Crank Balancing: calculation correct?

I had to install +20 Emgo pistons in a 850 Mk2. I found out they were slightly heavier than the standard AE's (originals).
I wondered what this would mean to the balance factor. A friend of mine has a nice balancing jig, so I gave it a go.
Did some calcs first, had not done that before, and I really wonder if they make sense.
Can someone check them for me? Especially this compensating for oil I did.
The jig looks like in the pictures below. Thanks a lot, Bennie.

Balancing a 1974 850 Mk2 crankshaft with +20 EMGO pistons

Recip weight (Mrecip)
- piston, incl. rings/pin/circlip: 402 gr (original STD from original AE: 388 gr)
- conrod small end: 126 gr
Total: 528 x 2 = 1056 gr (STD: 1028 gr)

Rotat weight dry (Mrot,d)
- conrod big end: 318 gr
Total: 318 x2 = 636 gr

Rotat weight wet (Mrot,w)
- rotat weight dry: 636 gr
- measured volume sludge trap: 120 cc. Spec. weight oil: assume 0.85 kg/dm3
weight: 120 x 0,85 = 102 gr
Total: 738 gr

CALCS WET
Factory balance factor wet (BFw) is 52%. Bob weight then to be:
(BFw x Mrecip) +Mrot,w = (52/100 x 1056) + 738 = 1287 gr (STD: 1273)

CALCS DRY
Compensation to be added to bob weight for oil: 102 gr
Total bob weight: 1287 + 102 = 1389 gr (STD: 1375)
Corresponding balance factor dry (BFd):
(BFd/100 x Mrecip) +Mrot,d = 1287 >
BFd = ((1287 - Mrot,d) / Mrecip) x 100 = ((1287 - 636) / 1056) x 100 = 61.6 %. (STD: 62,0 %)

TEST DRY
Balancing the dry crankshaft in a balancing jig showed a bob weight of 1395 gr was needed.
Means an actual BFd of (1395 - 102 - 636) / 1056 = 62,2 %.

Note:
The Mk3 manual mentions a BF of 52% wet, and 63% dry, the latter with a bobweight of 1335 gr.
I had a old Mk3 crank, put it in the jig, I did not measure in detail but it showed it needed a little less bob weight than the Mk2 one. Don't know how much dirt is in the trap though.

Looks like you have the calculations right.
I really doubt you will be able to tell the difference in balance factor in a Commando frame. [I know I never can with that small a change]
If you want to keep the vibes to a minimum I would suggest you work on the pistons around the bottom and sides of the pin bores- you should be able to get them to match the stock pistons pretty easily. Jim

OH goodie someone doing the experiment for me, to see if upping the BF, which tends to make the orbit more circular by increasing the horizontal component, might be smoother than factory, which just might of scrimped on the least amount of metal to get by with isolastion, considering that solid mounts move BF up for less damaging vibration and Norton upped the BF a tad for the 850 anyway.
I vote that you'll like it after getting the side to side balance hulled out.

Nice work slimslider, I have JS piston and rod kit, this reduces the reciprocating weight by approximately a third over stock. On Jim's recommendation I did not alter the BF and the new pistons makes a BF of late 60's
Apparently the production racer models were running a 67 degree factor, so I believe the higher number is suited to higher rev's.
It would be good to try and trim down the pistons, even lighter would help make a smoother ride, and you can reset the BF with your rig. In JS 's earlier piston lightening experiments, he drilled holes in the skirts to lighten stock pistons before coming up with the JE pistons and Carillo rods.

HTH cheers Richard

Guess what, JS also lightens even more his new light piston sets in his own ride, get with it and catch up with his raised BF 750. My 920 was BF's a 77% - going by my witness marks on cradle to get more circular orbital, but JS pistons/rod shot the BF in the upper 90's, so Ken Canaga bored out the flywheel to bring back to upper 50's but with screw in plugs can try BF over 100 someday. I've a report of one isolastic Cdo with 94% BF that was said to run smooth.

stockie2 said:

Apparently the production racer models were running a 67 degree factor, so I believe the higher number is suited to higher rev's.
HTH cheers Richard

Click to expand...

Factory spec sheet for Production Racer says 62% balance factor. Isolastics in PR also had different configuration than standard Commando.

Ken

Jim, thanks for checking the calcs.
Yes, I thought about trimming the pistons back to the original weight but doubt if I have the proper machining skills and equipment...
Then again, I just checked on the scale and was amazed what 14 grams represents:
it is the weight of a 37 mm long steel 8mm rod.
In aluminium, it would mean even more volume to remove.
This crank is for a standard street bike, I think I will leave it as it is, like you say, probably won't notice the difference.
Thanks again, Bennie.