Crank Balancing

comnoz said:

With a single or 360 degree twin shaking forces are at their minimum somewhere between 50 and 65% balance factor -depending on rod angles and other geometry. It has nothing to do with RPM.

Shaking forces are at there maximum at 0 or 100 percent balance factors.

Determining the best balance factor with iso mounts has nothing in common to finding the best factor for a solid mount motor.

Least shaking forces are always going to be the desire with a isolastic mount. So with a Commando that is going to be 53 to 56%.

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So you are telling me that changing the balance factor does not change the revs at which the vibes occur ? When the crank is spinning at idle speed, if the bike does not rock backwards and forwards, it will usually vibrate like hell at 7000 RPM. Isolastics are simply a quick-fix. As was the hole which was bored into the flywheel of the Atlas crank when it was used in the Commando. Isolastics don't change what is happening inside the motor, if the balance factor is unsuitable for the revs at which the crank is being used. They simply provide a soft landing for the whole motor and gearbox.

Alan
Just because the vibrations are different in amplitude at different revs does not mean a different balance factor is appropriate.
Balance factor, I believe, is not dark magic associated with frequency - is is simply a trade-off of reduced vertical (well, 15 degrees off-vertical for commandos) for an increase in fore-aft vibes.
I believe Jim has given sage advice in his post #39 above.
Do you have anything concrete - not yours or someone elses' feeling or opinion - that scientifically supports different balance factors for different rev ranges?
Cheers
Rob

So without storing up a hornet's nest... what steps can I take to make sure all components.. rods caps, bolts shells, pistons, wrist pins, rings, nuts & circlips are equally balanced in weight..?

You can weight each bit and when possible mix and match to get closer to same weight on left and right. I wouldnt
swap rod caps but I have no proof on that. Get a good scale. You might be surprise how each piece is a bit different
in weight. I think there is a few vids on You Tube of this.

swapping caps is out of the question as there a machined pair

I would not swap anything. Weigh each piston assembly (complete). Weigh each assembled rod big end. Weigh each assembled rod small end. There are other threads on this forum and on-line articles on how you go about weighing the rod ends.

acotrel said:

So you are telling me that changing the balance factor does not change the revs at which the vibes occur ?

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That is what I am saying -when using isolastic mounts.

The vibrations are always there. Changing the balance factor just changes the direction of the shake.

Raising the RPM always increases the amplitude of the vibration.

If the balance factor is correct, the amplitude of the shake decreases as the revs reach near the top of the operating range. With isolastics the direction of the shake can change but with a rigidly mounted motor, it cannot. So raising the balance factor in a road Commando would probably be a pointless exercise. However - have you actually tried it ?

There is another aspect which has not been discussed. When you are on the limit with a race bike, the difference between crashing and riding perfectly relies on the connection between your brain and the rear tyre contact patch. Losing the front is a rarity. I've had discussions with my A-grade friend about the difference between Manx Nortons and twin cylinder bikes of the same capacity. His contention is that the Manx is better because of the way the rear tyre behaves between power strokes - the power delivery of the twin is too smooth. What effect do you think the isolastics and the low balance factor have on handling ?

acotrel said:

. However - have you actually tried it ?

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Yes, I have. From 40% to 90 %

There is obviously something I don't understand about the isolastics. The motion of the engine cradle is relative to the rest of the bike and the road beneath it. When it moves energy is transmitted to the rear tyre contact patch through the transmission. So what you are getting must be considerably different to what happens with a rigidly mounted motor. When you tried moving the balance factor, did the 52% factor turn out to be the optimum for high speed work ? There is something about this which ties in with rubber-mounting of Yamaha two-stroke motors to stop frame-cracking. They never seemed to be as fast as when unmodified. Perhaps the lack of vibes gave the impression that the bike was slower ?

I actually could not feel any difference from about 45 to 65 %. Above 65% the footpegs vibrated more at highway speed.

Below 45% the shake at 2000 became uncomfortable but the engine still smoothed out at higher speed.

And yes, vibration does make a bike feel faster.

PS, when I went to 80% on my racebike the cases failed sooner -although I only tried it once so I can't say that was the only reason.

Ok, my 2c here.
My bike is a slimline Featherbed with a PW3 850 Mk2a engine. The crank had been cheese-holes balanced to a higher factor a long time ago, quite possibly at Dunstall as there were various Dunstall bits (including cams) on the bike when I bought it. The fastest speed I GPS-clocked this bike is 213km/h -130Mph on the Dijon circuit straight. I have 18" wheels and 22 teeth sprocket which computes to about 7800rpm. Hard to tell precisely as my tacho is not electronic and wobbles madly over 6500. The vibrations were perfectly acceptable, and certainly not of an unpleasant footpeg/clipons buzzing nature.
I don't know the exact balance factor of my crank, but obviously the work has been professionally done and I suppose it to be around 72%. This bike is also one of my road bikes and I made over 20000km with it in the last 5 years.
The only times I felt unpleasant vibrations on it were:

- when my carbs became out of sync because of a loose throttle cable adjuster. The vibrations were horrendous above 5000 and nasty from 4000.

- when a couple of engine mounting bolts became loose over time. That felt like the entire bike and rider were about to disintegrate in green peas sized bits.

- when my rear wheel became misaligned, but those vibrations were kind of smoother and not as bad.

In normal fast use, ie. up to 7000rpm, this bike is remarkably smooth for a big twin.
Its suspensions probably help: Manx dampers on the front and Fournales oleopneumatic units on the back. The improvement in out-of-corners traction when I installed those over regular Hagon units was 'Impressive'. They also weigh less than 2lbs each so the unsprung weight in the back is lessened by a good amount.

The second experience relates to Yves wonderful 920cc Seeley with Maney/JS engine. Yves recently asked me to redline it at 7000rpm which I duly did on 2nd, 3rd and 4th gear of his silky 5 speed TTI box. The entire bike feels like a 'Prophet Class' flying carpet, as smooth as my first lover' skin at all speeds.

My point in telling this? Do a thorough check of your entire bike for misalignments or other gremlins before thinking the crank factor is the (only) cause of your unpleasant riding massage.

You could also exchange it for a Honda Four I suppose.

Jagbruno said:

My point in telling this? Do a thorough check of your entire bike for misalignments or other gremlins before thinking the crank factor is the (only) cause of your unpleasant riding massage.

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Jagbruno - I agree with you completely
The engine, including it's balance factor, is only the force creating the vibration - not the way vibration feels to the rider.
The way the the motorcycle responds to this force (buzzing, etc) is down to how well you have set the rest of the bike up.
Using reamed, "fitted" bolts and studs on anything structural is a good start but tightness and integrity on all other fixings is vitally important in controlling vibration.
I believe too many riders blame "balance factor" when the fault really lay with how well the rest of their chassis is set up - or not!
Cheers
Rob

I agree with that. My Mk3 Seeley frame has a curved bar in front of the motor instead of the usual ladder. The curve is there so I can get the head off the motor while it is still in the frame. It also allows the front of the frame to open slightly when the motor is vibrating. If it was inelastic, it would probably break. All bolts are a close fit.