Heavy flywheel equals more torque? (2014)

Dances with Shrapnel said:

Yes, I vaguely recall the Triumph factory experimenting with a huge depleted uranium flywheel. The bike was so fast that they lost control of it and it was last seen going past the dark side of the moon.

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Sounds like somebody mis-remembering the osmium flywheel experiment that set off the Area 51 rumours.

Grown men were weeping; policemen were turning in their badges...

acotrel said:

A Triumph Bonneville fitted with the lighter Saint crank, never performs as well as it does when fitted with the heavier standard crank. And what I am talking about is both straight line go and ride-ability !

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My ‘68 T120 has the light crank. I’ve also raced Triumphs with the light crank. I couldn’t disagree with you more.

Hi all,
Further to my my previous post I have made a calculation example using figures approximately resembling a Commando 850 flywheel. The example shows how a lighter flywheel leaves more usable torque at the rear wheel during the acceleration phase and the magnitude of the additional torque to be expected.

Regards,
Knut Sonsteby

Aw shucks Knut, now you went and proved it with math. One should also factor in the translational aspect of same or greater torque acting against a lighter motorcycle due to a lighter flywheel. It's a double dip enhancement.

Nice work and thank you.

mdt-son said:

Hi all,
Further to my my previous post I have made a calculation example using figures approximately resembling a Commando 850 flywheel. The example shows how a lighter flywheel leaves more usable torque at the rear wheel during the acceleration phase and the magnitude of the additional torque to be expected.

Regards,
Knut Sonsteby

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I rather agree with mdt-son. Spinning up the flywheel is storing energy. And accelerating the bike is storing energy too. One is rotational and the other is just in a "straight" line. Because the flywheel is directly coupled, it doesn't suffer from chain snatch and the small amount of play in the gears. So the flywheel produces a smoother result in terms of power pulses. But the effect is more or less the same. The great need for a flywheel effect is when you are either not in gear(idling, starting, changing gears, etc.), or you are just starting and haven't built enough stored energy through forward motion of the bike to carry you from power pulse to power pulse. With a heavy flywheel much energy is used to spin up the flywheel (storing energy). If you try to accelerate quickly you are quickly pumping a lot of energy into the flywheel. With a heavy flywheel a wider ratio box will spin up the flywheel more slowly. And of course it depends on how broad your torque band is. When I was an over the road trucker there were always those who noted how smoothly their truck seemed to glide over the small undulations in the road with a heavy load (undulations you don't even notice at all in a car). With a light load there was a lot more shifting. It made it seem as if the engine was working harder with a light load. But at the end of a run more fuel was expended with the heavy load. And that meant more energy was used.

Dances with Shrapnel said:

Aw shucks Knut, now you went and proved it with math. One should also factor in the translational aspect of same or greater torque acting against a lighter motorcycle due to a lighter flywheel. It's a double dip enhancement.

Nice work and thank you.

Click to expand...

Thank you, Dances.
The weight advantage of a lighter crankshaft will indeed improve acceleration, although hardly noticeable. Using the figures in the example and weight figures for a fully tanked Mk3 Roadster, delta-M = 2.5 kg / (233+75) kg * 100% = 0.81%. Proviso identical pulling force, the acceleration figure will improve by an additional 0.8% .
The combined effect on the bike's acceleration due to freed-up torque during (angular) acceleration and the weight advantage now totals 5.2%, which is a noticeable figure.

Now, what about the disadvantages of a 20% lighter crankshaft in a roadster? Who has a Steve Maney crank fitted to a road bike and would like to comment on his observations? According to Steve, his crankshafts are 20-25% lighter than a standard crankshaft, which matches the weight figures I have used in the example. Lower weight does not necessarily translate into a lower moment of inertia, but in this case I think it does and Steve backs this up on his home page.

Anyone?

Regards,
Knut

I know that riding on public roads is different from road-racing, however the combination of the heavy crank and the wide ratio gearbox in the Commando is absurd. - Obviously built to a price. I tried racing with that gearbox and it was absolutely bloody useless. Even on public roads, it might be useful to sometimes quickly pop down one gear to pass a car ? You need at least five gears with a low first and the upper four close together, to get any sort of performance. On a cost/benefit basis, the good gearbox probably delivers more value that all the engine mods put together.

A Seeley with a commando engine, is still a commando. Actually, I HAVE ridden a near-standard Commando. The only difference is the weight. This is about the engine - NOT about the bike. Instead of having a go at me, why don't you post something constructive ? - You obviously know better !

When you race-change up through a close ratio gearbox with the standard 850 crank, acceleration is extremely rapid because the revs stay high and the crank has a lot of inertia. If you find yourself in the situation where you are balked in a corner, and the revs drop, the heavy crank is very slow to recover. The way the motor is ported can have a large effect. Similarly going down through the box, with the standard box it is impossible to be smooth because the steps between the gears are too large and it is difficult to match the revs. So the answer is simple - if you cannot afford the decent gearbox, buy the light crank. But if you buy both, you might be buying a pig in a poke - there is usually no easy way back

mdt-son said:

Thank you, Dances.
The weight advantage of a lighter crankshaft will indeed improve acceleration, although hardly noticeable. Using the figures in the example and weight figures for a fully tanked Mk3 Roadster, delta-M = 2.5 kg / (233+75) kg * 100% = 0.81%. Proviso identical pulling force, the acceleration figure will improve by an additional 0.8% .
The combined effect on the bike's acceleration due to freed-up torque during (angular) acceleration and the weight advantage now totals 5.2%, which is a noticeable figure.

Now, what about the disadvantages of a 20% lighter crankshaft in a roadster? Who has a Steve Maney crank fitted to a road bike and would like to comment on his observations? According to Steve, his crankshafts are 20-25% lighter than a standard crankshaft, which matches the weight figures I have used in the example. Lower weight does not necessarily translate into a lower moment of inertia, but in this case I think it does and Steve backs this up on his home page.

Anyone?

Regards,
Knut

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Getting back on track here:

Steve Maney crankshaft - not a roadster application but a street going Seeley and really no comparison because other things were done to the bike. Best I can describe is a Commando road racer which started race life with an unfettered factory crank and flywheel. After some time during a season freshen up, the stock flywheel was trimmed per someone's instruction book and it was then noticeably quicker acceleration (seat of the pants) with better drives out of corners and then finally boom - fragmented flywheel. Never even think about lightening a cast iron flywheel. I cannot recall the specifics but at least a pound or two of metal was removed from the perimeter.

If you built a 750cc version of a Paton 500, would it be faster or slower than a 750 Commando ? With a Commando there are two limiting factors - the valve gear and the strength of the bottom end. A lighter crank with a shorter stroke places less stress on the cases, but higher revs are the norm, so the valve gear becomes the limit.

Getting it back on track...again:

mdt-son said:

Thank you, Dances.
The weight advantage of a lighter crankshaft will indeed improve acceleration, although hardly noticeable. Using the figures in the example and weight figures for a fully tanked Mk3 Roadster, delta-M = 2.5 kg / (233+75) kg * 100% = 0.81%. Proviso identical pulling force, the acceleration figure will improve by an additional 0.8% .
The combined effect on the bike's acceleration due to freed-up torque during (angular) acceleration and the weight advantage now totals 5.2%, which is a noticeable figure.

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Running this through a couple of kinematic equations indicates the magnitude of the 5.2% advantage over say 5 seconds in terms of greater velocity and greater distance covered. Very significant.

On a breezy day the heavy crank might be superior - the bike doesn't get blown backwards so easily. If you've ever ridden a two-stroke around a corner into a head-wind, they feel as though they have stopped.

On a breezy day the light crank might be superior - the bike doesn't get blown backwards so easily. If you've ever ridden a Norton Commando with a lightened flywheel around a corner into a head-wind, they feel as though they're breaking the sound barrier.

That is interesting. With the steering geometry I use on my bike, I get on the gas extremely early coming out of corners and the rear tyre always loses traction - it wheel-spins. Because the bike is much more vertical, it doesn't really matter. I am actually surprised that a commando 850 engine with the heavy crank can even be raced. With the wide ratio box, it was absolutely useless. With the close 4 speed box, 1st gear is hopeless in clutch starts - there is a risk of blowing it to smithereens when you wind the crank up and dump the clutch. I ran with the lead group in four races the last time I raced - lost the start every time but one - in that one I was upfront and the motor gave up. Do you think that if you got a steam roller with an extremely heavy flywheel and fitted a 20 speed close ratio gearbox, we could break a land speed record with it ?

Realize this was all hashed out during Commando hey days mostly by east coast shops and builders like RON FRATTURELLI http://nortoncommando.com/ (let scrolling images sink in) who offered two steel flywheels both about 4.5 lb lighter one stock dia. for the smoother less secure road racers and one 1/2" small dia. for dragsters and jerky tire squealing flat track racers. Ms Peel got the last of the 1/2" smaller kind. To timid ole hobot - the most appreciated/noticed feature was distinctly easier lighter sense to flick/toss/tip/tilt/turn handling. Also if actually enjoying-employing this benefit > requires more brake skills as engine drag by down shifting is also distinctly less. Like wheels there is no down side to lighter-ness in the range Norton can still idle but trickier to tame yet more exciting if pilot and cycle adaptable. Current Mz Peel crank all welded up and nitrated weigths about 25 lb in smaller dia. flywheel and may allow clutch drops in 4th for 1/4 m drag efficiency hopefully matching tire grip with power curve (on sucked down forks) like a couple of chopper hot shots with lighter race level cranks said worked best for them.

My '68 P!! dragster build two inches low in front had very lighted flywheel for National drags yet idled at 600 but would smoke street rear w/o brakes and run through gears w/o moving a bike length on just throttle snap no front brake drag needed nor wet spot like drag strip often have, pashaw.

Acrtelt untamed=non-trinked or swash plated isolastic Commandos are not in same solid state league as Seely's turn load tolerance so they may benefit by heavier flywheels 'security but bet you'd like the sense I describe. Tamed Commandos don't need the extra drag delays. hobot level kix create loud tire sounds which I often mention but hardly ever hear from others on forum to ponder significance of above.

Before Mz Peel's crank assembled I had local water jet shop make a CNC file of it to reproduce in ya favorite alloy as Ron is a decade older than most of us so not supplying new flywheels as cheap factory hazard upgrade.

acotrel said:

Do you think that if you got a steam roller with an extremely heavy flywheel and fitted a 20 speed close ratio gearbox, we could break a land speed record with it ?

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No.

yes it would break a speed record - for its class of craft and would accelerate to top quicker too with photographers walking ahead of it to record the feat. Of course this would require the flywheel spun up before dropping the clutch or would be slower accelerating if had to start from engaged idle throttle snap.

Commando/Norton twin cranks range from factory ~28 lb to Maney and others ~18.5 lb.

I've had some rev up contests with modern inline 4's like R1's or Hyubrusa's spotting them to start snap at 4000 rpm to 12-13K, vs Combat 2000>7000, to find about same on factory crank but Ms Peel would hit redzone before I could finish a contest fast throttle snap so a instant later heard the moderns hit rev limiter. I had 2 Nortons that did not go varrOOOMMM leaving overlookers drawn closer ->they went BLAAPPP* that startled people stumbling and falling back involuntarily stunned. At 2004 BikesBluesBBQ event 300,000 with TV noise editorials, TV crew needed example so when they nodded to me on Ms Peel 2-1-long dong open megaphone at soft idle, BLLLAAATTT, gal dropped microphone and cameraman lost control of camera as they involuntarily recoiled - expecting HD type thundering not space fabric rips through head. Maybe archive available hm.

acotrel said:

Do you think that if you got a steam roller with an extremely heavy flywheel and fitted a 20 speed close ratio gearbox, we could break a land speed record with it ?

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Only on a breezy day if the steering geometry is such that you can get on the steam early without it standing up provided you are turning into a head wind. You might have a chance.

acotrel said:

That is interesting. With the steering geometry I use on my bike, I get on the gas extremely early coming out of corners and the rear tyre always loses traction - it wheel-spins. Because the bike is much more vertical, it doesn't really matter. I am actually surprised that a commando 850 engine with the heavy crank can even be raced. With the wide ratio box, it was absolutely useless. With the close 4 speed box, 1st gear is hopeless in clutch starts - there is a risk of blowing it to smithereens when you wind the crank up and dump the clutch. I ran with the lead group in four races the last time I raced - lost the start every time but one - in that one I was upfront and the motor gave up. Do you think that if you got a steam roller with an extremely heavy flywheel and fitted a 20 speed close ratio gearbox, we could break a land speed record with it ?

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You do realize, don't you, that many Commandos with stock "heavy" cranks have been very successfully road raced with stock "wide ratio" 4-speed gearboxes, particularly back in the '70s and '80s? They might not suit your riding style, but they clearly worked well for others. Back when I was still racing Commandos, my friend Fred Eiker won a lot of road races on his 850 Commando with stock gearbox and un-lightened crank, including the 1986 La Carrera II Mexican road race. He won on high speed tracks like Willow Springs as well as tight, twisty ones like Sears Point. Fred averaged 111.19 mph for the 125 mile race on Mexican roads from San Felipe to Ensenada. And that was before we had all the currently available good stuff, like Maney cases and cylinders, billet cranks, TTI gearboxes, etc, and running on period tires. I was lucky, my PR came with a Quaife 5-speed, and it was an advantage over a stock 4-speed, but not as much as you believe. I also raced a MK3 with 920 engine and stock 4-speed for a while, and it never blew up anything. It just had to be ridden a little differently. Would it have been better with a 5-speed? Probably, but it was certainly adequate without it.

Ken