Crank balancing

[Goffy]

I'm new to this forum, so hope you can help.
I've had my A10 for 30 odd years so know hat it should do.
At the last rebuild I had the crank balanced at Bassett Down with the rods which he match weighed & pistons to what he said was the std. factor, 56%. It was smooth to 50MPH, at 60MPH vibrated badly and at 70MPH it was unrideable. I took it back & he checked his work and said it was good.  He recommended balancing it to 65% as this was the factor, "in his notes" for racing.  This was done and it is better but still much worse than before, it is unpleasant to ride.
The crank that I used on this rebuild I haven't used before.  My questions is:
Is it possible to have a crank that shows up on the balancing machine as balanced but still vibrates badly?

[Josh Cox]

Hi Goffy,

Welcome.

Sorry, can offer no help on the crank balancing front, sure there will be plenty of info from some of our senior members ( in terms of knowledge   ).

Have been told 62% is the magic number.

[beezalex]

Goffy, there is no way to actually balance a parallel 360 degree twin.  All you can do is compromise between fore/aft and up/down directions for the vibration.  This is what the balance factor does.  It is really only possible to determine this factor through trial and error since it depends on many things including chassis dynamics, motor character and the riders perception.  For A10's it seems to lay in the 60% range and usually, the higher the rpm, the higher the balance factor since fore/aft vibration seems to be more tolerable to both rider and chassis.

The other thing that seems to help, particularly if you tend to run at higher revs is to dynamically balance the crank with the desired balance factor since dynamic balancing will reduce or even eliminate the 2nd order rocking couple vibration that tends to make the bike feel "buzzy".  This seems to be more of an issue with A65's since the tend to be revvier.

Finally, making sure your packing shims are between the motor and frame, no powdercoat between the two, solid installation of the head steady and generally careful attention to all mounting points can make a significant impact.

Sorry, but no "magic". 

"They all do that".

[Richard]

Hi
Goffy
Welcome to the forum.

I did wonder reading your thread that although the crank was balanced were the conrods weighed at both ends to see if one small end was lighter than the other the same with the pistons or were the rods and pistons just weighed as a hole.
I had my A65 crank assy balanced by John who used to be with Basset down for years but is now on his own in Chisledon, and he had to make a small end bush to fit in one rod to get them correct as the rods were different weights at the top of the rod, the pistons also need to be treated the same way ie balanced wight wise.
Thats about all I can offer as I am no engineer.
It is good to have you on the forum with your knowledge of the A10 and range of goods you do many answers to the electrical problems can be answered
Richard

[trevinoz]

Goffy, I would try another workshop and get the assembly dynamically balanced.
Standard is 54% and racing 65% according to Eddie Dow.
I had a crank balanced by a supposed expert and it was terrible. When done dynamically by a bloke who knew what he was doing it improved out of sight.
It still vibrates but at normal [legal] speeds it is quite OK.
Trev.

[coater87]

 Hi Goffy,

 If it were mine, I would try a couple of things before I went nuts. If you have a tach, take the bike up to the same RPMs at a slower speed (different gear)- if there is no problem its not the motor. As was said earlier, I would check every motor mount, including the tranny if it still does it before I pulled the crank again.

  If you do have to pull the crank, you can check a couple of things yourself, first is weigh both rods (the entire rod, bearings and bushes). I know a few times we have found that a shop "hacking" at this will do the bigs/littles separate, and that should work if done properly. But sometimes it is not, and if they don't do a re-check on the entire rod at the end, things can get off by quite a bit.

 Please let us know what you find and good luck!

 Lee

[Josh Cox]

FYI
Here: http://www.ajs-matchless.com/article_read.asp?id=26

Balancing.
Balancing seems to be a field the average motorcycle owner is reluctant to dig into. It might, on the first hand seem to be a sophisticated task, but it's not. There are given criteria to carry out the job from. Combined with some judging. The latter which makes this a very interesting modification to do.
And for those who are preparing race engines, sharpen your eyes. It's better to shiver from horror instead of being shaken to death by the engine. It will keep the bottom ends together.
To start with an end, the balancing factor is the difficult bit, and the one which must be judged. It can vary from 50% to 90"%. Roadsters are usually in the 60% to 70"% region, and racers 70% to 85%.
As you can see from the list below, factors are varying. These are parallel twins, hence they can be seen in relation with single cylinder engines.

Royal Enfield  65%  
Triumph twins  78%  
B.S.A.  70%  
Commando (rubber isolastic)  52%  
Commando (tight shimmed or bolted)  70-75%  
Commando (bolted to frame)  82%  
Weslake  80%  

The effect of the values simply explained are:-

The lower the percentage, the more vibration at high revs. (Heavier pistons give this effect).
High values give more vibration at low revs.
Con-rod geometry, combustion/compression pressures influence the factor in such a way that it is advisable to compare one engine type with another, and you must be aware that it is impossible to make our engines vibration free through the whole rev-range. There must be a compromise. So, how are you using your engine? 2000 - 4000 rpm cruising or is it a 5000 - 7000 rpm sort of job?
How do we find the necessary values? First you have to judge your old crank in two ways, one reciprocating part, and one rotating part.  

Lyder Moen, Braathens lnd. Div., 4050 Sola, Norway
Issue 382

[duTch]

So Goffy, I'm in crank balance mode, what've you come up with ??

[olev]

Gday Dutch,
You can read about Goffy's adventures on the link.
His bike is the one with teeth.
Seems like his motor has shat itself again.
It would be interesting to know what Steve McFarlanes balancing man did?


http://www.norbsa02.freeuk.com/goffya10.htm

ps: I've bought a bit of electrical stuff off him in the past with no problems.

[KiwiGF]

Hi just a thought to throw in the mix, the balance figures quoted for the A10 has anyone measured the std components to check the figure of 54% really is std?

Also l suspect that figure of 54% would not necessarily apply when non std weight pistons and rods are used.

[duTch]

Yup Kiwi,definitely a few different figures to process??

 Thanks Olev,will check it out, I thought I recognized Goffys' name from somewhere!- Haven't seen you on here for a while? cheers dutch
 (Laverda concourse 12th,may run into you if I'm/you're there?)

  uh-Oh I thought it coincidental this should show up now, till I realized I dug it out of the archives!!

[Topdad]

Good morning Goffy, thought that may be you! welcome, as for my tuppence worth I had mine done dynamically by SRM and it has been fine so I would also suggest a change of workshop. As an aside many thanks for your help when buying various bits and pieces from you ,the petrol taps are excellent  Best of luck BobH.

[Hubie]

G'day Goffy,

I too am a satisfied customer with a full set of 6V LED's throughout my Flash!  As for balancing, hard to help from here in Oz, but I had mine balanced to 65% and she's great on the freeway, a little bit of buzz but not more than one would expect.

Cheers,
Hubie.

[olev]

Gday Dutch,
Can't make it to the Laverda Concourse this year. Have to go up the bush that weekend.
Pity they moved it from under the bridge - makes parking easier though.
Maybe we should slip over to the national BSA do in September and thump a bit of culture into the NSW brigade.
It won't be easy. They don't appreciate good beer.
and we can't talk about state of origin football ... or Australian swimming.
politics and religion over a bucket of cheap shiraz sound alright ??
cheers

[KiwiGF]

Hi hubie are you using STD weight pistons and rods?