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bushmechanic February 3rd, 2014 11:26 PM

Balancing an 80mm 650 crank
for the theory on this download Phil Irvings fabulous book Tuning for speed

just to show you how easy it is to check the static balance factor of a single cyl crank

I could have used fancy parallels at work but i wanted to show that anyone can do this at home in their backyard.

the 2 rails need to be level and parallel .. check they are level with a spirit level and sight them for the parallel bit

then hang different weights off the rod till it is nice and neutral

try it both ways to make sure the rails are level

then weigh the parts

little end of the rod weigh like this

bushmechanic February 3rd, 2014 11:33 PM

calculations STOCK balance Factor
Little end of rod = 137 gm

bob weight = 148 gm

piston + pin + rings + circlips 369gm (all OEM 2008 "650" parts)

(reciprocating weight) x (balance factor) = (bob weight) + (little end of rod)

(369+137)xBF = 148 + 137

Balance Factor = 0.56

bushmechanic February 3rd, 2014 11:37 PM

Titanium pin
I want 0.625

if i use my Trick titanium Ti pin it reduces the weight by 30gm

Balance Factor is then 0.599

so I need to do some grinding :twisted:

berglsmerg February 4th, 2014 01:44 AM

Re: Balancing an 80mm 650 crank
Some careful work the with a hand scraper should do the trick. 8)

bushmechanic February 4th, 2014 02:01 AM

Re: Balancing an 80mm 650 crank
been sharpening the scraper all afternoon

its looking pretty good

berglsmerg February 4th, 2014 03:50 AM

Re: Balancing an 80mm 650 crank
9 iron :twisted: delicate work indeed.
reciprocating mass is reduced to 503 (396+137) - 30, and a factor of .625 gives 314g minus the small end weight of 137g to end up with a 177g bob weight. Looks like it will be a pork chopped (ha ha) crank
Are you hoping to move the vibes lower down in the rev range with the .625 factor ?

Editied for this frigging fruit phone sabotaging me again.

plumbbob February 4th, 2014 08:41 AM

Re: Balancing an 80mm 650 crank
Cheers Bushie,

I'll try it on my 02 crank tomorrow and see what results I get.


Dr_C February 4th, 2014 11:50 AM

Re: calculations STOCK balance Factor

Originally Posted by bushmechanic
Little end of rod = 137 gm

bob weight = 148 gm

piston + pin + rings + circlips 369gm (all OEM 2008 "650" parts)

(reciprocating weight) x (balance factor) = (bob weight) + (little end of rod)

(369+137)xBF = 148 + 137

Balance Factor = 0.56

I thought that the calculation should use the whole conrod weight, not just only the little end? Or is this method considered "close enough"? Those of us still having the counter balancer in use, will also get a contribution to the reciprocating mass from the counter balancer... Years ago, I set up the whole crank, rod, piston and counter balancer in "Working Model". It was interesting to analyse, but I soon realized that I didn't know what theoretical performance gave the best practical result. So I just had to return to what BF worked best I a particular engine and frame... So much for trying to be "clever"! :wink:

bushmechanic February 4th, 2014 01:55 PM

Re: Balancing an 80mm 650 crank
Hi Matts (both of them :) )

that sounds like an interesting model !

the formula in tuning for speed is indeed very basic and I think to be absolutely "correct" even the stiffness of the crank itself would have to be considered because these ones flex so much.

but as you pointed out in the end as with most seat of the pants stuff we just try something and then see how it works .. or in this case feels.

so I see this formula is just a means of providing some repeatability

and yes this is primarily useful for removing the counterbalancer.

Im running about 0.55 ATM and its not as good as 0.58 so im just going to try going a bit further in that direction. different bike different piston etc so just a place to start.

if you wanted to remove the CB without re balancing the 80mm crank I think it would feel OK if you spent most of the time above 3-4000 rpm.

be interesting to see what your crank is plumbob .. apparently it feels horrible without the cb so whatever it is would be worth noting.

just a quick note to say all the con rods I have are different by a few grams here and there and so are the cranks and considering the difference 3% makes you really should measure your own crank to be sure.

plumbbob February 5th, 2014 09:37 AM

Re: Balancing an 80mm 650 crank
1 Attachment(s)
Had a bit of a weigh up today,

[attachment=0:ochj7dnc]crank side.jpg[/attachment:ochj7dnc]

little end of rod 119g

piston, pin etc... 354g

bob weight 178g

(473x119) x BF = 178+119

Balance Factor = .628

SO that should be good if i remove the balancer right?

At the moment with the balancer it has real harsh vibes between 4 and 5000rpm.


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