The BSA A7-A10 Forum

Technical (topic titles must be descriptive) => A7 & A10 Engine => Topic started by: beezalex on 20.07. 2009 14:08

Title: A10 Excessive Vibration? (Crank Balancing)
Post by: beezalex 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 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?
Title: Re: A10 Excessive Vibration?
Post by: rocket man 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
Title: Re: A10 Excessive Vibration?
Post by: beezalex 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.
Title: Re: A10 Excessive Vibration?
Post by: rocket man on 20.07. 2009 20:52
sounds like you might have to strip it down again
Title: Re: A10 Excessive Vibration?
Post by: coater87 on 20.07. 2009 21:34

 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.

Title: Re: A10 Excessive Vibration?
Post by: trevinoz 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.
Title: Re: A10 Excessive Vibration?
Post by: beezalex 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.
Title: Re: A10 Excessive Vibration?
Post by: muskrat 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.
Title: Re: A10 Excessive Vibration?
Post by: rocket man on 21.07. 2009 20:13
yes dont loosen them too much your motor might set off before you do  *smile*
Title: Re: A10 Excessive Vibration?
Post by: A10Boy on 21.07. 2009 20:29
75% is std A10
Title: Re: A10 Excessive Vibration?
Post by: Richard 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
Title: Re: A10 Excessive Vibration?
Post by: muskrat 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.
Title: Re: A10 Excessive Vibration?
Post by: Richard 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?
Title: Re: A10 Excessive Vibration?
Post by: trevinoz on 21.07. 2009 22:38
             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.
Title: Re: A10 Excessive Vibration?
Post by: trevinoz on 21.07. 2009 22:52
              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.
Title: Re: A10 Excessive Vibration?
Post by: Richard 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.
Title: Re: A10 Excessive Vibration?
Post by: olev 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.
Title: Re: A10 Excessive Vibration?
Post by: muskrat 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
Title: Re: A10 Excessive Vibration?
Post by: beezalex 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.
Title: Re: A10 Excessive Vibration?
Post by: A10Boy 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%
Title: Re: A10 Excessive Vibration?
Post by: trevinoz on 22.07. 2009 22:48
Eddie Dow states that 54% is standard and 65% for racing.
Title: Re: A10 Excessive Vibration?
Post by: olev 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?
Title: Re: A10 Excessive Vibration?
Post by: beezalex 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?
Title: Re: A10 Excessive Vibration?
Post by: Brian 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.