Flywheel/clutch lightening

Discussion in 'Airheads' started by tsADV, Mar 4, 2017.

  1. tsADV

    tsADV Been here awhile

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    I'd love to get it balanced professionally, and LB's workshop looks impressive. However, the shipping cost from and to the UK would probably be more than what they'd charge, despite the parts being lighter now.

    One reason I took on this project was for the fun of working on it myself. It would obviously have gone smoother if I had better facilities & tools at hand . . .
    #21
  2. disston

    disston ShadeTreeExpert

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    I figured out from the early part of your thread that this was to be a home brew attempt. It's nice to have the solid professional machine tools to build this stuff but ameture builders have been doing great work for years with much less. Good luck with it. We sometimes don't know what we can do till we try.
    #22
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  3. bmwrench

    bmwrench Long timer

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    You can build a balancing stand. All you need to do is make up an arbor to securely hold the flywheel and a good wheel balancer. I have a couple of arbors-one for twins and one for Ks. The feedback has been good.
    #23
  4. chasbmw

    chasbmw Long timer

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    There will be lots of people in theUk who could balance your flywheel /clutch pack. A dynamic balance would be better than a static I would guess. Best of all to include the crank?


    Sent from my iPhone using Tapatalk
    #24
  5. disston

    disston ShadeTreeExpert

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    I think it's a different ball game once you throw in the crank. Cranks are balanced with Bob weights attached. The weights mimic the effect of the rods and pistons. To be sure the assembly is balanced and the engineers calculate the addition of the flywheel and everything else, in our case the alternator rotor. Some engines are balanced with the addition of a vibration damper on the front, not ours. Others have balance weights attached to the flywheel. I think in our case a neutral flywheel, as far as balance goes, is what's needed. Static balance may be all that is needed. If you want to balance with the crank then dynamic would be needed. There are people that do that. It seems half science and half art tho.
    #25
  6. tsADV

    tsADV Been here awhile

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    Update:
    That's what I were thinking as well. Having looked at a few spare and used flywheel & clutch parts, I could only find signs of balancing carried out on the flywheel and cover plate, so I assumed they were the major potential causes of imbalance between the crankshaft and gearbox.

    After DIYing something resembling a balancing arbor, and sliding on a couple of bearings on either side of the flywheel, I managed to balance out first the flywheel, and then the flywheel plus clutch cover plate to within roughly 1g with plasticine.
    [​IMG]
    [​IMG]
    Then it was just a matter of drilling out some of the holes opposite the heavy spot until the assembly didn't move by itself, or stopped randomly after being spun.

    I've now done a couple of hundred miles with the lightened flywheel & clutch cover plate installed, and cannot notice any significant vibration. Fortunately the new clutch parts cured the clutch slipping I experienced with the modified pistons etc. Hopefully the flywheel will stay in one piece . . . .
    #26
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  7. disston

    disston ShadeTreeExpert

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    Congratulations.

    Next you'll be making a trip to the Bonneville Salt Flats.
    #27
  8. Geezerrv

    Geezerrv Been here awhile

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    When I was having aircooled VWs balanced they didn't use bob weights since they're horizontally opposed. Started by balancing crank then added flywheel, clutch cover and crank pulley balancing each as it was added to the whole. Could get it done to half a gram at an RPM i can't recall. With balanced pistons and rods end to end made a huge difference from stock.
    Funny went to above mentioned site and they've got a bug crank on the page.
    #28
  9. bmwrench

    bmwrench Long timer

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    The trouble with lightening the clutch parts is that you get to do it again when replacement time comes.
    #29
  10. CanadaBiker

    CanadaBiker Been here awhile

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    There is another style of pressure plate that is approx. 377 grams lighter from factory. I had both styles in a box of /5 parts I found in a barn stash. The lighter style is just a flat ring without the raised edges or raised webbing. I put the lighter one in my mostly stock '70 build. But I plan to build the '73 as a bit of a go faster bike, so maybe I should have saved the lighter ring for that bike... or find another flat ring.

    Unless I've got something wrong?

    IMG_9473.jpg

    IMG_9474.jpg
    #30
  11. drhach

    drhach We can't stop here, this is bat country!!

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    What do you get from all of this? What's the benefit?
    #31
  12. Kiwiabbo

    Kiwiabbo Adventurer

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    In a lot of ways it’s just as easy to convert to the later flywheel assembly. Hav3 just done this with my 90. The difference between the two versions is quite a lot. The clutch action is now way lighter and revs out very nicely.
    #32
  13. eight90eight

    eight90eight Long timer

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    A heavy flywheel evens out power pulses. Eases clutch engagemeant moving away from stop.

    Can feel power pulses more so with lightened flywheel, ya know, vibration. However, less run on when throttles are chopped, quicker throttle response, some would say, quicker acceleration?, some would say, racier, some would think, and so forth. And it looks cool when you tear down for service, for pictures. Lookie here? A lightened flywheel. That being said I got two of'em and don't find it a problem, yet. You pays yer dues and you takes yer chances. Nothin's certain in this lifetime. FWIW, too sense, IMHO.
    #33
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  14. ME 109

    ME 109 Long timer

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    I've run my 81 without the clutch components attached. I was amazed at how quickly the engine spun up, like a two stroke.
    All that zip means nothing tho, in the real world.
    #34
  15. chasbmw

    chasbmw Long timer

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    (Apologies, forgotten that i had posted pictures earlier in the thread) Airheads until 1981 had heavy flywheels, then BMW fitted the later lightweight 'clutch carrier', the main reason to do this was to both quicken the response of the engine and to greatly improve the quality of the gearchange, which becomes much quicker and easier to get right without the mass of the heavy flywheel to contend with. On my 82, with a correctly adjusted clutch, clutchless changes are possible going both up and down the gearbox.
    My R90/6 also has a lightened flywheel, i'm trying to remember what the weight is, I think that its just under 3Kgs, which certainly improves the sluggish original gearchange. its covered around 40K miles and has done a couple of trackdays.


    [​IMG]
    #35
  16. tsADV

    tsADV Been here awhile

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    If the introduction of the "webbed" clutch cover plate coincided with that of the R90 engine, the new cover plate may have been designed to cope with the R90's higher torque? But since BMW engineers seem to have over-engineered many parts, the earlier flat plate may perhaps be rigid enough to cope with the R90 engine output. That said, if your go-faster-mods include a tuned R100 engine, the early style clutch cover plate may not be up to the task - ?

    Not sure if the early flat clutch plate is compatible with the /6 on flywheel, which I think is lighter (at around 3.6 kg) than the (early?) /5 flywheel. I seem to remember having seen around 5 kgs for the latter, which may be wrong.
    #36
  17. tsADV

    tsADV Been here awhile

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    Very true - but then you also need to either change to an '81-on gearbox, or replace the input shaft of your pre-81 'box with one from the later ones.
    #37
  18. Kiwiabbo

    Kiwiabbo Adventurer

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    Yup did that too. Had spare parts around so no biggie. I wanted to keep it in a smooth case for period reasons. I have a standard 90s as well and there is a lot of difference between the two bikes. Engine braking is also very difference. You end up using the clutch more with the lighter flywheel as there is no roll on as such. Mainly in traffic situations, motorway stuff.
    #38
  19. bmwrench

    bmwrench Long timer

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    I think a lightened flywheel is a nice compromise between the stock boat anchor and the '81 on piece. I'm scheming to make the '81 on clutch work with the old flywheel.
    #39