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Flywheel Clutch Balancing Information Help Please

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#16 ryomini


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Posted 29 June 2016 - 04:10 AM

Van man

thanks for sharing very interesting -  I couldn't see your pix but maybe my browser


I agree with you the fact that the clutch pressure plate and flywheel have to be bolted together for balancing

and then have to be re assembled with the clutch plate for use means that there is going to be some error


however it does seem that it is still well worth balancing - in fact the addition of the clutch plate alone is going to

upset things and more so as it wears


if you can get it ALL balanced up at once - whole crank assembly and even the harmonic balancer even better

but since one has to change clutches from time to time it's not essential for the budget builder


my personal difficulty is I have built an engine - the lightened flywheel being part of that particular build

so high lift cam, ported head, stronger springs, lighter flywheel - it would be a pity if I have to drop a part of what I was trying to do


I finally found a motorcycle machine shop nearby but haven't had time to go and see them - and I am currently working out a way to do a static balance myself

where I am located almost nobody but nobody seems to do dynamic balancing and every garage I contacted sends balancing work far away to be done - all adding up dramatically to costs


do let us know how you get along - seems like you are on a very good path to success with your motor

again thanks so much for sharing that

Edited by ryomini, 29 June 2016 - 04:12 AM.

#17 MontpellierVanMan


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Posted 29 June 2016 - 11:17 AM

In view of the little info about balancing cranks I could find, here's a quick low-down on how one firm do it - this is Julian Escuret of Escuret Rectification in Montpellier, France, in action on my A+ 1275 crank.




The crank is driven via a UJ bolt that replaces the Mini crank-damper bolt, and initially without any flywheel/clutch.






The BORGO machine has 3 speed ranges - 450, 900, 1400.






You start at 450 and balance the crank on the LH and RH ends (there are two "soft" or "floating" bearing carriers), then go up to 900 and carry out the same procedure, then 1400 and repeat.




The idea is that each speed throws up different harmonics and if you can balance out all the wobbles at all 3 speeds then you're on your way to a better engine ; don't ask me why it doesn't run at 8.000 rpm - probably because the electronics can't cope, and if the crank flew off the supports it would kill someone whereas at 1400 you only lose an arm and the workshop window.




The read-outs show current angular crank position, and the "imbalance" measured and the angle at which it occurs.









You then drill out a certain amount of metal at the given angular position - ie : automatic centre-punch followed by - in our case - a 5mm bit in an air-drill.




The depth of the hole is a seat-of-the-pants judgement by the operator, based on what radius he's drilling at (flywheel webs are of varying section) and how big the number on the top line is that he's trying to correct.




Julian never over-shot his drilling and took at most 3 stabs at the hole to chase the imbalance figure down to zero.


This is the 450 rpm read-out when we started :




ie - 8 "units" out at 89° on the LH end, and 13 "units" out at 17°


After some hole-drilling, we get :




ie - we're down to zero on the LH end, but still 10 to go at 24° on the RH end.


Bit more drilling to get this :




Both LH and RH down to zero.


So then it's up to 900 rpm :




and now we see we need :




11 off at 70° and 12 off at 30°


and at 1400 it was 08 off at 70° and 21 off at 35°




and once it's zero LH and RH at 1.400, you go back to 900 - and maybe correct something again - and to 450 - and maybe correct etc etc


Once we were happy with just the crank, I built up the Primary Gear, washers, circlips etc and fitted the Flywheel, Cluch Back-Plate, INCLUDING the Back-Plate and all the (numbered and marked) straps and bolts and spacers, all bolted-up with the flywheel bolt and stepped washer.


Now this is likely to raise a few screams, and I'll be interested to hear the reasoning - as long as you understand what follows.




The Back-Plate has the potential (1.77kg here) to be fitted in various positions that can be at least 0.2mm between each other, which (1770 divided by 0.2) is 350 gramme.mm of imbalance, so I prefer to centre it as best I can using feeler guages and believe that by NOT subsequently removing the strap-to-flywheel bolts and by using the feeler-gauages again I can build back up the flywheel and clutch assembly to that same position when I assemble the engine definitively. The best we could get was 1.05, 1.10, and 1.15 - but bear in mind these are repeatable gaps.








The idea that the clutch plate "never ends up in the same position" is true, but a) it weighs bugger all, b) it's industrially manufactured from symmetrical and regular identical parts, c) it's of a relatively small diameter, and d) any wear it suffers will be symmetrical.


So I believe including it in the balancing can never be more inaccurate than the balancing would have been without it (think about it ?) - and its presence enables you to clamp the back-plate (fat, heavy, big diameter) accurately and firmly in the position in which it will be running when the engine is running.


The flywheel had already been balanced by itself, by the supplier, and the small tweak seen here that was required to finalise the balancing of the complete crank/flywheel/clutch assembly would seem to confirm that the back-plate cannot be assumed to have no effect on the flywheel/clutch balance.




What was most noticeable, and Julian confirmed this, was that as the crank got better and better balanced, it took longer and longer to slow down after being spun on the machine.




Here you can see the finished crank complete with all the clutch components spinning at 1.400 rpm, with all the imbalances shown at zero - this is as good as you can get on this machine.


Plus the guy who does the work gets to look happy with what he has done !




Does any of this matter on a car that will only be revved to 6.500 rpm ? Not sure, but it sure is satisfying .................



#18 ryomini


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Posted 30 June 2016 - 07:59 AM

Van Man

woooooooooooow that is a beautiful picture story

machine made by the Swiiss can't go wrong there

and an experienced operator

looks like you will have the smoothest engine out there - I am jealous


I took about 1.5grams off my pressure plate and 4grams off my flywheel - but it was very rustic, very very rustic


but like you I also found that in the end you spin the wheel and it spins for ages and ages so it also helps with fuel economy and not just balance / engine vibration

I am very pleased for you and thanks for showing how it should and can be done :)


I noticed that you have two quite good dimples in the harmonic balancer - so that is balanced as well !


In one way I am fortunate as the SPI flywheel and clutch always go back in exactly the same position - there are 2 dowel pins to this effect.

#19 ryomini


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Posted 02 July 2016 - 02:11 PM

Oh yes and here's a shot of the rustic way - this machine I believe was not manufactured in Switzerland to a high spec - the tolerance in the cones alone makes this exercise almost futile




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