Strange Frame (2)

olChris · Mar 12, 2014

So here we are a few months on and its still "work in progress" here.. (some of you may remember chapter 1)

My question could have been answered a thousand times before, but i missed it..... Balance factor.... I am currently in a "unknown frontier" with the 750/1972 crank in 3 pieces on the bench. Yes i got several reference sources, but my comprehension seems inhibited...
The general concensus for a plodders engine would be 65-66% from my research..

Does 2/3rds (66%) represent to sum of reciprocating parts, eg piston, rings, rod, gudgeon,clips, shells, all the stuff hanging off the crank.

Which means to me that the crank counterweight is 33% heavier than the recprting parts ???? Or have i got it wrong.
My trailer case has new pistons and bearings etc from the PO and i would like to determine what sort of balance factor i have "inherited". If its way out i can arrange for it to be rectified... As you are all aware when you take it to the "right people" it is immediately $500 whether it +/-. and you get what you get...

JimC · Mar 12, 2014

You might want to check with Raber's on the price of balancing. It was about 15 years ago, but IIRC it was around $100. They actually sent the work out to Kaeding Performance. I never knew the factor, but any vibration above ~2000 RPM was gone.

laurentdom · Mar 12, 2014

Hi Chris,

Have a look at Jim Schmidt's (jsmotorsport.com) web site / Technical / Balance Information.

Quite interesting!

Using his conrods + pistons + camshaft (all in standard specs) in my 750 Commando engine in a Featherbed chassis (i.e. without isolastics) made it far less vibrating than a 500 cc Dominator engine.

Laurent

dynodave · Mar 12, 2014

"Does 2/3rds (66%) represent to sum of reciprocating parts, eg piston, rings, rod, gudgeon,clips, shells, all the stuff hanging off the crank."

Almost correct...you only use the small end weight of the rod.

To be more correct... the engines "imbalance factor" what ever % , is chosen to attempt to control the vectors of thrust into the frame due to its ability to holistically "resonate" at different frequencies.
1. It's the frame that resonates not the engine. The engine vibrates as in it imparts thrust without actually resonating.
2. The engines imbalance is calculated, calibrated and set. In the old days the results were judged empirically.
When I can afford to buy ($3K) a FFT analysis vibration meter, I would like to do some testing of featherbed resonance
The nuance of terminology is quite important here or no meaningful discussion will happen.

acotrel · Mar 12, 2014

If you tap a thread in the hole in the opposite side of the flywheel to the journals and fit a steel plug with loctite, and punch the ends of the thread, you should find the balance factor comes out to be around 70%. With a rigidly mounted motor, the bike will shake while idling, however with a few revs it will be extremely smooth. I would avoid drilling the flywheel if possible. You need a set of machined straight edges and a level - read 'Tuning for Speed' , it is not difficult. All I did the last time was make a steel frame, grabbed one end in the vise with the other end supported with a threaded steel rod so I could adjust the level of the ground straight edges.
I suggest the balance at low revs is not important, if it is out of balance at high revs the bottom end cops a beating and a race bike usually runs at higher revs than a road bike.

lcrken · Mar 12, 2014

If you wade through this thread, you'll find all sorts of info and mis-info about Commando balance factors

commando-crankshaft-porn-t8365.html

Ken

Hobot · Mar 12, 2014

The down and dirty is likely the 750-'72 crank and parts are already close to factory best load averaging 52=54% dry BF and as the isolastics are very forgiving on BF just send to dynamic balancer, of which there are a handful out there and they can tell ya what BF to aim at or find it ended up with while turning as smooth as can be. The only builders that have to worry a wart on BF are rigid mounted engines and some experimenters like Jim Schmidt and me seeking a more circular orbital for some extra smooth sense rigid or rubber mounted. If you put in JMS pistons/rods then BF would rise at least into upper 80's [standard BF for rigids] so crank drilling required to bring back near the optimal load avergaging BF and likely a bit sooner rpm isolation onset. Main decision is how many extra 100's $pent to land between factory 750-850 range or live a bit on either side, undetectablly but nawing and nawing on ya trying to just ride around in peace what with all of us harping on ya getting it just right.

Fast Eddie · Mar 12, 2014

olChris said:
So here we are a few months on and its still "work in progress" here.. (some of you may remember chapter 1)

My question could have been answered a thousand times before, but i missed it..... Balance factor.... I am currently in a "unknown frontier" with the 750/1972 crank in 3 pieces on the bench. Yes i got several reference sources, but my comprehension seems inhibited...
The general concensus for a plodders engine would be 65-66% from my research..

Does 2/3rds (66%) represent to sum of reciprocating parts, eg piston, rings, rod, gudgeon,clips, shells, all the stuff hanging off the crank.

Which means to me that the crank counterweight is 33% heavier than the recprting parts ???? Or have i got it wrong.
My trailer case has new pistons and bearings etc from the PO and i would like to determine what sort of balance factor i have "inherited". If its way out i can arrange for it to be rectified... As you are all aware when you take it to the "right people" it is immediately $500 whether it +/-. and you get what you get...

Balance factor is a big blah, blah, area! Seems like the actual factor chosen is of less importance than ensuring the crank assy is properly dynamically balanced. My own 850 crank was miles out!

Whatever... frame looks well cool in silver Chris!!

Hobot · Mar 12, 2014

Ok with Fastie's guick extra note, you crank just needs standard dynamic balancing for factroy supplied horizontal imbalance but the BF can't be off the normal range if normal parts involved. BF can be set w/o dynamic balancing, just static. No one whoose spent for dymanic balancing has complainted it wasn't worth it, but neither me nor my buddy has bothered and we don't feel bothered riding going the shade tree down and dirty route.

Rohan · Mar 12, 2014

olChris said:
Which means to me that the crank counterweight is 33% heavier than the recprting parts ???? Or have i got it wrong.
.

Yes, you have it around the wrong way. (putting it politely).
A 66% BF (Balance Factor) means the counterweight is 66% of the reciprocating parts.
Maybe the replies above are too polite ?

BTW, the lower part of the conrods is considered to be rotating, and the upper part of the rods is counted as reciprocating parts.
If you don't like the balance factor so calculated, just include more/less of the rods ??!!

BTW2, the std BF for an isolastic Commando is usually quoted as 52%. (?).
It would be wise to stick to that, and let the isolastics do their work ??

olChris · Mar 12, 2014

Rohan said:
olChris said:

Which means to me that the crank counterweight is 33% heavier than the recprting parts ???? Or have i got it wrong.
.

Click to expand...

Yes, you have it around the wrong way. (putting it politely).
A 66% BF (Balance Factor) means the counterweight is 66% of the reciprocating parts.
Maybe the replies above are too polite ?

BTW, the lower part of the conrods is considered to be rotating, and the upper part of the rods is counted as reciprocating parts.
If you don't like the balance factor so calculated, just include more/less of the rods ??!!

BTW2, the std BF for an isolastic Commando is usually quoted as 52%. (?).
It would be wise to stick to that, and let the isolastics do their work ??

:idea: :idea: :idea: Thanks Rohan, for clearing up my confusion... I will go through the motions of a static balance (Tuning for Speed) as an exercise.. From the few comments above it seems to me that the BF can be variable from 52% - 66%. So if i find my crank within that range i might just leave it unmolestered..

olChris · Mar 13, 2014

Took a while, but im now aware that "reciprocating" and "rotating" parts are two different animals in the B/Factor theory...

I learnt something today....

Hobot · Mar 13, 2014

The basic take home is a 360' crank single with two pistons is gonna vibrate no matter what, so BF just determines weather if knocks ya more up/dn or more fro & aft. Lower BF makes orbits more oval vertically while higher BF its more horizontal. BSA did research that showed least forces on our type crank engine bearings occurs in the mid 50's dry BF. Isolastics can take up a wide range of BF but not very much side to side imbalance so the importance of getting each side's piston/rods close as practical , under a gram off, usually .5 gram and dynamic balancing from there.

acotrel · Mar 13, 2014

The design of the isolastics is nothing like a conventional race bike with a rigidly mounted motor. In the early 70s the market competitors were smooth running Japanese bikes. The isolastics made a silk purse out of a sow's ear. Some of us have tried rubber mounting the motors un Japanese racing two strokes - stops the frame from cracking, however the bike always feels slower. Try it !

olChris · Mar 17, 2014

Whilst waiting for parts to arrive i have been contemplating "wheel alignment" after assembly with tyres and mechanicals are in situ...
Due to my inexperience with m/cycles i wonder how people align their rear wheel with the C/Line of the bike after the puncture, chain replacement or any other adjustment has been completed...
When relocating rear wheel after its removal it can certainly be replaced "by eye" or markings on the swingarm, but how do you know if its in line with the front wheel..
Its easy to do with a skeleton and true wheels as the above photo, but once i fit tyres and mechanicals the ability to align with "straight edges" is impossible...

Or am i "overthinking" it ???

pete.v · Mar 17, 2014

olChris said:
Whilst waiting for parts to arrive i have been contemplating "wheel alignment" after assembly with tyres and mechanicals are in situ...
Due to my inexperience with m/cycles i wonder how people align their rear wheel with the C/Line of the bike after the puncture, chain replacement or any other adjustment has been completed...
When relocating rear wheel after its removal it can certainly be replaced "by eye" or markings on the swingarm, but how do you know if its in line with the front wheel..
Its easy to do with a skeleton and true wheels as the above photo, but once i fit tyres and mechanicals the ability to align with "straight edges" is impossible...

Or am i "overthinking" it ???

Assumoing all wheel offsets are proper, yours probably being centered or close to it, I have a 6' piece of 2" by 2" aluminum angle stock. With the bike on the sidestand, because the centerstand is in the way, it is easy to hole it up to the front wheel and visually see a gap at the rear wheel and adjust so to touch on all points.

This is, of cousre, if your front and rear wheels are the same size or else you will have to compensate for the difference (a bit more of a hassle but do-able).

acotrel · Mar 17, 2014

I usually run a string line around the back of the rear tyre, under the bike, then around the front of the front tyre and back to the rear. If you capture a matchbox on both sides at the rear of the rear wheel rim you can check the clearance at the front of the rear tyre, and the rear of the front tyre. Once you have got the wheels in line, if you tighten the rear chain adjusters, then whenever you have to adjust the rear chain all you have to do is count the number of turns on each side as you pull the wheel back. That is unless your bike is like my Seeley. The guy who had the frame years ago obviously stripped one of the adjusters from the aluminium round which fits up the swing arm, and replaced it with the wrong thread. So a few days ago I bought a piece of 3/8 UNC allthread so I can make both sides the same. Unfortunately the correct tap costs $30, so I haven't fixed it yet. And I cannot figure what else I could use that tap for.

olChris · Mar 22, 2014

I was just reading elsewhere about short and long instrument pods... I inherited 2 long pods (MK3 type) in my initial baskets of goodies and have since sourced "short" original instruments which will be used... To save the messing around fitting short gauges to long pods i will source 2 short pods eventually and sell long pods..
The question is, what "protects" the rear of the instruments when fitted to short pods ???... Is it left open to the elements or is there some sort of cover??

L.A.B. · Mar 22, 2014

olChris said:
The question is, what "protects" the rear of the instruments when fitted to short pods ???...

Nothing, they were, as you say, left open to the elements.

jseng1 · Mar 25, 2014

laurentdom said:
Hi Chris,

Have a look at Jim Schmidt's (jsmotorsport.com) web site / Technical / Balance Information.

Quite interesting!

Using his conrods + pistons + camshaft (all in standard specs) in my 750 Commando engine in a Featherbed chassis (i.e. without isolastics) made it far less vibrating than a 500 cc Dominator engine.

Laurent

I also ride a featherbed and I no longer think about vibration. The big advantage besides comfort is the reduced stress on the crank & cases - no more cracks. I've seen enough cracked cranks and cases (less often than cranks but still a problem) when I was balancing cranks to know that its a serious concern for a Norton and those parts can be very expensive. The best way to reduce that stress and vibration is lighter pistons - its the most important upgrade.

Strange Frame (2)

olChris

JimC

laurentdom

dynodave

acotrel

lcrken

Hobot

Fast Eddie

Hobot

Rohan

olChris

olChris

Hobot

acotrel

olChris

pete.v

acotrel

olChris

L.A.B.

Moderator

jseng1

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