Balancing an 80mm 650 crank

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bushmechanic

bushmechanic

Joined
Jun 13, 2007
Messages
3,871
Location
south east WA Australia
for the theory on this download Phil Irvings fabulous book Tuning for speed

http://tuningforspeed.com/


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

12301382734_c8e1ea477e_c.jpg


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

12301109813_d1225e7960_c.jpg


12301530076_4b78a25c54_c.jpg


then weigh the parts

little end of the rod weigh like this

12300948945_6b93268c2d_c.jpg
 
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
 
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:
 
Some careful work the with a hand scraper should do the trick. 8)
 
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.
 
Cheers Bushie,

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

Carl.
 
Re: calculations STOCK balance Factor

bushmechanic said:
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
Click to expand...
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:
 
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.
 
Had a bit of a weigh up today,

[attachment=0:eek:chj7dnc]crank side.jpg[/attachment:eek:chj7dnc]

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.

Carl
 

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Thats interesting Carl

ive seen the 02 OEM BF specs at 40% (simon) and 51% I think that was weeds calculations

in any case Ned and weed both said the vibes sucked when the CB was just removed without a re balance

weed reckoned it vibrated horribly at idle and just got worse as the revs increased

I think the whole crank design and the weight distribution of the rod are some variables that make it a bit difficult to compare the 82 and 80mm cranks so i don't know how yours would go.

did some grinding this afternoon... pics coming
 
Im showing how to do 1/4 of this the backyard bushmechanic way

use heaps of electrical tape to seal off the gap between the sides of the rod and the crank (keep the grinding debris out) and wrap it up in some woolen socks

marked out the bit to get rid of

12341076284_32e42dc3a7_c.jpg


this is about the minimum you need to remove to get the stock setup closer to 0.6

12341074764_1a3ff268fc_c.jpg


used the 5 iron with a semi so i could hold the camera with my free appendage

if you angle it like this the sparks don't try to melt the stuff protecting the big end

12341069564_de2f97e84b_c.jpg


finished 1/4 of the pin end of the crank i don't reckon you'd want to take any more off than this :twisted:

12341060704_7e81b33656_c.jpg
 
new bob weight with just one corner missing 188gm

so i got rid of about 40gm (effective wt not actual wt)

4 corners means the final bob weight will be around 310gm

then I'll need to take some metal off the fat end of the crank to bring it back, reducing the overall weight of the crank by a bit.

for the rest of this job Im taking it apart and making a smaller diam dummy big end pin to be able to reassemble and check the BF without wearing out the crank pin bore

and I'll see if I have any milling cutters hard enough to make some sparks :twisted: .

just wanted to show that you could do the whole thing with a grinder and without taking anything apart.

Im going to lighten the con-rod a fair bit as well
 
Hi Bushie, :cheers:

That is what I had in mind for compensating for the Carrillo rod. (grinding those corners off)

How much weight could one remove from a crank before it is to light? ie detrimental to performance.

Do you use the lighter flywheel/stator that came out on the FC model?

Cheers spanner
 
Hi Spanner

Im not sure, its always a bummer if the bike is easy to stall

the RFS enthusiasts who play with this stuff run 78 -79mm stroke and can get down to 3.8 kgs (rod plus crank)

I like the idea of the lighter crank and reciprocating parts because it reduces the loads on everything

the FC flywheel I don't think would reduce any of the main loads by any useful amount .. so if it does stall too easy I guess the flywheel is a good place to add weight.

cheers
Bushie
 
plumbbob said:
Had a bit of a weigh up today,

[attachment=0:1r10ii37]crank 650 resized 2.jpg[/attachment:1r10ii37]

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.

Carl
Click to expand...


what Piston is that Carl ? seems nice and light.. or is it a light little end pin ? what weight are the rings ?
 
You could have posted this a month ago while my engine was apart. Thanks for the tip on the kibblewhite springs though, the 650 loves em!
 
Hey Bushie

The piston and rings are 283g and the pin is 70, forgot to weigh the rings separately.

I'll try and get some pictures up later.

Carl
 
so a about 260gm for the piston ... jebus thats gotta be the lightest berg piston ever

thats why your "OEM" balance factor is higher than weeds

could be worth a try

he went for 70% last time, ill find out how that went

FE 650 good to hear the Kibble whites work i reckon compared to the other junk they are sensational and you can go to a 16mm lift cam if you need to ... the others are more limited

i did a thread on balancing years ago where i copied weed and welded some stainless weights onto the fat end of the crank
 

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