Preparing a road legal FXR150 for Bucket racing

[thealmightytaco]

I should've said flywheel effect at low revs where a road bike can stall at manouvering speeds. If you're moving at almost an idle, in particular, a wheighty flywheel gives inertia to the engine to decrease the chances of stalling. Once the flywheel is lightened the engine has less mass to spin up when you accelerate it.
Once the engine has accelerated it makes no difference to the engines outright power.
I would be curious to see what it does to the torque curve on a dyno, I suspect it could be similar to gearing an engine down.
E.g. it allows the bike to accelerate faster but does'nt give it more outright power.

Yes true, once the flywheel has inertia it doesn't need to steal any more to get moving, so it would affect the curve more than peak anything.

I should stop pretending to be an expert on this, I just remember the basic principles of inertia etc from Uni. I also remember the flywheel exists to smooth out the revolutions to as you say prevent stalling, and also to smooth out the otherwise even more jarring accel and decel on the conrod and piston etc, therby preventing fatigue and ultimate failure. I think, might have to check that. But in other words, I'm pretty sure to take it out would cause more winning, but also more expenditure I believe.

[thealmightytaco]

Double post.

While I'm here though, just remembered higher revs may have been the thing that reduced forces on conrod, not flywheel at all. As piston and rod race back and forth faster and faster at higher revs, the force of the gas pressure on the piston is reduced thanks to the fact the piston is racing away from it, rather than hanging about and getting squashed. At some point the conrod obviously can't handle the direction change though and it fails anyway, but higher revs do reduce fatigue of conrods.

Hmmm, amazing what sticks in your head sometimes.

[F5 Dave]

Once you have got the flywheel spinning you have converted it into potential energy so the energy hasn't been lost. Of course the same can be said about the wheels, tyres, valve stems clutch & whatnot.

I'm not saying that lightening the flywheel is not a good thing, just there is no free lunch & certainly no free hp.

[F5 Dave]

Double post.

While I'm here though, just remembered higher revs may have been the thing that reduced forces on conrod, not flywheel at all. As piston and rod race back and forth faster and faster at higher revs, the force of the gas pressure on the piston is reduced thanks to the fact the piston is racing away from it, rather than hanging about and getting squashed. At some point the conrod obviously can't handle the direction change though and it fails anyway, but higher revs do reduce fatigue of conrods.

Hmmm, amazing what sticks in your head sometimes.

Erm practically that theory on fatigue sounds very shaky. If your tutor was trying to be clever I think he was under-thinking the effect on a dynamic engine.

So the argument is that the gas pressure is what causes the damage & having the same pressure alone for shorter time at high revs is better than having it there longer but many less times is counter intuitive, but the fact that higher revs causes the combustion chamber to be smaller as the rod is stretching/bearing clearances being used up would increase gas pressure. That is why you have squish clearance, also there is the tuned range that will increase pressure at certain frequencies as well. But plain old lubrication, bearing acceleration, Youngs modulus of the material & so on will kill the rod as the revs go up pretty effectively.

Thing about Tutors & other academics is that they are always trying to prove that they are smarter than the next person. What they should really do is research their opinions on the internet where the real experts are.

[Buckets4Me]

Thing about Tutors & other academics is that they are always trying to prove that they are smarter than the next person. What they should really do is research their opinions on the internet where the real experts are.

can i use that quote ?
it's a good one

[Ray LeCheminant]

Sorry to jump in here guys.....

Scored GN250 carb today on TM, $40.00 - except, before I got a chance to say, "hey 'll arrange pickup" - he sticks on a Fastway courier for $20.00 - sheesh - bargain just got expensive

Now back on topic.....many many moons ago, I used to race TQ' midgets, and ran a GSXR750J motor....std practice was to remove said flywheel (amongst other things)

Generally speaking, and actually witnessed this in practice......revs qwicker and harder....break faster and messier.....(bad engrish)

[Ray LeCheminant]

Sorry to jump in here guys.....

Scored GN250 carb today on TM, $40.00 - except, before I got a chance to say, "hey 'll arrange pickup" - he sticks on a Fastway courier for $20.00 - sheesh - bargain just got expensive

Now back on topic.....many many moons ago, I used to race TQ' midgets, and ran a GSXR750J motor....std practice was to remove said flywheel (amongst other things)

Generally speaking, and actually witnessed this in practice......revs qwicker and harder....break faster and messier.....(bad engrish)

[saxet]

Sorry to jump in here guys.....

Generally speaking, and actually witnessed this in practice......revs qwicker and harder....break faster and messier.....(bad engrish)

Sums it up nicely and applys to most tuning, a bit like my engine over Easter.

[koba]

Once you have got the flywheel spinning you have converted it into potential energy so the energy hasn't been lost. Of course the same can be said about the wheels, tyres, valve stems clutch & whatnot.

I'm not saying that lightening the flywheel is not a good thing, just there is no free lunch & certainly no free hp.

Now back on topic.....many many moons ago, I used to race TQ' midgets, and ran a GSXR750J motor....std practice was to remove said flywheel (amongst other things)

Generally speaking, and actually witnessed this in practice......revs qwicker and harder....break faster and messier.....(bad engrish)

A good mod for some rally engines was to put a flywheel from a Van on it. E.g. 4k Starlet engine with a 5k van flywheel.

In that application the stored energy being available to stop the engine bogging under sudden loadings can be more valuable than a quick spinup.

I guess it's question of matching the mas to the application.

Anyway...

[Scotty595]

Sorry to jump in here guys.....

Scored GN250 carb today on TM, $40.00 - except, before I got a chance to say, "hey 'll arrange pickup" - he sticks on a Fastway courier for $20.00 - sheesh - bargain just got expensive

Gutted about the shipping! $20 seems rather expensive doesnt it? just go n chuck it in and have a play with it

[Ray LeCheminant]

Yeah in hind sight, given he had already shipped it without me paying a cent, I should have said - "ahhh....NO bud....send it on a camel"

[thealmightytaco]

Erm practically that theory on fatigue sounds very shaky. If your tutor was trying to be clever I think he was under-thinking the effect on a dynamic engine.

So the argument is that the gas pressure is what causes the damage & having the same pressure alone for shorter time at high revs is better than having it there longer but many less times is counter intuitive, but the fact that higher revs causes the combustion chamber to be smaller as the rod is stretching/bearing clearances being used up would increase gas pressure. That is why you have squish clearance, also there is the tuned range that will increase pressure at certain frequencies as well. But plain old lubrication, bearing acceleration, Youngs modulus of the material & so on will kill the rod as the revs go up pretty effectively.

Thing about Tutors & other academics is that they are always trying to prove that they are smarter than the next person. What they should really do is research their opinions on the internet where the real experts are.

Gas squish is but one of several factors, and not necessarily damaging but up till the danger end of the revs is one of the more dominant forces present. And indeed in an engine that revs higher may well have tighter clearance and more gas pressure than a larger pistoned lower rev engine, but they'd both do well to be driven nearer the top end of the revs (if only considering conrod life, ring life would be another story, hence all this is just an interesting aside that I remembered yesterday after confusing it with the flywheel effect). An engine will only rev as high as it has been designed for though, very shortly above that the force of having to change the direction of the piston becomes the immeasurably over-riding factor, the conrod then tearing in two or some such equally devastating operation.

She actually had graphs of the strain on the conrod for a particular engine (mitsi something?) revving at different speeds, it forming a convoluted oval with a big bottom hoop on it for low revs, the whole thing tightening into a little circle a the revs got higher. She didn't have the graph with the thing ready to snap but it's pretty well documented in shards of metal all over the world I believe.

[thealmightytaco]

Once you have got the flywheel spinning you have converted it into potential energy so the energy hasn't been lost. Of course the same can be said about the wheels, tyres, valve stems clutch & whatnot.

I'm not saying that lightening the flywheel is not a good thing, just there is no free lunch & certainly no free hp.

Indeed it hasn't been lost, but it's been stored in a wheel rather than being used by you to go forwards faster.

[F5 Dave]

can i use that quote ?
it's a good one

Not a bady huh? knock yerself out.

[F5 Dave]

A good mod for some rally engines was to put a flywheel from a Van on it. E.g. 4k Starlet engine with a 5k van flywheel.

In that application the stored energy being available to stop the engine bogging under sudden loadings can be more valuable than a quick spinup.

I guess it's question of matching the mas to the application.

Anyway...

. . . . . . . . .


Yes road engine is about making it idle nicely but You can go too far in some cases. But at least we are starting to talk in terms of aceleration of revs rather than some change in power. The only way that occurs is if there is loss through friction where it is turned into kinetic energy:heat. Balancer shafts can fall into this category as they aren't symmetrical so they can cause some drag, but often that is irrelevant.