R65 motor vibration, 3500 - 4500 rpm

[allred]

I am aware of comments and conventional wisdom, from internet and magazines, etc, regarding what is generally considered a normal characteristic of the R65 motor, specifically that it develops a low magnitude vibration between 3500 and 4500 rpm.  Mine certainly does, and always has done so.  At slower revs it is quite smooth, and the same as it approaches 4800 rpms and higher, all the way to redline.

The vibration is not greatly evident nor objectionable, just an increase in the "buzz" felt in the hand grips and foot pegs.  In 5th gear it is most noticeable from 50 to 60 mph, which is the upper end of my favorite
back road riding speed.

Has any one else noticed or experienced this R65 trait?  If so, has anyone attempted some remedy for it?

While it certainly doesn't stop me from enjoying this bike, I am wondering if a fine balancing of all rotating and reciprocating parts of the motor might smooth out this characteristic.  I mean balancing flywheel, clutch, crankshaft, connecting rods and pistons.  

I like to tinker, and there is a long winter coming soon.  If these steps would smooth out the mid-range "buzziness" of the motor, I don't think there is one other thing I could think of to improve on my R65.  

What do you think?

[nhmaf]

My "buzz" becomes most noticeable in 5th gear from about 57MPH - 62MPH.  It is almost completely gone just a few MPH higher or lower than this range.  

some of the usual suspects/remedies involve making sure that the engine mounting bolts are properly torqued, and making sure that the carbs are balanced as well as you can make them.  sometimes vibration can come from timing jitter from worn timing chain or tensioner.

[Adrian]

Hi there - mine also vibrates at theat rev band more or less. Once I had the carbs balanced and the mechanic took very great care to get it right and the vibration dissappeared for a few days before returning - I was never able to repeat this experience even though I have my own balancing guages. Certainly the vibration becomes significantly worse as the carbs go out of tune after about 3 months or so. Adrian ......

[Barry]

Even the smaller 473cc engine has the same traits of 2 vibration periods - a little roughness of no consequence around 3000 rpm and then some vibration a little below 5000 rpm.

Carb balancing is certainly critical but in the end you can chase your tail on this and not achieve perfect smoothness through both bands of vibration.

I believe it is not as simple as mechanical engine balance as there will also be frame related resonant issues affecting the magnitude of the vibration. I did have some success playing with the engine mount torque settings. Note the 95 on bikes have lower torque figures. Why is that ? I used those lower settings together with a small difference front to back with some success as reported in an earlier thread.

http://www.bmwr65.org/cgi-bin/yabb2/YaBB.pl?num=1311767047/7#11

[Barry]

Looking back on this and previous threads I see that the reported high frequency vibration band differs significantly from one bike to the next with the band starting anywhere from 4000 rpm up to 5000 rpm. I can imagine having vibration in the low 4000 range would be real annoying as that is an everyday commonly used rev range. Mine didn't start until 4500 rpm or a little above and I found that reducing the engine mount torque moved the the band upwards to 5000 rpm as well as reducing the magnitude some. That effectively resolves the issue for me as I rarely exceed 5000 rpm.

[tvrla]

Mine, with the 850cc kit, vibrates between 4K and 4700 RPM. It was much worse before the carb adjust and synch.

One thing I realized concerned the flat slide Mikunis that are on it. With Bing CV carbs throttle cables only need to be synched off idle. But the slide carbs need to be synched at higher RPMs, being directly controlled by the cables rather than vacuum.

Made a big difference!

But that said, it's correct - you won't get rid of it all just with mechanical balancing. I have had bikes that were practically vibration free, and I don't know why that was. But even the larger bikes tend to have a vibration band right around 4K.  

[Mike V]

I find the vibration in my bike near ±4800 to ±5100 rpm, based on my tach. This was an uncomfortable discovery for me (in the beginning) after my total restoration.  I had no experience with the R65's charcteristics until then.  At first I thought I had overlooked something in my assembly of the top end or tuning until I did some research. Let me start by saying the vibration in my bike is minimal but noticable, and up to and beyond that rpm range the motor is like silk.  After some research and discussions with gurus I learned the R65 motor has an "oversquare" design.  Simple definition is the bore is greater than the stroke.  These bikes like to spin and the charcteristic of a short stroke (horozontally opposed) engine seems to have a vibration trait near that range, from my understanding.  I seem to think the R65 has a 2-stroke style power band in contrast to it larger and stronger siblings, much different than the 247 style traits.  I've grown to be comfortable with it and actually enjoy the "pipy" power band.

I love these bikes, very fun to ride and they seem to do everything well for me, vibration or not.

[montmil]

... With Bing CV carbs throttle cables only need to be synched off idle. 

I'm going to take the contrasting view to Wirespokes thoughts on synching throttle cables "just off idle".

The last item on my Bing CV carbs synchronization regimen is to run the rpms up to 3-3.5K, snug down the twist grip lock -that's what BMW put it there for- and use the threaded cable adjusters on the carbs to insure each Bing is drawing the same vacuum at higher revs. It really does make a difference.

My $4.00 yardstick manometer pays off again. Yes, the little over-square engine will still tingle a bit but it's now as smooth as it's ever going to be. I either ride above, mostly, or below the vibration range.

I do wish the R65s had maintained the two-cable system of throttle actuation as on many of the "slash series" bikes. Less monkey motion in the system.

[tvrla]

I used to rev the engine up to three or four K and synch the throttles there, but after a while I noticed there wasn't much difference between the two, so stopped doing it. Now I just synch the carbs off idle or about 1500 - enough to get the stretch out of the cables.

The 79 and 80 R65s did have dual cables full length. I've converted my 83 to duals. It's funny, some guys swear by the single pull and others by the duals. I prefer the duals.

[Mike V]

I used to rev the engine up to three or four K and synch the throttles there, but after a while I noticed there wasn't much difference between the two, so stopped doing it. Now I just synch the carbs off idle or about 1500 - enough to get the stretch out of the cables.

Couldn't agree more.  Not sure where this thread is headed, engine vibration or carb balance.  My opinion is vibration at the rpm level we're talking about is not related to carb sync., in any large degree at least, unless things are grossly unbalanced.  Vibration or shudder during acceleration (lower rpm's) in the transition zone from idle circuit to mid-range "is" - in my opinion.  Pretty much apples and oranges in my mind.  Those fitted with non-CV carbs may have an argument.

I feel the small vibration at ±4800-5100rpm is just an inherant trait of our engines and their design.  Granted, it can be limited or minimized to some degree with very well balanced carbs and well tuned engine including well maintained connection points to the frame. But carbs that are out of balance (inconsistent throttle pull) are pretty easily diagnosed getting up to higher rpm's.

I think the resonance and feeling through the handlebars of engine vibration vs. unbalanced carbs are different.

Not to beat a dead horse here or anything but I believe the vibration we feel at ±4800-5100rpm is something we live with and accept.  If there is a fix; I'm all ears.

I find the vibration much easier to live with than the crappy side stand and center stand.

[nhmaf]

+1 with Monte - it may be a slight tweak, but I think it may make some difference to the 4.8K RPM vibration level - I also adjust the Bings for equal vacuum draw at ~4K RPM.  I then recheck it at idle - as long as it it reasonably good at idle, I either leave it where it as after the 4K adjust, or I split the difference.   I am more bothered by vibration at cruising speed than when stopped at a stop light, so I tend to adjust the carbs accordingly.

[Barry]

Just to add one more odd ball comment on reducing vibration. I once read a reccomendation to torque up the engine mounts while the engine is  running. I recall that it came from A BMW maintenence shop.

[MrRiden]

When you suggest fine balancing of all rotating and reciprocating parts you bring up an interesting topic. Years ago I was set up to balance 650 triumph twins. What I learned was that I has to balance to a percentage of the reciprocating mass and not to the full weight. I've never seen what the figures are for our bikes but I seem to remember using 87% in the Britt bikes. I'm sure there are some more adept that could tell you exactly how to balance to move the vibration up or down the rpm scale. Of course the vertical twins had a 360° crank. I'm now curious how to balance a 180° (assuming that's what ours are) crankshaft. Hmmmm

[Ed Miller]

Airheads have 360 degree cranks, too.  Both pistons hit tdc at the same time, just on opposite strokes.  

Most people never seem to have to take their airheads apart far enough to need to balance stuff.  Unlike our beloved Brit twins.  

[Barry]

.....to balance to a percentage of the reciprocating mass and not to the full weight.  

I'm no expert on this but my understanding with the Triumph (which essentially had the same mechanical balance as a single cylinder engine) is that the weight used to balance the Pistons/Conrods at TDC would cause an out of balance when the pistons were at half stroke so the percentage approach was used as a compromise to give the best overall effect.

Airheads are fundamentally different in that theyhave in theory perfect primary  balance because of the opposing pistons. I would describe an airhead crank as 180 Deg.

When allred talks about mechanical balancing he is trying to achieve that theoretical perfect primary balance by making sure the pistons and conrods are exactly the same weight so the opposing forces cancel each other out.  I know little about doing this to an R65 airhead in practical  terms except the the factory tolerances for pistons and conrods were half of the the earlier mainstream airheads so maybe the scope for improvement is not so great.