650 Norton Vibration and Balance Factor

skipsoldbikes said:

To repeat a post

I'd be very worried about putting quarter shim(s) under corner(s) of the end housing. It will increase the end float of the armature. That will increase the radial play of the CB end of the armature. That will make the firing points unpredictable. Even if quarter shim(s) are taken out from under the opposite corner(s) of the end housing, the poor old bearing won't be running true. And I can't see how any of this can cure the problem, unless it's some second-order effect.

The reason for uneven firing intervals in a K2F is if the cam ring is not symmetrical about the axis of rotation of the armature. That can be caused by:-
- - - - - - - - -
the end housing being incorrectly machined, but in my experience that is rare
- - - - - - - - -
the armature axis being off-centre from the cam axis, because the bearing insulator isn't holding the outer race central in its recess.

In my experience, the last reason is usually the one (unless the cam ring is loose).

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You've given 2 causes,where the cam housing and bearing aren't square to the armature axis.One is the most common cause of timing error,in your experience.I don't dispute those findings.
It's always worth fitting another insulator (your most common cause),to see if things improve.

Surely it would then be better to shim the housing to get the bearing and housing square to the armature axis.It was meant to be square,but often isn't.
If a 0.002" shim was used on one side,a full 0.001" of shimming could be taken out to maintain bearing clearance.Even 0.002" or 0.003" of shim on one side can make an enormous difference to timings.

The other causes of timing error you listed are the more obvious ones,and the first to be dealt with.After all these more obvious faults are fixed,people still have timing error.Getting the bearing and housing square to the armature axis can fix it,and won't cause any other problem that I know of.The cumulative error on all these parts is quite likely to be 0.002" or more,and probably was when they were new.

skipsoldbikes said:

To repeat a post on this thread from Feb. 14th:

Re: 650 Norton Vibration and Balance Factor
by KenF » Fri Feb 15, 2013 4:19 am

I'd be very worried about putting quarter shim(s) under corner(s) of the end housing. It will increase the end float of the armature. That will increase the radial play of the CB end of the armature. That will make the firing points unpredictable. Even if quarter shim(s) are taken out from under the opposite corner(s) of the end housing, the poor old bearing won't be running true. And I can't see how any of this can cure the problem, unless it's some second-order effect.

The reason for uneven firing intervals in a K2F is if the cam ring is not symmetrical about the axis of rotation of the armature. That can be caused by:-
the cam ring not itself being symmetrical, but in my experience that is rare, even with a worn cam ring, unless somebody has stoned it
the cam ring being loose
the end housing being incorrectly machined, but in my experience that is rare
the armature axis not being fixed, either because the end float is incorrect so that there's radial play between the inner race and the outer race, or because the insulator is shot so that the outer race can move in the end housing
the armature axis being off-centre from the cam axis, because the bearing insulator isn't holding the outer race central in its recess.
In my experience, the last reason is usually the one (unless the cam ring is loose).
Ken
Posts: 3
Joined: Wed Feb 13, 2013 8:15 am
Skip Brolund

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And did you ever get hold of a brand new Lucas cam ring – not a pattern- as I did in the late 1960s and check the timing of the two cams on a K2F cam ring onto a milling machine dividing head to see how accurate they where :?:
I did, and I can tell you it was 41/2 degrees out –and so, the best of British engineering :!: :shock:

Here is an update on the vibration issue that I had with my 1966 650SS. To summarize the original post, I had rebuilt the engine and used .060 over JP pistons and the resulting engine seemed to vibrate quite a bit more than I had remembered (although my other two machines, a 1985 BMW R80, and a 1992 Ducati 900SS, may have spoiled me with their smoothness). I had difficulty with the left ring/cylinder seating and after about 150 miles I had a partial seizure. Thinking it was a good excuse to get rid of what I thought were the heavy JP pistons I pulled the engine down for a light hone and top end job. I sourced some 1960's era 060 over Hepolites pistons via Ebay. Advertised as 650 pistons, they turned out to be 99 pistons with the longer skirts. Following the advice of listmember Benjamin Gradler I trimmed the skirts of the 99 pistons to match my existing 650 pistons to insure flywheel clearance. Here was the shocker, the JP piston and pin weighed 370 gms, and the Hepolite 99 piston and pin weighed 307 gms! What a difference! The 99 piston is said to give another point in CR when used in a 650 so to play it safe I added a .020 copper base gasket under the cylinders to knock the CR back to a theoretical 9.5:1. I was worried about the balance factor as it had been balanced at 82% with the heavy JP pistons so who knows what the result would be with these lighter pistons...maybe 85 to 90%? I decided to leave the lower end alone and give it a try. The rings seated perfectly from the first kick. This time I only used a spray of WD-40 on the pistons/rings/cylinder. The vibration level is significantly (almost drastically) reduced! I am still in the running in phase but so far I observe that there is a slight rough patch at 48-50 mph (stock gearing) then glass smooth at 55 to 60 mph. I have not ventured much pass that but I am extremely pleased with the result. In addition, the engine is quite and responsive, and so far appears to be the sweetest Norton engine that I have owned in the last 35 years. Lesson for me is that light pistons are very important in these motors!

raleypc said:

Lesson for me is that light pistons are very important in these motors!

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The trouble is that you have altered both the piston weights AND the balance factor,
so we don't know for sure which is the more important.
Although you'd have to think that heavier pistons can't be good.
It is interesting that the higher BF is still smooth though. ?

Been contemplating pistons for an early 500cc dommie.
Stock pistons are 285 gm, and all the replacements are nothing like that. (How did Hepolite keep their pistons so light ?)
Although early dommies are so heavy (mit plunger frame) that vibes don't even seem an issue....

I just back calculated a balance factor making some assumptions on small end and big end weights (from Jim Schmidts old Race Manual) and it looks like if I started with 82% with the old pistons I am now up to 95% with the new pistons! Not too much data for this configuration.

Reading Jim Schmidts' current JS Motorsport webpage he states that even if the flywheel counterweight is not changed, " lightweight pistons will always reduce the vibration since the higher balance factor reduces the up and down shaking of the pistons (reciprocating weight), but the fore and aft shaking (rotating weight) stays the same..." This was stated relative to a Commando engine but is there a reason that it would not apply to the featherbed?

If it was that simple, motorcycle engine makers over the past +century+ or more would have chased lighter and lighter pistons ?

While this has been the general trend, only one carried it to extremes (?).
Indian in the early 1930s had quite light flywheels and pistons. (vtwin)
Dismal failure - they recalled them and fitted heavier.

Torque delivery and driveability improves with weight, to an extent ?).

It would be nice to know what the builder of Jay Leno's 650 SS did to make it so smooth, but we can't ask him, he is deceased.
It strikes me that although Leno says it is very smooth, and I believe it is, he might not be running much above 65 MPH. At 65 MPH my 650Ss feels great. At 70, not so great. Perhaps the answer is to stick to old roads and moderate speeds with this bike, rather than spend a fortune trying to make it smooth at a little higher rpm. A little extra vibration when running up to 80 or so to pass is quite tolerable for a short blast, then back down to 65.

One other low cost idea is to replace the seat squab. It seems like it might be original and is compressed to rock hard in the rider area. Most of the vibes are coming thru the seat, the bars are quite still at highway speed. I noticed today/that when I move toward the back of the seat, which has thicker foam and also has not been compressed, the seat vibes disappear. The later 650ss models (mine is a 68) had a tapered seat, thick at the rear and about an inch thinner at the front. I might forego originality and replace it with an earlier seat type if the foam thickness is greater under the rider
Glen

650SS said:

Hmmm...I would like to hear how you can correct the magneto to 180 deg firing without reprofiling the cam ring. It my simplistic view there are two "lobes" on the cam ring, one for each cylinder. What am I missing?

Thanks for your thoughts!

Paul R.

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Rather than stone the cam profiles, it is preferable to "shift" the cam ring in its housing to achieve even firing on both cylinders. This can be done by sanding one side of the cam ring and shimming the other side with cellophane tape. It does not take much (maybe .002 in) to correct 8-10 degrees of mismatch. A degree wheel is mandatory....the spoke in the hole is too crude. Also, remove the center screw mounting the points plate, then one can "buzz" the points to determine when points break.

Slick

Rohan said:

If it was that simple, motorcycle engine makers over the past +century+ or more would have chased lighter and lighter pistons ?

While this has been the general trend, only one carried it to extremes (?).
Indian in the early 1930s had quite light flywheels and pistons. (vtwin)
Dismal failure - they recalled them and fitted heavier.

Torque delivery and driveability improves with weight, to an extent ?).

Click to expand...

Fitting lighter pistons to a 650cc Triumph twin adds wings to the bike. Every time the pistons reach the ends of their stroke, they have to reverse, and the forces they exert are a squared relationship with the accelerations. Yes, motorcycle manufacturers have always chased lighter pistons. It is one of the major advantages of having 4 valves per cylinder, - you don't have to push that heavy domed piston crown around to get high compression ratios. All you need is enough mass in the crown to distribute the heat, and thus stop a collapse.
And I repeat - balancing the motor so that it is smooth at high revs is more to stop self-destruction, than it is to give rider comfort.
Torque delivery and driveability has much more to do with crankshaft mass than the weight of the pistons. If the pistons are lighter, you don't have to drill so many holes in the flywheel to get an acceptable balance factor. In a rigidly mounted four stroke motor anything over 70% is usually great at near peak revs.
Isolastic engine mounts don't change what the crankshaft is doing to the bearings and cases, they just give less resistance to the vibes. THAT is about rider comfort.

acotrel said:

And I repeat - balancing the motor so that it is smooth at high revs is more to stop self-destruction, than it is to give rider comfort.

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Yes, you keep repeating this.
But can't quote a reason, or even a single source, to back it up...

It has previously been discussed that a balance factor of ~53% gives the least average loading on the main bearings.
So why aren't all 360 degree parallel twins balanced to this then - and by extension all single cylinders too,
since they are essentially the same config.

The reason that 53% isn't universally adopted of course is that not all chassis are happy with that BF - ie they vibrate, in resonance.
So, the makers rebalance the motor to a different BF, to stop these resonances, to make it comfortable FOR THE RIDER.
Comprendez vous ??

And as for the lightweight pistons and cranks.

There is a big difference between slightly lightening pistons/cranks for racing purposes,
and all out lightening them. ??

MotoGP bikes are probably the current pinnacle of motorcycle engine design,
and you could imagine them being the absolute lightweight in internal design.
Multicylinder design makes this easier, of course.
But for say a Commando for road use, that may not be so usefull.

I've ridden both a Guzzi and a trailbike with the flywheel near completely removed, and while they were fun to play with - on a dry day in good conditions - they revved like a MX 2 stroke - for common road use they would have been lethal in poor conditions or any lapse in concentation.

And then there is the question of longevity of use....

??

If you are giving a bike a belting, it will last a lot longer with light pistons - less likely to pop the end off a rod. Bikes with ultra light cranks can be nasty pieces of garbage to ride. The light crank can give very quick response however require that you ride 110% every second of the day. With a heavy crank and the right gearing and steering geometry everything becomes smooth and calculated.
With my commando engine bike I never have anxiety about riding it fast. With my old short stroke Triumph the anxiety was terrible before every race meeting and nerve-wracking during it.
I'm not kidding when I say that I crashed at my first 5 race meetings, and once 4 times in one day - one time at extremely high speed. ( I blamed myself, and kept getting back onto the bike. ) I finally got in control of it, however it was never pleasant.

I continue to be pleased with how smooth my 650SS is with what I have calculated to be a 95% balance factor. I had installed the lighter pistons thinking that I would likely be disappointed with the result and need to tear down and rebalance the lower end over the winter... but it runs great as is! Maybe the featherbed frame is more sensitive to the vertical vibration than the fore and aft vibration which a high BF would produce? The machine is qualitatiively smoother than the mostly original Atlas I owned (no suprise!), the Dunstall Atlas (balanced at 82% by Leo Goff and very nice), and the previous builds of this particular 650 (first at 70% by vintage racer friend, then at 82% with heavy JP pistons). So Rohan is right in that since piston weight and BF changed I can't really draw a conclusion....but it sure runs great! The machine is stock with correct Norton flat bars except that I filled them with lead shot and mounted bar end mirrors in an attempt to tame the vibes with the previous heavy pistons. I have about 400 miles on the new pistons so cruising speed has been kept to 55 mph with brief excursion up to 70 mph (4100 rpm) so I can't really comment on high RPM vibs at this point. I am using Nicholsons recommended break-in scheme for 500 - 750cc machines where he recommends 50 mph cruise for the first 200 miles , adding 5 mph at each 200 mile increment up to 1000 miles.

How was it at 4100 rpm? Mine is very buzzy at this rpm.

Glen

I did a seat of the pants vib survey on the drive home today on my 650SS. With stock gearing it is glass smooth up to about 47 mph, then a bit of a resonance between 47 and 49 mph (2800 rpm), then smooths out to just a very minor level of vibration felt through the bars (not quite glass smooth as I previously posted) which remains constant through 70 mph (4100 rpm) up to 80 mph (4700 rpm), which is as fast as I thought I should briefly accelerate to while still in the break-in period. I could start to feel just a slight bit of vibration through the pegs at 70 mph and above. The mirrors are clear and for me, the vibration is really quite minimal for vertical twin. I feel could cruise all day at 65 mph, which is a about as fast as I want to go on an unfaired motorcycle for any length of time. The handlebar resonance point would probably change if I removed the lead shot that I added, but I am very satisfied with the current comfort level as is. While dealing with the previously installed heavy JP pistons, I did have both wheels balanced, verified equal timing for both cylinders, and made sure that the primary chain was adjusted correctly, so I am sure all that helped.

Rohan, How many Manx Nortons do you know about which raced with a 52% balance factor ? Perhaps you should offer your advice on balancing cranks to Andy Molnar or Fred Walmsley ? Why do you think they use large flanged steel sleeves in the cases to house the main bearings ?

acotrel said:

Why do you think they use large flanged steel sleeves in the cases to house the main bearings ?

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Because magnesium (Elektron) cracks easily ?
And the factory cases were cast too thin around the bearing bosses ??
And if the pistons run even a shade dry, the forces are concentrated on the area around the bearings ???

Thus giving quite a history of cases split clean in two through there.
I've seen several sets of cases in post-race teardowns turned to scrap by this, just walking around the pits.

The modern replica cases are often thickened up to counter this.
Lordy only knows why the factory didn't ever see fit to do this.
And are also done in RZ, rather than Elektron, to toughen them up ?
Plenty of spares on the shelves maybe ??

As has also been mentioned on this forum,
REPEATEDLY,
the engine balance factor selected has more to do with not allowing the frame to resonate with the engine vibes.

But maybe we should repeat the advice given to riders of the old longstroke (600cc) manx sidecar racers
"rev it until you start seeing double or triples of everything and change gear,
the tacho won't be visible by then.
If you start seeing bats flying away, they are the cylinder head fins,
and you have buzzed it too hard...."

The required torque characteristic and usable rev range of an old sidecar is considerably different to a solo. If you use the lower balance factor then rev the tits off the motor, of course it is going to vibrate at high revs and crack the frame, and chuck all the attachments off. While the crankcase is doing that with a rigidly mounted motor what do you think it is doing to the cases and bearings ? With isolastics the whole engine/gearbox assembly move in the opposite direction to the load until the rubber is fully compressed on each cycle. So you will get away with a low balance factor. That doesn't mean the bike will be fast. With isolastics and any rubber mounted motor deliver less power will be delivered through the rear tyre. I would not attempt to race a bike which vibrated excessively at the top end of the usable rev range. When my bike is in full flight, I rev it between 5,800 RPM and 7000 RPM continually and it is super smooth. However you would not attempt to ride it slowly. In traffic it would be a horror. Balance factor is not about 'rider comfort', it is about destruction of the whole bike. You choose what you are going to use the bike for, and balance the crank to suit. It is not rocket science. Buy yourself a 650cc Triumph Saint and find out for yourself. It is as useless as tits on a bull.

acotrel said:

Balance factor is not about 'rider comfort', it is about destruction of the whole bike.

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So if a low balance factor is about the bike vibrating more in the up-and-down plane,
(with lower average loadings on the bearings)
and higher factors have it more fore-and-aft = where the rider feels it less

then why would any factory go for the higher balance factors,
as have oft been applied to many a brit twin..

You will have a tough job convincing us that isolastiics absorb large gobs of power.
Or that Commandos are slow as a result !!
Put your money where your mouth is, and quote some figures.
Otherwise we may be tempted to think you just made that up on the spur of the moment,
more FUD.

??