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commando balance factor #64402
01/22/06 2:34 am
01/22/06 2:34 am
Joined: Jan 2006
Posts: 20
michigan
J
jpn33 Offline OP
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jpn33  Offline OP
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J
Joined: Jan 2006
Posts: 20
michigan
I recently bought a 1974 850 from an ex-ahrma guy that had stuck a 750 top end on the 850's bottom end and wondered if the stock balance factor between the two were different. I would think the pistons would have different weights,if only slightly. Glen


OVBSAOC #1654
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Re: commando balance factor #64403
01/22/06 3:55 am
01/22/06 3:55 am
Joined: Apr 2005
Posts: 243
Northern Virginia
V
VA Rebel Offline
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VA Rebel  Offline
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V
Joined: Apr 2005
Posts: 243
Northern Virginia
I've had House of Balance (Baltimore) do a couple of Nortons. The factor they used was a little different between the 750 & 850, but not by much.

Re: commando balance factor #64404
01/23/06 1:17 am
01/23/06 1:17 am
Joined: Aug 2001
Posts: 1,269
Hamilton, Mass. USA
D
Dave Comeau Offline

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Dave Comeau  Offline

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Joined: Aug 2001
Posts: 1,269
Hamilton, Mass. USA
Unfortunately vertical twin engine balancing (actually frames) is one of the most misunderstood concepts going.

data
The 750 pistons range 312-357 grams

The std 850 is around 400 grams

920 pistons go 439+ grams

So 52% of the 750 weight and 54% of the 850 weight don't really coincide much. I'll conviently ignore rod contribution for now.

For a steet norton commando don't waste your money or give it another thought.

"In a featherbed"....I would pay an airplane ticket anywhere in the world, and a week off from work to meet the "PHD" that can explain it to me.
I'll even buy the FFT spectrum analysis vibration meter so he can show me. (about $3000)
Then we'll give it a run a run on my dyno.......and measure it.

Oh yes, flame suit on...I did 7 years working for an industrial instrumentation Co. where I did vibration analysis as part of the instrument installation process. ON one trip I even did an anlysis of an in flight Beoing jet where the pilot through the stewardes asked me to put my instrument away for fear of "upsetting the other passengers". LOL laughing
laughing
dave


dynodave
BSA 3 1961-1963
Ducati 3 1992-2002
Norton many 1951-1975
87 Serv-Equip 100HP MC brake dynamometer,
Re: commando balance factor #64405
01/24/06 11:57 am
01/24/06 11:57 am
Joined: Sep 2002
Posts: 538
belgium
ludwig Offline
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ludwig  Offline
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Posts: 538
belgium
Dyno , what's the point you want to make ?

Re: commando balance factor #64406
01/24/06 3:36 pm
01/24/06 3:36 pm
Joined: Aug 2001
Posts: 1,269
Hamilton, Mass. USA
D
Dave Comeau Offline

Crew Chief
Dave Comeau  Offline

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D
Joined: Aug 2001
Posts: 1,269
Hamilton, Mass. USA
Quote:
Originally posted by ludwig:
Dyno , what's the point you want to make ?
ludwig & Glen
Is there a difference? Then the answer is POSSIBLY NO Possibly yes, why not ask the person you bought the engine from, what were the "balance weights" and %'s.
The (%) information alone that most people declare is inadequate for it to be meaningful and useful. It needs to be a total mass as well as a %.
Obviously 52% of the 750 mass and 54% of the 850 mass are different. Things to check while you're in there, is it a 750 crank in 850 cases? or a 750 top end on a 850 crank. Certainly the 850 cases do help it stay together better if stressed.
371 grams, if a stock 750 crank or 432 grams, if a stock 850 crank or neither, if creatively rebalanced to a "racers" preference confused

Balancing a crank to (x?) weight is a reasonably simple mechanical process,
however if that "installed" (x?) weight representing a % of reciprocating components imbalance is not a good one for the particular frames natural resonance(and there are several), than it vibrates(and they all do).
Right or wrong, commando's isolastics can accomodate a lot of vibration at these resonances. While rigid frames resonate at frequencies that defy easy mathmatical analysis. Imperical measurement with instruments such as vibration analysis instrument with FFT analysis software is maybe the only way to get useful data short of through a riders left and right cheek, hands, feet and observation of the tach.

I thought it was clear enough...over a few grams on a bitza commando engine, my suggesation is don't waste your money rebalancing or give it another thought. drive it.

Then again
Why second guess a "AHRMA" racer? In addition to campaigning my own sports car for 5+ years, winning 70+ trophies and several club championships. I've mechaniced and pit crewed for several bike "racers" for a good part of 10 more years. I've soon learned that being a "racer" doesn't necessarily make them a mechanic.

sorry, Well, on second thought, Yes.... tear it completely down and go over it all, it is likely to blow up any second now. laughing


dynodave
BSA 3 1961-1963
Ducati 3 1992-2002
Norton many 1951-1975
87 Serv-Equip 100HP MC brake dynamometer,
Re: commando balance factor #64407
01/24/06 5:31 pm
01/24/06 5:31 pm
Joined: Sep 2002
Posts: 538
belgium
ludwig Offline
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ludwig  Offline
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Joined: Sep 2002
Posts: 538
belgium
Dyno ,
I realise that arguing about the % of balance is like arguing about the gender of the angels ( do they use this expression in English ? )
As I said in another post about balancing , I have a very good manual that is adamant that the DRY balance factor for a Commando should be 63% , resulting in an actual 52% ( with oil )
Discussions about the balance factor in a featherbed are endless .
Anyway , whatever you do , vibrate it will .
There is also torsional vibration and vibration generated by crank flex .
Not much whe can do here , except a different (solid) crank , with more mass in the cheecks instead of the central flywheel .
But there is another kind of vibration that can be very destructive : If the gravity axis does not coincide with the rotation axis .
( an unbalance at one ( left ) side , compensated by an equal unbalance at 180 deg. on the other ( right ) side .
This fault can only be detected by dynamic balancing .
I hope this makes sense , My technical english is not that good , but I'm shure you know what I mean .

Re: commando balance factor #64408
02/08/06 11:19 am
02/08/06 11:19 am
Joined: Aug 2001
Posts: 1,129
Lucan ON Canada
J
jfligg Offline
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jfligg  Offline
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J
Joined: Aug 2001
Posts: 1,129
Lucan ON Canada
Hi Guys
I am new to Nortons. I have had BSAs for years. I am now wondering is balancing a Norton Commando crank on a new rebuild with larger pistons a waste of money? Thanks Jeff

Re: commando balance factor #64409
02/08/06 11:50 am
02/08/06 11:50 am
Joined: Dec 2004
Posts: 9,375
Scotland
kommando Offline
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kommando  Offline
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Joined: Dec 2004
Posts: 9,375
Scotland
If its going into an Isolastic frame why bother, the low rev shakes you get is more affected by the hardness of the rubber buffers.

Re: commando balance factor #64410
02/08/06 11:59 am
02/08/06 11:59 am
Joined: Aug 2001
Posts: 1,269
Hamilton, Mass. USA
D
Dave Comeau Offline

Crew Chief
Dave Comeau  Offline

Crew Chief
D
Joined: Aug 2001
Posts: 1,269
Hamilton, Mass. USA
Quote:
Originally posted by kommando:
If its going into an Isolastic frame why bother, the low rev shakes you get is more affected by the hardness of the rubber buffers.
clap clap clap
Well at least somebody get's it!
That is why I love the original style iso rubbers and I find even norton andover doesn't offer them any more confused
Now the search is on to buy all the NOS early style iso rubber.


dynodave
BSA 3 1961-1963
Ducati 3 1992-2002
Norton many 1951-1975
87 Serv-Equip 100HP MC brake dynamometer,
Re: commando balance factor #64411
02/08/06 12:07 pm
02/08/06 12:07 pm
Joined: Dec 2004
Posts: 9,375
Scotland
kommando Offline
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kommando  Offline
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Posts: 9,375
Scotland
Dave I was going to ref your site on iso rubber hardness but not at home currently, I have kept my orginal soft rubbers and instead of going full MK3 with hard rubber I will be going Hemmings adjusters on old soft rubber.

Re: commando balance factor #64412
02/08/06 4:37 pm
02/08/06 4:37 pm
Joined: Oct 2004
Posts: 307
Boulder, CO
debby Offline
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debby  Offline
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Joined: Oct 2004
Posts: 307
Boulder, CO
Dave, I have a full set of UOS old rubber I can sell you laughing

Don't think you'd want them though - the outer bushes had dry rot and the limiting bushes were worn out. I bought the Norvil vernier kits. I'm gonna put those in and live with the vibration. At least I'll have a nice tight chassis...

Debby

Re: commando balance factor #64413
02/08/06 5:49 pm
02/08/06 5:49 pm
Joined: Aug 2001
Posts: 1,269
Hamilton, Mass. USA
D
Dave Comeau Offline

Crew Chief
Dave Comeau  Offline

Crew Chief
D
Joined: Aug 2001
Posts: 1,269
Hamilton, Mass. USA
Debby
A local friend was going to conduct an experiment with the new MKIII iso's and drill a series of 1/4" or 3/8" holes in them to try and soften the spring rate in them. His bike project has stalled and unfortunately it may be some time before I ever get the results.
I only found out a few days ago while cruising the norton andover price list that old style iso's are no longer listed. I also though for sure that norvil had them but now I'm not so sure.


dynodave
BSA 3 1961-1963
Ducati 3 1992-2002
Norton many 1951-1975
87 Serv-Equip 100HP MC brake dynamometer,
Re: commando balance factor #64414
02/10/06 3:44 am
02/10/06 3:44 am
Joined: Sep 2004
Posts: 7
Rockingham Ontario Canada
T
timberline Offline
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timberline  Offline
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T
Joined: Sep 2004
Posts: 7
Rockingham Ontario Canada
I know a number of people have played with rephased cranks for Commando and other 360 degree vertical twin engines in an attempt to tame vibration. Is that a sensible approach for someone who has a spare slimline rolling chassis and wants to build up a reasonably brisk cafe bike?

On the other hand, if the relatively slim-walled tubing of the featherbed chassis is such a significant contributor to vibration through resonance, couldn't that be dimished by injecting something like expandable foam into the tubes to damp resonance?

I've seen several featherbed style alloy tanks which have cracked in several places, often 'round the petrol taps and seams in the bottom, as a result of vibration on hard-ridden bikes. Those tanks were all aftermarket jobs without the original proper internal baffling, so the large relatively flat expanse of the tank bottom was free to vibrate like a drum head or guitar top in response to engine revs. It doesn't take long for short-amplitude high freqency vibration to work harden the less exotic grades of aluminum. Adding self-adhesive foil sound deadener sheets sold by sportscar parts outfits like Victoria British etc.to the tank bottom can work wonders damping the vibes. Maybe a similar concept might help the old trellis as well.

KH

Re: commando balance factor #64415
02/10/06 10:39 am
02/10/06 10:39 am
Joined: Sep 2002
Posts: 538
belgium
ludwig Offline
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ludwig  Offline
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Joined: Sep 2002
Posts: 538
belgium
When I bought my first Norton (1970 ) ,Some old timers recommended filling all the frame tubes with lead !!! . never did it though .

Re: commando balance factor #64416
03/11/06 3:55 am
03/11/06 3:55 am
Joined: Jan 2006
Posts: 20
michigan
J
jpn33 Offline OP
BritBike Forum
jpn33  Offline OP
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J
Joined: Jan 2006
Posts: 20
michigan
Wow this thread is still going! I've been in the shop building engines and not on the internet for like a month. Thanks for all the info. My question was more out of curiosity than anything. The "racer" I bought the engine from was a hack. The rule I live by is that I don't trust anything I didn't build. Lead in the frame tubes is about the funniest thing I've ever heard. Glen (about to fire up my new 72 combat)


OVBSAOC #1654
Re: commando balance factor #64417
03/11/06 5:35 pm
03/11/06 5:35 pm
Joined: Feb 2006
Posts: 63
Lancaster, Ca
F
freakinhungover Offline
BritBike Forum
freakinhungover  Offline
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F
Joined: Feb 2006
Posts: 63
Lancaster, Ca
Quote:
Originally posted by ludwig:
When I bought my first Norton (1970 ) ,Some old timers recommended filling all the frame tubes with lead !!! . never did it though .
Thats funny, I JUST read a profile on someones bike the other day where he listed one of his "mods" as filling his bars/frame full of bb's!! I know theres bar weights sometimes but thats rediculous.


Moderated by  Dave Comeau 


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