R60/5: Lightened flywheel worth it?

Discussion in 'Airheads' started by backdrifter, Oct 28, 2012.

  1. backdrifter

    backdrifter Wannabe rider

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    I'm rebuilding a 1973 R60/5. It will be a pleasure bike, but as my only bike, I can also see it going to the track from time to time. It's been a long time since I did track days and I miss it. I don't mind some added vibration (I used to ride KTM singles :D). Other than that, what are the considerations towards and against a lightened flywheel? Does it have a large effect on engine response? I'm looking specifically at San Jose BMW's CC Products modification that reduces weight by 36%.

    I should mention that this will add about a month and a few hundred dollars to my project, but the engine and tranny are out, and if I'm ever going to do this, I'd like to do it now.

    Thanks in advance for any input.
    #1
  2. Biebs

    Biebs BMW Airhead

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    Lose some low end torque / grunt. These engines like rpm's but lose of low end power might not be good. Don't want to be 5000 rpm coming out of a corner when the heavier flywheel rotational mass will give you more low end torque.

    Money / time better spent on top end valves / pistons. :freaky

    I may be completely wrong here but just my 2 cents. :eek1
    #2
  3. Big Bamboo

    Big Bamboo Aircooled & Sunbaked

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    Do you know about "blocking the crank" to keep it from moving forward when the flywheel is off?
    #3
  4. backdrifter

    backdrifter Wannabe rider

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    Thanks for the response, Biebs, that certainly makes sense to me.

    Yes I do, but I always appreciate little reminders like this. I'll be replacing the rear main seal, clutch plate, and spring, so I'll already be in there.
    #4
  5. disston

    disston ShadeTreeExpert

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    I have been told that because it is a 600 motor this is one of those mods that will give you all of the disadvantages of lightening the flywheel and none of the benefits.

    Of course I come to this info second hand. Still I'm inclined to think that it is wisdom.
    #5
  6. Max Headroom

    Max Headroom lost in the '70s

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    I haven't tried this with an R60/5, but I like what the post '81 flywheel and clutch has done for my R90S. It loves to rev now, and the later clutch is much lighter to use.

    I also like the light flywheel when fitted to an R65 for the same reasons.

    Bear in mind that heavy flywheels don't produce torque - they create inertia which is quite different. Instead,, it's the engine which produces torque. Light flywheel = less inertia = revs faster.

    My $0.02

    YMMV etc
    #6
  7. supershaft

    supershaft because I can

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    Lightened flywheels do make a difference but not near as much as what MH is talking about. A lightened flywheel is still a LOT heavier than the later no flywheel setup. Plus, not only do the clutches have easier pull, they are way less likely to slip. I think the no flywheel setup is MUCH better on any model but your crankshaft has the wrong size flywheel bolts for that setup. IMO, a lightened flywheel will be better but not a lot better compared to the later setup.

    There are a lot of popular misconceptions out there about flywheel weight. IMO, most often there biggest advantage is controlling wheel spin. We simply do not have that problem. Grunt? They slow everything down. Your bike accelerates slower and decelerates slower That isn't 'more grunt' but they do have more inertia. For instance, your full lock turning around in a parking lot and you hit a ridge in the tarmac that you didn't see. The heavy flywheel bike resist stalling better for the flywheel's inertia. There's your 'more gunt'. That's why tractors have heavy flywheels. I hope that helps?
    #7
  8. chasbmw

    chasbmw Long timer

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    I think that an 60/5 at a track day, would be positively dangerous, the speed differential between you and the other bikes would be too much.....

    I have a lightened flywheel on my R90/6, it gives a better gear change than stock , but not nearly as good as the' no flywheel ' 1982. No vibration issues
    #8
  9. craydds

    craydds Long timer

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    I need a new real main seal and clutch/pressure plate, also. I was thinking about the Southland clutch rebuild, but tell us all about the late model "no flywheel" set-up. I have a '75 R90S. Is it a simple parts replacement process? Buy the late model clutch carrier, clutch plate, etc., and "plug-n-play"? Any machining involved?
    #9
  10. Biebs

    Biebs BMW Airhead

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    Here is the quote of the day:

    "That's why tractors have heavy flywheels."


    Tractors do have heavy flywheels just to help them plow through stuff.

    So as stated in my first post " I may be totally wrong in my advice."

    The crankshatf on the R60/5 is pretty heavy - a lightened flywheel my help shifting try it let us know :freaky
    #10
  11. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    The flywheel is on the "engine side" of the clutch. The clutch plate iis on the "transmission side" of the clutch. The inertia of the clutch plate-- and whatever residual drag is left over-- is what affects the gear change behavior. Flywheel has nothing to do with it (unless you shift without pulling in the clutch, but that's a different story).

    Urban legend.

    --Bill
    #11
  12. supershaft

    supershaft because I can

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    That would be true IF the disengaged clutch completely seperated the tranny from the engine and the clutch was disengaged at exactly the right nano second every shift but that isn't reality. The engine's inertia does effect input shaft speed during regular clutched shifts and that effects shifting. We don't shift perfectly and disengaged clutches are not completely disengaged.
    #12
  13. Max Headroom

    Max Headroom lost in the '70s

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    *Psst*. Don't tell Voltaire that the post '81 flywheel won't work with 10mm bolts, 'cos that's what he's running in his racebike . . . :rofl
    #13
  14. supershaft

    supershaft because I can

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    I know it will work but you have to have the right someone set it up.
    #14
  15. Boxer Metal

    Boxer Metal Mad Scientist

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    I like the heavy flywheel in my 73 R60/5
    #15
  16. Bulldust

    Bulldust Bulldust

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    I love the heavy flywheel on my R75/6 I think it suits the smaller capacity bikes.
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  17. backdrifter

    backdrifter Wannabe rider

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    Thanks for all the comments, gents. So there are some people leaning both ways, but what I haven't heard is a resounding "yes, do it - there are no down sides". I haven't even ridden this bike yet, so I don't think there is a need to start modifying things like this until I have, so I'm going to pass (at least for now).

    Thanks for the responses!
    #17
  18. Renner

    Renner combustophile

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    that won't be happening with the stock flywheel :D
    #18
  19. Airhead Wrangler

    Airhead Wrangler Long timer

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    You might (probably will) decide that it's not the ideal candidate for track days.
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  20. backdrifter

    backdrifter Wannabe rider

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    I certainly don't doubt that, but unfortunately as my only motorcycle for the foreseeable future, it will be the only candidate.
    #20