Heavy flywheel equals more torque? (2014)

A spinning wheel with a cast iron rim will produce a twisting force (torque) at the spindle. A wheel with an aluminium rim of similar dimensions as the cast iron rim and spinning at the same speed will produce a lower torque because of it's lower mass at the rim.

Engine should be going back together tomorrow but I've got a last minute question. I now have a Steve Maney lighter flywheel fitted to Mk2a crank shafts/journals with Carrillo rods.
I've still got the 150,000 mile Mk3 crank with standard flywheel and 35,000 mile aluminium alloy billet rods. But I'm wondering how noticeable the difference will be between the two regarding engine 'drive-ability'. Anybody had any experience of light compared to heavy flywheels?

If I'd known the time it was going to take etc etc I may well have done things differently but I am where I am. Hind sight's a wonderful thing.

After reading all the threads here...heavy is better...get a 1/2 ton flywheel and you will have no vibes, Fitting it into commando cases my be of an issue. :lol:

A rotating flywheel having more mass than another, both spinning at the same RPM, has more energy than the other, not more torque.

The engine must be under a load to produce torque, and the torque is independent of the flywheel weight. This said, the drivability and vehicle acceleration is affected by flywheel weight. Heavier flywheel = more drivability, lighter flywheel = greater acceleration..

Slick

A very interesting question.
My 750 Commando has a Maney crank & my 850 had, until recently, standard crank. I have now fitted a Maney crank to this also as part of the rebuild. The 750 is easier to stall on setting off but the is more responsive but still very driveable. The other two bikes that spring to mind are my airhead BMWs. A 1979 R100RS which has a very heavy flywheel & 1994 R100R Mystic (stupid name but great bike) which has a light weight clutch carrier in place of the flywheel. This bike is MORE drivable than the earlier model. This could be down to different size ports/cams etc so is not comparing an exact like for like. Either way I wouldn't worry too much. Just build & enjoy.
Martyn.

Aright, just how light is a light weight Maney flywheel?
Its the combustion pressure that makes torque, which can be spent on spinning up a heavier flywheel slower or lighter flywheel and cycle quicker. If engine lacks combustion torque to get along as easy as you like - increase the flywheel mass energy storage to suck some inertia off to get moving - till fuel heat can replace it fast as it may. Reciprocating mass has a lot to do with how heavy a wheel needed not to stall on throttle let offs to shift or low throttle take offs. If you or the cycle can't handle the traction testing response of more dragster like on/off throttle response on far over leans, or into braking more for turns than into power steering them - then you'll have more security to go around faster on more massive flywheel dampening than on a more responsive power character. Comnoz and others prefer a heavier flywheel both for the dampening of the crank whip on bearings and cases while some like Ron Fratturelli or me on the P!! or Ms Peel like em 5 - 7 lb lighter and smaller OD. too.

Chug chug chug > vaarrrROOOOMMMM vs boP boP boP > bBLATTT!, if ya know what I mean.

Magic marker darkened area is where Ron says ya can remove more for more responsive throttle reaction...

Matchless said:

A very interesting question.
My 750 Commando has a Maney crank & my 850 had, until recently, standard crank. I have now fitted a Maney crank to this also as part of the rebuild. The 750 is easier to stall on setting off but the is more responsive but still very driveable. The other two bikes that spring to mind are my airhead BMWs. A 1979 R100RS which has a very heavy flywheel & 1994 R100R Mystic (stupid name but great bike) which has a light weight clutch carrier in place of the flywheel. This bike is MORE drivable than the earlier model. This could be down to different size ports/cams etc so is not comparing an exact like for like. Either way I wouldn't worry too much. Just build & enjoy.
Martyn.

Click to expand...

Hi Martyn,

You say the 750 is easier to stall - is that in comparison to before swapping cranks? I can't remember stalling my 850. One feature of the standard Commando that I like is the instant acceleration from a set of lights simply by dumping the clutch and opening the throttle. Having said that the majority of riding I do is open road. This wasn't a planned rebuild and I've made some hasty decisions to try and get the bike back on the road asap (track day at Cadwell on Saturday!) I'll go with the lighter crank and see how it goes but may well be rebuilding in the winter - but this time with standard crank/flywheel and JS rods and pistons.

Simon.

I didn't know Steve sold his flywheels separate from the crank cheeks......

Or did you get it used?

hobot said:

Aright, just how light is a light weight Maney flywheel?
Its the combustion pressure that makes torque, which can be spent on spinning up a heavier flywheel slower or lighter flywheel and cycle quicker. If engine lacks combustion torque to get along as easy as you like - increase the flywheel mass energy storage to suck some inertia off to get moving - till fuel heat can replace it fast as it may. Reciprocating mass has a lot to do with how heavy a wheel needed not to stall on throttle let offs to shift or low throttle take offs. If you or the cycle can't handle the traction testing response of more dragster like on/off throttle response on far over leans, or into braking more for turns than into power steering them - then you'll have more security to go around faster on more massive flywheel dampening than on a more responsive power character. Comnoz and others prefer a heavier flywheel both for the dampening of the crank whip on bearings and cases while some like Ron Fratturelli or me on the P!! or Ms Peel like em 5 - 7 lb lighter and smaller OD. too.

Chug chug chug > vaarrrROOOOMMMM vs boP boP boP > bBLATTT!, if ya know what I mean.

Magic marker darkened area is where Ron says ya can remove more for more responsive throttle reaction...

Click to expand...

Hi Hobot,

Maney cranks are 17-18lb, standard Norton are 22-24lb but don't know the difference in flywheel weights. Looking at the picture the Maney flywheel is lighter than the magic markered flywheel. Fair enough the combustion creates the downward force to spin the crank but the heavier flywheel will create more torque than a lighter flywheel at the same speed.

Torque = r x F [where r = dist of mass from axis (radius) F = force = mass x gravity. Gravity = metres per second squared which comes from; metres per second (speed of rim of flywheel) divided by rpm which is revs/minute or seconds, which gives metres per second squared.]

so you get Torque = metres x kg x m/s/s (m/s/s = metres per second squared)

which gives you Torque = Nm [N(Newton) = kgm/s/s m(metre)].

Hopefully the light flywheel wont create an on/off throttle response - I like the smooth pick up and drive off a closed throttle the standard set up gives.

SteveA said:

I didn't know Steve sold his flywheels separate from the crank cheeks......

Or did you get it used?

Click to expand...

I bought it complete off a long time customer of Steve's. It was surplus to requirements as he'd gone for a 750 short stroke engine with a new Maney crank.

Al-otment said:

Matchless said:

A very interesting question.
My 750 Commando has a Maney crank & my 850 had, until recently, standard crank. I have now fitted a Maney crank to this also as part of the rebuild. The 750 is easier to stall on setting off but the is more responsive but still very driveable. The other two bikes that spring to mind are my airhead BMWs. A 1979 R100RS which has a very heavy flywheel & 1994 R100R Mystic (stupid name but great bike) which has a light weight clutch carrier in place of the flywheel. This bike is MORE drivable than the earlier model. This could be down to different size ports/cams etc so is not comparing an exact like for like. Either way I wouldn't worry too much. Just build & enjoy.
Martyn.

Click to expand...

Hi Martyn,

You say the 750 is easier to stall - is that in comparison to before swapping cranks? I can't remember stalling my 850. One feature of the standard Commando that I like is the instant acceleration from a set of lights simply by dumping the clutch and opening the throttle. Having said that the majority of riding I do is open road. This wasn't a planned rebuild and I've made some hasty decisions to try and get the bike back on the road asap (track day at Cadwell on Saturday!) I'll go with the lighter crank and see how it goes but may well be rebuilding in the winter - but this time with standard crank/flywheel and JS rods and pistons.

Simon.

Click to expand...

Please do keep us posted on how the light crank performs and feels Simon.
I've always favoured a heavier crank, probably cos of how I was 'schooled' with what Dave Nourish and Degens used to tell me, and others.
However, I once rode a Triumph with a very light crank and very trick top end with very light valve springs. That thing zipped up like a racing Honda, I only rode it in a practice session so do not know if it actually was any faster than other bikes I rode, but its always stuck in my mind cos it was so much FUN!
So much so in fact, that I had recently been thinking that I wish I had gone for a light Maney crank I my current build!

Al-otment said:

Fair enough the combustion creates the downward force to spin the crank but the heavier flywheel will create more torque than a lighter flywheel at the same speed.

Torque = r x F [where r = dist of mass from axis (radius) F = force = mass x gravity. Gravity = metres per second squared which comes from; metres per second (speed of rim of flywheel) divided by rpm which is revs/minute or seconds, which gives metres per second squared.]

so you get Torque = metres x kg x m/s/s (m/s/s = metres per second squared)

which gives you Torque = Nm [N(Newton) = kgm/s/s m(metre)].

Hopefully the light flywheel wont create an on/off throttle response - I like the smooth pick up and drive off a closed throttle the standard set up gives.

Click to expand...

Do heavier wheels and tires create more torque when spinning than lighter wheels? Absolutely not. They do not create torque, but when spinning have stored energy. Neither will a heavier crankshaft or flywheel create more torque, but has more energy than a lighter one (same rpm).

The torque produced by the engine causes acceleration...both the acceleration of the vehicle and pilot, in getting both to move down the road at increasing velocity, and the acceleration of all moving parts in the engine itself, including the increased speed of rotation of crankshaft and wheels plus tires. A heavier flywheel, or heavier wheels and tires, resist the rotational acceleration, with the result that linear acceleration is reduced. Look at it another way: the engine's torque has to be put to a dual use - linear acceleration (getting the parts to move down the road) and angular acceleration (getting the parts to spin up). Increasing rotating mass (flywheel, wheels, tires) sucks off more torque for the spin up, thereby producing less linear acceleration, perceived as less throttle response.

The heavier flywheel improves drivability (or tractability) because it tends to remain at speed, having more energy than a lighter one. It is Newtons Law: an object in motion tends to remain in motion, and a heavier object has a greater tendency to remain in motion than a lighter one.

Al-otment said:

Matchless said:

A very interesting question.
My 750 Commando has a Maney crank & my 850 had, until recently, standard crank. I have now fitted a Maney crank to this also as part of the rebuild. The 750 is easier to stall on setting off but the is more responsive but still very driveable. The other two bikes that spring to mind are my airhead BMWs. A 1979 R100RS which has a very heavy flywheel & 1994 R100R Mystic (stupid name but great bike) which has a light weight clutch carrier in place of the flywheel. This bike is MORE drivable than the earlier model. This could be down to different size ports/cams etc so is not comparing an exact like for like. Either way I wouldn't worry too much. Just build & enjoy.
Martyn.

Click to expand...

Hi Martyn,

You say the 750 is easier to stall - is that in comparison to before swapping cranks? I can't remember stalling my 850. One feature of the standard Commando that I like is the instant acceleration from a set of lights simply by dumping the clutch and opening the throttle. Having said that the majority of riding I do is open road. This wasn't a planned rebuild and I've made some hasty decisions to try and get the bike back on the road asap (track day at Cadwell on Saturday!) I'll go with the lighter crank and see how it goes but may well be rebuilding in the winter - but this time with standard crank/flywheel and JS rods and pistons.

Simon.

Click to expand...

Yes, as you and Texasslick said, the heavy flywheel spinning at a fixed speed has more stored energy thus can deliver more torque at that instant. However, once under way in acceleration mode the heavy flywheel is no different than any other component subject to angular acceleration (tires/wheels, transmission gears/shafts, primary drive and flywheels), i.e., the heavier it is the more energy is required to accelerate it.

Although I have no 1st hand experience to relay, IMHO driveability would be most influenced by flywheel weight in getting the bike in motion from a dead stop, but once underway where the engine is developing good torque, the lighter flywheel should be a non-issue, and as noted by others, would make the bike more responsive because it would rev quicker.

Vintage hog guys often employ T&O Torque Monster Flywheels (same mass as HD OEM flywheels) for just such purpose, i.e., they like to let the clutch out with minimal throttle and have the bike launch. However, once underway the bikes are not what you'd call "snappy"

Edit - Texasslick posted while I was drafting the above so apologies for any redundancy

I have a 750 Japanese copy of a Brit vertical twin, shorter stroke AND lighter flywheel... so easy to stall while taking off compared to 850 Commando. Once underway, the flywheel effect is not noticeable, just the short stroke less low speed torque.

Ok Maney cranks/flywheels are in the proven 5-7 lb range of effective lightening but still dang massive compared to most moderns even HD's. I was warned by many [on 3 other lists yrs prior to this one opening] Peel would suffer low idle bog downs ease offs and stalls on shifts but remember how tame my P!! was, 600 rpm idle, no throttle 4th gear lugging though Fla sand paths about 12 mph with tires splitting the sand like a boat wake or plow though sod yet didn't have to down shift to start tire spin to rooster tail up a steep off road - like any desert sled worth its fuel should do. Peel behaved perfectly on and off road in low traction and heavy cargo no problemo yet in 1st and 2nd could out drag the moderns which lifted up too much while Peel just leaped forward, to about 90 then shifted and they'd out horse power us. But by then was in opens so Peels points proven by the time it took their hp to catch up and pass, almost time to lean over again hehehe. I've closely observed vintage drag bikes and race bikes in crowed pit rows to see they were just fine down low and timid operation. You will not experience any 2smoke like on/off throttle response or stalling, just same ole nice smooth Norton throttle control that reacts more like you want the instant ya want it. Main things that may stall-bog your hot rods Cdos is cam profile and CR and educated throttle hand. The effect of flywheel as gyro's on low and hi speed handling comes into play of course but no one will believe my sense of it but gear rpm selection matters more than wheel gyro's, on and off road.

This is ancient pondering thread and one extreme example I remember was a 25 ft dia water wheel turning like 25 rpm would develop 12,000 lb ft torque but only 12 hp so would jerk your teeth out on a cycle up to about 12 mph then only water pressure to slowly work up from there. 7.5 lb less over all bike mass is worth about one hp more acceleration freedom and easier to pick back up too.

Kinetic and or stored energy.
My experience of light fly wheels on twins is this.
The engine will spool up faster including down shifting (especially on narrow tyre bikes) and will be more lively in general under acceleration.
For the same constant engine rpms in top gear it will be happier with the heavier rotating mass,the same on roads that are undulating.
The camshaft specification (and stroke ? ) plays a part also.
Its only a problem when you go to far (weight reduction) on a road bike with the effect it has on the ability to cruise/tour.

Edit.
I think the stand out test is top gear and the lowest comfortable rpm's as compared to before (stock)

Simon,
When I rebuilt the 750 with the Maney crank I also fitted a RGM close ratio gear set. 1st gear as standard is 2.56:1. The C.R. 1st is 2.3:1.( Both these figures are from memory) This doesn't help matters of the stalling variety, but the lighter crank does require more care with the throttle. You will soon get used to this however. The effect once underway is as I said before, quicker throttle response.
If you are going to use the bike for despatch riding I would put the standard crank back in otherwise use the lighter crank, get it dynamically balanced to standard spec. & use genuine Norton ISO rubbers. Vibes will not be a problem.
Martyn.

When I first raced my Seeley 850, I tried to do it with the standard gearbox. It was impossible to do anything smoothly, and downshifts had the potential to chirp the rear wheel. The close ratio 4 speed box was infinitely better however the high first gear stuffed up most clutch starts - had to really cook the clutch to get going. Once the heavy fly wheel is spinning, it tends to keep spinning. On a downshift you have to lose a few hundred revs to match the gears - easier with the close box. On the way up the heavier shaft weight tends to cause a lag in acceleration with a wide ratio box. I've bought the 6 speed TTI box to get around the problem with the lower gears off the start line in races.
In 650cc Triumphs, the Saint had a light crankshaft - if you are building a hot Triumph avoid buying that crank. Motors that use it never perform as well as those with the heavy crank. When you are racing, the crank inertia is very important, the gearbox must get the most out of it. The commando has a strange motor, it is totally different to racing a bike with a short stroke. You might think that a light crank will get you there quicker, however if you spin up a heavy crank and climb up through a set of close gears, there is very little lag after each gear change.

texasSlick said:

Do heavier wheels and tires create more torque when spinning than lighter wheels? Absolutely not. They do not create torque, but when spinning have stored energy. Neither will a heavier crankshaft or flywheel create more torque, but has more energy than a lighter one (same rpm)...............

The heavier flywheel improves drivability (or tractability) because it tends to remain at speed, having more energy than a lighter one. It is Newtons Law: an object in motion tends to remain in motion, and a heavier object has a greater tendency to remain in motion than a lighter one.

Click to expand...

The engineering/mathematical formula; Torque = Force x Distance is accepted universally.
Force = mass x metres/sec/sec (units of gravity) when you increase the mass you increase the torque.

I used the term 'drive-ability' only to try and describe if the bike would still pull away with relative ease and not require slipping the clutch for example. 'Drive-ability' was the wrong phrase, it is subjective and not quantifiable. I should have only mentioned torque.
Newton's law you mention is his first law which deals with inertia and also explains why a greater mass, at constant rotation and at a distance from an axle will create a greater torque than a lighter mass.

Al-otment said:

Newton's law you mention is his first law which deals with inertia and also explains why a greater mass, at constant rotation and at a distance from an axle will create a greater torque than a lighter mass.

Click to expand...

Flywheels do not generate torque though.
In internal combustion engines, they simply store a little of the rotational energy.

The TORQUE come from the fire in the engine,
and the hot gases pushing against the piston....

BMEP in jargon....

acotrel said:

When I first raced my Seeley 850, I tried to do it with the standard gearbox. It was impossible to do anything smoothly, and downshifts had the potential to chirp the rear wheel. The close ratio 4 speed box was infinitely better however the high first gear stuffed up most clutch starts - had to really cook the clutch to get going. Once the heavy fly wheel is spinning, it tends to keep spinning. On a downshift you have to lose a few hundred revs to match the gears - easier with the close box. On the way up the heavier shaft weight tends to cause a lag in acceleration with a wide ratio box. I've bought the 6 speed TTI box to get around the problem with the lower gears off the start line in races.
In 650cc Triumphs, the Saint had a light crankshaft - if you are building a hot Triumph avoid buying that crank. Motors that use it never perform as well as those with the heavy crank. When you are racing, the crank inertia is very important, the gearbox must get the most out of it. The commando has a strange motor, it is totally different to racing a bike with a short stroke. You might think that a light crank will get you there quicker, however if you spin up a heavy crank and climb up through a set of close gears, there is very little lag after each gear change.

Click to expand...

I know of other folks who have removed the lighter Triumph flywheel in favour of the heavy one. I put the lighter flywheel in my T140 and it's transformed the bike into a real rev-rocket in comparison to what it was.
I'm not going back - even for a pure road bike. It's made no noticeable difference to the likelihood of stalling, but it has a 5-speed box with decent ratios for the road- 1st is way lower than on a Commando.

The primary purpose of a flywheel - as already said - is to store energy, hence it will tend to resist the dissipation of said energy by trying to maintain a constant speed, resisting acceleration/deceleration forces.

I have an engine sitting under my bench waiting for the day I can afford a Maney crank - he won't sell just the flywheel any more. If I could get a drawing I'd get some manufactured myself.