Crank balance factor

Schwany · Jun 2, 2024

Regardless of what Comstock says, the vibrations of my P11 set up like it is does not get worse as RPM increases. The time between pulses gets shorter and less intense. I stop thinking about vibration and start thinking about where the POPO might be hiding and where my next brake point is. For some reason my brain tells me the engine is getting smoother when I'm pushing the RPMs. Definitely could be delusions of grandeur. Whatever the case it is always fun to get on the throttle in 2nd gear and stay on it through 3rd. That childish act still makes me feel 35 years younger.

worntorn · Jun 2, 2024

Similar thing on my Maico 490. For a long time I never noticed how bad the vibrations were because I was always adrenalized, busy hanging on in the trails and fire roads. One day I took it down a long straight road at about 70 mph and 9000 rpm. It vibrates something awful!
Vibrations on the bike start to get annoying at about 4000 and just go up from there.
When you are riding it aggressively for short periods of time as I tend to, the vibrations don't seem to be a bother or even noticeable.

Here's a bit on what happens with the crank imbalance as rpm increases

Weighing in on Balancing Work - Engine Builder Magazine

Engine Balancing: Achieve smooth performance & reliability. Learn the importance & tips for balancing various engine types.

www.enginebuildermag.com

Schwany · Jun 2, 2024

My bad I was describing an engine that has a balanced crank not an imbalanced crank.

hybridracer · Jun 2, 2024

Schwany said:
Regardless of what Comstock says, the vibrations of my P11 set up like it is does not get worse as RPM increases. The time between pulses gets shorter and less intense. I stop thinking about vibration and start thinking about where the POPO might be hiding and where my next brake point is. For some reason my brain tells me the engine is getting smoother when I'm pushing the RPMs. Definitely could be delusions of grandeur. Whatever the case it is always fun to get on the throttle in 2nd gear and stay on it through 3rd. That childish act still makes me feel 35 years younger.

It sounds like you have resonance somewhere in the chassis and as the forcing frequency ( the engine speed ) rises above that resonance frequency, the vibration that you feel reduces in amplitude.

worntorn · Jun 2, 2024

Schwany said:
My bad I was describing an engine that has a balanced crank not an imbalanced crank.

There is always some imbalance, we just pick the imbalance (bf) that works best.
I believe that frequency of vibration increases in direct proportion to rpm whereas amplitude of vibration increases exponentially in relation to rpm.
The combination of big vibes coming really fast sure takes the fun out of things. On the Vincent this starts to be a problem at about 80 mph. The simple solution on that bike is just to slow down a bit to 70 mph where the vibes are minimal. The wind blast is also just about plenty at 70 mph, especially for all day riding.
Irving understood that vibration increased with RPM and that the bike was intended for touring, so he overgeared it to keep rpm very low at cruising speed. It's still not quite as smooth as the rubber mounted Commando 920 or 850 at 70 mph.

Glen

ashman · Jun 2, 2024

Al has only one set mine, on the race track and it's been so long since he has even been there, he think the track is the only place you can fang a Norton or any other bike, I fang my Norton all the time just doesn't need to be flat out in top gear there are the lower gears and 19 tooth front sprocket where the power and torque is, he should come riding with some of my mates I been riding with for over 50 years now, up in the ranges we ride, it's always a race to the next corner, children will have fun lol.
A well set up Featherbed up in the ranges is a great play area, along the east coast of SE Queensland we have some of the best roads for fanging and tight twisties up in the ranges, but the last few years the cops are hitting the most popular roads closer to home so we ride further a field, we don't fang on the highways keep to the speed limit or a little above, but we don't sit on the major roads too long as there are plenty of back roads around.
I can leave my place and within 15 minute be on some great back roads heading out to the mountain ranges, but on the weekend the closer to my city gets very popular with the weekend warriors and the cops know it, being retired weekdays are best, but seems a lot of retired bikers are thinking the same, but places like NW of Brisbane on the WE Daybour and down the coast around Mt Tambourine are very popular with bikes as well cops.
My Norton is built for these roads, the tighter the better, but my Thruxton also love these roads and after 50 years I am still fining roads I haven't played on.
Balancing a Norton crank won't get it perfectly smooth but it makes it pretty dam good and only get a few bad vibrating points through the rev range but they always smooths out.
So Al you have told us many many many times you don't ride on the road and most on here only ride on the road with only a few track racers, but the race guys have their bikes set up for proper racing, I had my fun on track days at Lakeside Raceway back in the 70s, its not far from my place, but I prefer to ride hard up in the twisties with my mates and have brekkie at a nice quiet country town somewhere, better than riding around circles all day.
I will say no more, but it's a good subject about hard mount Commandos/Featherbeds and one that is well set up is a very fun bike that handles so good.
Some have had good success and others not so good, but it's a good subject no matter what.

Ashley

Schwany · Jun 2, 2024

hybridracer said:
It sounds like you have resonance somewhere in the chassis and as the forcing frequency ( the engine speed ) rises above that resonance frequency, the vibration that you feel reduces in amplitude.

Certainly, a high possibility.

worntorn · Jun 2, 2024

Resonance was also discussed in that thread. That certainly could account for the " going smoother at higher rpm " effect.
The engine itself just vibrates more and more as rpm rises but the rider might feel something else.
Here is the post, so that I don't misquote

Jim's comment-

"Engine balance factors can be changed to change the plane of vibration. IE, 0 % balance factor would mean the engine shakes fiercely up and down but no forward to rear shake. 100% balance factor would mean the engine would shake very little up and down but it would shake fiercely forward and rear.

Any solid mount motor is going to have a certain frequency that it resonates at. When it resonates then the vibration will be amplified greatly. Many times you can reduce the tendency for a frame/engine assembly to resonate by changing the plane of vibration.

It has normally been found that a featherbed frame/engine assembly would resonate somewhere over 5000 rpm with the stock balance factor of around 65%. People who wanted to use the engine at a higher rpm would commonly raise the balance factor to around 80% so the engine's plane of vibration shifted more to the horizontal plane. This allowed them to rev the engine higher without exciting the resonance. Then the bike would shake more at lower rpm so it is a trade off.

Not all frame/engine combinations react the same as the featherbed bike. Raising the balance factor may make the resonance problem worse at high rpm in some cases. About the only way to find out is to cut and try or find someone who has a similar setup and copy it. Sometimes things as simple as handlebar bend and footpeg placement can affect the resonant frequency.

So no -you do not make the engine vibrate more or less at a certain rpm by changing the balance factor but you can certainly change how that vibration affects the bike at a certain rpm by changing the balance factor and thereby feeding the forces into the chassis in a different direction. Jim

acotrel · Jun 2, 2024

ashman said:
Al has only one set mine, on the race track and it's been so long since he has even been there, he think the track is the only place you can fang a Norton or any other bike, I fang my Norton all the time just doesn't need to be flat out in top gear there are the lower gears and 19 tooth front sprocket where the power and torque is, he should come riding with some of my mates I been riding with for over 50 years now, up in the ranges we ride, it's always a race to the next corner, children will have fun lol.
A well set up Featherbed up in the ranges is a great play area, along the east coast of SE Queensland we have some of the best roads for fanging and tight twisties up in the ranges, but the last few years the cops are hitting the most popular roads closer to home so we ride further a field, we don't fang on the highways keep to the speed limit or a little above, but we don't sit on the major roads too long as there are plenty of back roads around.
I can leave my place and within 15 minute be on some great back roads heading out to the mountain ranges, but on the weekend the closer to my city gets very popular with the weekend warriors and the cops know it, being retired weekdays are best, but seems a lot of retired bikers are thinking the same, but places like NW of Brisbane on the WE Daybour and down the coast around Mt Tambourine are very popular with bikes as well cops.
My Norton is built for these roads, the tighter the better, but my Thruxton also love these roads and after 50 years I am still fining roads I haven't played on.
Balancing a Norton crank won't get it perfectly smooth but it makes it pretty dam good and only get a few bad vibrating points through the rev range but they always smooths out.
So Al you have told us many many many times you don't ride on the road and most on here only ride on the road with only a few track racers, but the race guys have their bikes set up for proper racing, I had my fun on track days at Lakeside Raceway back in the 70s, its not far from my place, but I prefer to ride hard up in the twisties with my mates and have brekkie at a nice quiet country town somewhere, better than riding around circles all day.
I will say no more, but it's a good subject about hard mount Commandos/Featherbeds and one that is well set up is a very fun bike that handles so good.
Some have had good success and others not so good, but it's a good subject no matter what.

Ashley

I would like to ride your bike. I suggest the weight distribution might be better than it is with many other bikes. When you accelerate in a corner, the weight being lifted off the front can feel scary, and make you slower. I have ridden my own 500cc Triton, my mate's 650 Triton and a geniune 1961 500cc Manx. The Manx was the slowest, but it wins hands-down. It can be ridden extremely fast in corners, and I think mainly due to where the engine is in the frame. The Seeley is a better deal, but different.
Tom Phillis did well on the Domiracer, however he was an exceptional rider, I think it might have been a pig.
The Seeley 7R AJS was the best ever British 350. A Seeley G50 might be superb. I think somebody should go into production making them. A Molnar Manx would have to be more expensive. The only problem with those bikes are their brakes. To have a competition, there might need to be a change in the way of thinking. Landsdowne is real racing and really great to watch.

Crank balance factor

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