Calculating Balance Factors a simple example

[Brian_Walker]

Hi All,
Being new to the forum I wasn't sure if this sort of post was appropriate as many of the more experienced members would consider this a little too basic.

As I was calculating my VCH crankshaft balance factor I thought I would share the method I use with a few bibs and bobs around the shed a my good wife's kitchen scales.

The general principle is shown below with a few photographs illustrating the process. (The formula below I entered into a spreadsheet I set up so I could model different piston weights I did it this way as different gudgeon pins can be used to reduce weight with oversize pistons)

Example is with my new 485 gm 12.5 : 1 Piston

Calculating Static Balance Factor
Symbol Description Weight grams
WP Weight Piston = 485
WC Weight Conrod = 207
WG Weight Gudgeon + Circlips=113
BW Balance Weight =278
TRM Total Reciprocating Mass = WP+WC+WG= 805
BF Balance Factor = (WC+BW)/TRMx100= 60.25

I had previously balanced my crank at 67% but with a lighter piston, if I use this new piston I will need to mill out some of the lower skirt area to lighten it a little if I want to maintain 67% (Phil Irvings magic number)
Weigh Conrod (Note from photo conrod is parallel to the scale height.= WC
Gudgeon Pin and Circlips =WG
Weight Piston= WP

Set up crank on two saw horses that have a few angle bits screwed to them. (make sure the whole contraption is level side to side and back to front so the crank doesn't go walkabout.)

Keep hanging weights to a wire hook suspended from the little end bush until the crank will stay in any position without moving. ( if the bigend is going to the top there is not enough weight if it going to the bottom there is too much weight.
Weigh back the wire with its bit and pieces and record at Balance weight = BW
Then apply the above formulae, I will not cover drilling and or adding weights as from what I understand 57-67% values have been measured on any number of Ariel cranks and I would not start drilling holes willy nilly without understanding exactly where they need to go and how deep or how added weights will be retained.
I hope some find this useful.

[chris.shearwood]

Brian,
I like the detail of your step by step explanation. Thanks for posting.
Regards, Chris

[Keith.owen]

I will try to understand this!!

Keith

[Steven.Carter]

Very handy to know, I've never realy understood balance factors but this I'm sure will help me understand.

[alan.moore]

Thanks for posting this Brian, very informative and provides a clear way to check the balance factor. My VH engine is just at the stage of going back together. I am undecided as whether I should check what the balance factor is...if its out then that's another job to get done but you've set me off thinking I would like to know what it is....decisions decisions

Cheers
Alan

[nevhunter]

WC ,weight of conrod is the weight of the small end with the rod HORIZONTAL and that part could be considered as part of the TOTAL reciprocating weight (That which goes up and down) which you factor by the" magic number" ie the balance FACTOR expressed asa %, to get the actual weight you balance to.. Some part of the rod weight is considered to be going round and round. (the big end part) so you don't worry with that in your calculations... The small end is already there as an actual weight when you hang the assembly on level edges and is part of the total you form with your little bits. Nev

[john.whiting]

When a single cylinder engine crank assy is "balanced",you swap some of the up and down vibration for fore and aft vibration....which the frame is better able to cope with......you depend on the frame being more rigid in one plane than another.So the "balance factor"depends on the construction of the frame.

[nevhunter]

IT does indeed vary with the frame. Bike engines in TQ cars used very different factors than when the same engine was installed in the original bike frame. Nev

[cmfalco]

WC Weight Conrod = 207

weight of conrod is the weight of the small end with the rod HORIZONTAL

Nev is quite right and, although your photo shows you have indeed used the weight of the small end rather than the entire conrod for 'WC', as a helpful suggestion it would avoid causing mistakes for people who read your post in the future if you edited it to change this symbol and description (e.g. to WSE Weight Small End of conrod).

[alan.moore]

So... to be clear I assume that when you weigh the small end of the conrod you need to make sure the centre point of the big end and the centre point of the small end are at precisely the same height ?