Balance Factor

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acotrel

acotrel

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Let us put this rubbish to rest. The crankshaft in my Seeley 850 is balanced at 72%, I believe the standard commando factor is 58%. To get from 58% to 72% I fitted a threaded steel plug in the hole in the commando flywheel. The plug was about 26mm dia by 40mm long (about 0.5 Kg). My Seeley has a rigidly mounted motor and does not vibrate much at 7000 RPM, however at idle the bike rocks backwards and forwards. If my bike had isolastics they would have to cope with the weight difference at idle, just as the isolastics in a standard commando have to cope with such a weight difference at 7000 RPM if it the 58% factor is used. The isolastics are a cheat used to solve a problem. If they were not there, the effect of the weight difference would be transmitted through the bearings and cases, and meet a fixed obstacle. However the standard commando crank if revved to 7000 RPM must still bend more due to the difference from the non-vibratory 72% factor of a crank in a rigidly mounted motor. So if the rubbers in the isolastics are too rigid, the cases and bearings must cop a beating while the crank is flexing unduly. I suggest that if you use a standard balance factor crank with the correct density rubber in the isolastics, you have the same result at about 3,000 RPM, as you have with a rigidly mounted motor with the 72 % balance factor at 7,000 RPM. However when you rev the standard 58% balance factor commando to 7,000 RPM it must vibrate , and while it is doing it the crank holding bolts are at full stretch, the crank bending, and the superblend bearings are coping with the barrel rollers not centralized. If you balance the crank to 72% in a standard commando, it would shake badly at idle, however at 7,000 RPM the crank would not be bent because of the weight difference, and the loads on the bolts and journals would be less.
Surely somebody on this forum must have tried to dynamically balance a commando crank at high speed, and shone a strobe light on it ? I'm sure if you spun a standard crank at 7,000 RPM, you would see the flywheel grow. I believe the major limiting factor to revving my 850 is the piston weight. Force equals mass times acceleration, and the accelerations in relation to the revs are squared relationships. Jim Schmidt's long rods and light pistons are a good answer to getting a higher rev limit, The longer rods give lower acceleration rates at the top and bottom of the stroke, and the inertia of the pistons which loads the rods is less.
 
acotrel said:
Let us put this rubbish to rest. The crankshaft in my Seeley 850 is balanced at 72%, I believe the standard commando factor is 58%.
Click to expand...

The manual states 63% dry (no oil), 52% wet (crank shaft full of oil). This is for a road bike and, like a lot of things, is a compromise. Race bikes can be balanced for max revs because that's what they do most of the time. Plus the Commando's got rubber mounting, which isolates the majority of vibes (providing there's clearance) except when the engine revs are producing vibrations which are near or at the natural frequency of the rubber, typically between 2,000 and 3,000rpm. This is when unacceptable vibration is transmitted.
 
Comfort to the rider doesn't mean comfort and less stress to the crank.
Engine balance factor is selected for rider comfort,even at the expense of extra loading on the crank.Vertical imbalance is usually more noticable to the rider,and a high balance factor reduces that.If the cylinders were horizontal instead of near vertical,a low balance factor would reduce vertical imbalance.

A high balance factor makes for big imbalance and stress in the horizontal plane.
If the wet balance factor on a Commando was 64.9%,you'd have equal imbalance at TDC and at around mid-stroke.
Changing the balance factor from there will increase stress in one plane (so maximum imbalance increases),while it decreases in the other plane.
 
acotrel said:
Let us put this rubbish to rest. The crankshaft in my Seeley 850 is balanced at 72%, I believe the standard commando factor is 58%. To get from 58% to 72% I fitted a threaded steel plug in the hole in the commando flywheel. The plug was about 26mm dia by 40mm long (about 0.5 Kg). My Seeley has a rigidly mounted motor and does not vibrate much at 7000 RPM, however at idle the bike rocks backwards and forwards. If my bike had isolastics they would have to cope with the weight difference at idle, just as the isolastics in a standard commando have to cope with such a weight difference at 7000 RPM if it the 58% factor is used. The isolastics are a cheat used to solve a problem. If they were not there, the effect of the weight difference would be transmitted through the bearings and cases, and meet a fixed obstacle. However the standard commando crank if revved to 7000 RPM must still bend more due to the difference from the non-vibratory 72% factor of a crank in a rigidly mounted motor. So if the rubbers in the isolastics are too rigid, the cases and bearings must cop a beating while the crank is flexing unduly. I suggest that if you use a standard balance factor crank with the correct density rubber in the isolastics, you have the same result at about 3,000 RPM, as you have with a rigidly mounted motor with the 72 % balance factor at 7,000 RPM. However when you rev the standard 58% balance factor commando to 7,000 RPM it must vibrate , and while it is doing it the crank holding bolts are at full stretch, the crank bending, and the superblend bearings are coping with the barrel rollers not centralized. If you balance the crank to 72% in a standard commando, it would shake badly at idle, however at 7,000 RPM the crank would not be bent because of the weight difference, and the loads on the bolts and journals would be less.
Surely somebody on this forum must have tried to dynamically balance a commando crank at high speed, and shone a strobe light on it ? I'm sure if you spun a standard crank at 7,000 RPM, you would see the flywheel grow. I believe the major limiting factor to revving my 850 is the piston weight. Force equals mass times acceleration, and the accelerations in relation to the revs are squared relationships. Jim Schmidt's long rods and light pistons are a good answer to getting a higher rev limit, The longer rods give lower acceleration rates at the top and bottom of the stroke, and the inertia of the pistons which loads the rods is less.
Click to expand...

So hands up who needs to rev up to 7k in the shed on your hobbyhorse........ When mine revs to about 3k on the concrete floor, it tries real hard to get off the carpet square but it cant........ GMF'd who would try to rev a std crank to 7k. :shock: :shock: :shock:

Balance Factor
 
So hands up who needs to rev up to 7k in the shed on your hobbyhorse........ When mine revs to about 3k on the concrete floor, it tries real hard to get off the carpet square but it cant........ GMF'd who would try to rev a std crank to 7k. :shock: :shock: :shock:

Balance Factor
[/quote]

What do you mean it only tries to get off the carpet square, my 750 Atlas with engine revved on the centrestand would walk backwards with every twist of the throttle :!: :shock:
 
Bernhard said:
What do you mean it only tries to get off the carpet square, my 750 Atlas with engine revved on the centrestand would walk backwards with every twist of the throttle :!: :shock:
Click to expand...

Use a thick rubber mat instead of carpet.
 
If I rev the Combat while on the center stand at about 3K the whole bike starts to go into a death wobble, scary stuff.

I think the whole balance factor debate boils down to what you use the bike for.

Normal road bikes balanced for comfort and race bikes balanced for high RPM life.
 
bwolfie said:
If I rev the Combat while on the center stand at about 3K the whole bike starts to go into a death wobble, scary stuff.

I think the whole balance factor debate boils down to what you use the bike for.

Normal road bikes balanced for comfort and race bikes balanced for high RPM life.
Click to expand...


Plus One, Brent.

So when you fit the dramatic cam and the big ports, what are you doing when you move the power band up higher on a road bike? The was no way I was going to race with a standard commando crank at up to 7,000 RPM without changing the balance factor and even if I used isolastics , I would still do it if I wanted to rev the motor. There was a magazine article about a guy with a Seeley Commando which vibrated badly and it mentioned one rider in the old days who machined up a piece of depleted uranium to get a decent balance factor. If you can feel that bad vibration, the cases and bearings are copping a beating. When it happens at low revs, it is not so destructive . If it happens at high revs on a bike fitted with isolastics, you might not feel it so much however that doesn't mean bottom end damage is not happening.
There was not one production racer two stroke in the seventies which had a rubber mounted motor even though many cracked frames. We have fitted rubber mounts to TZ350 Yamahas, and while they are more comfortable, they never seem to go as hard as they do when the motor is rigidly mounted. The Maxton Yamahas even had a schraeder valve in the frame, and it was pressurized so that if it did not hiss after a race , you knew you had a crack somewhere . I think it was Charlie Williams who won a Senior IOM TT on a 350cc Maxton Yamaha in the 70s. A guy I knew brought a Maxton 350 back to Australia in the 70s, however I don't believe it was ever raced much. I saw it at the Broadford Bonanza a couple of years ago. I don't think many people would know what it is about - the swing arm is longer than standard, and the handling is much better.
 
I still worry when I see 7,500 RPM on some gear changes. I think about those heavy 850 pistons and the aluminum rods getting fatigued. My feeling is that the 6,300 RPM limit quoted for 650cc Triumph engines is more sensible, however my motor gets up there too quickly to observe that limit when I use the close gearbox and methanol fuel. It hasn't let go YET. If it happens near the end of a straight, it could mean a really big crash.
 
While we're at this "Let's put this rubbish to rest" malarkey, Superblend bearings? Barrel shaped rollers? I don't think so
 
gripper said:
While we're at this "Let's put this rubbish to rest" malarkey, Superblend bearings? Barrel shaped rollers? I don't think so
Click to expand...

Have you got a better option ? If it works I will use it. I was reading an old Roger Bacon book on the Triumph T120 Bonneville last night and it mentioned that the unit construction cases were too rigid and caused bearing problems that did not occur in pre-unit motors. Seems strange to me.
 
I too am sick & tired of hearing this tosh about barrel shaped rollers. I've measured the earlier Ransome & Marles 'Superblends' & also the FAG type. The rollers are parallel with a slightly larger end rad. to stop them digging in to the outer race. The barrel shape rollers are single & twin row self aligning bearings. A different thing altogether & used to accept misalignment between bearing bores / housings.
Martyn.
 
BSA paper was concerning parallel vertical 360 twins in the late 60's early 70's. I still ain't found it but did find this practical review of it..

I decided to use a balance factor of 58%. Let me explain what the considerations for this very number were:
Assuming the weight of the original con-rods and pistons the crank must have been balanced to exactly 54%, which is the original factor used by BSA (as stated in Eddie Dow's tuning sheet).
For an engine with 42mm crank throw radius and a rod lenght of 165mm, using 54% results in the lowest average value for the resulting force over the crank angle (Fmittel/Fmax/% in the spread sheet, sorry it is in German). The polar diagram bottom right shows the force progression in vertical and horizontal direction, the red graph representing 54% and the green one 58%.
The average resulting force is slightly better with 54%, but the peak values are smaller at 58% (graph not visible on the screenshot, sorry), furthermore the amplitude is smaller in vertical direction, while the bigger amplitude in horizontal direction isn't felt that badly than vertical vibration.
I found 58% to be the best compromise between peak and mean amplitude, experience will show how well it is going to perform in the BSA frame.
Click to expand...
Balance Factor
 
Matchless said:
I too am sick & tired of hearing this tosh about barrel shaped rollers. I've measured the earlier Ransome & Marles 'Superblends' & also the FAG type. The rollers are parallel with a slightly larger end rad. to stop them digging in to the outer race. The barrel shape rollers are single & twin row self aligning bearings. A different thing altogether & used to accept misalignment between bearing bores / housings.
Martyn.
Click to expand...

+1

Just about every rolling element bearing comes with specifications for allowable misalignment. "Superblend" - just a marketing moniker; probably hatched by Norton to smooth over public perception when the Commandos began to frag their bearings.

As for the acotrel's diatribe on balance factor - it's pseudoscience. Yes, please put this rubbish to rest.
 
While we are here, this really is pseudoscience.
A balance factor of 72% is putting a BIGGER load on the bearings than a BF of 63% or 58% or 52% that a Commando roadster uses. (I thought it was quoted as 52% ?).
Since more out-of-balance weight is 'added' to the flywheel to get it up to that 72%.

That means cranks would bend more at higher BF - but since when does that cause more crank problems, or "the big bang" ???
The CombatS problems were a variety of maladies, coupled with some under-specified bearings.

Find an engineering 101 textbook and have a read...


acotrel said:
Let us put this rubbish to rest. The crankshaft in my Seeley 850 is balanced at 72%, I believe the standard commando factor is 58%. To get from 58% to 72% I fitted a threaded steel plug in the hole in the commando flywheel. The plug was about 26mm dia by 40mm long (about 0.5 Kg). My Seeley has a rigidly mounted motor and does not vibrate much at 7000 RPM, however at idle the bike rocks backwards and forwards. If my bike had isolastics they would have to cope with the weight difference at idle, just as the isolastics in a standard commando have to cope with such a weight difference at 7000 RPM if it the 58% factor is used. The isolastics are a cheat used to solve a problem. If they were not there, the effect of the weight difference would be transmitted through the bearings and cases, and meet a fixed obstacle. However the standard commando crank if revved to 7000 RPM must still bend more due to the difference from the non-vibratory 72% factor of a crank in a rigidly mounted motor. So if the rubbers in the isolastics are too rigid, the cases and bearings must cop a beating while the crank is flexing unduly. I suggest that if you use a standard balance factor crank with the correct density rubber in the isolastics, you have the same result at about 3,000 RPM, as you have with a rigidly mounted motor with the 72 % balance factor at 7,000 RPM. However when you rev the standard 58% balance factor commando to 7,000 RPM it must vibrate , and while it is doing it the crank holding bolts are at full stretch, the crank bending, and the superblend bearings are coping with the barrel rollers not centralized. If you balance the crank to 72% in a standard commando, it would shake badly at idle, however at 7,000 RPM the crank would not be bent because of the weight difference, and the loads on the bolts and journals would be less.
Surely somebody on this forum must have tried to dynamically balance a commando crank at high speed, and shone a strobe light on it ? I'm sure if you spun a standard crank at 7,000 RPM, you would see the flywheel grow. I believe the major limiting factor to revving my 850 is the piston weight. Force equals mass times acceleration, and the accelerations in relation to the revs are squared relationships. Jim Schmidt's long rods and light pistons are a good answer to getting a higher rev limit, The longer rods give lower acceleration rates at the top and bottom of the stroke, and the inertia of the pistons which loads the rods is less.
Click to expand...
 
Rohan scan Crankshaft Porn again to see how many take mass off the center flywheel and use heavy metal [depleted uranium less dense or costly] slugs in the cheeks closer to bearing support.

Here's a visual that sort of does and does not apply to our cranks, expect our cranks can turn way faster...
http://www.youtube.com/watch?v=k4BB7967z3w
 
Steve there are 1000 ways to skin a cat in this game.
And 1000 different thoughts on which is better.
But the actual balance factor makes bugger all difference to the reliablity. ?

If I've got 2 identical engines, but one has 52 % BF and one has 72 % BF, which is 'stronger' ??
Thats what this discussion boils down to...

BTW, thats about 4 or 5 oz the difference, in a flywheel mass of something like 25 pounds combined...
 
Matchless said:
I too am sick & tired of hearing this tosh about barrel shaped rollers. I've measured the earlier Ransome & Marles 'Superblends' & also the FAG type. The rollers are parallel with a slightly larger end rad. to stop them digging in to the outer race. The barrel shape rollers are single & twin row self aligning bearings. A different thing altogether & used to accept misalignment between bearing bores / housings.

Martyn.
Click to expand...
gripper said:
While we're at this "Let's put this rubbish to rest" malarkey, Superblend bearings? Barrel shaped rollers? I don't think so
Click to expand...

the subject of barrel shaped rollers has indeed been done to death here. all advice regarding "Superblends" on this post are absolutly correct. there is no "Superblend" made especially. thats why they are called CYLINDRICAL roller bearings. . there are also barrel roller bearings. they also dont take as much speed and are not suitable.

All reputable bearing manufacturers (FAG, SKF, NTN etc), have the modified end radius to prevent roller edge loading when misaligned. so you dont even have to mention the word superblend when ordering. it is a given. For all sizes of CYLINDRICAL ROLLER BEARINGS]. the thing that matters is the "E" suffix. this means maximum capacity . . also the brass cage but the steel cage will suffice quite well for a street bike not flogged at high RPM. NJ306 E M1 (plus clearence suffix), is all the bearing people want to know . They will not recognise the term superblend.also NF306E M1 is very suitable.

I like the NF style because I can get the inner race off very easily with no shaft damage.

The internal clearence I will not go into here. 2 main choices, Normal or C3, this has also been done to death.

So endeth the sermon again, and again and again. RIP Superblend.
best wishes Bradley
 
Ok sure Rohan I'll bite, up to some threshold of engine rpm BF don't make a rat's ass on reliability for most of us, but some racers cracking cases and poor design BSA crank bearings mileage might matter engine wise.

It made economic sense as well as some mechanical relief for Norton mid 50's BF as less material needed in cranks and maybe cases smaller w/o spending on metal slugs. Main reason is frame and pilot protection but isolastics are forgiving to a range of BF so my fascination is 1st discovering that nothing hooks up THE Gravel climbs like my Commandos where everything can over power tires trying to go faster till ain't, Commando out does my 4wd mudder ice spiked tires 411 lock up differentials all breaking loose too dam soon. PU can hit 65 barely my Trixie '72 and Peel could top 75 only backing off d/t sharp turn in time. So how can that be? I also noticed Peel held power in lean better than anything else too. Only thing I can think of is the pair of iso's are taking up the thrust spikes in a front/rear direction. Peel will start near 60 BF then will try 77, then 98, and maybe 110 by heavy metal screw plugs.

Superduper blend unique claims wree dispelled decades ago. My two and a whole bunch of long hard run Combats still had like pristine non superdupers when opened 10's 1000's miles use and could be run more.
 
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