Author Topic: A10 Excessive Vibration? (Crank Balancing)  (Read 4573 times)

Offline beezalex

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A10 Excessive Vibration? (Crank Balancing)
« on: 20.07. 2009 14:08 »
So, after having my regular transportation Karmann Ghia smashed up in a wreck, broken the footpeg off my rickman, busted a rear wheel seal on my truck and broken the oil rings on my A50, I decided I needed to pep up the ol' plunger A10 for transportation.  The main problem was that it wasn't charging, so I pulled the dynamo, tested it, nothing was wrong so I re-installed.  It subsequently started charging again.  This was when I noticed that the lower front motor mount bolt was kinda loose...then on further inspection saw that both rear gearbox mount bolts were gone.  Well, I thought I'd tighen up the motor mount bolts and install some fresh rear bolts with extra loctite and I'll be good to go.  Now, what I can't seem to explain is that, while this thing vibrated before, now it's just absolutely intolerable.  All engine mountings are in place, the bike seems to run OK, has lots of power (thought maybe at first it might be on one cylinder) and the vibration goes away at high revs.  The motor has maybe 2000 miles on a complete rebuild and was OK before...so my question is, what could make it vibrate so bad?  I guess I need to go through timing and valve adjustment, etc., but I don't see how any of those could cause excess vibration.  

Any thoughts?
Alex

Too many BSA's


Offline rocket man

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Re: A10 Excessive Vibration?
« Reply #1 on: 20.07. 2009 20:13 »
did you have the crank balanced i had mine dynamicaly
balanced wich removes a lot ove the vibs also are all
your engine mounting bolts good and tight

Offline beezalex

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Re: A10 Excessive Vibration?
« Reply #2 on: 20.07. 2009 20:33 »
I statically balanced it and it had been more or less OK.  The irony in all of this is that the vibration became substantially WORSE when I tightened all the motor mounts.
Alex

Too many BSA's


Offline rocket man

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Re: A10 Excessive Vibration?
« Reply #3 on: 20.07. 2009 20:52 »
sounds like you might have to strip it down again

Offline coater87

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Re: A10 Excessive Vibration?
« Reply #4 on: 20.07. 2009 21:34 »
 Beezalex,

 When I first started learning about the A10s, the first thing that shocked me was how little isolation the driver has from the engine/gearbox on these bikes. Very thin rubber on the pegs, and hand grips (not thinking seat here). No rubber bushes for the motor or box, nothing at all. Weird isn't it?

 If it were mine I would do the methodical thing in my own way- first I would pull out one of those box bolts and try it, then both and see how it goes. If that had no effect I would put them back in and loosen that motor stud and try it. That would let me know pretty much where the vibe is coming from- engine or tranny.

 Then I would start looking from there. I highly doubt if something major was left out of a fresh build, I would think it has to be something simple that was missed 2000 miles ago.

 Keep us informed on what it turns out to be.

 Lee
Central Wisconsin in the U.S.

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Re: A10 Excessive Vibration?
« Reply #5 on: 20.07. 2009 23:17 »
Alex, What balance factor did you use? A mate of mine statically balances his A10s with good results.
Personally I prefer dynamic balancing as the metal is removed from the exact place required, not just the centre line of the flywheel.
Trev.

Offline beezalex

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Re: A10 Excessive Vibration?
« Reply #6 on: 21.07. 2009 13:36 »
It is balanced to 58%.  As I said, it's not ever vibrated nearly this bad.  Now, there's a chance I never put those gearbox bolts in after the rebuild *red* but why would it get so much worse.  I took it for another spin last night and it definitely seems like it's a chassis resonance type thing.  Once I get above a certain RPM, it really smoothes out.  I would guess that if something in the motor was really unbalanced, it would just get worse with increasing RPM.  So, I think I'll follow Lee's suggestion for now and one by one loosen the mounts and see what changes...I should also add, along those lines, that I had to do a fair amount of prying to get the holes for the gearbox bolts to line up enough to start.
Alex

Too many BSA's


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Re: A10 Excessive Vibration?
« Reply #7 on: 21.07. 2009 14:23 »
g'day Alex, yep those two are mungrels on mine as well. 58% sounds a bit low. Does anyone here know what the std factor is for A10's & A7's. I did my '56 SS at 76%, the faster it went the smoother it got. Don't loosen too many bolts, you may have to go back and pick up the motor. *eek* The gearbox flange can crack if things get too loose.
Bheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
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Offline rocket man

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Re: A10 Excessive Vibration?
« Reply #8 on: 21.07. 2009 20:13 »
yes dont loosen them too much your motor might set off before you do  *smile*

Offline A10Boy

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Re: A10 Excessive Vibration?
« Reply #9 on: 21.07. 2009 20:29 »
75% is std A10
Regards

Andy

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Richard

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Re: A10 Excessive Vibration?
« Reply #10 on: 21.07. 2009 21:46 »
So that those who do not understand % of balance (I never said it was for me) can someone explain what is meant by 56% or 75% and how do you balance it statically and what is the difference dynamically and of course in simple English so non engineers can grasp the meaning.
I had vibration on my plunger and the two bolts underneath were never fitted once I had slackened off all the bolts and removed the head steady I was able to line the gearbox up and fit them, once all was retightened and the head steady refitted it sorted the vibration out
Richard

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Re: A10 Excessive Vibration?
« Reply #11 on: 21.07. 2009 22:08 »
Simply its like balancing your wheel. With a crank you are counterbalancing the weight of your rod and piston. The % factor arrives because if you balance at 100 % the crank will be out of balance at 1/2 stroke. So 75% is a happy medium. Different motors use different factors, and a different factor to std will move the sweet spot up or down the rev range. Use two sets of identical scales, place the big end with bearing and bolts on one, the little end and piston on the other. Take the big end weight and add the % of the little end, eg, 50g + 75% of 100g = 125g. Now make a strip of lead & clamp totaling 125g and wrap around crank pin. Balance crank as you would a wheel. Simple ay.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
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Muskys Plunger A7

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Re: A10 Excessive Vibration?
« Reply #12 on: 21.07. 2009 22:29 »
Thanks for that I think I understand  *dunno* but what stops the lead from flying of the crankpin when its running? ???
or do you mean that you reduce the weight of the small end?
Richard

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Re: A10 Excessive Vibration?
« Reply #13 on: 21.07. 2009 22:38 »
Muskrat,
             The standard factor is 54% and 65% was recommended for racing. It is probably more important to properly balance the engine than worry about a few percentage points. The engine will always vibrate, it's the nature of the beast, but balancing will minimise it and the factor will determine the rev range at which it occurs.
Just my thoughts.
I had my Atlas balanced to 70% and it is a joy to ride, not at all like the reports from the 60s when they were new.
Another factor in vibration is the frame. Different frames require different balance factors, or so we are told.
Trev.

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Re: A10 Excessive Vibration?
« Reply #14 on: 21.07. 2009 22:52 »
Richard,
              Weights are fitted to the crank pin only while balancing. With the weights of the calculated mass attached to the crank, the assembly is placed on level knife edges and allowed to turn. The heavy point will be at the bottom when movement stops. Weight is removed from , usually, the flywheel until the assembly will stop anywhere when spun, not at a heavy point.
The piston assemblies are weighed and machined if necessary to identical weights and metal is removed from the ends of the rods to make them the same. That is static balancing.
If the dynamic method is used the piston and rod are treated the same and weights are attached to the crank pins the same but the crank is the mounted in a balancing machine and spun with the out of balance points being calculated. With these machines you will find that the metal may have to be added or removed from the flywheel, not necessarily on the centre line, and the bob weights. In my opinion a far more accurate way of determining the balance.
  Trev.