Commando Crankshaft Porn

Are those Omega pistons lightened?

Swoosh,
I don't think they're especially lightened - they're not as light as the JS norton pistons. A complete 73.04 mm bore piston is about 319 grams.
These Carillo rods have conventional little end bronze bushes in them but I guess they could be called long rods since they are 6.5" on a 88.5mm stroke ( =1.87ratio)
The reason this engine was being rebuilt was because when I switched it to a 90 deg crank I didn't realize I had to recalibrate the tach. When I did check it with our instruments at work I figured I'd been turning the engine to 10,000 -10,500 rpm through the gears. The bottom end stayed together through this abuse for 1/2 a season ( the long stroke meant it was a silly piston speed) but the rather tired timing side crankcase cracked first.

figured I'd been turning the engine to 10,000 -10,500 rpm through the gears. The bottom end stayed together through this abuse for 1/2 a season ( the long stroke meant it was a silly piston speed) but the rather tired timing side crankcase cracked first.

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Gosh impressive to think how the rush of that felt, but helps make me feel rather a timid abuser.

[quote="RennieK"Once you have your intake, exhaust and combustion chamber maximized it will be capable of producing a fixed amount of energy or power. [/quote]

There is a problem with that line of thought when you are talking about a four-stroke engine because there is a long pause between power pulses. If it were possible to build an engine with zero mass, it would not run because there would be no stored energy to turn the crank through the strokes that consume power to pump and compress.

The Norton tuner John Gregory of Sunset Motors fame once had a 500cc Norton road-race bike. He once tried a very light crank for it that was welded together into one piece, just three pork-chops. He ended up having to add weight to it to get it's top speed back.

It is easy to find top road-racers quotes about how the most time is saved on the fastest parts of a track. In high gear the engine has the least power-pulses per say, 100 feet, and is balancing itself against the highest resistance, wind, rolling, road etc..
In high gear at high speed the flywheel stores the energy fed to it by the crankshaft at regular intervals but the times that the power is taken from the flywheel have nothing to do with the operation of the engine at all. changes in wind direction, wind gusts, a bend in the track or traveling up or down hill will give the bike with a heavier flywheel and more stored energy a big advantage over a bike with a light flywheel with less stored energy to draw from to fight the mentioned obstacles.

A heavy flywheel would also give a rider more control to charge harder out of the turn leading onto the fastest part of a track than a rider who is riding a bike with no flywheel, which would make him have to modulate the throttle a lot more to keep the power pulses and increasing power-curve of the engine from overwhelming the traction of his rear tire.

There is an older(sorry) AMA expert dirt tracker named Jim Challingsworth who ran a Triumph Cub, a rigid Goldstar and a Trackmaster Triumph through the 1960's. If you talk to him he will tell you that he can not find a flywheel heavy enough to race with. He is all for building power at high-rpms with big cams and carbs etc.. but he wants that heavy flywheel to get the best control and traction out of the turn for the charge onto the straight.

The acceleration might be better up through the lower gears with a light flywheel, but the only race you would win would be the one on dry pavement in a straight line from a rolling start that ended when you shifted into high gear.

I have been down that road also, to the extreme in lightweight cranks. Sounded hotter- went slower. The only upside I have found was when you carried them across the shop.

I have seen and worked out the SAE formulas for the correct crankshaft inertia to extract the most power from a particular bore and stroke ratio. Norton was obviously aware of them as their stock crank fit the formula perfectly.

Now if they would have used nodular iron for the flywheel and a little larger PTO shaft they would have had a good thing. Jim

RennieK said:

Once you have your intake, exhaust and combustion chamber maximized it will be capable of producing a fixed amount of energy or power.

Click to expand...

beng said:

There is a problem with that line of thought when you are talking about a four-stroke engine because there is a long pause between power pulses. If it were possible to build an engine with zero mass, it would not run because there would be no stored energy to turn the crank through the strokes that consume power to pump and compress.

The Norton tuner John Gregory of Sunset Motors fame once had a 500cc Norton road-race bike. He once tried a very light crank for it that was welded together into one piece, just three pork-chops. He ended up having to add weight to it to get it's top speed back.

It is easy to find top road-racers quotes about how the most time is saved on the fastest parts of a track. In high gear the engine has the least power-pulses per say, 100 feet, and is balancing itself against the highest resistance, wind, rolling, road etc..
In high gear at high speed the flywheel stores the energy fed to it by the crankshaft at regular intervals but the times that the power is taken from the flywheel have nothing to do with the operation of the engine at all. changes in wind direction, wind gusts, a bend in the track or traveling up or down hill will give the bike with a heavier flywheel and more stored energy a big advantage over a bike with a light flywheel with less stored energy to draw from to fight the mentioned obstacles.

A heavy flywheel would also give a rider more control to charge harder out of the turn leading onto the fastest part of a track than a rider who is riding a bike with no flywheel, which would make him have to modulate the throttle a lot more to keep the power pulses and increasing power-curve of the engine from overwhelming the traction of his rear tire.

There is an older(sorry) AMA expert dirt tracker named Jim Challingsworth who ran a Triumph Cub, a rigid Goldstar and a Trackmaster Triumph through the 1960's. If you talk to him he will tell you that he can not find a flywheel heavy enough to race with. He is all for building power at high-rpms with big cams and carbs etc.. but he wants that heavy flywheel to get the best control and traction out of the turn for the charge onto the straight.

The acceleration might be better up through the lower gears with a light flywheel, but the only race you would win would be the one on dry pavement in a straight line from a rolling start that ended when you shifted into high gear.

Click to expand...

There's considerble anecdotal evidence here and I am glad that the point was made that Challingsworth's applications were it is common racer knowledge that in dirt, a lighter flywheel mass would promote too much wheel spin. Some of what is presented is somewhat conflicting with "heavier is better" but clearly to a point. The flywheel stores the energy but if the energy is not all stored in the flywheel, where does it go? .....acceleration.

On asphalt most anyone would give their left testicle for more useful drive out of a turn and that would be diminsished with a heavier flywheel; what road races are won on a flat out up hill against the wind. An appropriate balance must be met between application, rider ability and practical engineering design limitations. I am not a proponent of light as possible but there is an appropriate balance for every instance.

This all gets back to another thread about the merits of a brake versus inertial dyno which I am not going to get into here.

>This all gets back to another thread about the merits of a brake versus inertial dyno which I am not going to get into here.<

My dyno showed more power with the light flywheel until I engaged the eddy currant absorber. Then it showed more power with the heavy flywheel. Jim

comnoz said:

>This all gets back to another thread about the merits of a brake versus inertial dyno which I am not going to get into here.<

My dyno showed more power with the light flywheel until I engaged the eddy currant absorber. Then it showed more power with the heavy flywheel. Jim

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Yes, exactly. They are showing or measuring two slightly different things (regardless of what they are calling it), both useful information. Just curious, what were the differences and where; an engines power and torque performance are not a point estimate.

If we had the ways and means it would be neat to do a series of power & torque curves through several flywheel weights to see what happens and where.

If we had the ways and means it would be neat to do a series of power & torque curves through several flywheel weights to see what happens and where.

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If I can get Ms Peel's act together this year she's set up to do this already so want to dyno at Jim's or Texas Big D. I want as much power as anyone but more concerned on how it delivers power when tire traction is iffy. I will not need a dyno station to know for sure. oops, can only test BF not much crank total mass changes.

Dances with Shrapnel said:

comnoz said:

>This all gets back to another thread about the merits of a brake versus inertial dyno which I am not going to get into here.<

My dyno showed more power with the light flywheel until I engaged the eddy currant absorber. Then it showed more power with the heavy flywheel. Jim

Click to expand...

Yes, exactly. They are showing or measuring two slightly different things (regardless of what they are calling it), both useful information. Just curious, what were the differences and where; an engines power and torque performance are not a point estimate.

If we had the ways and means it would be neat to do a series of power & torque curves through several flywheel weights to see what happens and where.

Click to expand...

I did tests with top gear acceleration runs. I was testing with inertia only at 450 lbs simulated bike weight. Then I was repeating the tests with reduced inertia and the computer controlling the acceleration rate with the brake. Also set at 450 lbs simulated weight.

The differences were in the 3 to 5 horse range and pretty well though-out the power band. Unfortunately I no longer have any records as this was done pre-fire.

Other things I have noticed:

Most modern bikes show higher readings with inertia only. Most Nortons show about the same power whether inertia only or with eddy currant control.

Under steady state RPM control most bikes normally show less horsepower than in an acceleration run.

Under steady state RPM control the heavier crank always shows more power.

How much of this difference is caused by the dyno operating system I do not know.

Dances with Shrapnel said:

RennieK said:

Once you have your intake, exhaust and combustion chamber maximized it will be capable of producing a fixed amount of energy or power.

Click to expand...

beng said:

There is a problem with that line of thought when you are talking about a four-stroke engine because there is a long pause between power pulses. If it were possible to build an engine with zero mass, it would not run because there would be no stored energy to turn the crank through the strokes that consume power to pump and compress.

The Norton tuner John Gregory of Sunset Motors fame once had a 500cc Norton road-race bike. He once tried a very light crank for it that was welded together into one piece, just three pork-chops. He ended up having to add weight to it to get it's top speed back.

It is easy to find top road-racers quotes about how the most time is saved on the fastest parts of a track. In high gear the engine has the least power-pulses per say, 100 feet, and is balancing itself against the highest resistance, wind, rolling, road etc..
In high gear at high speed the flywheel stores the energy fed to it by the crankshaft at regular intervals but the times that the power is taken from the flywheel have nothing to do with the operation of the engine at all. changes in wind direction, wind gusts, a bend in the track or traveling up or down hill will give the bike with a heavier flywheel and more stored energy a big advantage over a bike with a light flywheel with less stored energy to draw from to fight the mentioned obstacles.

A heavy flywheel would also give a rider more control to charge harder out of the turn leading onto the fastest part of a track than a rider who is riding a bike with no flywheel, which would make him have to modulate the throttle a lot more to keep the power pulses and increasing power-curve of the engine from overwhelming the traction of his rear tire.

There is an older(sorry) AMA expert dirt tracker named Jim Challingsworth who ran a Triumph Cub, a rigid Goldstar and a Trackmaster Triumph through the 1960's. If you talk to him he will tell you that he can not find a flywheel heavy enough to race with. He is all for building power at high-rpms with big cams and carbs etc.. but he wants that heavy flywheel to get the best control and traction out of the turn for the charge onto the straight.

The acceleration might be better up through the lower gears with a light flywheel, but the only race you would win would be the one on dry pavement in a straight line from a rolling start that ended when you shifted into high gear.

Click to expand...

There's considerble anecdotal evidence here and I am glad that the point was made that Challingsworth's applications were it is common racer knowledge that in dirt, a lighter flywheel mass would promote too much wheel spin. Some of what is presented is somewhat conflicting with "heavier is better" but clearly to a point. The flywheel stores the energy but if the energy is not all stored in the flywheel, where does it go? .....acceleration.

On asphalt most anyone would give their left testicle for more useful drive out of a turn and that would be diminsished with a heavier flywheel; what road races are won on a flat out up hill against the wind. An appropriate balance must be met between application, rider ability and practical engineering design limitations. I am not a proponent of light as possible but there is an appropriate balance for every instance.

This all gets back to another thread about the merits of a brake versus inertial dyno which I am not going to get into here.

Click to expand...

Well put DWS. There is a 10lb. range between various cranks posted on these threads. What is the tipping point? At what point is a crank too light or too heavy?

Jim who has a lot of experience with Norton cranks seems to favour the stock weight which is about 24 lbs. Using the stock weight as a reference or "norm" we could say anything heavier is a heavy crank and anything lighter is a light crank. We obviously can't just keep adding weight to gain power, there is a point where it becomes disadvantagious. Do we always want to store maximum power in the crank? How often do we call on ALL this stored power and how often is it not being utilized or required?

beng said:

There is a problem with that line of thought when you are talking about a four-stroke engine because there is a long pause between power pulses. If it were possible to build an engine with zero mass, it would not run because there would be no stored energy to turn the crank through the strokes that consume power to pump and compress.

Click to expand...

Other than your very last paragraph you've made some good points beng but I find the above hard to follow as well. There has to be enough energy from combustion to compress for the next power stroke after all that energy is what provides the stored power to the the flywheel in the 1st place AND compresses the fuel at faster and faster rates and eventually drives the bike with passengers. I would think the power from a 375cc cylinder burning high octane fuel at 9:1 compression will easily spin the engine enough to re-compress a new fuel air mix for the second and other subsequent firings specially on a zero mass engine. By this consensus a rocket ship would require a flywheel.

To translate this last conflict of concepts, if there is no mass in a flywheel to carry the piston power stroke on past BDC, then the rotation is over after one power stroke stops at BDC. Staggered firing order of multi pistons could over come this.

Multi Pistons = TWO should be enough . No sense in getting overly complicated .



http://www.ctie.monash.edu.au/hargrave/pearse1.html

The following URL presents an interesting overview on crankshafts - materials of construction.

Seems like there are a few opinions out there in terms of which is better; billet or forged. I am taking it all with a grain of salt as these are suppliers opinions.

Nice table of comparable tensile strengths for the various crankshaft materials of construction at the end of the article.

http://www.popularhotrodding.com/tech/0 ... ewall.html

For reference, the Norton twin crankshafts are constructed of cast steel cheeks and cast iron flywheels.

hobot said:

To translate this last conflict of concepts, if there is no mass in a flywheel to carry the piston power stroke on past BDC, then the rotation is over after one power stroke stops at BDC. Staggered firing order of multi pistons could over come this.

Click to expand...

Of course a crankshaft with no mass is not a possibility for without mass kinetic energy can not exist, and that is why once an outside force stopped moving something with no mass, as when the exhaust valve popped open and the piston had to start doing the work of pumping again, it would stop.

I was just introducing a way of thinking that I use to help me see why things work. Comparing extreme examples of physical properties of a mechanism can help make subtle things more obvious.

For instance the fact that a Norton with a stock weight flywheel seems to vibrate less and run faster on a track and those with light flywheels seem to vibrate more and are slower. If you look at extreme examples it may help you see why. Think of a crankshaft with the weight and mass of styrofoam compared to one made of some material that would let it weigh 500 pounds.

Previously we were trying to figure out the merits of heavy and light crankshafts and I said that the only race a light crank would win would be one through the lower gears in a straight line from a rolling start. This is because from a standing start, drag racers can rev the engine to a high rpm to launch and the stored energy in a heavy flywheel might help the bike half-way down the quarter mile strip as in the case of the 1960's Fuel Drag bikes that would use only one or two gears in the transmission and the spinning rear tire as a "clutch".

Even shifting up through the gears on a stock 850 Commando the energy from the flywheel helps acceleration so much upon shifting into second gear that it will lift the front wheel off the ground with a heavy rider on board. For a bike with a styrofoam flywheel the only energy available for acceleration, or to fight hills and wind gusts would have to be coming from combustion itself.

If you had a Norton crank with 500 lb mass spinning at 7000 rpm, then you could disconnect the ignition, close the throttle, make your run down the quarter mile dragstrip and have enough left over to get you back to the pits.

So the energy stored in a heavy flywheel running up through the gears out of a turn might slow it down a bit when the flywheel is storing, but upon the shift to a higher gear it is giving back and at high speed where friction and resistance begin to equal the horsepower the engine can produce, it is a help to be able to have the energy coming smoothly from a flywheel than for a few degrees every crankshaft revolution because the resistance exists throughout 360 degrees of crankshaft rotation.

Switching subjects. Since 360 degrees seems to be what works the best with a Commando's isolastics I would stick with it. It is certainly the stiffest design for a two-bearing twin crank as it has the shortest path between main bearings, while switching to 180 or 90 degrees makes the "spring" that is the crankshaft longer and more springy, which with the same weight would just make it move more.

I was thinking of the twisting force of the Norton crank when the bike is shifted or the clutch dropped. Not only would this introduce torsion, but as the crank twisted because of it's length it would also bend into a spiral which is going to cock the flywheel sideways and make it so the bending force on the journals is not only in a plane with the reciprocating force and the extra weight added for "balance factor", but it will also bend the journals at a right angle to the reciprocating force.
When the crank winds up in torsion like this, which it must do quite a bit, it would cyclically grow and shrink in length, which is one more reason to have roller bearings on each end to let it move.

To eliminate all the stress for torsion possible you would want the crank to have a full circle flywheel on it's drive side, a porkchop in the middle doing ALL the work of balancing the throws and reciprocating assembly and it's needed out-of-balance "balance factor", and on the timing side of the crank there would be nothing at all except for the arm going from the journal to the timing shaft. this way with changes in rotational acceleration there would be the least flexing and bending over the length of the shaft possible, there would be no rocking couple still, and stored energy to be drawn from would come mostly from the full flywheel right next to the drive side main bearing. Put mallory metal slugs the entire way around the outside of that flywheel.
Now when you drop the clutch off the line and shift gears the entire shebang should be more reliable and have less bending and subsequent vibration because it is not being induced in the first place, and once produced, because the crank is lightest furthest from where energy is drawn from it, will have less mass and inertia keeping it out of shape. It will fall back in line more quickly in higher gears and run more smoothly when power is drawn from it in a more smooth manner.

Using the way of thinking introduced above, if you had all the weight of the crankshaft furthest from the drive side, when you drop the clutch or change gears etc., it will wind the crank up like a rubber band and bend wring the shit out of it , cocking the journals, the rods and getting the flywheel going side to side in a manner from which it would not be easily stopped.

That is what goes on in my head when I am trying to fall asleep......

Lighter cranks always worked best for me. I would take about 4 lbs off a stock crank as described in my race manual. I was then able to shift quicker because the rpm is more instantly adjustable and the rear wheel had less tendency to slide when down shifting. Saving a fraction of a second with each shift could add up with each lap. And each pound removed from the bike just made it that much faster. From memory - a stock crank was 24 lbs and I cut them down to 20 - still not as light as a Maney crank.

I have found that generally, lighter worked better for me and other riders of my bikes but as others have pointed out there is a practical limit. You sacrifice drive out of the corners and for vintage road racing it seems like drive out of the corners has more utility from what I have been told and from my own experience. This could all probably be better illustrated with time, velocity acceleration charts.

As for "beng" there's a fair amount for me to agree with but as for the concept of placing all flywheel mass towards the drive side of a Norton crankshaft, that would be disasterous in my not so humble opinion. This was gone over in this thread or a different thread (ad naseum) with a lot of passionate discourse. I would suggest looking at most any vertical twin larger displacement four stroke twin (w/o central bearing and see where the flywheel is. Surely they did not all get it wrong. What comes to mind is most all of the British vertical twins as well as Dave Nourish's cranks which are shown somewhere here.

The mass in the center provides dampning to bending moments generated by the various forces. Without that central mass these forces will have their way in a bigger way with flexing the crank at high rpm.

Dances with Shrapnel said:

As for "beng" there's a fair amount for me to agree with but as for the concept of placing all flywheel mass towards the drive side of a Norton crankshaft, that would be disasterous in my not so humble opinion. I would suggest looking at most any vertical twin larger displacement four stroke twin (w/o central bearing and see where the flywheel is. Surely they did not all get it wrong.
The mass in the center provides dampning to bending moments generated by the various forces. Without that central mass these forces will have their way in a bigger way with flexing the crank at high rpm.

Click to expand...

Yes, they did get it wrong, that is why they are no longer in business and we are still trying to fix their cranks. Norton did exactly what I am talking about in the 1950's on works bikes, putting a large full circle flywheel right next to the main drive side bearing and otherwise having as light a crank as possible, and they won a world roadracing championship.

A body set in motion will tend to stay in motion, so once a heavy flywheel is flopping around in the middle of what is essentially a spring, it will not dampen oscillations, it will amplify and keep them going longer.
No matter what weight of crank you like, it is better to have most of it's mass close to where energy is drawn from it, otherwise you are forcing it to wind up and unwind along with all the gyrations that it brings.
Making the spring lighter away from it's mounting points will change the vibrations to a lower amplitude, smaller movements in other words, and it will raise the frequency, exactly like the piece of alloy steel held between two points on a string musical instrument. If you fasten a piece of lead shot to the center of the string it will greatly increase the time it takes for the vibrations to stop, but if you put the lead shot next to a mounting point, you will keep the same weight or mass in the string or crankshaft, but the string will transition to higher frequencies and they will not last as long because the section of the string that has the most movement has the least mass.

Because the bending and moving of our parallel twin cranks causes so much damage to the bearings and crankcases of our engines, making those movements as small in amplitude and duration as possible is moving in the right direction.

There are a couple lines of thought concerning this. Some say the largest twisting force on a crankshaft is the force between the rod journal and the flywheel on the power stroke. Others say it is between the pto and flywheel.

beng said:

Dances with Shrapnel said:

As for "beng" there's a fair amount for me to agree with but as for the concept of placing all flywheel mass towards the drive side of a Norton crankshaft, that would be disasterous in my not so humble opinion. I would suggest looking at most any vertical twin larger displacement four stroke twin (w/o central bearing and see where the flywheel is. Surely they did not all get it wrong.
The mass in the center provides dampning to bending moments generated by the various forces. Without that central mass these forces will have their way in a bigger way with flexing the crank at high rpm.

Click to expand...

Yes, they did get it wrong, that is why they are no longer in business and we are still trying to fix their cranks. Norton did exactly what I am talking about in the 1950's on works bikes, putting a large full circle flywheel right next to the main drive side bearing and otherwise having as light a crank as possible, and they won a world roadracing championship.

A body set in motion will tend to stay in motion, so once a heavy flywheel is flopping around in the middle of what is essentially a spring, it will not dampen oscillations, it will amplify and keep them going longer.
No matter what weight of crank you like, it is better to have most of it's mass close to where energy is drawn from it, otherwise you are forcing it to wind up and unwind along with all the gyrations that it brings.
Making the spring lighter away from it's mounting points will change the vibrations to a lower amplitude, smaller movements in other words, and it will raise the frequency, exactly like the piece of alloy steel held between two points on a string musical instrument. If you fasten a piece of lead shot to the center of the string it will greatly increase the time it takes for the vibrations to stop, but if you put the lead shot next to a mounting point, you will keep the same weight or mass in the string or crankshaft, but the string will transition to higher frequencies and they will not last as long because the section of the string that has the most movement has the least mass.

Because the bending and moving of our parallel twin cranks causes so much damage to the bearings and crankcases of our engines, making those movements as small in amplitude and duration as possible is moving in the right direction.

Click to expand...

I will try to be kind here. If I understand your post above:

1.) The fall of the British motorcycle industry is because of the use of a center flywheel and
2.) Norton had a winning combination of end flywheel because they won one championship in 1950 but changed to a center flywheel, some success.

By the way, are we even talking about a vertical twin 360 firing order on the 1950 works bike? If it was a twin did it have a center bearing (which would make a center mass unecessary and maybe even preclude fitting one there - but you would still need some flywheel)

It seems like the BSA, Norton and Triumph vertical twins all have center flywheeels yet the only bike of notoriety for crankshaft breakage is Norton. My undersatnding is that Triumph and BSa may very well have used steel forging but Norton used cast steel cheeks and cast iron flywheel. From my experience, the flaws that lead to failure on a Norton twin crank are materials of construction. STeve Maney crankshafts do not break as the cheeks are billet and the flywheel is steel. So in essence, since the solution to breaking Norton cranks is better materails of construction, the center flywheel has no bearing (no pun here, eh?) on the matter. From what I see, a center flywheel mass is essential to mitigate the bending moments.

As for dampning vibration, if we had an rediculously heavy center flywheel I would like to say we agree that this would smooth the bike out tremendously and would not spiral out of control like some little kid loosing his or her grip on a merry-go-round.

I suppose there could be some bad combination of center flywheel mass and crankshaft stiffness which could spiral out of control but I have not seen that yet.