R100 Smoother?

Discussion in 'Airheads' started by ccmickelson, Oct 10, 2021.

  1. getonyourbikeandride

    getonyourbikeandride Been here awhile

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    Well the correct method of balancing rods is not just laying them on a gram scale. You need two scales and some polished round stock stands that you tare out. This gives you the weight of both the small and big end of the rod. Both of which are important and should be adjusted as needed. The rod is set horizontally with the rods supporting the ID top of each end.
    Think of it this way. You can weight you bike on a truck scale and get the total mass. But if you put the front and rear wheel on its own scale you get total mass plus front end weight and rear end weight. So you have the F/R balance numbers.
    #21
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  2. Arktasian

    Arktasian Feelin the BMEP

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    I'd like to add - beyond the consideration of the engine internals there are the following that can influence how a bike (airhead) feels:
    a) I've run a dial indicator around the locating tabs for the transmission into engine bellhousing and found runout, perhaps wear - in the order of around 3-4 thou. I corrected with a metal compound. (this factor tends to have a detrimental effect on disc splines and input shaft bearing life also)
    b) Driveshafts - I did a u joint replacement last winter, and paid very close attention to all the shaft surfaces, yoke bores, and their machined surfaces. I suspect most use the external machined surface to set joint trueness, when measured however they were not congruent with the shaft OD, nor the centerline of yoke that bolts to tranny. Being as how the shaft contains the bulk of the rotating mass, I trued things to that and mounting centerline and not the yoke external machined faces. Again, this was a slight deviation around 5-6 thou, probably could be ignored without too much risk.
    I've not had any engine components balanced, the kits are Siebenrock, but rods, crank, etc. all stock.
    I'd have to offer, she's smoother to higher level with the above sorted.
    #22
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  3. ccmickelson

    ccmickelson MonoMania Supporter

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  4. tsADV

    tsADV Been here awhile

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    At the time I was planning my R100 upgrades, I got a good deal and bought one of the Siebenrock "upgrade" kit. Which I never installed, since used original parts worked for me. Now, after getting it down from the attic, I've weighed the piston (sans rings) both in at 446 g, and the wristpins at 106 g - which should be approx 150 g lighter than the same BMW R100 parts. I think they are better balanced than the stock parts, and thus shouldn't contribute to vibration.

    I also read up on the experience of some German colleagues. One web page with lots of detailed info is https://www.zeebulon.de/Mot/bmw_allg.htm#KolbenGewicht, which also includes weight measurements of different upgrade pistons etc - for those who read German (or use online translation services).

    Also, a search at one of the German airhead forums came up with this thread:
    https://forum.2-ventiler.de/vbboard...ibrationsproblem&highlight=Siebenrock+gewicht, about a 3-4 kRPM vibration in a Siebenrock upgraded engine. One of the replies mentions that the vibes disappeared after the owner replaced the timing chain. Not sure if that may be a common problem of the post-'81 R100's ?
    #24
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  5. thetubespoke

    thetubespoke Long timer

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    Sorry to hijack the thread, but I hadn't heard this before. I had a R75/5 that had a vibration I just couldn't handle. I've been fine with R50, R80, R90, R100, but this R75 bugged me so much. We took the pistons out and they were within a gram of eachother. Ended up swapping for an R80 set, turning down those R80 jugs, and the vibration was worse, if anything.

    Is bearing clearance a possible issue then on the old R75 motors? I guess rod balance is also possible.

    Everyone talks about how smooth the R75 is, but it was the least smooth Airhead I ever had by a long shot.
    #25
  6. bmwrench

    bmwrench Long timer Supporter

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    The /5 crank clearances tended to be pretty tight. You missed balancing the connecting rods, whose imbalance is often terrible.

    The /5 runs more ignition advance than later engines. This can also contribute to vibration.
    #26
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  7. Jim K in PA

    Jim K in PA Long timer Supporter

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    Yes. I have a set in my R100T.
    #27
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  8. OldCrustyStuff

    OldCrustyStuff Adventurer

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    Fwiw - I put a set of buds pistons in my R100RS and it noticeably decreased vibration.
    #28
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  9. NDGuerre

    NDGuerre n00b

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  10. hunterkey

    hunterkey Been here awhile

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    Like other say, I balanced my engine (within 0.5 grams on both sides) I balanced the rods, wrist pins, and pistons individually.
    Beyond that, I'd say a proper carb sync, and keeping the motor mount bolts tight is key.
    Lastly I keep it above 4k 90% of the time, and now that I have a super trapp, it's above 5k the other 10%m of the time haha

    Jokes aside, I'd say that my bike feels as smooth and the throttle response is just as progressive as the R80 I own.

    If you're still not satisfied, you may like the older style flywheels. IMHO that makes the bike feel less sporty though.
    #30
  11. kdstalker

    kdstalker Been here awhile

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    I have an R80g/s, never had any vibrations. A workshop changed the crankshaft front bushing.
    Capture d’écran 2021-10-23 165045.jpg
    Since that day I have vibrations......
    Obviously I tried all the remedies (new carburetors, new BMW oem cylinders and pistons, balancing of connecting rods etc) :deal
    #31
  12. limeymike

    limeymike Who Me?

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    A thread about the vibrations of airheads is a close second to an thread about oil. My R100 does have various amounts of vibrations at differing RPM's, every airhead I have ridden does also. Does a perfectly balanced one exist? I am sure if starting from scratch on an engine build, selecting the perfect components, checking and double checking tolerances within 0.00005mm. Perfectly setting up the ignition and fuel supply, having perfectly installed and tightened all frame mounts etc. But the person that does all this in pursuit of perfection is not the person I want to be sat next to at the camp fire.
    I like to ride airheads because of what they are, if I don't want to feel the engine I will buy a Goldwing or similar. Some say the 800cc engine is smooth or at least smoother, maybe I rode some bad ones but I could tell no difference. My 100RS vibrates, it vibrates more around 3000-4000rpm, at 4500 and up it is as smooth as it is going to be. When I am heading out on a trip, great roads and a camp fire with good bourbon waiting for me I really don't give a shit. Just my thoughts.
    #32
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