flywheels in the GYT-R catalog and note the relationship of weight added to the inertia mass gained, note this:

The off road flywheel for an '06-'09 adds 9.21 oz. for an inertia mass of 6.15 kg-cm² (stock inertia mass=3.8 kg-cm²), whereas the same flywheel for an '03-'05 YZ450F adds 6.38 oz. for an inertia mass of 6.2 kg-cm² (stock inertia mass=3.6 kg-cm²). The newer flywheel adds roughly 50% more weight to net slightly less inertia mass. Why? because the rotational inertia of an object depends a great deal on its form. A 4" flat disc will have less inertia than a flat 6" disc of the same weight because this distance of rotating mass from center is a factor. Likewise, a wheel consisting of a heavy ring on light spokes will have more inertia than a flat disc of the same weight. Simplistic rules don't work.

A motor with a lighter flywheel can accelerate from a lower rpm than a motor with a heavier one.

No, it cannot. If you go down to a low enough RPM with an engine that produces very little power at that speed (like a two-stroke), it can be said that an engine will pull more readily with a lighter flywheel, but if it will pull with a light one, it will pull with a heavy one also. The engine will produce exactly the same power with either. Only in the case that the available torque could not overcome the additional inertia, but a YZ450 has enough torque, even at speeds below idle, that this just isn't going to come up. Even the much larger flywheel from a WR doesn't produce this effect.

In fact, in the particular case of the YZ450, it actually pulls harder at low RPM with a heavy flywheel than with a light one. The reason is that below about 3500 RPM, the YZ has so little rotating mass that it cannot conserve its momentum between power strokes, and will loose so much of it that it will begin bucking, loosing power to its own compression stroke. With more inertia, it rolls smoothly through the stroke, conserving its energy, and pulls harder.

The difference is that you are talking about an engine that had enough flywheel inertia to begin with, and I'm talking about a YZ450, which clearly has far less than it needs for low speed work.

Your statement that the heavier flywheels for the YZ450 make no noticable difference in acceleration tells me you have no idea what you are talking about. The heavier flywheels have a very noticeable effect on acceleration that makes the bike slower. A YZ450 with a heavier flywheel will lose a drag race to one with a lighter flywheel.

Nice theory. In actual practice, running my son's bike against mine, the two pull dead even from a rolling second gear start (it varies a little back and forth, but by and large, there's no difference). The only difference between the bikes is I have the GYT-R flywheel, and his is stock. They ran the same before I added the flywheel, too. What you've overlooked here is that the flywheel on the YZ450, weighted or not, represents a comparatively small percentage of the total rotating mass of the engine. The extra inertia simply doesn't require a large enough portion of the available torque to be noticed in this way.





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