Balance factor

bazzee said:
I find these images a little bit arousing.

What's with the imperial spanner?  :)

bazzee
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At 9/16 it is probably a bit small for the burred nut on the die. I assume that was done with a shifting spanner and the rubber mallet we see featuring in other photos. 2 of my personal favourite tools by the way. Useful for fucking just about anything :D
 
weight of the bearing cage included in the reciprocating weight? if so then the numbers look right

i'm curious why you chose 60% with the stock length rods
if i'm thinking right (never really certain) 60% would better suit a very long rod engine

going back to piet's post:

"The stock 750SFC crank is balanced to 58%

The 180? twins will most probably be similarly balanced to the SFC, as the 180 configuration equals itself out nicely

Triples are a whole different kettle of fish....

180s have the outers balanced to 35% and the centre to 0% while the centre LH and RH web are balanced to -20% and +20% respectively, obviously to counter the rocking couple induced by the uneven cylinder centre spacing. "


if the factory was using different factors for the outers from the center would using the % change from 35 to 60 work on the center?
if the outers are treated as a 360 twin should the center be balanced as a single?
 
G'day Red

I'm at a bit of a loss as to what you're trying to do there mate. Maybe I'm not seeing the method of your balancing process.

Your bob weights look like they clamp onto the crankpins and don't rotate. Since they're asymetrical weights, the centre of gravity of the bob weights won't be concentric with the crankpins. Eccentric weights might be OK if you're doing a static balance and the weights are free to rotate and hang down vertically in line with the crankpins. The static friction between the weight and pin might be a bit much in your case though, without roller bearings. You'd be better off hanging them on bits of string.

If you're doing dynamic balancing with those weights, they'll be all over the place.

Apologies if I've missed the point entirely. In that case I'd be very curious know how your balance process works.

Cheers,
Cam
 
Cam,
Good questions and truthful answer is I don't know, never done this before hence the postings here to get the experts to comment.
I looked at the bob weights that they use on car cranks, lets say a small block Chevy and based my design on them.
They just clamp on, couple of Vee blocks and draw bars.

Will find out tomorrow when I front up at the balancing works.
Will quizz the man at the counter extensively.

Scott, had to choose something to start with so went with 60%.
Factory setting at 35/0/35 does not apply for a race engine as you are operating in a different rpm range.
We don't get 35/0/35 on this crank anyway as it was a 120 crank, it would be 35/35/35 across the webs, same as Crispin's current crank and it is running fine.
Did not notice any undue vibrations with this set up, it is reportably smoother then the actual 180 cranks though I've not ridden the spaceframe.
Web 3 is different between the 180 cranks and the 120 cranks, there is more metal removed on the counterweight hence the different balance factor.

I asked my mate that builds race engines mainly for go carts and he said 60%.
Am seeing him tomorrow morning anyway so will quizz him again.

Weight of the big end bearing is included with the rotating weight so is entered at 100%.

Thanks Keith, I think my reasoning is correct.
Get well soon.

Red
 
Hi Red - Not done any dynamic balancing personally, only static, but I think I would make the dummy weights circular so they themselves are balanced - maybe too risky to take chances. If you are unlucky enough to have to add weight to the opposite side of the crankpin on any web, there is a material called "Mallory metal" - probably has OZ equivalent - that is about as heavy as lead but stronger and will even thread OK. Had to do this on my 90 degree twin after shaving a load of weight off. Project looks brilliant - best of luck - Phil
 
Thanks Andy, I think that is the first thing in this thread that I have been able to follow with any certainty.

Dave
 
Red

You may be able to modify your bob weights to look something like the diagram below. It doesn't matter what shape they are as long as they're radially symmetric, which keeps the CG in the centre. You could add weight by stacking washers under the bolt heads. As long as they're added in opposing pairs, the CG will stay in the middle.

Forget about Mallory metal if you want to add weight - use uranium. It's nearly twice as heavy as lead, and it'll keep the engine warm  :D

Cheers,
Cam
 

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Hi Red,

Your calculations are the same as how I see them to get to the factor that you desire!  This is the general practiced method of calculating.

48121examplebob_00000021113.jpg

(Note, this is for a V8 with 2 rods/pistons on each crank throw)
This is a 50% balance factor.
I read that the % of weight you calculate is the Balance factor! NOT the weight you "Don't" calculate as some theorise!

Here is a pic of a crank setup to balance,

BanksBalancing_1lg.jpg


I too look at pics and think if it is going to be completely accurate and it to be honest can not possibly be "completely" accurate but I can't see another sensibly reasonable way of doing it and it seems to work for everyone else so it works well enough, especially as we are at best "educated guessing" our factors anyway, and if everyone does it the same then we are all relative to one another so it is pertinent to our investigations.
Good luck tomorrow!

Cam,
Please don't think that I was in any way being negative to your theory, I very much appreciate and value your comments!  I think actually we think along the same lines most of the time. Especially with regards to Bumble Bees  :D

Julian,
 
Red,

Your bobweights are completely wrong..... as Cam and Phil noted, they need to be symetrical around the crank pin, ie, same CoG.

I use an adapted crank pin for my static balancing, if the conrod oil supply bore is not placed in exactly the same place each time, it throws the calculations out the window!  And that's only a couple of grams.....

Guess trying it out will tell if your balance factor works.  :D

piet
 
Hi Red and all

as I haven't found anybody near me who would even consider tackling dynamic balancing of a Laverda triple crank, I have been following this thread with great interest, and doing some basic internet reading. Here is a link to a plain language article that made sense to me, plus it talks about the various crank balancing machines available. I was especially interested in the brief discussion about "overbalancing" a racing engine to allow it to run smoother at higher rpm.

http://www.enginebuildermag.com/Article/1209/balancing_equipment_a_weighty_matter.aspx

Paul LeClair
 
"Overbalancing" is of course changing the balance factor....something most car engine builders are extremely reluctant to do - probably because they don't understand it - and generally have no need for anyway.

The best reading on the subject IMO is the simplest - look at any of Phil Irving's books for a clear and concise explanation.
 
just back from the Balancing Works, sort of like the mechanical version of the Skunk Works

so, end result
my bob weights suck so need to redo them, for one they have to be symetrical as some have pointed out
also, they need to be thinner as they cannot touch the webs or they alter the balance again  :o
touchy little suckers

need to work on a drive method to turn the crank on the balancing fixture which I'll get done today
he also would like another standard crank, stripped and rebuilt minus the conrods so he can reverse engineer the factory balance factors
will work on that today as well, have plenty of cranks here but no spare 120 cranks which does cause me a problem on web 3.

so, crank will be delivered tomorrow morning to the balancing guru and pick it up this coming Friday afternoon
strip it over the weekend and deliver it to the coaters next Monday morning to have it's oil shedding coatings applied

coming together

Red
 
redax5 said:
...also would like another standard crank, stripped and rebuilt minus the conrods so he can reverse engineer the factory balance factors
will work on that today as well, have plenty of cranks here but no spare 120 cranks which does cause me a problem on web 3.

Red
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Hey Red, as discussed re Web no.4 being a 120 web with a different balance factor than a standard 180, I can only re-iterate that the Spaceframe is one very free revving 180 engine, before we set the rev limiter to 8800 rpm Blair used to be rather content telling us he was looking after the machine by limiting the revs to 9000 rpm and this was when we still ran alternator and starter motor baggage on the bike. It doesn't have the same real rough spot at 4200 rpm that most 180's have and is definitely the smoothest I have ridden. Most 180's start to really tell you they are trying hard above 7000, but this engine has been quite different.

Not sure what the impact of the different balance factor on the 120 web is to the overall balance factor config and i'm sure if reverse engineering the 180 crank it would be a different setup again, but I do know that with the 120 web at number 4 it damn well works and perhaps we've stumbled upon something good. Maybe it's like overbalancing and helps the middle cylinder by having some opposing weight factor at top dead helping it tip over ??? I remember you said this web was 38% but in which direction + or -, given the standard 180 has no.3 at +20 and no.4 at -20. If no.4 web on the 120 is +38 then the center balance factor rather than being zeroed out is at 58% on my engine, OR if it is -38 then the factor will be -18. No.4 is bigger web on the 120 which suggests it's -38 eg. off-setting the weight of the center conrod piston assembly???

Fascinating stuff and don't know what it all means other than to say what we have at the moment is a good one.

Cheers Crispin


 
julian said:
Cam,
Please don't think that I was in any way being negative to your theory...
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No worries here Julian

I didn't think you were being negative - just having an interesting discussion. Life would be boring if everyone looked at problems from the same point of view and all had the same opinion.

I don't think we disagree in principle either. But if you (or anyone else) do disagree, don't be afraid to say so. That's how we learn things. You don't need to be diplomatic on my account either - I'm not that sensitive. I deserve to be called an idiot if I express an idiotic opinion  :D

Cheers,
Cam
 
Crispin,

If you were a good guy you would strip your engine down for us all so that crank and rod/piston assembly can be logged for all of us to copy when needed!  Anyone would do that for a mate!  :P

So when can Red expect delivery?  :D :D :D

Julian  :D
 
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