Page 7 – Airheads Beemer Club

Discussion of Tank Slappers

Hedz:

The past several days on this topic has resulted in a host of opinions ranging from almost agreement to diametric opposition. It needs some clarification. There is a lot of flesh at stake… The causes and cures have been published in AIRMAIL tech pages more than once and many years ago in BMW News in the late 1970’s in feature articles on the subject. But for you neophytes and latecomers here it is in a nutshell……

First and foremost, never let the problem manifest into a worsening condition. The progress from normal handling to dangerous deterioration is usually slow. Don’t ignore the warning signs and get it fixed before it fixes you so nobody can fix you. You will usually get some early warning signs before the big event of the tank slapper happens.
The tank slapper is of course the worse, almost always preceeded in miles and time with a lesser degree wobble mishandling. Weaving is an entirely separate phenomenon.( discussion forthcoming)….
The most dangerous combination for the wobbling and tank slapper is a handlebar mounted wind screen arrangement, a solo lightweight rider, a significant load at the rear of the machine rear of the rear axle, diagonal headwinds, and of course, steering head bearings too loose or worn and notched. The more of these ingredients in the act, the greater the chance of disaster-and the event may happen without warning.

The primary instigation of wobble is a physical resonance set up in the frame and steering geometry that once starts, feeds the accumulated resonant energy back into itself to accentuate the problem. This is what makes it so difficult to squelch once commenced. A rigid frame and steering coupling (tight steering head preload) will avoid the resonance,by absorbing the energy needed to create and manifest the problem, but a small amount of liberty in movement of the steering is needed for continuous self correction of tracking versus road aberrations as the machine moves along. The proper preloading of the bearings is a compromise-to allow enough movement for corrective needs and not so much as to allow resonance to initiate. Kind of a tightrope act.

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Front Forks – Alignment, Braces, Cleaning, etc.

PRELIMINARY & INTRODUCTORY INFORMATION:

Because the rear tire wear condition has such a MAJOR effect on handling, I recommend the steering be cleaned, lubricated, and adjusted, only at tire change time, after a new rear tire is installed, and perhaps 100 miles or so is on it.

Be extremely cautious about using BMW parts books sketches; same for Haynes and Clymers manuals. Various parts may be shown on any particular sketch or drawing that are only used on some models or some years, and, in some instances, are no longer used at all. The order of assembly, or placement, may not be as on the sketches.

You may want to read/review the following article by Brook Reams, it is lengthy, but has many photos in it that I do not supply in my article; and Brook has a somewhat different approach here and there. Review his article, you may want to do that more than once ….and maybe once more after reading my article.
http://brook.reams.me/bmw-motorcyle-rebuilds/1973-bmw-r755-rebuild-project/31-bmw-r755-install-and-align-front-forks/

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Front Fork Oil

Discussion of Oil types and characteristics.

The following article SHOULD BE USED WITH: http://bmwmotorcycletech.info/viscosity.htm. That article has information that REALLY SHOULD BE READ…either NOW, or, after reading the below article!

I prefer Spectro’s fork oils and suspension fluids. They are of good quality, have low stiction, wide temperature range (excellent viscosity index), & the viscosities can be depended upon. Due to how specified, & lack of stiction fighters in some oils used commonly in our front forks & other characteristics, you are better off with a real fork oil like Spectro’s ….especially the full synthetic or part synthetic fork oil. For fork oils & suspension fluids, the various manufacturer’s do not agree on measuring viscosity; sometimes they don’t do more than give some sort of approximate SAE grade value. Viscosity measurements & temperatures are vastly more accurate between manufacturer’s for engine & gear oils; not so, apparently, for many suspension & fork oils. Because of these various things, and other reasons I won’t bother to get into, I highly recommend you stay with one manufacturer, this is particularly so if you are trying different viscosity grades.

I have not yet done extremely long period testing to find out what oils, or ingredients (esp. mineral vs synthetic), are causing the deterioration of the ‘bumpers’ at the bottom of the forks. Until someone does such a test, or an accelerated test (increasing the temperature?), it is best to simply change the fork oil at reasonable intervals.

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Transmission Input Spline Cleaning, Lubrication, Hints

Applicability: All BMW Airhead models; with some useful information for any splines-driven dry-plate clutch.

Why do it: (1) Smoother shifting; (2) Avoid wearing out expensive parts with expensive labor to repair damage; (3) Avoid suddenly spline failure

When to do it: Depends on year model and conditions you ride in, but probably every 12000 to 35000 miles.

What are you going to do: Unfasten the transmission, move it slightly backwards, clean and lubricate the transmission input shaft splines, and then reinstall transmission. You will probably do other work at the same time, described in the text that follows.

NOTE: While an adequate job can be done by just moving the transmission backwards, a 100% job means removing the transmission from the motorcycle. THAT can be put off until you have another reason for removing the transmission.

Required Skill level: Lower intermediate or better

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Throwout Bearing Maintenance

…cleaning, inspecting, and servicing the throwout assembly on ’74 to ’84 airheads every 10K. Would someone be willing to describe this process in enough detail so that I would have half a chance of performing this myself successfully?

Sure… The reason I suggest doing this, is due to the nature of the layout of the rollers in the bearing. Before the ’74’s (/5 and earlier), BMW used a ball bearing throw-out assembly. They went back to this after 1984. If you referrence a price list, you will see that the ’74 to’84 roller set up is a LOT cheaper. The rollors are laid out radially in a circle.This insure that as the rollers attempt to roll, one end will go faster than the other and will scrape on the two “thrust pieces” on each side.

The bearings do fail (usually indicated by a sudden need to take up slack in the clutch cable). Now, the failed bearing, the two thrust pieces and the clutch thrust rod are turning as a unit. The clutch end of the thrust rod can bore into the forward pressure plate of the clutch assembly. Surprisingly, the bike can still be operated while this is going on, with the rider noting strange shifting and odd clutch noises. This happens often enough that I carry a spare set up as insurance for my wife’s ’78 R100/7, when we tour. (I have, of course, the vastly superior /5)

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Spline lubricants

Airheads need scheduled lubrication on transmission input shaft splines (never grease the clutch disc splines themselves); and, rear wheel cup splines and associated rear drive output splines on the twin-rear-shock models.

Numerous lubricants have been tried in many different climatic and riding conditions, over many years. Some newer lubricants are still being tested. There does not seem to be any magic, perfect lubricant for these places. BMW has specified quite a few lubricants over the years, such as Staburags, Optimoly, ETC. The author has never believed these lubricants were as good as some others, at the times BMW had those recommendations. The author, and others, have done long-term testing on various lubricants. This article you are reading no longer lists these lubricants, nor recommended lubricants, as the latest information will be found HERE:
http://bmwmotorcycletech.info/chemicalsetc.htm where the author keeps the information up-to-date. The spline lubricant information is at item #6A in that article.

Later model (exact year and models are unclear, probably late 1980’s) transmission input splines are SUPPOSEDLY nickel-plated and do not require cleaning and relubrication quite as often, but 30,000 miles seems the practical LIMIT, and for earlier ones perhaps 20,000 is the limit. Best you do it before these mileages, at least once, and then, upon inspection, adjust the interval for the next clean/lube. Once a spline shows rusting, you are LOSING METAL! The transmission input splines are fine-pitch and not very deep….you do NOT want them to fail!

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Re-Gearing an R80/7

Charles Bachmann, charles_bachmann@mgv.com writes:

Hi Airheads,

I am a fairly new owner of a 1978 R80/7. It is a great, fun bike. However, I find that at speeds over about 60 mph, the thing turns higher rpm’s than I would like. I am considering going to a higher gearing.

Thanks for any feedback/advice!

Charles Bachmann
Raleigh, NC

I know how you feel. I’ll bet you’re always looking for 6th gear.

A 37/11 drive is the same that came on my ’74 R60/6. At 60 mph I’m showing at least 4000 rpm. Kinda high. But it’s only a R60. On the other hand, it’s the same drive that came on my ’83 R80RT. This was not acceptable to me. I replaced the 37/11(3.36:1) with the next higher-up 32/10 (3.20:1) from an R75/5. The drive housing has a different look & design, but everything fits fine. It was better, but still too revvy for me. Unlike you though, the ’83 R80 has lower compression than your ’78, therefore less horsepower too. I dare not try a higher drive because I have already lost a fair chunk of power.

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Clutch – Maintenance

Warning! Warning!! WARNING!!!
Under no circumstance should the flywheel or clutch carrier on any model be removed without first blocking the crankshaft from moving forward!! http://bmwmotorcycletech.info/flywheelremovalwarning.htm

You must block the crankshaft! Do not take a chance on serious damage to the engine!

Many have removed & replaced an Airhead flywheel or clutch carrier without blocking the crankshaft with no problems. My advice, very strongly given here, is to take a minor amount of time & make & use at least my simple crankshaft blocking tool. If you do not use some sort of crankshaft blocking tool, & your crankshaft should happen to move forward enough (does not take much pressure nor does it have to move far!) to have one or both of the thrust washers fall off their pins, you could cause very serious damage as you attach & bolt-up the flywheel or clutch carrier. Even if you do not cause serious damage, if the forward thrust washer drops, you may be unable to get it back on its pins without a very considerable amount of work. Do not take a chance!

Since it is a must, in my opinion, to mechanically block the crankshaft from moving forward at any time during the process of removing or replacing a flywheel (or clutch carrier as it is called on 1981+ including some 1980 models), you need to know how to best do that.

This can be done in several ways. I recommend you do not use ‘a towel’ between the front cover & the alternator rotor, as is sometimes done. I recommend a simple & neat method …just make a tool out of a piece of Allen wrench material, weld a disc (fender washer) on one end, making the length such that the Allen end fits into the alternator bolt, & the disc end presses against the inside of the outer timing chest cover. Usually 3/4 inch overall. The length should be such that there will be some light pressure applied by the cover to the tool, the cover being screwed back towards the engine lightly (but not touching the engine case). The tool should be just long enough that the cover can not be fit fully home all the way. Obviously you don’t want to tighten the outer cover very much & you do not need to!

A further treatment of how to make this tool, & a photo of it, is in http://bmwmotorcycletech.info/tools.htm , as item #8A. Note that making the tool requires a very small amount of welding. If you do not have a welder, or do not wish to have the tool made that way, you can just use a piece of allen wrench. The tools article also has photos of the clutch disassembly & assembly tools …you may need three if you have an early model; although three common bolts work well.

At this point I strongly suggest you go look at: http://bmwmotorcycletech.info/flywheelremovalwarning.htm

Whatever tool you use, be sure it cannot rotate such as to loosen its pressure on the inside of the outer cover if you rotate the flywheel in either direction. The advantage of my tool is that it is unlikely to allow the alternator bolt to loosen if the tool is made and installed correctly, as you rotate the flywheel. Be sure the tool is not ‘captured’ by raised grooves on the inside of the cover, etc. Think!

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The Dell’Orto Carburetor

If you can stomach all this reading and absorb and understand it all …you could become a DellOrto expert!

Be sure to see the information on the floats, it does pertain to Airheads, this is in section 3.2.2. See also near the very end of the long article below that you are reading, where there is my information on the pump setting and float setting, as pertains to BMW Airheads specifically! There are hints there too!

The following information article came from, I was told, the .startwin.com site, and was sent to me in a zip file, which I have unzipped, modified, and put below. I have been unable to find the author or person who might, or might not, have copyrighted this information, so as to gain official permission to place it here. This article, whatever its original source, now appears to be public property. I tried to make sure about that, so, on 10-06-2003, I sent e-mails to van Star Twin Motors, the Startwin.com folks, asking about use and copyright. There has never been a reply. I last viewed their website in November 2017, and there is nothing about the carburetors.

I have added my own comments to the article prefixed by ***, and underlined. I have corrected many typographical errors and misspellings in the original article …and changed to U.S. type English spelling and usage. I have also eliminated some in-article hyperlinking, etc. There are places that my comments now are not all that clearly identified, as I wished to eliminate red color I used in earlier versions of this article, which resulted in gaudiness and an unprofessional look to the article; so this latest version generally uses asterisks and underlines for personal comments by me as well as for normal emphasis. I have also modified how the original’s photos are formatted and placed, and added borders, and I also prevented wrapping of text around the photos.

The following informational article can be quite useful for those trying to understand how any carburetor works. While the information does not deal directly with Constant Velocity carburetors, it does deal with needle & slide carburetors, & a vast percentage of the information applies to all types.

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Alcohol-Proof Float Kit

The MAIN advantage of these kits is that you will likely never have to replace floats. That does NOT apply to the float needles, of which there were two basic types. The VITON tipped ones have the same easy-to-lose fine wire clip as on the stock carburetors. That clip insures positive float needle operation when the needle should be leaving the needle seat and allowing fuel to flow into the bowl.

If the original floats (which DO AGE) are already really bad, and/or the float needle is already bad, things will certainly improve with PROPER installation of these PRICEY kits! Installation is, however, somewhat tricky. IN MY OPINION, installation of these kits when the old parts were worn, often considerably, is the HONEST & TRUE reason that SOME find improvements with fuel mileage after installing the kits.

Adjustment of these dual-independent float kits is more involved than with the stock floats. These kits can cause $$$$ problems, so, please read all of this article.

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