Brakes

This long & extensive article covers both disc & drum brakes. There is a lot of information for 2-wheelers, some for sidecars & tugs. There is a complete discussion about brake fluids & bleeding. Much is applicable to any hydraulic brake system. Included is squealing information for motorcycles, especially Airheads, other bikes, Bulletins for K-75, Etc.

Warning: working on brakes is serious business. Read this entire article, perhaps more than once, before you begin work on your brakes. If you do not feel competent, take your bike to a qualified shop.

Broken cable or lever operated brake switch? (NOT the hydraulic switch). Brake pedal bolt not contacting the switch properly? (and it’s not a bent tube at the frame): http://bmwmotorcycletech.info/brakeswitches.htm

Regarding the hydraulic pressure activated brake switches: Those switches, originally 34-31-1-233-959, were replaced with 61-31-1-244-334. If you haven’t access to the BMW part, you can probably substitute the switch used on old VW Beetles. Napa carries that switch as number SL143.

FRONT DRUM BRAKES:

An article written by Duane Ausherman discusses assembly & adjustment of the 1955-1976 front drum brakes, with some applicability to the rear drum brake, & drum brakes after 1976: http://w6rec.com/

In my article that follows, below, I have numerous sections where I get into things Duane did not, regarding the drum brakes.

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Rear Drive Ratios

Variables for the chart are your actual on-the-road tire diameters (modest effect); tire pressures/temperature (tiny effect). The variation in UNLOADED diameter of a 4.00-18 rear tire versus a 120-90×18 rear tire is about 15 mm in the WORST case I know of. However, the actual rolling circumference varies little (probably about 2%, but could be larger). Hence the values shown below are THEORETICALLY fairly accurate, and some are taken from a BMW chart dated 1978, others are calculated, and some are actual test data.  Values are theoretical; and do NOT account for tire slippage and tire variations, nor for speedometer and tachometer variations.  Because of these factors, rpm is LIKELY higher for a true speed as charted.

See notes at end of this article!!  The speedometer ratio is printed on the dial of most speedometers.  The author’s website discusses things in much more depth, and includes expanded ratios for earlier models, etc…..link at the end of this article.

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Driveshaft Bolts, the split lock-washers…..and, push-starting cautions

The four TRANSMISSION OUTPUT FLANGE BOLTS CHANGES;…split lock washers, updates, …and….pushstarting! © Copyright 2021, R. Fleischer https://bmwmotorcycletech.info/drvshftboltstoolstorque.htm 47 Background and History on these 10 mm 12 point bolts and lockwashers used at the transmission output flange: The earliest bolts for the /2 & the Airheads (transmission output flange-to-universal joints)  had steel split-type lock-washers, & were […]

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Throttle & Clutch Cables

In my shop I saw many control cable failures from these things.

1. Throttle cables on the Airheads: Left cable failing at the carburetor, due to the throttle cable being bent as owners checked the oil dipstick. Do not bend the throttle cable at the left carburetor when checking your oil. There is no need for the oil dipstick to be overly-tightened. Bending the left throttle cable is a prime cause for that left cable to have increased friction, possibly spread some coils on the wrapped sheath (& making that carburetor difficult to synchronize, if bad enough), & eventually you might break an inner strand …usually where you can see it between the throttle lever on the carburetor, & the sheath. A single strand found broken (You do inspect these cables regularly, don’t you?), will usually cause other strands to eventually break from the same reason why the first strand broke ….this will …or can …result in total cable failure in as little as few hundred miles or so.

2. The bushing at the clutch lever at the handlebars is a replaceable plastic part and as it wears the result is the lever can move up and down & also allow angular motion. If worn enough, the stranded core of the cable will start rubbing, or even catching, on the sharp edged guiding slot in the lever. Eventually a strand breaks, failure comes soon as more strands break. The bushings are easy to replace and not expensive. If your new bushing does not finger press into place, heat the lever first. The Nylon-like bushing is 32-72-1-232-662 and has been used from 1976 onwards. That bushing may need light reaming for a good fit to the pin. If you do not have a 8 mm tapered ream, you can use very carefully selected drill bits, to progressively remove a quite small amount of material, a few thousandths at a time …until the pin fits properly …an easy, but not loose, push-sliding fit. The lever has a recess, and in that recess must be a waverly washer, 32-72-1-230-871. I recommend the sharp edge of the slot in the clutch lever be filed smooth. Be sure the crimped end of the cable that fits into the clutch lever at the handlebars is not fouling the lever.

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Lubing Steering Head Bearings

Cleaning and lubrication of the steering head bearings should be done at a scheduled mileage/time but most let it go until the steering has a ‘notch’ felt in the straight ahead position. This procedure was developed specifically for a 1983/1984 R100RT, but is similar for all Airheads. I advise you to read this procedure through before beginning. Cleaning and regreasing MAY WELL eliminate “notchy-ness” that SEEMS to indicate need for new bearings and outer races. It is best, but not mandatory, to do this procedure after installing new balanced tires, as road crown and tire squaring wear, and balance, might have an adverse effect on trying to make final on-the-road adjustments. This is not hardly just for the front tire….most riders do not know that a squared-off REAR tire is THE most common cause of wobble and weaving from the tires. The author has usually, but not always, done this procedure to his own bikes after installation of a new front tire, but when the wheel is off the motorcycle. If your REAR tire is not squared-off considerably, it will be OK for the final procedure, which are riding tests to get the preload adjustment ‘just right’.  It is possible to do this procedure with the front wheel in place, usually that means having the front wheel hanging over the edge of a curb, or the centerstand is in use and on a piece of wood, so the front end can be dropped a couple of inches.

Cleaning and lubrication of steering bearings is not at all difficult, but if a bearing is found truly bad, replacing them is more labor intensive, as part of any fairing must be removed, and possibly brake piping, cables, etc. Contrary to some popular belief, our BMW steering head bearings of the tapered ‘Timken’ style may well last over 200,000 miles. If the bearings/races/shells are in good condition and properly greased and adjusted, the steering will be light, smooth, without any straight ahead notch. You likely will not find out if the bearings are really bad, that is, in need of replacement, UNTIL you first try cleaning and greasing.  In a SHOP situation, the bearings are not cleaned and lubricated and then adjusted to see if any notch is gone. In a SHOP situation, labor is too costly for that.  A shop can not take the time to clean and regrease, and then find out that the bearings really are bad, so a shop always replaces a notchy bearing.  YOU, on the other hand, don’t need to do that…..and will often save a LOT of money, and a considerable amount of labor saving is possible.

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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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