# Will an '05 yz450f motor fit on an '03?

The relative speed of the internals will make a big difference (in regards to engine braking) do to "moment of inertia"...

This has been one of the best posts I have ever read. Finally something better than "what oil do I use" and "why is my engine popping"

The relative speed of the internals will make a big difference (in regards to engine braking) (due) to "moment of inertia"...
The term you are looking for is actually not "moment of inertia", it is kinetic energy. That it increases with speed is, by itself, true. As with any mass in motion, the kinetic rotating inertia increases roughly with the square of the speed. The simplest representation of that is:

Energy=1/2 the Mass x the square of the Velocity

Moment of inertia is how that basic principal of inertia is applied to rotating objects is fairly complicated:

The method of calculating inertia also varies somewhat for different rotating shapes:

But in this particular case, you need to understand that the inertial weight of the clutch is never directly applied to the crankshaft because there is in both cases a reduction ratio between crank and clutch in which the clutch is at a significant disadvantage as a driving force to the crank. Any force of inertia within the clutch of the '05 is reduced by the primary ratio to .377 of what it was. Also remember that the speed of the clutch is only 6% higher than it was in the earlier bike, and that even with the engine turning 10k, the clutch only spins at 3770 rpm (compared to 3540 on the '03). Whatever small increase in inertia we gained there is going to be reduced to just over a third of that by the primary ratio. While it's theoretically correct to say that it will increase inertia at the crank, the difference is not going to be noticeable.

The other thing that the above discussion leaves out is that it is still the rear wheel driving the engine during deceleration, and that the overall gearing between the wheel and engine is the same. The gains in inertia at the clutch are offset in part by the increased load the rear wheel now bears in driving the clutch faster, and in any case, neither are more than a small fraction of the force required to dead spin the coasting engine.

If you still think it makes a difference, I can only offer you this:

Just a personal observation...from the pants feel for the two bikes I cannot feel any difference in engine braking between 04 and 05.
The gains in inertia at the clutch are offset in part by the increased load the rear wheel now bears in driving the clutch faster, and in any case, neither are more than a small fraction of the force required to dead spin the coasting engine

does the reduced final gearing (13/50) not make it easier for the wheel to spin the faster moving countershaft sprocket?

does the reduced final gearing (13/50) not make it easier for the wheel to spin the faster moving countershaft sprocket?

No, just the opposite. It makes it easier for the engine to drive the wheel, and more difficult for the wheel to drive the engine.

The term you are looking for is actually not "moment of inertia", it is kinetic energy.

No actually it was "moment of inertia" I was talking about. The initial impact on traction and the rate at which the motor picks up revs is highly dependant on many things INCLUDING greatly, the moment of inertia of the parts that are accelerating.

Thats why a lot of times when you "bump start" a bike the rear wheel skids for a second then begins to turn. Im not commenting on the "amount" of effect that can be detected by the highly inaccurate "booty dyno" but the fact that the forces do exsist...

I can start using really fancy wording and make theings even more confusing for those reading this to disguise the parts I do not understand; but the point here is to educate not mutilate...

The fact is, no matter the means, if you spin parts faster that energy needs to come from somewhere (remember, Einstein, equal and opposite) If the wheel speed and engine speed of both bikes is equal, yet the internal gearing changes the speed of the clutch, shafts, bearings, etc. that energy gain or release has to come or go from somewhere!!

Thats why a lot of times when you "bump start" a bike the rear wheel skids for a second then begins to turn. Im not commenting on the "amount" of effect that can be detected by the highly inaccurate "booty dyno" but the fact that the forces do exsist...
You must certainly be able to understand that the reason the rear tire locks momentarily (at least we hope it will be momentary) is due to running afoul of the compression stroke, and has little to do with inertia one way or other.

The fact is that inertia in a rotating object is fundamentally the same simpler concept of kinetic energy that applies to all objects in motion, whether rotating or going in a straight line. Moment of Inertia is a set of algorithms used to quantify the simple underlying concept of kinetic energy in rotating objects of varied shapes, and the force applied to the axis of those objects, and gives you the means to take such things as radial distribution of mass into account. But it is still just a means of quantifying that force, and not the force itself.

The fact is, no matter the means, if you spin parts faster that energy needs to come from somewhere (remember, Einstein, equal and opposite) If the wheel speed and engine speed of both bikes is equal, yet the internal gearing changes the speed of the clutch, shafts, bearings, etc. that energy gain or release has to come or go from somewhere!!
"Equal and opposite" was Newton. And you're right, the energy to spin the trans at a higher speed does have to come from somewhere. Now tell me where it comes from under deceleration, and how that reduces engine braking.

the motor spins up the faster moving gears,so the difference in energy is probly given up in the accelleration torque curve. then the benefits shine on decel:thumbsup:

No, just the opposite. It makes it easier for the engine to drive the wheel, and more difficult for the wheel to drive the engine.

if the shaft speed were the same that would be correct but its faster

"Equal and opposite" was Newton. And you're right, the energy to spin the trans at a higher speed does have to come from somewhere. Now tell me where it comes from under deceleration, and how that reduces engine braking.

Sorry, it was late, I have a habit of mixing up stuff like that. That energy comes from the rear wheel, store in it while the bike was accelerating, as a gyroscopic inertial force...

the motor spins up the faster moving gears,so the difference in energy is probly given up in the accelleration torque curve. then the benefits shine on decel:thumbsup:

if the shaft speed were the same that would be correct but its faster

Remember that that rear wheel is doing the driving during decel. Regardless of where the shaft speed came from, the rear wheel is trying to maintain that speed under deceleration. There's no way you can suggest that it's easier for the rear wheel to spin something faster rather than slower. If that were true, it would take less horsepower to turn a given load faster than to turn it slower.

The rear wheel, as a driving member is seeing the final gearing the same way a bicycle rider sees gearing, the reverse of how the engine sees it. If you had two identical bikes, one with a 48 front and 13 rear, and one with a 48/14, which is harder to pedal at the same crank (pedal) rpm?

That energy comes from the rear wheel, store in it while the bike was accelerating, as a gyroscopic inertial force...
Precisely. But under decel, is it not true that the rear wheel, being driven by the inertia of the moving motorcycle, is now driving the transmission and clutch? Is it not then also true that if the transmission is now to be turned faster at the same road speed (which means you have the same amount of inertial energy to use), it will require more power from that store of inertia to do that?

That offsets some of the inertia gained within the clutch.

I'm not saying that the faster spinning clutch has no influence whatsoever on the overall rotating inertia of the complete engine assembly. What I am saying is that the net effect of that additional inertia on engine braking in this case is far too small to be perceivable between the '04 and '03 models:

> Inertia increases or decreases exponentially with speed, and the clutch is turning at only 40% of crank speed to start with, so it has much less inertia than it would have if it were on the crank.

> The clutch speed has been raised only 6%.

> On deceleration, the clutch must deliver what inertia it has to the crank, but does so at a mechanical disadvantage of more than 2.5:1, further reducing the effect.

> The mechanical load on the rear wheel under deceleration is unchanged because the overall gear ratio is unchanged.

That's my point.

Remember that that rear wheel is doing the driving during decel. Regardless of where the shaft speed came from, the rear wheel is trying to maintain that speed under deceleration. There's no way you can suggest that it's easier for the rear wheel to spin something faster rather than slower. If that were true, it would take less horsepower to turn a given load faster than to turn it slower.

>>> but its not a gain of energy its a loss,if the dang things already spinning faster its less difference between the two evils,the faster shaft speed is helping the tire to lose its energy <<<<

The rear wheel, as a driving member is seeing the final gearing the same way a bicycle rider sees gearing, the reverse of how the engine sees it. If you had two identical bikes, one with a 48 front and 13 rear, and one with a 48/14, which is harder to pedal at the same crank (pedal) rpm?

>>>>as you said before the overall gearing of the motorcycle isn't changing its just using different ratios for the same result,your example of the bicycle would change the mph at the same rpm. now if the driving force is from the wheel(as it is on decel) would the wheel not move the larger sprocket easier(especially as the countershaft is spinning faster to overcome that gearing difference you keep referring to but in reality is not there,due to the overall gearing being the same) as its circumference is greater(just as a valve wheel is easier to turn if its bigger)

Just a personal observation...from the pants feel for the two bikes I cannot feel any difference in engine braking between 04 and 05. I own the 04 and my riding partner owns the 05 but mind you this isn't with stock gearing. You guys got WAY too technical for me to understand lol.

What gears are you running in your 04?

On some straightaways I am tapped out and could use some more top end.

Mine is stock.

I'm not saying that the faster spinning clutch has no influence whatsoever on the overall rotating inertia of the complete engine assembly. What I am saying is that the net effect of that additional inertia on engine braking in this case is far too small to be perceivable between the '04 and '03 models:

> The mechanical load on the rear wheel under deceleration is unchanged because the overall gear ratio is unchanged.

Those 2 statements alone appear completely contradictory, am I wrong??

Now it seems you are agreeing with us that there is an effect on the engine breaking, you are just saying that it is so small its unperceivable??

I never stated how much effect it made, just that the theory of why is solid. Yamaha would not have gone the lengths had the outcome not been "perceivable"...

Those 2 statements alone appear completely contradictory, am I wrong??

Now it seems you are agreeing with us that there is an effect on the engine breaking, you are just saying that it is so small its unperceivable??

I never stated how much effect it made, just that the theory of why is solid. Yamaha would not have gone the lengths had the outcome not been "perceivable"...

exactly,i don't know much about the physical aspects at work;but i do know that yamaha did state this was done to "REDUCE ENGINE BRAKING" now whether or not its perceivable but some people are more perceptive than others:prof:

Now it seems you are agreeing with us that there is an effect on the engine breaking, you are just saying that it is so small its unperceivable??
Imperceptible? Yes, that's pretty much it. Because the clutch turns at such a relatively low rpm, it doesn't have much inertia in the first place, and because of the primary ratio, can only deliver 37% of that to the crank. Such a small increase in clutch inertia as would be caused by increasing the clutch speed by 6% would hardly be noticeable as flywheel inertia in the first place, and my experience with changing flywheel weights on YZ450's is that they have little, if any, perceptible effect on engine braking at all.

The mechanical load is that which the rear wheel is subject to in driving the engine as an air pump, and it is simply a product of the resistance offered by the engine and the gear ratio through which it is done. Since neither of those is changed, the mechanical load applied to the rear wheel remains the same.

I never stated how much effect it made, just that the theory of why is solid. Yamaha would not have gone the lengths had the outcome not been "perceivable"...
That's based on the assumption that this was done to decrease engine braking, as Bob contends. He's based that on what he says he read in some of Yamaha's ad material. These statements made in ads have to be taken with a grain of salt. Just as in the statement regarding the increase in rotating mass in the '04 model, a good many of them are the most enormous exaggerations, and true only in the strictest terms. The statement that the primary reduction ratio was done to reduce the load on the clutch itself came from some technical literature that I cannot cite for you, unfortunately, because I don't remember the source. Nevertheless, my experience tells me that ad copy is usually a poor source of technical information.

i guess we'll never know for sure;another one of lifes great mysteries!

will the great mystery of the reduced engine braking ever be solved.....will yamaha continue to force feed us crap until it pours from our ears......will gray ever stop beating his head against the wall from utter frustration....will mtnman ever just say "oh you must be right"? don't hold your breath folks; i'd say theres something to this moment of inertia but i don't know enough of what i'm talkin about,so i really can't argue the point any more:cry:

man o' man gray we really hijacked the bejeezus outa this thread!!!

we really hijacked the bejeezus outa this thread!!!
Ya think? I should give myself an infraction.

Maybe if you want to have some more fun, we could talk about what engine braking is; why four-strokes have it and two-strokes don't.

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