crankshaft bob weight

acotrel said:

Why do Commando crankcases break through the main bearing housings when the motor is revved to 8000 RPM ? Triumph 650 motors do not do that. The crank balance factor is set to suit the revs. Isolastics do not change what the crank does to the crankcases. As the revs rise, the counterweight on the crank is supposed to smooth out the internal forces in the motor. Isolastics are cosmetic...If I was going to race a normal Commando, I would raise the balance factor to avoid a blow-up.

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Commando's stroke is 89mm; Triumph 650's is 82mm. That's a big difference. It has nothing to do with balance factor. Isolastics DO change what the crank does to the crankcase. Draw a free body diagram of the system and you will see (or not.) Raising the balance factor on a Commando for racing would make it less balanced because it is optimally balanced taking into account the balancing effects of the freedom of movement of the whole power train assembly. Here's Google AI agreeing with me:

"You are essentially correct that the large mass of the engine assembly performs a function analogous to a counterweight in the overall motorcycle system. The Isolastic design fundamentally changes the dynamics of the vehicle's balance.
Here's why the balance factor is so different for the Commando and why your observation is scientifically sound:

• Balance Factor Differences: Rigidly mounted engines (like the BSA A65 or Norton Atlas) typically require a high balance factor, often in the 70% to 80% range, to achieve acceptable rider comfort by minimizing vertical vibration at typical cruising speeds. The Norton Commando, however, uses a much lower balance factor, generally around 50-52%, which is optimized for the Isolastic system.
• The Role of the Assembly's Mass: The specific, low balance factor chosen for the Commando means the engine's internal counterweights only partially mitigate the reciprocating forces of the pistons and rods. This results in a substantial residual shaking force within the engine unit itself. The engine/gearbox/swingarm assembly is allowed to move in a small, circular orbit "around" the crankshaft's center of rotation, in opposition to the piston forces.
• Dynamic Mass Principle: Because the entire powertrain assembly has a significantly greater mass than the internal reciprocating components (pistons and rods), its small, controlled movement effectively "soaks up" the momentum of the internal forces through the principle of conservation of momentum. The large mass of the engine assembly, therefore, acts as a dynamic absorber or an effective counter-mass for the system as a whole, preventing the forces from reaching the frame and rider.

So, while the Isolastic mounts themselves are just rubber components that allow movement, the entire powertrain assembly they support acts as a heavy counter-mass, allowing the use of a lower internal balance factor optimized for this specific application."

I will never ride with a drum front brake again. I am not bloody stupid, You can only ever ride as fast as your front brake will allow. With a race bike, you are always braking on a lean, and you need to know what your front brake is going to do.

hhh said:

Commando's stroke is 89mm; Triumph 650's is 82mm. That's a big difference. It has nothing to do with balance factor. Isolastics DO change what the crank does to the crankcase. Draw a free body diagram of the system and you will see (or not.) Raising the balance factor on a Commando for racing would make it less balanced because it is optimally balanced taking into account the balancing effects of the freedom of movement of the whole power train assembly. Here's Google AI agreeing with me:

"You are essentially correct that the large mass of the engine assembly performs a function analogous to a counterweight in the overall motorcycle system. The Isolastic design fundamentally changes the dynamics of the vehicle's balance.
Here's why the balance factor is so different for the Commando and why your observation is scientifically sound:

• Balance Factor Differences: Rigidly mounted engines (like the BSA A65 or Norton Atlas) typically require a high balance factor, often in the 70% to 80% range, to achieve acceptable rider comfort by minimizing vertical vibration at typical cruising speeds. The Norton Commando, however, uses a much lower balance factor, generally around 50-52%, which is optimized for the Isolastic system.
• The Role of the Assembly's Mass: The specific, low balance factor chosen for the Commando means the engine's internal counterweights only partially mitigate the reciprocating forces of the pistons and rods. This results in a substantial residual shaking force within the engine unit itself. The engine/gearbox/swingarm assembly is allowed to move in a small, circular orbit "around" the crankshaft's center of rotation, in opposition to the piston forces.
• Dynamic Mass Principle: Because the entire powertrain assembly has a significantly greater mass than the internal reciprocating components (pistons and rods), its small, controlled movement effectively "soaks up" the momentum of the internal forces through the principle of conservation of momentum. The large mass of the engine assembly, therefore, acts as a dynamic absorber or an effective counter-mass for the system as a whole, preventing the forces from reaching the frame and rider.

So, while the Isolastic mounts themselves are just rubber components that allow movement, the entire powertrain assembly they support acts as a heavy counter-mass, allowing the use of a lower internal balance factor optimized for this specific application."

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The Google robot is spouting a popular myth. The balance factor affects the way the motor spins-up, It is not about rider comfort except in the case of the Commando. A Norton 650ss is designed to run smooth at 100MPH, like a Triumph Bonneville or a BSA Super Rocket. If you ride a normal Honda CB750, it is almost vibration-free, but it is piece of garbage - like riding a brick. When the CB750 was released, many people in the UK were riding motor-scooters. The Commando 750 was about recapturing the market. In the old days, road bikes were cheap versions of race bikes. Motorcycles were about speed, not comfort. The first really successful 4 cylinder race bike was the Geoff Duke Gilera 4 - a CB750 is nothing like it. A Commando 750 is not designed for high speed, and does not need to be. Very people ever rode Triumph Bonneville 650s or Norton Atlases on public roads, as fast as they will go. When was the M1 Motorway built in the UK ? What was the Ace Cafe about ? Minorites are not what sales numbers are about.

Honda CB750s were successful in motorcycle road-races in Australia when races were push start. Riders used to push the button on the start line. Otherwise anything would beat them. I once saw a standard Norton Commando win a production race. I am surprised that a Commando was up there with the H2 Kawasaki. One of the first Atlas 750s to come to Australia, pulled its barrel flange off - that did not help sales. Remembering this stuff reminds me of my age.

Yes the balance factor affects many things on a bike, but the part you're missing is that in the Isolastic system, the whole engine, gearbox, cradle and swing arm (to some degree) act as part of the crank counterweight mechanism. The Commando wasn't designed to "reclaim" anything; it was to try to produce a "new" model with an engineering department that was behind on engine development. The Honda 4 had yet to be released.

When I bought my 850 Commando motor, the first thing I did was rebalance the crank. I did not try to rev it. One look at the crank was enough. I am amazed that the 850 motor is so good - it is extremely responsive to tuning, but it needs close ratio gears. If you lose revs on an up-change, they never come back, and you will be slow forever. I did not like threading the steel plug into the hole in the crank counterweight. It works, but is not the best answer. It probably takes experience to know when your gearbox is useless. In the past, I always raced with close ratio gearboxes - I know what should happen, and with the 850 motor, it didn't. I will not tell you how fast my Seeley 850 is, except to say it is fast enough. I probably will never get to race it again, but it would take something really good to catch it, on a short circuit. It is not me who is fast, it is the bike. Something so bad, should not be so good.

When I first rode the Seeley 850, it was revving and making noise without actually doing anything. I thought it was gutless. It is strange to increase acceleration by raising the gearing - most bikes are the reverse. Usually lowering gearing increases acceleration. Close ratios keep the engine on the boil. When your motor is used continually between 5,500 and 7000 RPM, the crank balance factor needs to be high. If you want to go fast,- spin it high and make it pull, and never lose revs - stay high in the power band. If it drops out the bottom, you will be slow. Lost revs never come back. A Manx racing gear cluster fits into an AMC gear box, but for clutch starts, you need the Commando first gear - once you are in second gear, you never go back to first gear during a race. Even the slowest corner on any circuit can always be taken in second, regardless of how high the overall gearing might be. With Manx ratios, my Seeley 850 goes around the slowest corner on Winton Raceway in second gear losing traction at the rear wheel. It is pretty funny. The last time I raced, I thought the Norton gearbox might give-up on the start line. It copped 6000 RPM in first gear, which was as high as second in a normal box, without spreading. Always look behind you when you are on the start line. If there is something really fast there, go around and start from the back. A bit of loss at the start of a race is easily recovered in the first corner, most riders back-off for it.

Al you keep talking about racing but have no experience riding Commandos on the road, my hot 850 with the balanced crank runs pretty good on the road and with the stock GB and gears, up shifting it recovers pretty quickly with the cam and head work as well that big spark and going through the gears I take it up to 7k RPMs regularly without losing any power even without CR gears, of course you never shift back to first gear when out riding, first is for taking off, 2nd and 3rd are the fun gears, most owners of Nortons ride on the road, not the race track, there are a lot of Commandos built for high revving running and reliability with the stock gearing.

jseng1 said:

Mentioning the balance factor is fine - but I want to see the bob weight - not just the balance factor. I want the bob weights of stock Commando cranks. We already know what the balance factor is for those. And I don't want it from a manual. I want the actual measured bob weighs to verify these balance factors.

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Jim, did you find these figures? 638 / 668 grams is the counterweight required for each crank cheek and 1/2 of flywheel bobweight (cast +- balancing weight). I would like to see the actual figures too, especially for the Mk3.

In the past I had the Mk3 flywheel digitized and will get the crank cheek digitized soon. This will allow a theoretical figure to be computed at least.

- Knut

The smallest amplitude of vibration will be with a 50% balance factor, the vibration will be equal up down and fore aft. The way we feel the vibration at different RPM is due to the harmonics of different parts of the frame.
The best ways to reduce vibration are to reduce reciprocating mass and dynamic balancing, static balancing cannot balance the crank end to end.
Spacing the crankpins at 180 degrees will cancel out reciprocating mass but introduce a rocking couple at twice RPM frequency. Spacing the crankpins at 90 degrees can also balance reciprocating mass, but unlike a 90 degree V twin a rocking couple will be introduced.

Have you ever noticed how much smoother a 500 single is than a 500 360 degree twin? that's due to the twin having nearly twice the reciprocating weight of the single

The balance factor only changes the direction of the vibration, the amount of weight out of balance remains unchanged and so the amplitude also remains the same. Lightening the conrods and pistons reduces the out of balance weight and also the amplitude. You use the balance factor % to reduce resonance in the frame the engine is mounted in, change the frame and you may have to change balance factor. The factories tested different balance factors in new frames and test rode in normal road conditions them until they found the best %. All very subjective and if you race the same combination of engine and frame, as the conditions are generally much higher revs, a different % will work better.

Effect of Balance Percentage
The "balance factor" (BF) is a compromise where a percentage of the reciprocating mass (usually 50-60%) is added to the rotating mass for balancing purposes.

• 0% Balance Factor (hypothetical): The engine would have maximum vibration forces in the vertical (piston motion) direction.
• 100% Balance Factor (hypothetical): If all reciprocating mass were balanced by a counterweight, the vertical forces would be cancelled, but an equally large new force would be created in the horizontal (at right angles to the cylinder axis) direction.
• Typical Balance Factors (e.g., 50-60%): A compromise is achieved where the resultant unbalanced force is directed at an intermediate angle, typically around 45 degrees, and its magnitude (amplitude) is constant regardless of the specific percentage chosen. The total force amplitude is not reduced, but its line of action is changed.

jseng1 said:

Does anyone know the actual Bob weight of a stock Commando crank? This is the weight placed on the journals to balance it. I want the dry weight. Not the percentage factor. For both 750 and 850. None of my cranks are stock.

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Definitely not may area but it seems to me that you could take a stock crank, dry and experimentally determine the bob weight by adding weight until that crank balances. I suspect that several would be required and then averaged to get a useful answer.

There is an old Triumph movie that shows Triumph cranks being balanced and the guys spent under a minute on each so it definitely was not rocket science in the factory and I have no reason to think Norton did it any more precisely.

The normal method of balancing the crank is to weigh all the bits which hang of the ends of the conrods, decide what balance factor would be desirable and hang that percentage in place of the pistons and gudgeons, then drill the side of the flywheel on the side of the crank at which the big ends are high, until the crank will run on level straight edges without halting. The balance factor is the percentage of the reciprocating weight which brings the crank into balance. A balance factor of 54% compensates for that percentage of the reciprocating weight by flywheel inertia, the force increases with revs. At 8000 RPM - the flywheel is trying to escape while the rods and pistons are trying to hit the head. If the balance factor is not high, the rods and pistons have more effect on the rate at which the motor spins-up. The balance factor is chosen to suit the intended purpose of the bike. With four cylinder motors, that problem does not really exist. 180 degree twins produce a rocking couple. 360 degree twins are better. Single pulses of power are usually better than twice as many for road racing. I have been told that it is about the way the tyre distorts. None of it is rocket science. In the old days when we raced single cylinder four strokes, the balance factor was the first thing we changed. Riding a Manx Norton in traffic would be like being continually booted up the bum.

acotrel said:

The normal method of balancing the crank is to weigh all the bits which hang of the ends of the conrods, decide what balance factor would be desirable and hang that percentage in place of the pistons and gudgeons, then drill the side of the flywheel on the side of the crank at which the big ends are high, until the crank will run on level straight edges without halting. The balance factor is the percentage of the reciprocating weight which brings the crank into balance. A balance factor of 54% compensates for that percentage of the reciprocating weight by flywheel inertia, the force increases with revs. At 8000 RPM - the flywheel is trying to escape while the rods and pistons are trying to hit the head. If the balance factor is not high, the rods and pistons have more effect on the rate at which the motor spins-up. The balance factor is chosen to suit the intended purpose of the bike. With four cylinder motors, that problem does not really exist. 180 degree twins produce a rocking couple. 360 degree twins are better. Single pulses of power are usually better than twice as many for road racing. I have been told that it is about the way the tyre distorts. None of it is rocket science. In the old days when we raced single cylinder four strokes, the balance factor was the first thing we changed. Riding a Manx Norton in traffic would be like being continually booted up the bum.

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My mate's 650 Triton has had all the bullshit done to it. It is amazingly quick. I rode it once - came around a corner onto a straight beside an H2 Kawasaki which all the gear, and blew it to the weeds. The guy chucked the H2 down the road behind me at the end of the straight - I could hear it banging and crashing. Nobody ever knows how fast a bike is until they race against or with it.

acotrel said:

My mate's 650 Triton has had all the bullshit done to it. It is amazingly quick. I rode it once - came around a corner onto a straight beside an H2 Kawasaki which all the gear, and blew it to the weeds. The guy chucked the H2 down the road behind me at the end of the straight - I could hear it banging and crashing. Nobody ever knows how fast a bike is until they race against or with it.

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Of course you did Al.....

I have observed, after 54 years of riding, there are those who ride fast, and then there are those who TALK about it.

concours said:

Of course you did Al.....

I have observed, after 54 years of riding, there are those who ride fast, and then there are those who TALK about it.

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"The older I get, the faster I was"

For several years from about 1969 until 1978. I rode my 500cc short stroke Triton in Allpowers C Grade races, mainly on Winton Raceway. I know how fast a Z1 or H2 Kawasaki can be. My bike badly needed a six speed close ratio gearbox. It only had 4 speeds close ratio. If I geared it high, it would stay up with the lead at the ends of the straights. I did not usually gear it low, because I would be passed when I was towards the ends of the straights and enter a corner behind the leaders going too fast. I am talking fact not bullshit. My Seeley 850 is not a Triton 500, it is an easy ride - much safer. 'The older I get the faster I was' was said to me by David White when he was leader of Motorcycling Australia. He only ever rode sidecars on speedway. GO and ride road race before you comment. I can outride almost anyone on Winton Raceway - I do not race to assist my ego. I do other things for that. I am usually a fast rider on a slow bike - the Seeley 850 is fast enough everywhere. Winton Raceway is now only academic -to get a decent ride, I need to travel hundreds of kilometres to another state and the racing is now stuffed anyway. Motorcycling Victoria has gone broke. When we race, we fund a bureaucracy. There is now only one small circuit in Victoria - at Broadford - have a look at it on Youtube. It is owned by Motorcycling Victoria. Most of the guys who run road races in Australia, ride sidecars.

In Australia, it is a really funny joke - the people who make money out of race meetings. It is not the clubs which become wealthy. My mate's son and grandson are now both road-racing. Club politics are driving them away. They are moving towards power-boat racing as crew. Judging from what is on Youtube, I think people in Europe might actually do things for the love of the sport. Many of the 'historic' motorcycles in Australia should even not be ridden to the pub.