Author Topic: A10 Excessive Vibration? (Crank Balancing)  (Read 4540 times)

Richard

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Re: A10 Excessive Vibration?
« Reply #15 on: 22.07. 2009 08:26 »
best left to someone that understands what they are doing then, I think we all have our limitations and comfort zones and I would not feel happy doing that job myself.
Richard

Offline olev

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Re: A10 Excessive Vibration?
« Reply #16 on: 22.07. 2009 12:05 »
I  think its time we sorted this balance factor out once and for all.
Does anyone have formal figures published by the factory?
Did the factory have a figure they tried to work to?
and how accurate does it have to be? Does a few degrees make a difference?
I weighed up the star twin when I stripped it. This crank is original but worn. The bare crank was set up on rollers with a tin can hanging off a big end.
Nuts, bolts etc were thrown in the can till the crank was balanced and could be parked anywhere without wanting to turn.
The weight of the can, bolts etc was 891 grams.
The weight of the big end part of the conrods c/w shells, nuta and bolts was 240 grams each (tot 480g)
so total rotating weight is 1371 grams.
The weight of the small end part of the conrods was 104 and 102 grams.
The weight of the piston, rings and circlips were 244 grams each (tot 488g).
now the interesting bit
The weight of one original gudgeon pin was 44 grams, the other was 65g.
The weight of the pins in a 2nd hand set of bsa brand +20 pistons is 66 grams each.
The weight of my new pins (actually tiger 100 pins) is 49 grams.
So the reciprocating weight varies from 782 grams (44g pins) to 826 grams (66g pins)
So if balance factor = rotating weight/reciprocating weight X 100 the balance factor varies between 57% and 60% just on the pins.

I was talking to BeezaBill (WA) earlier tonight and he told me I'd forgot to add the weight of the oil in the trap.
That will bring it back a bit more. So Trev even if yor mob can't play footy you just might be close with that 54% if you use light pins.
I'd love to know how many rpm it takes to hit the sweet spot there.
cheers

Online muskrat

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Re: A10 Excessive Vibration?
« Reply #17 on: 22.07. 2009 14:05 »
Nice calcs Olev, I think we are all close to right. yep I forgot the oil as well. Bill is a very smart man. I was told once by an elderly racer that 66% was std. How can we find out for sure ? Back to Alex's problem. If motor was fully rebuilt, what could have changed to make it vibrate after 2000 miles ? Something coming loose ? Thats why the mounts are loose or missing. First double check the tuneup and listen for any knocks or rattles. Then I would start at the clutch and work forward then right,then up checking every nut, bolt and bearing. If everything checks out ok we could then think about balance or something coming loose inside.
The other alternative is thicker grips and boots. In a car you would just turn the radio up.
Best of luck Alex
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, .
Australia
Muskys Plunger A7

Offline beezalex

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Re: A10 Excessive Vibration?
« Reply #18 on: 22.07. 2009 17:49 »
Thanks, hopefully I can get to it this weekend...I've got a lot of irons in the fire right now.

BTW, I got the balance factor from a)the "58%" stamped on the flywheel, b)several sources on the internet and c)the fact that the .040 over pistons, rings, and rods came out to 58%.  BTW, I could not find a BSA source for the balance factor, but service sheet 712X gives the balance weights as 18 oz. 10 drams (538 g).  Given those weights, it was pretty much right on 58%.

Also, the oil in the sludge trap doesn't really make any difference.  It adds a calculated 8 grams to the total big end weight of more than 1000 grams.  Less than 1% difference - I don't think it matters.
Alex

Too many BSA's


Offline A10Boy

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Re: A10 Excessive Vibration?
« Reply #19 on: 22.07. 2009 19:23 »
75% is STD A10, both my cranks, one from a flash and one from a spitfire are stamped 75%
Regards

Andy

1960 A10 - Black Golden Flash
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1974 Kawasaki Z1a
Yam XJR 1300

Offline trevinoz

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Re: A10 Excessive Vibration?
« Reply #20 on: 22.07. 2009 22:48 »
Eddie Dow states that 54% is standard and 65% for racing.
Trev.

Offline olev

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Re: A10 Excessive Vibration?
« Reply #21 on: 23.07. 2009 01:10 »
oops, got the maths wrong,  i think.
The balance is a chosen % of the reciprocating mass.
I should have taken the weight of the conrod bigends off the rotating weight, not added it on.
so rotating weight/reciprocating weight X 100 gives figures of 51% to 54%.

but aside from this, we can't have a standard balance factor with a range from 51% to 78% ..can we?
I don't think the factory stamped balance factors on the cranks, so the marks on Alex and A10boys may have been added by a machine shop.
Also, I'd be suprised if the balance was the same on my sedentry 500 plunger as a fire breathing 650 spitfire.
Has anyone got a formula or computer program to calculate this?
cheers

Offline beezalex

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Re: A10 Excessive Vibration?
« Reply #22 on: 23.07. 2009 13:46 »
Olev, since we have the balance weight masses from the factory, we could figure this out if we could get the masses of an OEM STD piston with rings and wrist pin as well as the big end and little end masses of the rod.

I'm pretty sure my rods hadn't been modified, but not 100%...anyone have these items that can weigh them?
Alex

Too many BSA's


Online Brian

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Re: A10 Excessive Vibration?
« Reply #23 on: 25.07. 2009 12:29 »
This topic did get a bit off track and I wonder if now some readers are wondering if the crank in their bike is any good or not and if they need to get it balanced or modified.

Whatever the original balance factor was a standard A10 or A7 crank will work perfectly well even taking into account rebores with oversize pistons. It can and has been argued that balancing may or may not improve things and maybe it does but it is not absolutely necessary and you run the risk of ruining a perfectly good crank and ending up with a motor that is not as smooth as an original one. BSA knew what they were doing when they designed the original cranks.

I have two A7's and two A10's and all run on original cranks and all are smooth with little or no vibration at normal cruising speeds, in the 60 to 70 mph region. My 53' plunger A10 is the best of them all, at 65mph there is no vibration, both mirrors are clear as crystal. It is .040" oversize.

What I am getting at here is that if you build a motor that is well assembled using all standard parts it will give many thousands of miles of service and be as smooth to ride as can be expected. Have a bit of faith in the original BSA engineers, they knew what they were doing.