Heavy flywheel equals more torque? (2014)

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oldmikew said:
Eddie , by better I simply was thinking in terms of performance. If there is some sort of straight before the first bend then a stock crank will catapult the bike off the line in a shorter time than a light one, however a light crank will have better engine braking which might mean the bike will handle better under heavy braking making it possible toget power on sooner coming out of say a hairpin like at Olivers Mount. So swings and roundabouts ?
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A light crank might, I repeat might get you quicker off the line if given enough engine revs, and will scream faster to the next upward gear change - I did lighten the crank of a 750 and that appeared to be the result !
 
I am more and more of the conviction that light crank and heavy cranks are simply DIFFERENT rather than better or worse.

I believe it depends largely on rider style and rider preference.

Don’t forget, by modern sports engine standards, even a light Norton crank is f-ing heavy!

I would imagine a rider raised on modern bikes would prefer a lighter crank in his Cdo.
 
A modern bike is very different from a Commando. Both have their strengths and weaknesses. You have a choice, you can win in the corners or you can win in the straights. A lot depends on how the bike handles and the power characteristics are a major factor. I usually ride my bike up and down through the gearbox, so that when I am cornering, I still have a heap of revs and use the gearbox to accelerate hard while the bike tightens it's line. If you do that with a modern bike, you get the hi-side. A modern bike is usually more suited to 'point and squirt' and the rolling distance in the middle of corners is usually longer. You can make a Commando behave like a modern bike, but you would lose a lot of inherent advantage. If you come out of a corner on a Commando beside a modern bike and have to rely on a straight line blast down the next straight , you will probably always lose. But if you turn under the opposition and get on the gas much earlier, you have the run on the other guy. The heavy crank avoids the situation where you get sat on your arse in the heat of the moment. Smooth power delivery is at a premium with a Commando. That is the reason I use a 2 into 1 exhaust and not separate pipes with megaphones and my carburation is always spot-on. Anything which causes jerkiness in corners can crash you.
 
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'I believe it depends largely on rider style and rider preference.'

I don't prefer to ride my bike a certain way, the bike dictates the way I ride. When you try to go fast, you adapt to the bike and use it's strengths to best advantage. I think a lot of guys believe modern technology can make an historic bike better - bike development was not a smooth continuum - there were leaps in technology. On a tight circuit a Commando can be a lot better than a modern bike. With a bike which is down on power, you can choose where you want to lose races - with a Commando it is usually at the ends of the straights. But the good thing is that by the time you get there, the other guy has to pass you at very high speed and many back off early.
 
Fast Eddie said:
I am more and more of the conviction that light crank and heavy cranks are simply DIFFERENT rather than better or worse.

I believe it depends largely on rider style and rider preference.

Don’t forget, by modern sports engine standards, even a light Norton crank is f-ing heavy!

I would imagine a rider raised on modern bikes would prefer a lighter crank in his Cdo.
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acotrel said:
A modern bike is very different from a Commando. Both have their strengths and weaknesses. You have a choice, you can win in the corners or you can win in the straights. A lot depends on how the bike handles and the power characteristics are a major factor. I usually ride my bike up and down through the gearbox, so that when I am cornering, I still have a heap of revs and use the gearbox to accelerate hard while the bike tightens it's line. If you do that with a modern bike, you get the hi-side. A modern bike is usually more suited to 'point and squirt' and the rolling distance in the middle of corners is usually longer. You can make a Commando behave like a modern bike, but you would lose a lot of inherent advantage. If you come out of a corner on a Commando beside a modern bike and have to rely on a straight line blast down the next straight , you will probably always lose. But if you turn under the opposition and get on the gas much earlier, you have the run on the other guy. The heavy crank avoids the situation where you get sat on your arse in the heat of the moment. Smooth power delivery is at a premium with a Commando. That is the reason I use a 2 into 1 exhaust and not separate pipes with megaphones and my carburation is always spot-on. Anything which causes jerkiness in corners can crash you.
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Am inclined to agree with Eddie's last post.. specifically that a rider raised on modern machinery would probably prefer a lighter crank but yes the commando has virtues that should not be discarded, better to have a first rate commando than a second rate point and squirt device.. However at the moment all that is available in the UK is a Maney crank and that has to be better than the alternative. What I need to do is walk some of the other hill climbs my nephew is likely to compete on. The only one I am so far familiar with is Olivers mount.. and that has a very very slow hairpin.
 
cjandme said:
What about Nourish didn't a son or someone take over his enterprise
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Sadly he sold the company to someone who many have subsiquently reported having big issues with. Not dealt with the new firm myself though, so can’t offer a first hand opinion.

It’s a shame, cos Dave was as straight talking, honest and helpful as they come.
 
I dislike long fast sweeping bends with my bike because it takes power to stay on line and the temptation is often to shut off halfway around. I've been giving myself nightmares watching the videos of this year's Island Classic. Most guys tend to shut off halfway around the sweepers which go on forever and take another bite. The really fast guys don't do that

 
oldmikew said:
So chaps ,if you were building a 750 for hill climbing , would you go for standard or light crank?
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Good points made on considering steel flywheel for racing.

Not sure what style of hill climb you are referring to here. Stateside, a hill climb is most typically held on a steep straight dirt hill of maybe several hundred feet in length though we do have the Pikes Peak which now is mostly if not near totally asphalt. My understanding is if it is a British style hill climb it is a closed asphalt road course on hill.

Assuming it's a British style you will generally want all the useful torque you can muster to the rear wheel so a lighter crankshaft rotational mass would be where I would recommend going. If it is in the dirt, there are other traction management aspects that a heavier rotational mass crankshaft may be beneficial. Best to consult the front runners in the sport.
 
If it is the Swiss style of hill-climb you are referring to, where you can probably keep the revs high around every corner, the heavy crank would be good - but for short sharp bursts where the bike almost stops in between, a light crank is better. With the heavy crank on a road-race circuit, it is more difficult to recover if you lose revs in a corner. You can't just spin the motor up and drop the clutch, it all happens too slowly. With the heavy crank, throttle response suffers, but if you keep it spinning high you get better acceleration and the bike copes better in windy conditions - a gust of wind makes many two-strokes almost stop.
 
Dances with Shrapnel said:
Good points made on considering steel flywheel for racing.

Not sure what style of hill climb you are referring to here. Stateside, a hill climb is most typically held on a steep straight dirt hill of maybe several hundred feet in length though we do have the Pikes Peak which now is mostly if not near totally asphalt. My understanding is if it is a British style hill climb it is a closed asphalt road course on hill.

Assuming it's a British style you will generally want all the useful torque you can muster to the rear wheel so a lighter crankshaft rotational mass would be where I would recommend going. If it is in the dirt, there are other traction management aspects that a heavier rotational mass crankshaft may be beneficial. Best to consult the front runners in the sport.
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Its British , and yes have decided to opt for a lighter crank
 
Probably the only advantage you will have with the lighter crank, will be for the first few metres off the start. For the rest, if there are any sharp rises in elevation (bumps) the light crank might be slightly slower. With the heavy crank, once it is spinning, it tends more to keep on spinning than a light crank does. Like a lot of these things, different bikes suit different circuits better. A lot also depends on the internal gearbox ratios and the overall gearing. You might find that very steep parts of the circuit require a couple of closer ratios, otherwise you might fall through the bottom of the power band.
 
I have no reason to believe that my Steve Maney cases/light crank, iron barrel, JSM rods/pistons, Fullauto head, Web cam 750 short-stroke would be unsuitable for hill climbing!

In the 750 class, it is 'probably' more suitable than a long stroke/heavy crank, but really speed up a twisty hill is more down to overall weight, clutch, gearbox, final drive ratios and big handlebars!
 
I think the lighter crank creates the obligation to keep the revs higher to maintain the same amount of kinetic energy being delivered through the gearbox. That means different gearing and a different inlet port size and exhaust system. However in the end, a motor with a light crank revving to 9000 RPM would probably give the same acceleration rate uphill as a motor with a heavy crank revving to 7,500 RPM. The difference might be that the lighter crank gives better throttle response ? And the need for that probably depends on the circuit.
 
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Dances with Shrapnel said:
Good points made on considering steel flywheel for racing.
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Assuming it's a British style you will generally want all the useful torque you can muster to the rear wheel so a lighter crankshaft rotational mass would be where I would recommend going. If it is in the dirt, there are other traction management aspects that a heavier rotational mass crankshaft may be beneficial. Best to consult the front runners in the sport.
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You imply that torque is lost when using a heavy vs. light flywheel. This is definitely wrong. Energy is conserved in the flywheel and the available torque at the rear chainwheel will be somewhat lower during the acceleration phase only.

Any motorsport which requires rapid acceleration and deceleration will benefit from a lighter flywheel. I would think motocross and road racing on tight circuits are such examples.
On the other hand, racing on circuits with varying slopes and few tight corners (IoM comes to mind) as well as some hill climbing tracks will benefit from heavy flywheels. The stored rotational energy will help prevent a large drop of revs. The same can be said if the rider encounters head wind gusts.

-Knut
 
acotrel said:
I think the lighter crank creates the obligation to keep the revs higher to maintain the same amount of kinetic energy being delivered through the gearbox. That means different gearing and a different inlet port size and exhaust system. However in the end, a motor with a light crank revving to 9000 RPM would probably give the same acceleration rate uphill as a motor with a heavy crank revving to 7,500 RPM. The difference might be that the lighter crank gives better throttle response ? And the need for that probably depends on the circuit.
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Unfortunately neither the motor nor the gearbox delivers any _kinetic_energy_ (the crankshaft stores kinetic rotational energy though). They deliver torque which is transferred to the rear chainwheel whereby a pulling force at the rubber/road interface is created. This pulling force is in balance with rolling resistance, a positive slope gradient, and any desire for acceleration of the bike.
At any specific time, the bike and rider as a whole posesses kinetic energy (E=0.5*m*v*v). Irrespective of crankshaft mass, higher revs yield more torque at the rear chainwheel and therefore, a higher pulling force to meet the aforementioned loads. Your supposition that a lighter crankshaft requires higher revs in general is wrong. It all boils down to the desired rate of dM/dt (i.e., how quickly do you want the torque and the pulling force to change with use of throttle and altering loads). I agree with you - this is dependant of rider style/preference and the circuit itself.

-Knut
 
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Mz Peel inherited Ms Peel's 1/2" smaller dia, 4.5 lb lighter flywheel. I noticed 4 things, quicker throttle response and quicker/more engine down shift drag but more so the easier to flick toss around and of course a bit less effort to lift/drag back to road grade. No effect on total torque just the way its delivered, more jerky vs smoother. Off road play requires on/off wheel spin control so lighter is better. On pavement allows more accurate instants of rear spin to skip out a bit for better turn line, yet re-hooks more predicable in nick of time. Crank/flywheel gives more gyroscope stability than road wheels do - so at times creeping along very lumpy loose humped/rutted paths I down shift to spin crank more for distinctly less nervous/twitchy - be it my commandos or modern Vtwin.

At extremes light cranks may prevent idle or cause stalls in short intervals of power cuts by clutch disconnecting bike momentum from engine to shift but restarts on engagement, which may upset grip if at limits. Drag Nortons do kill button/shift jabs at WOT power peaks to avoid this.
 
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 !
 
mdt-son said:
You imply that torque is lost when using a heavy vs. light flywheel. This is definitely wrong. Energy is conserved in the flywheel and the available torque at the rear chainwheel will be somewhat lower during the acceleration phase only.

Any motorsport which requires rapid acceleration and deceleration will benefit from a lighter flywheel. I would think motocross and road racing on tight circuits are such examples.
On the other hand, racing on circuits with varying slopes and few tight corners (IoM comes to mind) as well as some hill climbing tracks will benefit from heavy flywheels. The stored rotational energy will help prevent a large drop of revs. The same can be said if the rider encounters head wind gusts.

-Knut
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Well if you put it that way Knut, you are definitely wrong. I am not implying torque is lost when using a heavy flywheel, it's just being consumed for accelerating other things like the extra rotational and translational mass of a heavier flywheel - basic engineering dynamics. With a lighter flywheel, energy ends up being conserved in a faster moving motorcycle rather than a flywheel. I think you understand that from the second half of your first sentence but just got a bit excited.

Your second cluster of sentences is, in my opinion, just conjecture, not necessarily wrong or correct.
 
acotrel said:
I think the lighter crank creates the obligation to keep the revs higher to maintain the same amount of kinetic energy being delivered through the gearbox. That means different gearing and a different inlet port size and exhaust system. However in the end, a motor with a light crank revving to 9000 RPM would probably give the same acceleration rate uphill as a motor with a heavy crank revving to 7,500 RPM. The difference might be that the lighter crank gives better throttle response ? And the need for that probably depends on the circuit.
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Pure conjecture and gobblydegook!

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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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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