Crank pulley vibration dampers & harmonics. Update: Lightweight crank pulleys = Bad

Discussion in 'Engines' started by A.N. Other, Jan 18, 2010.

Tags:
  1. A.N. Other Banned after significant club disruption Dec 5th 2

    Joined:
    Oct 23, 2003
    Likes Received:
    447
    I stumbled over this pic from the VWM Rallye thread, and left me thinking what we've got here!

    [​IMG]

    To the best of my knowledge, that's original factory/Lehmann supercharged Group A G60 stuff, running ~ 250bhp.

    Comments and thoughts....
     
  2. s1m0n Forum Member

    Joined:
    Mar 7, 2008
    Likes Received:
    0
    Location:
    Northants
    My experiences of the vw 4 & 6 cylinder engines is pretty rudimentary but I do know a bit about managing harmonics in race engines.

    My little example was grossly simplified, I was just trying to illustrate the "dangers" of arbitrarily changing stuff without really understanding what effect your actions will have.

    Gross changes to the weight of the reciprocal components (and to the distribution of that weight) WILL have an effect, what that effect will be and how it will manifest itself is the question...

    Personally I would err towards a fluid damper on a 1.8t (like Bill) as it seems a rough old dog at the best of times! (the 1.8t not Bill :lol: )

    A properly balanced 16v seems to run beautifully up to 8k or so with a light flywheel and ally pulley, (personally I'd steer clear of knife edging a stock crank though - but that's just me ;) )
     
  3. RobT

    RobT Forum Junkie

    Joined:
    Oct 24, 2003
    Likes Received:
    975
    Location:
    Cheshire
    US F3 crossflow VW 8V - no crank damper and external trigger disk

    [​IMG]
     
  4. prof Forum Addict

    Joined:
    Oct 23, 2003
    Likes Received:
    18
    Location:
    Suffragette City
    Rob the theory of engine vibration and why harmonic dampers are used can't be argued with. Those pukka race engines, must have a different set of components/ better balancing which moved the dangerous vibration periods out of the range of the normal operating cycle.

    Pretty sure we did all this in A level physics, you must have covered this at Uni?

    Ideally the whole rotating assembly should be looked at by someone who fully understands this stuff and dampers with the correct properties created. However with the limited resources available to most of us, fitting a stock crank damper or something like Bill's seems to work with common rev ranges and the flywheel clutch assemblies out there at the mo.
     
  5. RobT

    RobT Forum Junkie

    Joined:
    Oct 24, 2003
    Likes Received:
    975
    Location:
    Cheshire
    Oh I understand fully well why they are needed in theory, but are just pointing out that there are an awful lot of properly built engines out there without them.

    As has been suggested, it seems that there are just some engines that have a particular collection of components, operating windows, and power characteristics that mean that they are required. Their use (or not) has probably been arrived at empirically via trial and error - a race engine was built without one and it didn't explode on the dyno, so the finding got propagated.

    There's a lot of proof that atmo race engines seem to get away without them more than turbo engines, but the mountune F2 turbo lump is the outlier to that trend....maybe it has a steel crank....we dont know enough about it presently to further that discussion.

    What becomes apparent though the more I read about this, is that OEM dampers are designed to work with OEM components. Firms who make 'aftermarket' dampers claim that they are specific for certain collections of parts. So if this is the case, why dont we see more catestophic engine failures when folks put an aftermarket 'light' flywheel and clutch on a car fitted with an OEM damper.....surely it wouldn't work anymore?
     
    Last edited: Feb 10, 2010
  6. badger5

    badger5 Club GTI Sponsor and Supporter Trader

    Joined:
    Oct 22, 2003
    Likes Received:
    15
    Location:
    nr Glos
    because not many of them produce higher power and utilise higher revs is my guess
     
  7. prof Forum Addict

    Joined:
    Oct 23, 2003
    Likes Received:
    18
    Location:
    Suffragette City
    this all makes sense, but lightening the flywheel often moves the bad vibration periods above the normal operating revs. I remember Vizard explaining it all in relation to the 4k revs a-series genade problem.
     
  8. A.N. Other Banned after significant club disruption Dec 5th 2

    Joined:
    Oct 23, 2003
    Likes Received:
    447
    Presumably CCC? I can try and dig it out if you've any recollection when it was.
     
  9. prof Forum Addict

    Joined:
    Oct 23, 2003
    Likes Received:
    18
    Location:
    Suffragette City
    [​IMG]

    in here

    however there will be some much more reliable "proper engineering" textbooks/SAE papers that would be more help....if you can understand them.

    However with the constant change of revs/load etc, there would be too many variables to calculate, well for my rusty brain to sort through.

    Where's trendy tramp when you need to rent a brain?

    i guess those race engines won't spend long at a constant speed, so harmful vibes may only be encountered briefly, whereas on a road engine you may hammer along at 4k all day
     
  10. Toyotec

    Toyotec CGTI Committee - Happy helper at large Admin

    Joined:
    Jul 26, 2006
    Likes Received:
    3,320
    Location:
    Creating Pfredstarke
    I think the grenade issue in Dr Vizard's book was when you tried to build an A series larger than 1380cc i.e. 1500 unit.
    As I said in my post and in the link supplied, torsional twist is the greatest away from the PTO or flywheel end. Your harmonic damper, if matched to the reciprocating parts of an engine, attempts to cancel this vibration that can result in failure.
    DMF or solid flywheels have other issues associated with them. The engine is trying to drive them not the other way around.
     
  11. prof Forum Addict

    Joined:
    Oct 23, 2003
    Likes Received:
    18
    Location:
    Suffragette City
    it was a tendency to vibrate badly at 4k , the 1500cc ones were just an amplification of the basic problem
     
  12. A.N. Other Banned after significant club disruption Dec 5th 2

    Joined:
    Oct 23, 2003
    Likes Received:
    447
    I've no A-Series knowledge.

    Is a 1500 done on crank or bore, or both?

    Does a bigger crank have an effect, by having mass attached further from its centre, force x distance etc?
     
  13. danster Forum Addict

    Joined:
    Apr 2, 2008
    Likes Received:
    15
    A series engines have 3 main bearings instead of 5 to locate the crank.
    And also the clutch / flywheel is similar to the vw 020 style on the older ones. The flywheel is right at the end on the crank by quite a way as there is a drop gear to get the drive to the gearbox in the sump![:s]

    Glad I do not have to work on them anymore.
     
  14. romaingirardlamamy

    romaingirardlamamy Forum Member

    Joined:
    Apr 1, 2010
    Likes Received:
    2
    Location:
    london
    Hi guys
    from what i know and to kind of confirm what toytec was saying i saw an m3 6 cylinder engine blowing a big hole in the bloc similar then the one you show on the thread and splitting the crank in half because the harmonic damper wasnt looking great ;
    we advised him but the owner didnt want to listen to us and just asked us to do the head on it to go to his race the next week end he finished his race on the truck
    i know for sure that bigger engine eg :6 inline or V 8 does need an harmonic damper to balance the torsional strenght on the crank this one being longer then our 4 cylinder
    our m3 4 cylinder engine rallye tuned and reving up to 8500 rpms didnt need one ,
    just plain pulleys
    now it is true that all those engine are NA the 4 cyl bm being anything between 2.0 and 2.8
    and develloping up to 350 bhp for the bigger one
    the other interresting things i got an old french magazine talking about the golf 1.6 16S oettinger reving at 8500 rpm and those one wasn t fitted neither with harmonic dampers
    and this was in 81
    now i m rebuilding one of those with a brand new 90.5 mm oettinger crank and planning to revs up to 8500 rpm and i really dont know if i should fit a damper or not
    i m thinking of putting one just for the peace of mind but because my engine is nowhere near standard i m affraid to have the opposite effect
    so i m following this thread closely
     
  15. A.N. Other Banned after significant club disruption Dec 5th 2

    Joined:
    Oct 23, 2003
    Likes Received:
    447
    Romain, I've just reading back through this thread, and it's probably worth fitting one.

    Having finally really taken this thread in properly (+ links, Toyo's being the best, here), a few questions stand out a mile.

    Why are we not losing engines the minute forged pistons are added, or cranks knife-edged? Cranks being lightened to the hilt must have a notable effect on the counterweighting behaviour, and likewise, modified or steel rods and aftermarket pistons, shifting critical speeds* about (* see Toyo link).

    In this recent crank weights thread, a fully knife-edged crank saw roughly 1.8kgs lopped off from a standard 9A crank. So that's 0.45kgs per counterweight and I assume a similar amount to theoretically lop off the piston/rod assembly to truly compensate? Where can that be saved from? A rod & piston weighs 1kg as it is [:s]

    Is knife-edging and alteration of reciprocating components an exercise in blind science? To what extent does it play a major part in harmonics? It seems to me we are living in a guessing zone, hoping the damper/lack of can cope with the set up, by and large it does - and occasionally it doesn't and a mega blow up happens [:s]

    Gillm's system 9A blow up here strikes me as worth a re-visit. Toyo pointed to det from the #2 rod apparance, but the blow up rattled the back of the block, and more to the point, there's a knife-edged crank sat in there - click - & which pistons? Comments?

    Also, surely increased compression and different BMEP has some effect on crank flexing and creates different or excerbated (or less?) harmonics compared to OEM?

    In some ways this is fascinating what we're getting away with (but it doesn't feel clever [:$]). Over the last decade, various 4 cyl engines have been tuned, from moderate to high levels on here and run repeatedly, and yet the apparent damper-related failure percentage (that we know about) is so low.

    • Crankcase failures are 1, maybe 2?
    • Flywheels off - 4 I can think of (two 1.8T {same car}, one n/a 16-valve and one n/a 20v)
    • Crank noses off? None?
    • VR6s? No spectacuar ones I know of (but less commonly bottom end modified?).

    As s1m0n said above, the KR/9A/ABF seems "not to mind" a billet pulley and light flywheel. It doesn't seem to mind much at all, quite frankly what with all that we've thrown at it :clap: [:*:]

    But at the same time, there's no shortage of 1.8Ts, well north of 300bhp, with and without intermediate shafts (internal and external water pumps), which have given good service.
     
  16. badger5

    badger5 Club GTI Sponsor and Supporter Trader

    Joined:
    Oct 22, 2003
    Likes Received:
    15
    Location:
    nr Glos
    1.8t's come with damper as std tho
     
  17. jimmy8v Forum Member

    Joined:
    Aug 8, 2005
    Likes Received:
    2
    Location:
    United Kingdom
    'a fully knife-edged crank saw roughly 1.8kgs lopped off from a standard 9A crank. '

    Chris, it's not a case of direct weight proportions. In simple non-rotating spring damper systems the natural frequency/crit damping is proportional to mass. In rotating systems its all about moment of inertia and therefore its a case of mass and location about rotation axis.

    Some have already touched on this, but I suspect the turbo engines are affected more because they tend to have lower redlines. The resonant frequencies for the crank/rotating assembly is likely to be closer to the operating window of a turbo engine, whereas NA will pull through it more quickly and into higher safer revs.

    The other thing is that there will always be 2nd order frequencies which vw may have designed to be much higher than normal operating range which a standard damper will not take into account. Basically you need to know the frequencies rather than just bolting bits on and hoping.

    It's not that difficult to work it all out, if you can model the bits in CAD/CAE any dynamics add on will tell you the natural frequencies, then you can either add damping/mass accordingly to move it around or just stay away from that rpm.

    Vibration free used to have some good docs on this stuff, but any dynamics/vibration text will cover most of this stuff. Speed talk also has many threads on balancing although its mostly concerning v8 stuff.
     
  18. A.N. Other Banned after significant club disruption Dec 5th 2

    Joined:
    Oct 23, 2003
    Likes Received:
    447
    Yes, accepted - I was merely quoting the headline outright weight, but this is clearly spread up and down the counterweight, with varying degreees of moment effect according to where the weight has come off.

    But it's still chunks of weight relatively speaking. So by taking a 9A, removing a lot of inertia from the crank and using (hypothetically) standard rods and pistons, where does that leave a) the counterweighting and b) the harmonics?

    I assume by 'redline' this is 'critical speed' in other terminology, or do you mean rev limit?
     
  19. A.N. Other Banned after significant club disruption Dec 5th 2

    Joined:
    Oct 23, 2003
    Likes Received:
    447
    From reading through the thread, I saw mention of there being no damper on some, and just the dual mass flywheel (Danster mentioned).

    Edit: here

    Ah ha... n/a 20vs only. Hmmm!!! This tells us something. I'm not sure what, because from the reading I've done, a DMF is far less effective for managing harmonics and it's at the wrong end of the crank to pick it up [:s] [:s]

    But this is to overlook a lot of the modfied 1.8Ts, which is really a generic point made above. Why are the mods not moving them into unacceptable harmonics zones? All 1.8Ts are 220mm deck heights? (crank centre-line to deck). ABF, 2E, AGG - all 236mm, with different rod lengths. Similar cranks to 9A though (if not the same).

    (This is just a really interesting topic, that's why I've bumped it)
     
  20. jimmy8v Forum Member

    Joined:
    Aug 8, 2005
    Likes Received:
    2
    Location:
    United Kingdom
    When I said 'redline' I was trying to say lower revs in general. Badly worded on my part.

    It's not as simple as just taking weight from the crank, even if you work out the change in rotational inertia, to calculate the dynamic balance of the rotating assembly you need to know all the operating conditions. Then you need to know what the lowest 1st natural frequency is and then compare that to the operating conditions, once you've done that you decide what to do.

    If a standard factory 9A rotating assembly has its 1st natural frequency at 8000 rpm (arbitrary number) then its very simple to balance the system and as far as vw are concerned there are no harmonic issues in the system because the engines never rev that high.

    If you then machine the crank and it gets lighter, most likely you will reduce flexural rigidity a bit. Now the lowest natural frequency might come down to 7600 rpm(more random numbers), still not really a problem for most people. But the original damper was designed to take into account the original crank set up and no longer works as vw intended.

    Now you raise the rev limit to 7800 rpm, and you change gear at 7600. Now you've started to encounter the natural frequency of the crank and everything starts resonating.

    Bill's damper would make sense here as an insurance policy, but I'd like to know what engine setup they designed it for. The problem is that as others have pointed out damping is very specific. If its over damped then the energy required to reach the steady state is high, and that applies every oscillation. If under damped then it might help a bit but its impossible to say. Generally light damping is good, but you need to know that the period between excitation is long enough for the damping to carry out the required effects, or they will build on each other exponentially.

    @Bill, do these dampers have any tech stuff distributed with them?

    The other thing is that lots of systems out there have operating speeds above their lowest natural frequency, this isnt a problem as long as this region is moved through quickly. Ship engines are a good example, if left at certain speeds you get masses of shaft whirl etc, but they dont operate there so it doesnt matter. What you dont want is to machine your crank, throw in light rods/pistons, mince the flywheel and then end up with a natural frequency bang in the middle of your power band.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice