Peter, you have covered a lot of issues along the path I would consider.  What more can you write about the history of the bike?

Was there any major transmission work done, final drive work, brake work?   What is the mileage on the bike?  

It is odd this happens in a narrow speed band.  I would expect a balance or mounting problem to get worse at higher speeds.  

LOL, give us as much information as you can, the more the merrier as not having the bike at hand makes forum diagnostic a bit fun.  Where are you located at?  Have you looked in the community for another pair of eyes to perhaps help?  St.

Since it is not on your list, I would check the bolts that secure the driveshaft to the transmission output flange. Just loosen the ribbon clamp on the transmission and pull the rubber boot back to access the bolts. If you do this, use the rear brake with the transmission in neutral to bind up the drive train. the bolts should be as tight as you can possibly get them. 

When you say up and down, are referring to a speed specific vibe?, regardless of what gear you are in? I am wondering about how to differentiate something in the chassis, or drive train as opposed to a vibration related to the motor RPM?, or specific to a particular gear selection? 35 to 40 mph can be maintained in at least 2 different gears. 35 to 40 mph is right about the speed I am shifting from second to third gear. 

Can you notice this more in the front or the rear?   

Maybe not the issue but sometimes over tight steering head bearings will make the bike "waddle or duck walk" from side to side to side at about that speed.  St.

The bike has 47K - the mono shock version. I've owned it for only a year (my other bikes an R90 and an R100GSPD). Neither of those bikes have this issue, funny how the R90 1976 is still the smoothest of the lot. 

I don't think there was major work on the transmission or final drive. I checked the fluids but will double check the final drive bolts.

The vibration feels up front, as in coming from the wheel tire and felt through the bars (vs the seat). I just replaced and adjusted the steering head – in fact I was hoping this was the issue since it existed before that work was done. 

I suspect it's not as speed related as I've said, but that's where I can feel it the most. I wish I could swap a wheel from one of the other bikes, but they're all different sizes. The R80 is the smallest. The alloy wheels are nice, but you can't true them up as easily as a spoked wheel. I will try some different tire pressures and maybe tighten the rear mono shock. It does not seem to matter which gear is selected. 

Maybe I'm too sensitive, but I'm planning a very long tour and prepping this bike. It's easier for me to handle than the R100GSPD. Ultimate touring airhead?

I'm in Arlington Ma and plan to bring it to the MA Tech day. 

Thanks for your thoughts.  

I like James' suggestion of checking the drive shaft output flange bolts.

• Modern "block pattern" tires do not tend to steer the bike like the older "ribbed" tire design did. So rim side-to-side is important, but not as important as it would be with ribbed tires. Having "new tires" installed would rule out issues internal to the tire, but we still need to know the tread pattern of the new tires you selected.

Here's some more suggestions....

• Check the runout on the front disc brake rotor(s). The OEM discs are bad to warp. This in turn may be caused by brake hoses aging out and restricting the fluid flow inside the brake hose. Which in turn holds the brake pad against the rotor. Brake rotors cannot be "ground" like on a car. If you have any runout, then may I suggest the floating rotors from EBC. They are an iron alloy and FAR superior in stopping power.

• Change the front fork oil. (Highly suggest Bel-Ray 7W.) This needs to be done once a year anyway. If you are in the habit of washing your bike or riding in the rain, then there is probably water in the fork oil. Fork oil is changed due to the accumulation of water, not because it's "worn out". 

• Have you checked your wheel bearings ? Jack up each wheel and grab the tire from the side at 6 and 12 o'clock positions. Do a push-pull full-strength wrestling match with your wheel. There should be absolutely zero discernable play in the wheel bearings. Front wheel bearings go bad because owners never have the grease re-packed. I've personally had rear wheel bearings on the "single-sided" bikes go bad even though they are washed in the final drive oil. Bearing replacement at either front or rear is a specialist job.

Hope this helps.

Thanks for the added information Peter.  

Richard gives a lot of good ideas to check and take care of.

When you had the new tires installed did you notice any damage to the wheel?   There is a slim chance you might have a bad tire.

Like brake discs Richard suggests testing for run out, tires and wheels can be checked as well.

Not to be a jerk but are the tires fully popped up on to the rim?   When you spin them do you see a "jump" of any kind?  Any strangeness in the tread pattern?  St.

Spinning the wheel on the bike I do notice some small bumps in the rim. The tire runs pretty damn good, but there are some low spots on the wheel.

Question. Will replacing the fork oil stiffen it up so these tiny bumps won't translate or be felt so much in the front-end? Well, I'll give it a try over the weekend. 

I still can't believe just a little out of true wheel isn't absorbed by the tire. 

Can you take pictures of the small bumps on the wheel with something to judge the size of them?  

With things spinning as fast as they are, and the forces involved, the truer the wheel, tire, brake rotors the better.  You would be surprised how little things can make noticeable issues.

By the way, did you have the issue before you changed the tires?  

One other thing are you sure the steering head bearings are tightened properly?  They should be tightened at assembly then checked after riding a bit, maybe 500 miles?

St.

Posted by: @16077

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► Spinning the wheel on the bike I do notice some small bumps in the rim. The tire runs pretty damn good, but there are some low spots on the wheel. I still can't believe just a little out of true wheel isn't absorbed by the tire.

• In a laced wheel, there will always be small imperfections, if not from the factory, then after 40 years. This is the advantage of cast or machined wheels. These later type should be perfect and stay perfect. 

• Ultimately it's the tire that matters, because it's the tire that contacts the road. The tire will tend to smooth out minor dings in the rim. So if you want to measure "roundness", "concentricity" or some other wheel quality, then do it at the tire's running surface.

• Speaking of tire running surface, you never answered the question about the type of tire you purchased. This is important info and I'm not asking just so I get to type.

Posted by: @16077

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► Question. Will replacing the fork oil stiffen it up so these tiny bumps won't translate or be felt so much in the front-end? Well, I'll give it a try over the weekend.

• The forks are one giant shock absorber. The stanchions need to be perfectly straight and parallel so that both sides act in perfect unison. With the fender, wheel and fork brace in place, the front end should be capable of gliding all the way to the compressed position and return to fully extended position with zero sticking or binding. Even a small amount will cause ride, steering control and braking issues. 

It is common on smaller bikes to hit a pot hole and temporarily twist the forks. That is to say, the 2 stanchions move out of parallel. This can happen on an Airhead too, but is not as common. And it can be very hard to see if you have the bellows type fork boots in place.

• The fork springs (which are dual rate) determine the load carrying capacity, ride height and "stiffness". Old springs will collapse, so fork springs should be checked to be the same length. Support the front of the engine to withdraw both springs.

• The fork oil determines the speed of the shock compression and rebound, which determines how well the tire follows the road surface. Generally, 7W oil mimics ATF and is preferred in modern forks. I have sometimes used 10W, but would not suggest using more viscous oils, unless the manual specifically calls for it. 

Generally, most riders ignore the front forks, but the forks are critical to ride, braking control, obstacle avoidance and road holding. Proper maintenance, frequent checks and annual oil changes are MANDATORY.

► Also note that tire mounting, tire positioning on the rim and wheel balance are equally as important, and constitute a complete chapter unto themselves.

Some more good information from Richard.  What air pressure are you running in the front tire?

I am wondering if you have not got a bad tire, broken belt or a bent wheel.

If you can make up a jig with a pointer to first look at the wheel as you spin it on the bike from the front.  Does it "wobble" from side to side Does the center of the tire track center?   From the side of the tire using again a jig, can you see any out of roundness as the tire is spun?  

In other words, is the tire and wheel perfectly round.  Does it track straight?  

Have you or the previous owner ever hit a pothole that may have damaged the wheel or broken a belt in the tire?  Again, it doesn't take much to mess things up.  St.

https://bmwmotorcycletech.info/instability.htm

The tires are Michelin Road Classic 100/90 - 18. They were a little difficult to find, but Max BMW had them. Originally Rusty sent new but older dated tires. I installed them and experienced this problem. I returned the front one and he graciously sent a newer replacement. Interestingly, the first one did not have a positioning dot. The replacement did - and it went by the valve stem. I balanced it carefully and am sure it's even on the rim. That said, I plan to take the front wheel off this weekend to re-balance and try to capture the tiny wheel dents. I've never seen a 35+ year old wheel that's perfect. I will grab a short video with the wheel on the balancing jig. 

Can't say for sure that this problem was there before the tire change. There were so many issues (terrible carb sync, 16 year old tires, awful loose fairing, older rusty mono-shock, notchy steering...). 

Since I did the steering head bearings I had to have the forks off. I elected to remove then as a unit trying very hard to avoid misalignment. I think there's no binding or stiction. Again, on the road front and back bounce is very even – and just standing with the brake on if feels smooth. Up and down is even. 

More to come this weekend...

Peter, I just want to rule out a damaged wheel/bad tire, some small nicks and dings are some times normal wear and tear.

As for fork alignment there are procedures to follow to assure proper alignment.  I would recommend perhaps looking at Brook Ream's website to see if he has one of his excellent videos on the subject.  St.

Richard, do you have a preferred procedure for fork alignment?   St.