R100 Smoother?

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

  1. ccmickelson

    ccmickelson MonoMania Supporter

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    Been wondering what the current thinking is on the notorious R100 3000-400 RPM vibration. Over the years I've read scores of threads from both ADV and other forums on this topic and there are varying opinions as to why this common characteristic exists. Some have postulated that the added weight of the pistons accounts for the increased harmonic vibrations and others point towards imbalanced rods and pistons, flywheel/clutch assemblies, imbalanced and misaligned crankshafts, mal-adjusted timing, carb balancing, etc.. Whatever the cause, it really seems to be a common characteristic of this particular displacement and engine configuration.

    I've had four R100's...an 83 RS, 82 RT, and two 93 RS's...all completely stock. The 83 RS, 82 RT and my current 93 RS each had the 3000-4000 RPM vibe. The 93 RS I let go approached R80 smooth but I have no idea why (I should have kept that one!). Of the R80's I've owned (an R80/7, two 87 monolever's, an ST and a g/s) all were very smooth. I'll also include a 75 75/6 in that bunch as it also had a very smooth motor.

    I should acknowledge that "smooth" can be somewhat relative and subjective. I once had a guy who had only ever ridden R100's and took a ride on my current RS and told me it felt "surprisingly smooth". I also rode my RS from NM to PA with a buddy who had a K75S during which we swapped bikes quite a few times. We both agreed the RS was markedly vibey in comparison but that the K75 was actually too smooth!

    I love the extra power I get from the R100 and 5000+ RPM is sweet, but the R80 engine has spoiled me and I find the 3000-4000 RPM vibration quite annoying and I'd like to eliminate or minimize it if possible. With this particular bike it really gets pronounced around 3500-4000.

    I'm wondering what the conventional thinking is on making a 1 liter as smooth throughout the rev ranges as all of the R80's I've owned. Please don't bother with the obvious...I already try to avoid riding in this rev range and I know how to properly tune an airhead. What I'm looking for here is a viable mechanical solution/remedy for this issue.

    Thoughts?
    #1
  2. MonzaCross

    MonzaCross When life throws you a curve, lean into it

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    I dont have any experience with an R100 engine, but I'm wondering if a certain comparison would help isolate the issue and point to connecting rods and pistons being unbalanced as the primary issue. Does anyone know if this vibration exists in the Siebenrock 1000cc kits? If the vibration is not a thing when someone upgrades to those kits and if otherwise /6 airhead crankshafts and clutch assemblies etc... are basically the same between R 90 and R100 models, then that could point to stock R100 connecting rods and pistons as being the unbalanced issue. Just a thought. If so, it wouldn't be the worst job in the world to pull rods and pistons and have them balanced.
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  3. Tripletreat

    Tripletreat Long timer

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    With the ease with which the one can work on the old airheads, a half-talented owner can easily balance the pistons, wrist pins and rods on an economic scale right at home. I have also had good results with bar end weights... the heavier the better. My R100RS was wonderully smooth - it responded well to those mods. I just bought a R100R that buzzes a bit around 3.5 K rpms. Sometime in the future I plan to see if an aftermarket ignition will minimize that with more precise spark timing.
    That said, I've wondered for a long time about the fact that the airhead engines and the K-bike engines (K75s excepted) have their strongest vibes around the same rpm range.
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  4. bmwrench

    bmwrench Long timer Supporter

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    Back when lightweight wristpins were available, replacing the stock pins, and balancing the rods and pistons made a big difference in vibration.

    The R80 and R100 cranks are the same, which changes the balance factor, so their sweet spots are at different RPMs.
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  5. ccmickelson

    ccmickelson MonoMania Supporter

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    This is what I’ve been leaning towards. Just hoping to hear of others’ experience in this realm before I dive down what could prove to be a very expensive rabbit hole. Hoping for more concrete evidence of the resulting performance from this or that modification. The top end would seem to be the most suspect but of course the entire rotating mass, including the clutch assembly is at play here. I too, am curious about the resulting smoothness of the Siebenrock kit. Really hoping someone chimes in with positive confirmation of this.


    Thanks for your input wrench. Do you know if the Siebenrock kits have lightened wristpins? What is your experience with these kits in terms of smoothness?
    #5
  6. melville

    melville Long timer

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    I've got a stockish R75/6 and an R100/7 with lightened flywheel and some other work done. Quite possibly there's been some balancing work on the rods, as they look shinier than I would expect.

    They both vibrate the pegs and bars around 4K rpm, particularly with a light load. They both smooth out about 4.5K, which is a slightly illegal speed on the R75 and a distinctly illegal speed on the R100 in top gear. My pillion notices the 4K vibes and I've made a point of not letting her know what speed that represents.

    The R75 vibrated at 3200 rpm until I got a single-fire ignition and was able to adjust the timing on both sides to the same level.

    The R100 has a harshness accelerating from 2500ish to about 3000 but that may resolve as it now has the ignition from the R75. The top end is going back together in the next couple weeks and I'll report back.
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  7. tennessee thumper

    tennessee thumper Long timer Supporter

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    CC, what's the mileage on the RS?

    Next time you do push rod seals you could balance the con rods etc...that's is to say if it's warrants doing con rod bearing shells etc...you are there already but...as you say, it can turn into the long and winding road.

    Also, I recall you are a stickler for carb balancing. Might be worth making sure your cables and pull are balanced at that RPM.

    If you don't have heated grips, bar end weights (as someone noted) would be an easy trial to see if that improves anything.
    #7
  8. Beemerguru

    Beemerguru Beemerguru...G/S guy

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    I balance the pistons, rods, wrist pins on every R100 engine I build...whether stock or hicomp, big valve, dual valve. I've had as much as 8gm difference between the sides due to mismatched rebuilds and about half that difference from the factory.

    Postage scale and Dremel/ginding wheel ...and take your time...I usually get each side to within .01gm. Some of bigger engines are as smooth as glass IF the carbs are also rebuilt and balanced.

    Makes a big difference in response, fuel mileage, and especially vibration in the hands..like close to none.
    #8
  9. bmwrench

    bmwrench Long timer Supporter

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    'Thanks fir your input wrench. Do you know if the Siebenrock kits have lightened wristpins? What is your experience with these kits in terms of smoothness?[/QUOTE]

    Piston and pin are lighter than stock. I've never installed Siebenrock's kit.
    IIRC, my pistons are lighter still.
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  10. tsADV

    tsADV Been here awhile

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    So all your R100's are/were the clutch carrier versions, as opposed to the earlier flywheel ones. The absence of an old-fashioned flywheel may be the reason some vibes are noticed between 3 and 4 kRPMs?

    I've upgraded two late 70-ies R80's to R100's. The first got 1st year RS pistons, which I ended up modifying to take external wrist pin circlips since I found it difficult to fit the 2 mm internal ones those pistons were designed to take. In the process, I balanced the pistons against each other, but did not take the con rods out. I also installed a slightly lightened (400 grams, not by me) flywheel which didn't seem to alter the running characteristics much. Result; OK running engine, I'd rate it as with the R80 pistons wrt vibration.
    The second engine got lightened and balanced pistons, balanced con rods (only over-all, not big/small end matched), as well as a lightened and balanced flywheel/ clutch pressure plate assembly (1200 grams). Result; very smooth and much more responsive engine, no noticeable vibration above 2 kRPM - I am very happy with that result.

    Before I started converting those engines, I looked around for info on how to minimise vibration, and I think I came across one article (by Tom Cutter ???) that mentioned that the main reason for R100 (?) vibes was found to be caused by imbalance of the front crankshaft bearing. If that is the case, it should affect all capacity versions of the airheads, but probably the post-'81 R100's would be affected most, due to the heaviest pistons and lowest "flywheel" mass.

    How much experience do you have riding a pre-'81 R100?

    Well, that was my 2 cent worth observations . . .
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  11. Beemerguru

    Beemerguru Beemerguru...G/S guy

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    • The Siebenrock kits do have lightened wrist pins and are balanced perfectly out of the box. Just built one last month..their parts are a work of art just looking at them on the table. And yes, pistons are lighter too.
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  12. bmwrench

    bmwrench Long timer Supporter

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    More accurately, it's the clearance in the front main bearing. Many later Airheads were built with largish clearance, and bring them down to near minimum clearance does decrease vibration. This is not an easy job.
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  13. Plaka

    Plaka Brevis illi vita est

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    My 1000 pistons are perfectly balanced, from the factory. I did a simplistic overall balance job on the rods. I don't use a postal scale, my digital one one goes to 1gm, not a hundredth of a gram. I use a balance beam. I don't care what things weigh, I care if they match and one can't easily add weight, only subtract it.

    I get a pronounced vibe around town but haven't noted the revs. It is a very narrow band (nothing like 500 rpm) and I pass through it so fast I just ignore it.

    The carb balance is extremely accurate and the needles have about 100k miles on them so barely broken in.

    The engine has a natural rocking couple. Noticeable at idle, especially if the idle is a bit low. But it is always there and it will have resonant frequencies at different RPMs. Only thing that will help it is lighter reciprocating parts. Thus the smoothness of the 500. (tho' the 500 also has a heavy flywheel)

    Tire wear and balance counts for a lot. new tires always feel so nice. The kinds of grips used count fr what is felt through the bars.

    I have read that engine mount bolt torque can figure. no personal experience.
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  14. ccmickelson

    ccmickelson MonoMania Supporter

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    I really appreciate all of the knowledgeable and thoughtful insight here fellas.

    Thanks, tt. Bike is just about to tick over 56K. Yes, I like to sync the cables at idle and also at speed…one of the many advantages of the amazing Harmonizer…so carb sync not a concern. Timing at advance is right on as well. I’ve also double-checked and re-torqued the engine mounts and ruled out wheels, transmission and driveshaft. In terms of setup and tuning I’ve left no stone unturned. I’m convinced this is an engine component balance issue as it likely was with the other R100’s I’ve owned. This has simply been the nature of this bike as long as I’ve owned it. The first 93 RS I acquired did not have this issue. Indeed that was a magical bike and probably a big reason for wanting to know the why here and resolve this issue.

    I do have OEM heated grips. Bar-end weights are not really necessary as the bike is not what I’d characterize as “buzzy”. The vibe is only discernible in that narrow rpm range. Most of the miles during my ownership were accumulated doing long-distance travel, riding in the sweet spot of 5K+ RPM, where the bike is smooth as glass, but often “a distinctly illegal speed” as @melville so eloquently puts it. :D

    tsADV, thanks for your input. Some great insight there from the motherland of freeheel skiing :-). Indeed, all have been clutch carrier versions with the exception of the 75/6 and I have not ridden a pre-81 R100 although hoping to one day take a spin on my buddy @radmeister’s 77RS :D.

    This is very encouraging info. Thanks, Bg.

    Yes, this does seem like a job for a pro. As this was probably the case for the R80’s, I’m wondering if lightening/balancing the reciprocating parts might mitigate/eliminate any influence this might be having on smoothness…

    Can you elaborate a bit on these pistons you mentioned?
    #14
  15. motu

    motu Loose Pre Unit

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    Seeing as the Airhead has pretty good primary balance, the rocking couple is what we feel, and will have the same frequency as a primary vibration. Balance of compents will be felt with the rocking couple.

    Trying to find a vibration in my R65 Mono earlier this year, I checked all the basic and obvious stuff, then pulled to gearbox to check the clutch etc. The R65 and R80 Mono's are identical bikes apart from the engine, and so go to ''1985 and later'' for specs. Putting my reading glasses on, only the R80 has the lighter torque for the engine mounts, retorqueing to the higher spec solved my problem.
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  16. bmwrench

    bmwrench Long timer Supporter

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    Here's a pic. JE makes them to my specifications.

    PISTON.jpg
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  17. ccmickelson

    ccmickelson MonoMania Supporter

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    Good lord, that’s beautiful. Are the wristpins lightened as well?
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  18. Beemerguru

    Beemerguru Beemerguru...G/S guy

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    If you're thinking of building an engine, I have a solid one that needs a home. A new engine I built for another project just became available as part of the package. High end parts with all the HP and torque you need for the build.

    The engine is a rebuilt low mileage R100 short block with all the studs seated in Big-Certs.

    It has:

    Oshmo hicomp R100 piston kit

    Oshmo lightened/balanced rods

    NGK Dual Plug Wire Set

    Dual Dyna 1.5 ohm coils

    Moto Israel 324 mid race cam

    New balanced clutch assembly

    Rebuilt R100 Dual Plug Big Valve Head - shaved for 10-1 comp

    EME Digital Ignition - 9 torque curve options - dual plug version

    With or without oil cooler.

    New Veleo starter

    Rebuilt Delorto carbs installed

    NOT maxed out for a full bore race engine. Built for torque and long life...think of it as loafing as you cruise cross country at slightly excessive speed.
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  19. tennessee thumper

    tennessee thumper Long timer Supporter

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  20. Beemerguru

    Beemerguru Beemerguru...G/S guy

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    I resemble that remark ;-)
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