Are we labouring this point about centre of masses being in different places on opposite sides? Understood here. Fine to move on.
Labouring!? Im doing more labour now if thats what you mean Going by my findings so far, they appear to be the same on both halves also. Ill put up pics in a while once I make my set-up a bit more ''professional looking''.
Ah, cutting them in half. Plan The Club GTI (West of Ireland divn.) R&D workshop is whirring and sawing as we write Staying tuned
Just had to eat something to keep my "concentration equilibrium" within exceptable parameters! It is like open university here. Any need to dynamically balance a crank is to correct differences in opposing moments.
Chris, I cannot begin to think of the increased rattles and harmonic issues that your 16v engine will have compared to my 8v! Just had a shave myself to go to town tomorrow. hmm, "The weight of facial hair thread"
I tried to get it back on topic, honest. This whole last page should really be in Brians crank sectioning one anyway? Feel free on my behalf to shift the relevant posts over and clean this one up if you wish as they are both good topics in their own right.
Ive been busy the last few weeks but I cant wait to find the thread where all this 8v-16v love between ye started Anyway, guess what, mass centres are the same on both the journal half, and the counterweight(s) Cut off snout> Linnished and checked> Finding the centre, a real balancing act, took a while to get it stable enough to take a few shots. Bar on the left is just a parallel guide to tally with my fulcrum lines(biro). Balanced on fulcrum wire> Fulcrum fell 31mm from centreline of crank(minus half the kerf width of blade)> The other weight was the very same> Onto the journal, standing on the fulcrum, took a while to get this settled but when it did settle it settled on 31mm, and plumb at that also. Getting ready for drilling> I want to be sure Im right with the fulcrum test, this will make it certain, drilled> Hanging, they all sit perfectly level, hard to get it on camera so you'll have to take my word for it> (wire held up to stop them spinning) 4mm foam sheet to stop journal spinning and also to display plumb. String is plumb and not cocked as it would be if the points it passes through were not in the mass centre> The above now shows that the mass centres are also In the same place on both the journal half, and the counterweights. Given that these are at the same point from crank centre on both, and also that both halves weigh the same, there is no question about it, the counterweights do not themselves hold any extra mass to counter act the reciprocating weight of the piston and conrod above it. In my opinion I cant Imagine knife edging the counter weights(removing mass) would be a good idea at all for the above reasons. Brian.
Well that's great info. It's difficult to visualise how the weights are hanging on the cord, but ignoring that, it points to the centre of mass being in the ballpark or better. It makes sense - how else do you spin something at 8,000 rpm and yet have it not vibrating to death? Post 70 onwards: http://www.clubgti.com/forum/showthread.php?t=212438&page=3
What have you trouble with visualising Chris? Id prefer not to take a wider pan picture because that will mean Ill have to get a bigger white backdrop to hid all the mess @ post 70>, it makes sense now. Brian.
It was top banter back then Brian. There are various threads full of shenanigans. Must have been the full moon or something. That last series of pics pretty much proves what you were trying to find out and shows that to all intensive purposes the moments on either side of the centreline are equal. I mentioned previously that, as I understand, any need to dynamically balance a crank is to correct unequal opposing moments which are causing the imbalance. What else could be causing the imbalance? As it is impossible to cut up a crank that one hopes to use in an engine to check for equal moments after any machining (knife edging for example), will dynamically balancing the crank not show up the differences and once corrected effectively equalise the moments again?
Yep thats right. Now, what I cant understand is this. The crank above has its mass at the same points in either side ok? So it was balanced in every way, as the tests show. Now, you knife edge a crank(shave the weights) and you send it to get dynamically balanced, it will static balance ok, but dynamically....I just cant see how its possible without them removing the same from the four journals as you did from the counterweights. The loads are then miles off per bank on the crank causing internal stresses through the mains.
I would guess they are using the shaved counterweight of no1 to equalise the shaved counterweight of no2 and the same for no3 and no4 so it wil dynamically balance out ok as equal weights have been removed from opposing sides of the crank centreline. But, as you say they are altering the opposing moments for each journal and it's counterweight, and moving them diagonally across the main bearing to the next journal if that makes sense. Not sure if this could be considered something to avoid or not though.
Thats static balance your talking about there, not dynamic. Correct on the last bit, I think it should be avoided! Why would they make the moments the same from factory otherwise?
Tis a can of worms to be sure! And I am keeping my tin opener holstered. I have mentioned this before with regard to the development process of an engine, I suspect when initial prototypes are built, they will be run on dynos with a huge array of sensors measuring all sorts of data to check for potential harmonics and the like which can be finessed out in the final design stages. My only real crank concern is if removing (not using it) the internal trigger wheel from my crank will have any detrimental effect. As I will use the 77.4mm stroke crank, I am hoping the shorter stroke will be far less likely to twist as the longer 86.4, 92.8 and 95.5mm ones. (Must weigh crank for Chris) As an extra point though. The mounting of an aftermarket (possibly heavier) damper on the already suspect crank nose arrangement of some VW cranks may be worth considering. The diesel folk now get a ratcheting alternator pulley to reduce load on the keyway of the timing belt to crank gear. Later 06x type cranks have gone to a "D" shape configuration for this pulley.
On topic. These early era cranks have smaller crankpins and therefore I would imagine, lighter moments!
Formula 1 cranks are ground with a difference sometimes upto 10 or 15 degrees from one end to the other. Once the engine is on full chat they all line. Cool huh? Most cranks we have only ever see 1-3 degrees of twist from one end to the other. Removing the trigger wheel will have no effect. Yes, the journals are smaller, you will have less weight to spin, and more important less drag on the shells are there circumference is shorter> less travel per rpm> less drag. My engine sim made 5 more hp at 8000rpm just by lowering the diameter of the big ends 3mm but tricking it and keeping the mass the same to get the drag result. This extra 5hp is made by having less drag on the journal.
