Engine balancing

[quixotic]

I think I might give it a try...especially after picking up the Haynes manual and seeing how easy it is to take out the con rods.  Initially, I'd just do the con rods and the pistons.  Then, if I wasn't satisfied, I'd do the flywheel.  Then, possibly the crankshaft.  

Has anyone done any of these?  Any improvements as a result?  

One thing that has been puzzling me is why would vibration only affect a certain rpm range?  Intuitively, I would think that if some engine part was out of balance, then the whole rpm range would be affected.  But obviously I'm missing something obvious (wouldn't be the 1st time [smiley=beehive.gif])

[mrbuck]

I once inquired of Bob's BMW about having Msbuck's '84 R65 engine balanced because she wanted to get rid of the 4000 rpm vibration.  I was told by their machine shop that they had never done a balance job on an R65 because "no  one ever wanted to spend that kind of money on an R65".  I was also advised that not even the larger engines responded 100% to the process.

At the time I was warranty manager at a BMW dealership and had good connections with BMWNA tech guys.  They advised first to loosen all engine  mounts and retighten as uniformly as possible.  Also suggested to check owners manual to see if the carb needle positions were correct.  Some owners had received notification and a sticker to put in their manual to move the needle up a position.  They strongly advised against using the rubber grommets some folks were putting on the engine mounts as this could cause the frame to crack.

Bottom line is that nothing has eliminated the vibration totally but Msbuck says after 22 years she has just gotten used to it!

[Tony Smith]

Has anyone done any of these?  Any improvements as a result?  

Yes and No is my answer.
 
you may recall that a few months ago I pointed out how easy it is to balance the reciprocating parts of our R65s, just make a beam balance and balance up like for like.

Will you feel a difference worth the effort - I doubt it. Like wise you can spend a fortune and have the crank, flywheel (sorry Clutch carrier) and clutch all balanced up and I still doubt you will feel any difference. The reason is that the primary source of vibration in a boxer twin is the rocking couple of the UN-counterweighted big ends. The rocking couple can set up harmonic vibration in the frame which for some reason seems to bedevil the R65 more than any other model.
 
The other source of high speed vibration - about which you can do nothing by the way, is flex of the crank shaft.
 
BMW's own investigation into crank flex (which first became a real issue in the R90s) was to decided that they could not do anything about it and sole response was to increase the internal clearance of the alternator rotor to stator. I have seen a film of a R90 engine that had the crankcase "windowed" and the crank painted with a fluorescent dye which was when running photographed using a xenon strobe as a light source.
 
The amount of whip in the crank was initially quite sobering until I recalled that the streets were not littered with BMW twins with broken cranks (in fact I have never heard of one breaking in normal service) and after my own R100 passed the 200,000km mark I kind of decided that BMW's view of the world, namely to simply provide enough clearance to allow the crank to flex without rubbing anyhting, was the right one.
 
But, the crank flex is also a source of vibration about which you can do nothing. I've never held an R65 crank next to a R75/80/90/100 one, but I suspect that the R65 crank is probably stronger by virtue of the shorter webs whilst retaining pretty much the same diameters everywhere.
 
If you get the idea that I think that there is little useful you can do about the particular vibration in BMW airheads, you are right.
  
I do however think that the idea of loosening all the engine mounts and then carefully bringing them all up to the same torque is a good one.
 
I discount the assertion that cone couplers are a bad idea. Firstly BMW fitted them as OEM eventually and secondly, I haven't seen too many airheads parked by the side of the road with cracked frames (cracked sub-frames yet, but that is a cat of a different colour).
 
I have ridden R65s with and without aftermarket cone couplers and mty own r65 has them OEM. My opinion can be summed up as "if my BMW didn't all ready have them, or if I had all the spacers handy to dispense with them, I wouldn't bother, but I don't have the spacers and the couplers were available within 5 minutes of my office, so the couplers stay.

[Barry]

One thing that has been puzzling me is why would vibration only affect a certain rpm range?Intuitively, I would think that if some engine part was out of balance, then the whole rpm range would be affected.But obviously I'm missing something obvious

Any mechanical system will have a point of resonance which is the natural frequency at which the amplitude of vibration reaches a maximum. Clever design tries to put that frequency out of the normal range of use. Sometimes they don't succeed think Tacoma bridge.

Ours happens to be somewhere in the region of 5000 RPM although it can be reduced/moved up the rev range a little by changing the coupling with the frame by aftermarket rubber mounts or to some extent by changing engine mounting torque settings.

The interesting question- is it the engine that has a resonant frequency at 5000 RPM or is it the engine frame combination. If as Tony suggests it's the latter then your supposition that engine vibration levels should be of the same amplitude throughout the rev range may be broadly correct and it's the frame that is resonating. I wonder if anyone that has done frame strengthening on an R65 has noticed a change in vibration levels.

 

 

[steve hawkins]

The only thing I had balanced was my 1979 R65 was the flywheel (heavy),  which was done as a part of a lightening exercise.  Not a great deal of material taken off, as I had a strict budget, but I felt that there was some improvement.  Not that my 1979 bike suffered much from the 5000rpm vibe in the first place.  But then it was well run in (at circa 100,000 miles) by that time.

Interestingly enough, my R100 has developed vibration, now that I am changing its configuration.  Removal and redistribution of weight in general, changing the handle bars (alloy renthal type things) and sub frame, etc, has now give me tingly hands after a long ride at speed.  
I am considering replacing the light alloy handlebars with a set of Honda Hornet steel bars that I have, which have bar end weights.  Trouble is the weight are designed to dampen the vibes from a 600cc 4 cylinder inline, not a 1000cc flat twin.

Something I am now going to have to look into further as I have not finished removing weight....Centre stand goes next, followed by anything else I can remove.

So I am also coming down on the engine/frame/cycle parts interaction, rather than blaming the engine alone.

Cheers

Steve Hawkins

[quixotic]

Thanks for the responses everyone.  This is getting curiouser and curiouser.  I'll have to check with the Motorrad specialist in Calgary, who is currently doing some balancing stuff for an R45.

[quixotic]

Here's a fascinating thread on the subject:

http://www.advrider.com/forums/showthread.php?t=420346

I like this quote:

 "It was weird revving the engine. I'd hear the noise, the bike would lean to the right, but I couldn't feel ANY vibration. Seriously, it was hard to tell the bike was running."

[decorn33]

I replaced the metal spacers between engine and frame on my '84 about 10 years ago with hard rubber spacers I made up myself.  My frame is just fine, and the harmonic between 4500 rpm and 5000 rpm was eliminated completely. I replaced the rubber spacers once already, just as a maintenance, and I will probably do it again in a couple of years.

[R65Singh]

I replaced those metal front spacers with home made rubber spacers and  there is a big difference. I am planning to change the rear ones too, just to see what happens.
A couple months ago, when I took my R50/5 engine out of the frame, I noticed that the spacers were made of aluminum, which is softer than steel.  It might be to reduce the vibrations getting to the frame.  
Just try putting some hard rubber spacers in there to see if it helps, make sure your carbs are well balanced with new slide needles.  
Brand new needles made the bike smoother in my case.

[quixotic]

I replaced those metal front spacers with home made rubber spacers and  there is a big difference.

What did you make them out of?  Can I just carve up a chunk of engine mount material from a car?

Given what Barry said above, rubber spacers might even mean that engine balancing would be worthless (ie, if the major issue was frame harmonics).

Would the R65 frame really be that different from the old slash/5 frames? (I used to have a toaster, and I don't recall any rpm bands of vibration that ever stood out).

[Motu]

So the lack of vibration in the R65 Mono would be just down to the frame and not anything BMW did with the engine ?

[R65Singh]

I used sway bar bushings which looked like this that are listed on ebay.
http://www.ebay.com/itm/Moog-K7275-Sway-Bar-End-Link-Thermoplastic-Bushings-/390862680049?pt=Motors_Car_Truck_Parts_Accessories&hash=item5b013b2ff1&vxp=mtr

I am not sure what car they were for.  Basically any solid hard rubber should do it. You might have to size them using a simple bench grinder and a couple of thin cup washers on both side.  I think I posted a picture on here a few months back.  Hope this helps.

[Barry]

Given what Barry said above, rubber spacers might even mean that engine balancing would be worthless (ie, if the major issue was frame harmonics).

Would the R65 frame really be that different from the old slash/5 frames? (I used to have a toaster, and I don't recall any rpm bands of vibration that ever stood out)

So the lack of vibration in the R65 Mono would be just down to the frame and not anything BMW did with the engine ?

What I was saying is it's the way the engine couples with the frame that might make a difference to how the engine and frame vibrate together. It's not as if the rubber bushes totally isolate the engine as there is still metal to metal contact but there is enough difference to change the coupling. The frame may have harmonics but it still has be excited by the engine vibrations so a smoother engine is always going to help.

On the mono's besides a different frame the only thing I know that's different is substantially (something like 40%) lower torque settings on the engine mounts which again may change the way the engine couples with the frame.

[quixotic]

I used sway bar bushings which looked like this that are listed on ebay.
http://www.ebay.com/itm/Moog-K7275-Sway-Bar-End-Link-Thermoplastic-Bushings-/390862680049?pt=Motors_Car_Truck_Parts_Accessories&hash=item5b013b2ff1&vxp=mtr

Thanks for that info.  I think I'll get these from Princess Auto:
http://www.princessauto.com/pal/en/Accessories/Sway-Bar-End-Link-Bushings/8449373.p

I didn't have any luck finding cone washers, so hopefully the ones shown in the photo would work.  

Would blue locktite be sufficient to keep the nuts on?  There's not much extra thread to work with, so maybe I should put a couple extra nuts on there with locktite, in addition to the original nuts tightened to 12 foot/lbs.

[Luca]

The jam nuts should really have all their threads engaged.  Then they can be snugged down nicely.

If you can't get jam nuts all the way onto the stud, I wouldn't trust threadlocker. I'd probably go for nylock main nuts or drill the stud to accept castle nuts and cotter pins... but that could mess around with getting the torque "just so."

Btw, I wouldn't consider split lock washers for fear that they'd crack.