Balance factors

robs ss · Dec 23, 2022

acotrel said:
What balance factor is used as standard for the 750 motor in the N15 and P11 ? - Do they also have isilastics ?

No they don't - but balance factors were/are used on bikes other than commandos, or maybe you weren't aware?
My jibe was about your post #77 - all about how you avoid riding on the road (again!) and the rest was what a beast your Mazda is.
How about sticking reasonably to topic.

Merry (& safe) Christmas!

johnm · Dec 23, 2022

johnm said:
When I was campaigning my 500 Domi twin years ago several friends suggested I go the 270 route. I didn't. Partly because I had heard and seen problems. Since my last post I also remembered a Matchless 500 twin down here that had been converted to a 270 crank. It was part of a well resourced race team running G50s. The 270 bike came to several meetings but never delivered great results for the significant expense and work that had gone into it. They have two separate camshafts with gear drive so that's probably not the issue with that engine however.

I'm very happy with Jim's longer rods and lighter pistons which have gone onto my Commsndo. Considering the same for my 500 although life is a bit busy and expensive just now with lots of family events, weddings etc coming up.

I should perhaps expand on my ride on the 270 crank Norton 600. It was Dick Huurdenmans own bike. He has passed away now but was very active in the NZ race scene from the 1960s through to about 2000. This is an article about his and Dallas Rankins exploits with Ducatis. Dallas owned British Spares in Wellington for a long time.

DUKES OF WELLINGTON: HAND BUILT WORLD BEATERS

Rankine and Huurdeman’s next project would get the nickname the Duke of Wellington. With weight-saving and handling performance...

amcn.com.au

I rode Dicks 600 only through the suburbs to no high speed experience. The engine had very little vibration, feeling more like the low speed rumbling of a V 8 car engine actually. It was in a soft state of tune and I think he used on his sidecar most of the time. You can see from the article that Dick was a bit of an innovator so exactly what he had done inside the motor is anyone's guess.

I never heard of any reliability issues so I think this shows that for street use at least 270 cranks would be an excellent way of reducing vibration in the British 360 twin.

ntst8 · Dec 23, 2022

Excellent article, thanks for posting the link.

ggryder · Dec 24, 2022

jseng1 said:
... your 50% thinking is rational and logical but it doesn't take into account that the "snap" at TDC and BDC....

Yes, you are right, the conrod angle effect increases piston acceleration at TDC (and reduces it at BDC) and the crankshaft balance weight doesn't see the same effect. Looks like if you balance the crank at 50% by the conventional reckoning you'll have an "effective" balance factor closer to 40% at TDC, and 75% at BDC.

Piston Motion: The Obvious and not-so-Obvious, by EPI, Inc.

lcrken · Dec 24, 2022

ggryder said:
Yes, you are right, the conrod angle effect increases piston acceleration at TDC (and reduces it at BDC) and the crankshaft balance weight doesn't see the same effect. Looks like if you balance the crank at 50% by the conventional reckoning you'll have an "effective" balance factor closer to 40% at TDC, and 75% at BDC.

Piston Motion: The Obvious and not-so-Obvious, by EPI, Inc.

Thanks for the link. There's a wealth of other engine design and function articles on the EPI site. It's a site worth exploring in much more detail.

Ken

jseng1 · Dec 24, 2022

ggryder said:
Yes, you are right, the conrod angle effect increases piston acceleration at TDC (and reduces it at BDC) and the crankshaft balance weight doesn't see the same effect. Looks like if you balance the crank at 50% by the conventional reckoning you'll have an "effective" balance factor closer to 40% at TDC, and 75% at BDC.

Piston Motion: The Obvious and not-so-Obvious, by EPI, Inc.

That's one of the best balance explanations I've read - and clears up the load diff between TDC and BDC. A bit up a thick read though and weighted down with math. I haven't come across "hypyotenuse" since high school.

acotrel · Dec 24, 2022

I once built a Suzuki T250 racer. I always think of port timings in a two-stroke as being as being similar to valve opening and closing points in a four-stroke. For those T20 and T250 motors, many guys tried to copy the port timings of the TD3 Yamaha.
If you draw a diagram of the rod and crank with the piston pin part of the way down the cylinder, you can work ot the angles which give you the distances from Top Dead Centre. If you know your maths, it is not difficult. At that time I had access to a computer which had a good high level language. I was able to print out conversion tables between crank degrees and piston distance from TDC.
My timings for the T250 were much less dramatic than most other guys were using. The motor was on methanol and the bike was extremely fast. The last time I saw the bike, the current owner told me it ad won 28 races and 4 championships. They have kept copying my port timings when they changed cylinder barrels.
The distance of the transfer port from the head in a TD3 is about 32mm. In my T250 it was 29.5mm - the two motors are different in concept about gas flow.
I don't think many guys ever do that calculation. It is also useful when changing ignition timings. It means you can use a depth gauge instead of a degree disc.

acotrel · Dec 24, 2022

johnm said:
I should perhaps expand on my ride on the 270 crank Norton 600. It was Dick Huurdenmans own bike. He has passed away now but was very active in the NZ race scene from the 1960s through to about 2000. This is an article about his and Dallas Rankins exploits with Ducatis. Dallas owned British Spares in Wellington for a long time.

DUKES OF WELLINGTON: HAND BUILT WORLD BEATERS

Rankine and Huurdeman’s next project would get the nickname the Duke of Wellington. With weight-saving and handling performance...

amcn.com.au

I rode Dicks 600 only through the suburbs to no high speed experience. The engine had very little vibration, feeling more like the low speed rumbling of a V 8 car engine actually. It was in a soft state of tune and I think he used on his sidecar most of the time. You can see from the article that Dick was a bit of an innovator so exactly what he had done inside the motor is anyone's guess.

I never heard of any reliability issues so I think this shows that for street use at least 270 cranks would be an excellent way of reducing vibration in the British 360 twin.

With my 850 motor, simply filling the stupid one inch hole in the counter weight gave a balance factor of 72 %. My motor idles very slowly with the bike moving slighty backwards and forwards. That is probably the reason Norton reduced the balance factor. When I ride the bike, at low revs, it does not feel different to any other. Above 5500RPM, it is dead smooth. The guys with the dufle coats and desert boots were never into speed. And those guys were Norton's intended customers for the Commando.
Only a university professor would drill that one inch hole into the Commando crank. When I first saw it, I felt sick. Car guys do not know much about motorcycles.

acotrel · Dec 24, 2022

acotrel said:
With my 850 motor, simply filling the stupid one inch hole in the counter weight with steel, gave a balance factor of 72 %. My motor idles very slowly with the bike moving slighty backwards and forwards. That is probably the reason Norton reduced the balance factor. When I ride the bike, at low revs, it does not feel different to any other. Above 5500RPM, it is dead smooth. The guys with the dufle coats and desert boots were never into speed. And those guys were Norton's intended customers for the Commando.
Only a university professor would drill that one inch hole into the Commando crank. When I first saw it, I felt sick. Car guys do not know much about motorcycles.

Did the N15 and P11 motors have that hole in the counterweight ? It is the ultimate in stupidity. With the high balance factor, the feel at low revs adds to the adrenalin rush. I absolutely love riding that bloody bike.
I would love to know what motor the 850 crank was used in when it did not have that hole. Filling the hole with a threaded steel plug is dangerous. - was it the Atlas ?
The only other alternatives are a new flywheel or a new billet crank.
What I did not realise when I was building the Seeley, was where I worked, I could have put a billet crank through the system as a legitimate job. It would have been perfect and cost me almost nothing. We had the best low alloy steel in Australia. We were making gun barrels for the 105mm NATO gun.
In Australia, there used to be very good places in which to work, if you were into motorcycles.

tricatcent · Jan 12, 2023

I have a Triumph 650 with a big bore kit, it has some cams a bit more duration than standard and about 10:1 compression ratio. This engine is installed in a Trackmaster frame. You can get quite a bit of vibration from this kind of machine, a powerful twin engine in a very light frame. The engine is installed in an unusual way. The rear engine plates are attached by bronze bushings to the frame, those bushings which allow the front of the engine freedom to bounce up and down are in line with the top of the chain. Above the head is a single isolastic mount from a Norton, with a few holes drilled in it. Near the front of the engine there is a valve spring that holds the weight of the engine. The balance factor on this engine is 0%. The iso system can only absorb vertical vibration, but because of the 0% balance factor that's mostly the direction of the vibrations from this engine. The chain tension since it pulls concentric with the pivot bushings doesn't tend to compress the iso rubbers. It works fairly well. I think it is not as smooth as a Commando but it's a big improvement over solid mounting. I have never heard of anyone else using a 0% balance factor, but that's how this engine runs. I have put over 20,000 miles on it.

ashman · Jan 12, 2023

Fast Eddie said:
WTF…?

Al is a expert has never rode a road going Commando but he seems to know all and add up all the years he haven't been on a bike, race or road but he seems to know all, sorry Al but somethings you come out with just don't add up, but balance a Commando crank and they will spin up a lot smoother than factory balance and a motors that has been blue printed is even snoother.

marksterrtt · Jan 14, 2023

tricatcent said:
I have a Triumph 650 with a big bore kit, it has some cams a bit more duration than standard and about 10:1 compression ratio. This engine is installed in a Trackmaster frame. You can get quite a bit of vibration from this kind of machine, a powerful twin engine in a very light frame. The engine is installed in an unusual way. The rear engine plates are attached by bronze bushings to the frame, those bushings which allow the front of the engine freedom to bounce up and down are in line with the top of the chain. Above the head is a single isolastic mount from a Norton, with a few holes drilled in it. Near the front of the engine there is a valve spring that holds the weight of the engine. The balance factor on this engine is 0%. The iso system can only absorb vertical vibration, but because of the 0% balance factor that's mostly the direction of the vibrations from this engine. The chain tension since it pulls concentric with the pivot bushings doesn't tend to compress the iso rubbers. It works fairly well. I think it is not as smooth as a Commando but it's a big improvement over solid mounting. I have never heard of anyone else using a 0% balance factor, but that's how this engine runs. I have put over 20,000 miles on it.

tricatcent, your frame mods sound like a simplified version of James Love's Vibratek anti vibe system as depicted in Tony Foale's Motorcycle Chassis Design: the theory and practice book. Triumph tried a version of this with a T140 using their AV (anti-vibration) modded frame. This system was discussed on the Access Norton forum back in 2013 and I believe Triumph balanced the engine at 55% but not sure why since In T. Foale's book the Love's system was based on using a 0% balance factor as yours does. It would be great to see a couple pictures of your frames modifications. Mark

jseng1 · Jan 15, 2023

tricatcent said:
I have a Triumph 650 with a big bore kit, it has some cams a bit more duration than standard and about 10:1 compression ratio. This engine is installed in a Trackmaster frame. You can get quite a bit of vibration from this kind of machine, a powerful twin engine in a very light frame. The engine is installed in an unusual way. The rear engine plates are attached by bronze bushings to the frame, those bushings which allow the front of the engine freedom to bounce up and down are in line with the top of the chain. Above the head is a single isolastic mount from a Norton, with a few holes drilled in it. Near the front of the engine there is a valve spring that holds the weight of the engine. The balance factor on this engine is 0%. The iso system can only absorb vertical vibration, but because of the 0% balance factor that's mostly the direction of the vibrations from this engine. The chain tension since it pulls concentric with the pivot bushings doesn't tend to compress the iso rubbers. It works fairly well. I think it is not as smooth as a Commando but it's a big improvement over solid mounting. I have never heard of anyone else using a 0% balance factor, but that's how this engine runs. I have put over 20,000 miles on it.

I had a visitor from Australia who did the same thing with a Norton roadracer in a custom frame - mounted the motor on rear pivot bushings so it could shake vertically on a front isolastic. Then he decided to go for a vibration reduction with lighter pistons etc.

Son of Siredward · Jan 15, 2023

Sir Eddy's Rocket was developed and ran a 270 crank with a 270/450 firing order. I ran the bike on the salt three different years, 2014, 2016 and 2018.
I never felt that the vibration was unbearable, in fact I feel that my street Atlas' and Commando have more vibration than the Rocket.
My father had many years of development and almost right out of the gate decided on a 270 crank to help support the inertia needed to spin and make power up to 10,500 RPM and not snap a crank.
This is a short stroke, long rod motor with a 73mm bore to make it 498cc.

We ended up with a balance factor of 55%, and went to a billet crank and reinforced cases knowing that both of these would be factors when pushing a pushrod motor to it's limits,
The comment was made about the sound, it is hard to put a finger on it, it is not a Ducati note, not Triumph and definitely not a Norton.

I fell in love with the project and lived it for 6 years after my father's passing to fulfill his dream. Still planning on going back one more time to push the record up again with my dad by my side.
I get emotional every time I listen to this video of the first time we fired the bike in Comstock's shop.

What does the group think it sounds like, enjoy!

gortnipper · Jan 15, 2023

Son of Siredward said:
What does the group think it sounds like, enjoy!

A fire breathing beast.

Sounds so damn good.

Kinda Norcati sounding.

acotrel · Jan 15, 2023

I suggest it is impossible to balance the crank of a 360 degree vertical twin and have it running smooth at all revs and still get reliable high performance without doing damage. The Commando attempt simply cosmetic. It was designed to make the bike acceptable to all sections of the market, except those who wanted to use it as a sports' bike. Isolastics do not stop a poorly balanced crank from shagging the cases and bearings, when you run the motor at the revs needed for racing. The Commando was not designed for very high speeds, and it did not need to be. As a road bike, it is adequate, as long as you do not really fang it. And why would you do that anyway ?
My Seeley 850 is used for only one purpose, so the crank is balanced to suit that purpose. It runs extremely smooth at 7000 revs. At 3000 revs, it is shit., But 3000 revs is not where damage usually occurs - the internal loads involved are much less. An out of balance crank at 7000 revs can really do some damage. 54% crank balance factor at 7000 revs is not smart. And a light crank is worse, than a heavy one.
When I built my bike, I did not believe in the heavy crank. But the heavy crank can be used in a way which is more effective than a light crank. Once it is spinning high, nothing stops it. So you need close gear ratios, and high overall gearing. All things work together in a motorcycle. Most cannot be dual purpose. With a normal Commando, you can almost race it, but the next step is a big one. I have only ever seen one normal Commando win a road race. The guy who rode it was an expert, who could win on anything..

acotrel · Jan 15, 2023

I have enough trouble timing the cam in a 360 degree crank 850 engine, without going stupid. How do you get the cam timing right for both cylinders with a 270 degree crank. With a 360 degree crank, when there is a difference in cam timing between the two cylinders due to the cam, you usually split the difference.

Son of Siredward · Jan 15, 2023

acotrel said:
I have enough trouble timing the cam in a 360 degree crank 850 engine, without going stupid. How do you get the cam timing right for both cylinders with a 270 degree crank. With a 360 degree crank, when there is a difference in cam timing between the two cylinders due to the cam, you usually split the difference.

Son of Siredward said:

Sir Eddy's Rocket was developed and ran a 270 crank with a 270/450 firing order. I ran the bike on the salt three different years, 2014, 2016 and 2018.
I never felt that the vibration was unbearable, in fact I feel that my street Atlas' and Commando have more vibration than the Rocket.
My father had many years of development and almost right out of the gate decided on a 270 crank to help support the inertia needed to spin and make power up to 10,500 RPM and not snap a crank.
This is a short stroke, long rod motor with a 73mm bore to make it 498cc.

We ended up with a balance factor of 55%, and went to a billet crank and reinforced cases knowing that both of these would be factors when pushing a pushrod motor to it's limits,
The comment was made about the sound, it is hard to put a finger on it, it is not a Ducati note, not Triumph and definitely not a Norton.

I fell in love with the project and lived it for 6 years after my father's passing to fulfill his dream. Still planning on going back one more time to push the record up again with my dad by my side.
I get emotional every time I listen to this video of the first time we fired the bike in Comstock's shop.

What does the group think it sounds like, enjoy!

Click to expand...

This was how we found the balance on the LSR 500 Rocket 270 degree crank.

Son of Siredward · Jan 16, 2023

I see that the video on the balancing of the crank to determine how much weight was needed utilizing clay was on a reply.
This is the old school way of balancing and it works. Pictures of the first crank that has the left hand throw rolled back 90 degrees and bolted back together and the notes my dad had on camshaft that was cut and also rotated to send off to have a new camshaft made. The last picture is the plugs in the billet crank we had to add to get the balance desired.

Centered and Pinned

acotrel · Jan 17, 2023

Phil Irving is usually correct in what he says in tuning for speed. However he might sometimes be taken too literally. If you read what he says about methanol fuel, many people seem to believe you should run it rich. That is not the true meaning of his words ' if you run methanol rich, you still get good power'. You DO still get good power - better than you can get with petrol - but not as good as it can be. Phil Irving was an engineer of the 1930s. The Japanese, Italians and Germans have come a long way since then. Kenny Dreer's design for the 961 Commando engine might be excellent - OR NOT. These days most modern motorcycles do not vibrate much, probably because most probably have balance shafts in the motor with the crank, to counteract the vibes.
Boring that hole in the old type Commando crank is simply butcherous - only a car guy would do that. Most of them simply would not know.
There is no simple and safe way to get the balance factor right for high speed work with a 70s Commando, unless you replace the flywheel or fit a billet crank.
For what it is a 70s Commando is a very good motorcycle. If you buy one, why do you want it ? Would you paint a moustache on the Mona Lisa ?

That spare set of crankcases I have in my shed were cracked through the drive side main bearing housing. The guy who did it used to race his mates on the Great Ocean Road in Victoria. His mates were riding Japanese bikes. The Commando was very good - for a while.

Balance factors

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