Heavy flywheel equals more torque? (2014)

Hi Rohan
The hammers are very,very closly linked to this subject


Your arm is the leverage, your shoulder being the axis, the hammer head is the flywheel rim and the weight [mass] ...grabbing the flywheel is the act of transmitting the force ie= the energy captive in the fly wheel...there your are, a new phrase CAPTIVE! all makes perfect sence to me...but i do like maths.
I think the original post Heavy flywheel makes more torque...as got us all talking .... some heaver than others :lol: :lol: :lol: :lol:
Enough ?

Rohan said:

john robert bould said:

..agree?

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I don't know !!
Hammers and grabbing hold of flywheels is getting off track a tad.
And a 2000 lb flywheel in a Norton is a bit of a physical impossibility too..

??

Clearly, flywheel weight has some influence on power delivery.
As near everyone has said all along, if in rather different ways.

But flywheels can't make torque on their own.
So the fire in the cylinder(s) is the ONLY provider of energy to give it energy/torque in the first place.
Open the throttle, and more torque flows to the back wheel.
Close the throttle, and less torque flows.
Couldn't be more clearcut.... ?

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Somehow we think that hammers are a whole another subject.
Different applications, no engine involved, different outcomes.

Tent pegs need a BIG hammer, picture frames need a small tacking hammer.
Simple......

Nothing has changed since page one,it is about kinetic energy.
Please lord don't let me go like this when I get old and senile. :lol:

L.A.B must have the patience of a saint.

72Combat said:

I have two similar BMW airheads, one has the heavy original flywheel, the other the later 'clutch carrier" one, the heavy one has a clunky gearshift, slow change and jacking effect when throttle released, complains when going over 6000 rpm. The other my race bike has a nice gear shift, no jacking effect and revs to 8000, Torque.....it will pull past the 1000cc Harleys going up the hill at the track.
I'd have one on a sidecar but otherwise I don't like heavy flywheels.
I must say I surprised when I first got a Commando how keen it was to rev, I always got the impressions that long stroke motors did not like it.

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Have you got the close ratio gearbox in the BMW ? The standard AMC box is useless for racing behind the 850 commando. With the American close ratio 4 speed box, the 850 commando engine with standard weight crank is perfect once you've got the bike rolling. If you give it heaps of revs in a clutch start and ride the clutch out, you get rapid acceleration at the risk of doing damage to the drive train. On one occasion I'd fitted the TTI 6 speeder and didn't know it had the wrong selector drum. I got a bit frustrated and spun the motor up and rode the clutch out - it took up and I almost ended up in the fence at the end of the pit road (in about 100 metres) . I would not ever have expected the bike to accelerate like that from a standing start in 5th gear. I'm still fairly quick-witted so I caught it in time, however you would not want to be a beginner and have that happen.
I have always had lightened cranks in my Triumph engines. In the 650s it didn't seem to make any difference, in the 500cc short stroker the bike was impossible anyway so it only compounded the problems. The Commando engine is totally different and I've come to love it. What surprises me about is that most of it seems so right.

72Combat said:

I have two similar BMW airheads, one has the heavy original flywheel, the other the later 'clutch carrier" one, the heavy one has a clunky gearshift, slow change and jacking effect when throttle released, complains when going over 6000 rpm. The other my race bike has a nice gear shift, no jacking effect and revs to 8000, Torque.....it will pull past the 1000cc Harleys going up the hill at the track.
I'd have one on a sidecar but otherwise I don't like heavy flywheels.
I must say I surprised when I first got a Commando how keen it was to rev, I always got the impressions that long stroke motors did not like it.

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Someone talking real world flywheels !

I had a 1000cc BM for a while ( same time as a Commando.)
Chalk and cheese.
BM was a great tourer, but no sports bike.
It had the heavy clutch - I used to joke that I'd worked tractors that had a better gearchange.
The manual also said "to test if you are in top gear, prod the gearlever. If it is firm, you are".
If you prodded the Commando gear lever like that, it would change gear !

Long stroke bikes from older days usually have mild cam timing, and granny type performance.
Nortons ditched that idea quite a while back, with the 1st of their parallel twins.... !

Time Warp said:

L.A.B must have the patience of a saint.

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This is obviously half an education for some.

We notice you deviate off into Kwaka H2's there for a moment.....

Do you use the clutch on the way up through your box ? - I don't most of the time. As the revs peak out, I think I back the throttle off slightly then stand on the lever. Coming down is where the problem is with the heavy flywheel, when you blip the throttle to match the revs, with a wide ratio box it takes a heap of revs and if you mismatch, the flywheel makes the bike do things you don't want. I think on a down change with my 4 speed CR box, it takes an increase of about 600 RPM to make the change. With the heavy flywheel and methanol fuel the change is so easy that I hardly notice it happening. With the standard box, it is revolting.
Sorry I can't be more definite about the revs involved in the gear changes, I tend to ride the bike without thinking and concentrate on where I am going and what I am doing with it.

Rohan said:

Time Warp said:

L.A.B must have the patience of a saint.

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This is obviously half an education for some.

We notice you deviate off into Kwaka H2's there for a moment.....

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A Kawasaki H2 is a good way to win races if you don't mind anxiety. There are a couple of H2R replicas with genuine frames around here, a few years ago I was trying to buy one and didn't succeed, so I am still alive.

Quote some flywheels weights, and stay relevant.....

I've seen the flywheels, but don't recall anything ever said about weights.
Not a concern to Kwacka pilots ?

Here is something which is relevant, it was on the website for the 961 Triton :

'To add to the complication, Britalmoto’s customer wanted more performance from the stock Triumph engine for his dream bike. So Britalmoto boosted capacity to 1087 cc using a big bore kit, Carillo rods, modified throttle bodies and bigger valves. The motor now puts out 96 rear wheel horsepower and 104 Nm of torque. '

I'm interested in the numbers, especially those of the standard 850 commando and the 1007cc Maney bikes. I notice that the 104Nm max. torque figure doesn't state the revs at which it occurs. There is a thing about this discussion which I find confusing. If the motor is set up to pull with a heavy flywheel, what is the basis for the comparison with a similar motor with a light flywheel ? The biggest factor affecting torque is the inlet port diameter. If you are building a motor with a light flywheel, isn't it expected to rev higher, so the ports would be bigger and the torque characteristics would be different anyway ?

Modern engine, and a Triumph to boot !
How is that relevant here ...
EFI too ?

Stock Hinckley Thruxton is quoted as 51 Ft.Lbs of torque at 5800rpm.
Thats about what was quoted a Commando puts out ? (1970s version).

69 Nm ?


This might come in handy :

http://www.unitconversion.org/energy/ne ... rsion.html

Who has got the numbers for the mods with different cams, ports and flywheel weights ?

Rohan, have you seen the dyno graphs of commando torque curves ? I am interested in how much the standard 750 motor drops off above 6,000 RPM compared with the combat 750.
Is there a website you know of which might show those curves ?

I found this however it's really the cam comparisons which interest me. dyno-tuning-results-t13649.html

Do you guys try different timings and exhaust configurations for the various cams you buy, or do you simply set the timings to manufacturers' spec. and jet up to that ? Personally I don't usually accept the specification as being optimum, I experiment to get better performance and often best doesn't occur when you simply install cams to the marks. I'm pretty butcherous, my cam sprocket has 3 keyways - two broached at random.

I bet LAB is on his jolly's! or just sits on the side laughing! ..this as been a great posting and no one got personal :roll: even though you are all wrong,,,",don your hard hat soldier"!

Time Warp said:

Nothing has changed since page one,it is about kinetic energy.
Please lord don't let me go like this when I get old and senile. :lol:

L.A.B must have the patience of a saint.

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" The biggest factor affecting torque is the inlet port diameter. If you are building a motor with a light flywheel, isn't it expected to rev higher, so the ports would be bigger and the torque characteristics would be different anyway ? "



things 30 ?? ports , or 28.5 , and 2S ' Combat ' ? ? Cam .



Consider the water wheel old son . :lol:

Fast Eddie said:

worntorn said:

If light cranks are the way to go for racing, why did Nourish make his racing cranks heavier than stock? The Nourish crank seems to be the golden standard in competition cranks.

Glen

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Dave Nourish was very clear on this, his words went something like this:

"a heavy crank increases torque at the rear wheel" (note: he never said it creates torque, just increases what gets to the wheel)

"and give better drive out of corners"...

"There's more corners on a track than straights lad... its torque that wins races"

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Pretty much everyone agrees that you need enough flywheel for a dirt bike. Perhaps Nourish is talking about drive out of turn with a dirt tracker where you don't want the rear wheel to break loose and spin away - losing traction and drive. A heavy flywheel certainly makes things smoother and makes the bike more pleasant to ride. But dirtbike rules don't apply to pavement. I've used stock flywheels and light flywheels. The main difference I noticed is that the lighter flywheels allow for quicker throttle response and faster gear changes. With a heavy flywheel you have to wait for the RPM to change before you can change gear. With a light flywheel You can adjust the RPM instantly and snick it into the next gear change without waiting. This saves precious time with every gear change and can save a second per lap. A heavy flywheel also causes the rear wheel to slide when you downshift and this is a problem. A lighter flywheel spins up quicker - there is less resistance. So you can go into a turn with the tires stuck to the pavement instead of sliding - and when your are really pushing there is a lot of sliding going on that you want to reduce. On the pavement you can dial as much power on out of the turns as you want unless you over cook it and crash. A heavy flywheel isn't going to stop you from overcooking it. I never noticed any downside in powering out of turns with a light flywheel. My crashes usually happened going into turns - not coming out. I mentioned Ducatti because they are the fastest twins. You can build motors more toward their design with shorter stroke etc and get more power. You can lighten the valve train which is another plus. You can reduce the overall weight of your bike - another plus. You can ask what the winners are doing and see if it works for you. Of course we are not going to turn our Norts into Ducks and everyone has their own opinion. But Ducks are worth looking at and learning some lessons. Maybe there aren't many Ducks on the classic tracks nowdays. But when I was racing - the air cooled ducks were the bikes to beat.

Note the light flywheel and short pistons in the photo. You can keep it light - or add more weight. And you can go to a dyno to see what makes more torque and HP - a lighter or heavier flywheel. Its better than accepting hearsay.

Matt Spencer said:

" The biggest factor affecting torque is the inlet port diameter. If you are building a motor with a light flywheel, isn't it expected to rev higher, so the ports would be bigger and the torque characteristics would be different anyway ? "

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??

Funny, we all thought it was the camshaft that was fitted that determines the torque output..
And compression ratio, combustion chamber shape, inlet and exhaust tuned lengths, carby size, muffler type, etc
Port size/SHAPE would be in there somewhere too, as would valve sizes, shapes and angles.
A whole combination of things, in fact, all working in unison if the engines designer/builder have done their jobs.

The red line on a Commando is 7000 rpm, and making the flywheel lighter isn't going to change that. ?
Or make it rev any higher - that would have to come from other factors.
At the riders peril - remember that 7000 rpm redline......

If you over port the cylinder head there is no easy way back, and we've all done it at some time. It doesn't matter what cam you use within reason, most race cams can be set up to make the motor pull, and the timings are usually changed to suit the exhaust. The trouble comes when you develop a lot of torque on an under geared bike. You would believe the bike is accelerating as hard as it can until you start dropping teeth off the rear sprocket and find the acceleration rate doesn't change much however the top speed increases considerably. I once made a set of chambers for a Suzuki two stroke which made it develop a lot more torque. It didn't go any quicker because I didn't increase the gearing and it was horrible to ride. What you find is that the increase in torque doesn't necessarily translate to more top end, unless you increase the gearing and use it. Then you run into other problems with clutch starts and twisty circuits. When I first built my Seeley 850, a friend said 'if you've got a torquey motor, you don't need a close box' - that is rubbish and I'm certain he has never used one. With that heavy flywheel it is essential. Matt's comment about difficult gear changes with the heavy crank confirms that. If you've got a wide box, on the down changes you must get too much increase in revs to match the internal gear speeds - the changes are always slower. A light crank makes an easier time with the gear changes with a wide box. To get the most out of either crank the CR box is necessary, with the heavy crank there is a lot of stored up energy going up through the box which close gears utilize better. With my Seeley there is no rev lag on the up changes. With a light crank, it always take a little longer to get going unless you have a severe top end motor.

Well, I put in my thoughts earlier, then stepped aside . Now the old mechanical engineering professor in me just has to chime in again.

The basics:

Burning fuel in the combustion chamber generates pressure.
Pressure acting on the piston area generates force. F = p x A
Force acting on the con rod, and crank throws generates torque. T = F x distance
Torque x RPM = horsepower (simple formula - conversion factors left out)

The flywheel does not enter into any of the above. The flywheel, per se, does not alter or affect pressure, force, torque or horsepower.

A spinning flywheel (think of one suspended on pillow block bearings, not connected to an engine shaft - forget how it got to spinning) has energy by virtue of its mass and rpm. More mass or greater rpm = more energy, but no torque, unless you want to quibble about the friction in the pillow block bearings which would cause the bearings to rotate if they were not restrained. Now here is where some may be getting confused: What if this spinning flywheel were to be suddenly clutched into a transmission shaft? It is intuitively obvious that the transmission shaft would have a torque applied to it for at least a brief time. But to say the flywheel generates torque is like saying matches generate forest fires....it is only the manner in which the energy of the flywheel, or match, is harnessed, that generates the result.

The function of the flywheel is to smooth out the inherently non-smooth pressure fluctuations in the cylinders, and the non-smooth reciprocating mass. In some cases, particularly single cylinder, 4 strokes, the flywheel provides the energy to keep the mass moving until the next power pulse occurs.

In general:

heavier flywheels = smoother engine operation, less "bucking" or surging.
lighter flywheels = faster changes in rpm, either up or down.

The above is basic physics.
I defer to those with racing experience, for specific effects of altering flywheels.

Prof Slick

An extreme example of a flywheel imparting torque on a driveline:

As a kid I watched my Father clearing land with various big Cats, mostly D8s. They also had a D7 which was not as heavy or powerful as the 8s but had a massive flywheel and relatively low horsepower.
The toughest thing a dozer can be subjected to is removal of an old growth Douglas Fir stump. Some of these are as big as a house when the rootball is included.
The method for getting the stump out is to get one or two teeth of the clearing blade under a root, then lift with the hydraulics as the machine moves forward in low gear, full throttle.
With the D7, it took about ten seconds for the RPM to drop from top governed speed of 1100 down to near stall out at fifty rpm or so at which point the operator throws the clutch , stops lifting on the hydraulics and lets the rpm build to full again.
Then the clutch is pulled in again, the flywheel imparts massive torque on the system all over again as the rpm is pulled down and tremendous pressure is exerted on the root.
This is repeated again and again until a chunk of stump about the size of a small car breaks free and comes out of the ground.
My father referred to this method of stumping as "using the Flywheel".
I think anyone can see the huge torque that is imparted on the drive line by the spinning mass of the 900 lb large diameter flywheel (dia is part of the equation as well)
The same machine equipped with a 7 lb aluminium flywheel like my 230 Ossa MXer had, would not be able to produce the huge momentary torque that this machine could produce because of it's flywheel.
The confusion is between momentary torque that can be produced by the kinetic energy of the flywheel and continuous torque output of the engine
Continuous torque output is independant of flywheel weight and size. Momentary torque output is greatly affected by flywheel weight and dia.
The diameter or location of thevweight relative to centre determines the moment of intertia. Big flywheel carrying weight far away from centre, big MOI. Same weight flywheel carrying weight close to center, lower MOI.