impact on acceleration of lighter flywheel.

[xsiliconkid]

We have seen all the ads, replace stock 4lb flywheel with “_Cyclecat's 0.5lb flywheel” and gain significant performance. Reducing weight of something that rotates, significantly improves performance seems to be a general rule of thumb. Is this true? If so, how much improvement? I wanted to answer that questions.

The rule-of-thumb of lightening a flywheel has a dramatic effect of performance due to reduction of rotational inertia comes from the car world. Motorcycles are tricky devices compared to cars. What works for a car does not necessary translated to motorcycles.

Is there a quantifiable advantage to lightening the flywheel of a motorcycle, even dramatically lightening to aid in linear acceleration?

The Readers digest answer:-)
Performance increase in acceleration -negligible in the lower gears… nothing in higher gears. For the analysis I used a Ducati 851/888 with a 3.5lb lighter flywheel from stock. The below analysis shows the lightened flywheel only reduces bike's effective overall mass delta by 4.5lbs in first gear and 0.5lb in 6th gear. A delta of effective mass of 4.5lbs on a 400lb motorycle is a little over 1% mass advantage over a 3.5lb heavier flywheel,

Take a small car - an early 70's mini being re-built by a friend and reducing flywheel weight by 11lbs over the stock flywheel (radius of 5.1"). This provides an effective overall 200lb weight or mass advantage in first gear (on a 1100lb car) - impressive! A delta of 18% mass advantage over a 11lb heavier flywheel

For acceleration a =force/mass and therein lies the answer, advantage on the car, virually no advantage on the motorcycle.

Long answer.....................
I sent the following message to my physics mentor, for checking......
----- Original Message -----
From: Graham Pattison
To: Chris

Sent: Monday, December 19, 2005 5:39 PM
Subject: Re: flywheel effect on overall mass- therefore acceleration

Chris, do me a favor and double check math and logic.
Also what about the lighter rotating mass effect on torque/HP .... work over time?
G
:

First, I'm invoking the 80/20 rule. Eighty percent accuracy can be gained with 20% effort, while the last 20% accuracy takes 80% of the effort…..We could go into a great detailed analysis- exactly where on the flywheel is weight removed ( as you move toward the center, rotational effect drops – less equiv static weight advantage) or use a formula to derive and determine the exact relationship between weight on the flywheel and weight of motorcycle. All this requires knowing the moment of inertia, and measuring, that's difficult and does not fit in my 80/20 rule..and....
As a motorcycle's flywheel's rotational energy is directly linked to the forward motion through the drive train the “equivalent static weight or mass” of the flywheel can be calculated with the following formula. Once the "effective" change in mass has been calculated the impact on acceleration can be easily derived.
We= (fw*0.5)(ftx*gr*fd)^2
( tr )
fw = change in flywheel overall weight
ft = flywheel radius
gr = gearbox ratio ( which gear selected)
fd = final drive ratio
tr = rear tire radius

Ducati 851. The flywheel weight saving is 3.5lbs with a radius of 2.953". First gear ratio is 2.953:1, final drive is 2.6:1, tire radius is 11.8 inches. In first gear, the Ducati "feels" 4.5lbs lighter and in 6th gear (0.86:1) less than 1lbs lighter.
Therefore it's not even worthwhile calculating the linear acceleration benefits on a motor cycle. AS the chart below shows – on a car with a larger heavier flywheel is different, even on just a 1,100 lb mini with a 10" diam flywheel
The following is actual real world data.
Graham


flywheel Flywheel gear final tire selected
eWeight weight radius ratio drive radius gear
save loss inches
4.51 3.5 2.953 2.47 2.6 11.8 1ST
2.31 3.5 2.953 1.76 2.6 11.8 2ND
1.35 3.5 2.953 1.35 2.6 11.8 3RD
0.88 3.5 2.953 1.09 2.6 11.8 4TH
0.68 3.5 2.953 0.96 2.6 11.8 5TH
0.54 3.5 2.953 0.86 2.6 11.8 6TH
mini
253.39 12.5 5.5 3.67 3.82 12.1 1st




RESPONSE-------------------------------------------------------------------------------
Original Message --------
Subject: Re: flywheel effect on overall mass and therefore acceleration
Date: Mon, 19 Dec 2005 19:04:29 -0500
From: Chris
To: Graham Pattison <graham@stargraham.com>

References: <43A72230.6090907@stargraham.com> <43A736B2.4010806@stargraham.com>


Actually, with the F=ma it all comes out in the wash when you look at energy.. that's the beauty of looking at problems from an energy perspective instead of from a force perspective.

The lighter flywheel requires less energy to spool up... period. The energy comes from the motor which gets it from the fuel (with a considerable efficiency hit). Changing the flywheel should not change the efficiency of the combustion, so any argument that suggests you are missing an element of engine torque would be misguided. (Changing the combustion chamber or camshafts on the other hand.....)

So.. to check your equation.. I lead off with:
KEf = 1/2 *m(bike total)*V^2 + (sum of all rotating bodies individual kinetic energies..) [ 1/2 * I * w^2 ]

What you are investigating specifically is the *comparable* mass change to linear energy requirements if the flywheel weight changes. So, we will compare the linear kinetic energy of the bike sum, to the kinetic energy of the spinning flywheel:

delta(1/2*mbike*vbike^2) = delta(1/2 * (I) * w^2)

This allows me to calculate the equivalent delta mass of the bike (assuming that the flywheel *didn't* change) when I change the flywheel weight. This is where the nitpickers will say you're introducing error, because your change to the flywheel is also changing the mass of the bike and must be considered. The linear effect of removing 3.5lb weight is, in effect, neglected effective weight loss for forward motion.

Moving on...
1/2(comparabledeltamassbike)*vbike^2 = 1/2 * (1/2 * deltamassflywheel * rflywheel^2) * wflywheel ^2
comparabledeltamassbike = (deltamassflywheel*rflywheel^2*wflywheel^2)/( 2 * vbike^2)

Sanity check:
as the change in the mass of the flywheel increases, the comparable bike mass increases.. CHECK
as the speed of the flywheel increases, the comparable bike mass increases..CHECK
as the velocity of the bike decreases, the comparable bike mass increases... CHECK

OK..looks good.


The engine will spin to redline or something slightly less in each gear and the bike will reach a certain velocity at the redline in each gear. Those are the numbers you should use...

Again, its the beauty of energy analysis - everything else comes out in the wash. Sure, there are energy losses to heat and friction.. but those don't matter because you're just comparing flywheel weight to bike mass weight.

For what its worth, you could use a similar analysis to show an equivalent 'weight loss' by increasing or better aerodynamics of the bike. Develop the equation for the work lost to friction and then compare it to your kinetic energy equation...You could come up with: "A 50% reduction in CEf is the same as a 20lb weight reduction at 40mph"... or something to that effect..You could continue on for all sorts of effects...
I think I now understand why you used the other variables instead of speed and angular velocity...

the concept is sound

-C

[6 Fingered Man]

Huh?

[xsiliconkid]

I started a new thread from a discussion over on a conti tire thread. probably should have said that and more:-), but this gave me brain cramp too...just dragging it up from my past....

bottom line. don't waste your $'s on lighter flywheels - not even for a race bike.

[Evil_Weasel]

i know radical sport bike builder thorstein durhban removes all the flywheels from his bikes. says they are only there so owners don't stall in parking lots. other than that all i know is less flywheel= quicker revs. if that effects bike acceleration i would imagine the engine spinning up faster would have more of an effect on acceleration than the actual removing of overall weight (even rotational weight) from the bike would.

best bet. buy lighter rims. the marchisinis i put on my 12R was the best performance mod i ever did to a bike. it handled better, accelerated and braked better and the suspension even worked better. it basicly improved all aspects of the bike, though the quicker steering was the most noticable and frankly amazed me. the 12R went from a high steering effort bus to not quite like an R1...but close.

[jcbell1007]

what about going to a total loss system like a number of people have done with their hawks? it involves removing the flywheel from the starter gear, and also the magneto. I think the weight savings is around 10 lbs at least. on such a light bike with comparitively low HP, you would think there would be some benefit.

edit: isn't less rotational mass a benefit as well (IE lighter rims, 520 chain, flywheel removal). Similar to the reason drag cars go underdive pulleys etc... I heard a figure at one time, that 1 lb of rotational mass loss might give you as much as 5 hp?

[xsiliconkid]

Lighter wheels do make a difference a couple of pounds is huge. When I raced my Ducati and switched to Dymag magnesium rims it started to actually feel like it would turn in... the same on my 851 road bike by going to Marchesini's

I heard the WSB guys use special light weight rims that are only good for one week end. ... Some went missing a few years back from the Ducati paddock area - all hell broke lose - if someone fitted them on their road bike it would not be long before they went down...

[xsiliconkid]

total loss is different, you have the added benefit of removing a magnetic induced braking system (alternator).
Even 10lb static weight - rotor, magnets, copper etc is not much rotating mass.. As it's all in close to the COR (Center of rotation). Again the real benefit is removing the alternator charging cct.
Removing or not adding mass that rotates is a huge benefit when you get to large radius items such as rear sprocket, brake rotors, wheels and tires- tires are at extremities, like 9 inches from the Center of Rotation and therefore every lb you save out there has a dramatic positive or negative impact.

It's not simple to calculate actual impact for complicated shapes like wheels... .
but like eveything there is a formula (I) = sum (mi*di^2). (mi).

[butcher bergs]

It's crazy to think that technology has come to a point where we can manufacture things that are strong enough to withstand the forces of racing for a determined amount of time and at the same time fragile enough to where the consumer population has no use for it.

[Kurlon]

So, does this mean the inverse theory, more flywheel weight leads to better pull out of corners is also false?

[intergalactic]

So, does this mean the inverse theory, more flywheel weight leads to better pull out of corners is also false?

Yes. It is false. My car started spinning the drive tires in 3rd gear up the hill out of turn 3 AFTER I lightened the flywheel.

Rule of thumb: 1 kg-m^2accelerated at 1 rad/sec requires 1 n-m.

[ssg]

Yeah less flywheel would remove linearity from power delivery, just think about a dirtbike…whats the first thing anyone does to an MX bike to make it more useable in the woods? flywheel…but take it off and all that power can break loose and it definitely revs out quicker.

"Speed" would be relative too I believe, not that I'm disagreeing in case it sounds like it…I just remember flipping flywheel weights around on my bike depending what I was riding in.

[seth399]

Yes. It is false. My car started spinning the drive tires in 3rd gear up the hill out of turn 3 AFTER I lightened the flywheel.

Rule of thumb: 1 kg-m^2accelerated at 1 rad/sec requires 1 n-m.

whats that have to do with it?? aha

Interesting thread, I see what Kurlon is asking though, weighted flywheels on mx bikes definitely make a difference. More noticable for a pipey 2stroke I suppose.

[intergalactic]

Yes. It is false. My car started spinning the drive tires in 3rd gear up the hill out of turn 3 AFTER I lightened the flywheel.

Rule of thumb: 1 kg-m^2accelerated at 1 rad/sec requires 1 n-m.

OK, some **quick** estimates of a 1.75kg/ 4 lb flywheel with 5" diameter gives a moment of inertia of less than .01, perhaps .006. (kilogram-meters squared)

But a bike engine can accelerate at what, 10,000 RPM per second unloaded? Call that 1000 radians per second. So the flywheel requires 6-10 Nm to accelerate. Call it 4-7 ft-lb of torque absorbed with no load.

Accelerating in 2nd gear with a bike that makes lets say 60Nm and an acceleration rate of 400 radians/second, the loss is perhaps 2.4Nm or 2-ft lb. Call it a 5% difference. Not nothing at all. But if the lack of flywheel breaks traction or transmission parts, that's an issue.

But if I were building engines, I would do it in a minute. An easy couple of percent improvement in the lower gears.

That being said, losing the flywheel in the diesel I worked on would definitely break parts, but also improve available first gear torque by perhaps 25%!!!

[Doc]

So removing flywheel and magneto is not a good thing?

[intergalactic]

So removing flywheel and magneto is not a good thing?

Removing them will allow for more available torque when the bike is accelerating. The faster the acceleration, the more extra torque available. For racing it is worth it. IMO.

[Paul_E_D]

A lot of professional race teams actually ADD weight to the flywheel or cranks of motorcycles. I think it aids stability on roll off with high compression high hp motors. Actually, they even do it on 125s at the world GP level. I think it might help at tracks with long straights.

Competing practices/theories on this one.

[MotoFreak]

Good read. No idea what the numbers mean , but very interesting.

[xsiliconkid]

<<so removing the flywheel etc is not a good thing?>>

Correct, not off the start line from a standing start.
The chances of stalling increases:-).
I know when I took something like 8 grams out of each piston/conrod - I had to launch my bike (highly built Duc 800SS) at 7Krpm-splipping the clutch until in second gear (bogged it down the first time), whereas before I would launch at 3.5K RPM dump the clutch and just let the torque curve pull me (remember the rate of acceleration directly matches the torque curve).

[brady]

the marchisinis i put on my 12R was the best performance mod i ever did to a bike...the quicker steering was the most noticable and frankly amazed me...

When I raced my Ducati and switched to Dymag magnesium rims it started to actually feel like it would turn in... the same on my 851 road bike by going to Marchesini's

the quicker steering is a result of the reduced gyroscopic inertia, correct? (I guess turn-in is just acceleration on a different axis, now that I think about it....)

[Doc]

<<so removing the flywheel etc is not a good thing?>>

Correct, not off the start line from a standing start.
The chances of stalling increases:-).
I know when I took something like 8 grams out of each piston/conrod - I had to launch my bike (highly built Duc 800SS) at 7Krpm-splipping the clutch until in second gear (bogged it down the first time), whereas before I would launch at 3.5K RPM dump the clutch and just let the torque curve pull me (remember the rate of acceleration directly matches the torque curve).

Good heads up. Looks like i am praticing Launches!

[scottieducati]

[Cerberus]

The lighter flywheel requires less energy to spool up... period. The energy comes from the motor which gets it from the fuel (with a considerable efficiency hit). Changing the flywheel should not change the efficiency of the combustion, so any argument that suggests you are missing an element of engine torque would be misguided. (Changing the combustion chamber or camshafts on the other hand.....)

A lighter flywheel does not JUST require less energy to spool up, but given the same input of energy (which it would have), it will accelerate (spool up) more quickly. This could be argued to actually increase the efficiency of combustion by reduction of parasitic inertial drain. This is the quintessential reason for using a lightened flywheel.

Presuming now; but i believe measurable torque output would not be effected since torque is a a measurement of force without respect to time, but effective horsepower (being a measurement of force over time) should be measurably better, resulting in better acceleration of the engine/vehicle in question.

Now, the effect may or may not be negligible, but your basis of determination using gearing ratios and total masses seems to discount or ignore the savings of inertial rotating mass, through a much larger rpm range (for most bikes) than the average car (again, presuming the mini shares a similar redline to other production cars in the same displacement class)

i think your mentor is wrong.. but I never took physics formally, i just love the subject

[xsiliconkid]

Love it!

Going back to the original statement," The rule-of-thumb of lightening a flywheel has a dramatic effect of performance due to reduction of rotational inertia comes from the car world. Motorcycles are tricky devices compared to cars. What works for a car does not necessary translated to motorcycles".

Ok, let me tackle this a little different. So given the above we are talking real world not clinical laboratory physics. Not point scoring, “micrometer on the end of yard stick stuff”. I'm sure not too many folk here are interesting in that from a motorcycle forum. I’m a pragmatist so 5 years ago I asked myself, “if I spend $400 for a light-weight flywheel what are the benefit?”
So before I laid down the cash to Cycle Cat I challenged the status quo- "it helps". Passing the results of that discussion with myself on, then as now might save someone else wasting their money.

Now Fast forward to today, someone said some motorcycle tires may weigh 7lbs more but impact is marginal (or something like that). It again got me thinking….What does 3.5lbs more mass spinning around on the end of a 9” radius mean?
In the real world is it statistically important or not?
What impact would that have on linear acceleration?

But back to this discussion.
Yes, we are talking about linear acceleration impact by focusing on one small part of the mass (the flywheel of a motorcycle) and calculating the flywheel maximum inertial rotating mass saving (lets say for arguments sake at max torque, max rpm, max HP -whatever you like) and let's take first gear.

I was taking the maximum "mass" savings and calling that effective static weight savings. You are acceleration the "whole mass" and by calculating the maximum rotating mass savings you can simply call that mass in the a=F/m ....
Sure there are other factors but they are even less significant to this discussion.

Now back to what might be very interesting.
How about looking at the real world of motorcycle wheels; lighter/heavier rims, lighter/heavier tires, lighter/heavier rotors... and the impact on acceleration.

[GNTurbo6]

I've heard 1 lb of rotational mass = 8 lbs of static (or whatever) mass