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Christian Bynum's Classic BMW Motorcycles: Conversion Bikes
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Greg Scallon's pristine conversion, mating a '60s /2 frame with a later '70s powerplant

A "/2 conversion" motorcycle is a hybrid BMW bike constructed from Münich-made and Spandau-made parts; the '60s /2 twin-loop chassis is outfitted with a '70s engine, electrical system, and drivetrain. A conversion bike can be a great way for Beemer enthusiasts familiar with the classic range of /5, /6, and /7 motorcycles to get in on the aesthetic qualities of vintage BMWs but with greater practicality, a lower front-end investment, and reduced long-term maintenance.

For most regular riders who can wrench to some extent but are not necessarily up to the most daunting and involved maintenance and restoration tasks, a stock /2 is much less practical for regular riding on today's streets and highways than a /5 or later machine. Despite their bombproof reputation and their ability to kickstart and run even with a discharged battery (thanks to the nifty magneto ignition system), /2s have some notable drawbacks. The older /2 engines roller engines have difficult-to-clean oil slingers rather than a modern oil filter. They use 6-volt electrics with more limited load capacity. Their engine power and torque characteristics are not in line with requirements of high-speed highway touring. Additionally, as the value for all-original /2s continues to rise, stock Münich-built bikes are quickly becoming too valuable for many owners to want to ride on a daily commute.

Thus, /2 conversion bikes enjoy a distinct popularity within the larger BMW motorcycle community because they have the vintage bike mojo and remain relatively practical at the same time. They are typically more expensive to rebuild than a later Airhead, but they have a style and a ride quality that's undeniable, and with the /5-/7 upgrades, they are as hearty and defensively aggressive on the street as the later bikes. For Beemer sidecar outfits, /2 conversions are especially favored, enjoying a sturdy chassis with a twin-loop frame and leading link fork, while having the horsepower and durable drivetrain of the '70s Airheads. Even though they are typically worth less than stock bikes in good original condition, these nicely restored conversion bikes and sidecar outfits hold there value in the used motorcycle market.

As hybrids, each conversion is an individualized work of art and engineering, a collection of unique mechanical solutions and creatively coaxed components put together to create a very special kind of riding synergy. Think how many scuttled /2s there are scattered across the globe -- languishing in disuse and exposed to the elements -- most with sound frames but seized old-style engines that will never be put back into sound working condition due to the increasing scarcity of powerplant parts. Why not make conversion bikes out of them?

I am currently assembling two conversion bikes:

A /2 conversion project is not for the faint of heart; even experienced Airhead riders and restorers may find the decisions and challenges of making parts not originally designed to work together perform in desireable fashion to be an involved process with many headaches on the way to its eventual long-term payoff. By virtue of age and limited production, the necessary /2 parts can be difficult to find and expensive to purchase (as new old stock, used, or reproduction), and there are certain key components in any conversion project that must be custom-fabricated or machined to specification, taking a typical conversion restoration a step beyond all but the most involved rebuilds of stock classic or vintage motorcycles. The major engineering decisions common to all /2 conversion projects, each with its own alternatives and trade-offs, are outlined below...

Frame

Virtually all conversion projects are based on 1960-69 twin-loop swingarm frames from the following BMW models: R50/2 R50S, R50US, R60/2, R60US, R69S, and R69US. Although adventuresome souls have attemped to adapt the chassis from earlier plunger-frame twins and single-cylinder bikes with mixed success, these conversions are rare and generally not recommended. There are some key changes in the frames for these model years. In 1964, strengthening gussets were welded into the twin-loop frames to better handle torque stress and shear loads from sidecar use; in addition to their visible reinforced areas just below the swingarm pivots, these later strengthened frames sport a shortened lower main frame tube that terminates well ahead of the welded-on rectangular tank mount. During the 1967 production run, sidecar lugs were deleted from the /2 frame; therefore, those looking to build a conversion outfit should look for a '64-67 frame that has both gussets and lugs. Conversely, projects destined to become solo bikes with smaller-displacement Airhead engines (500-750cc with output up to 50HP) can use virtually any available /2 frame as a sound foundation.


Front Suspension and Steering

The Earles leading link fork is the heart of the /2s legendary ride, and the old-style front end is highly sought-after for many conversion bikes. In addition to the unmistakable vintage styling and visual appeal of a front swinging fork, the Earles front end trades off some quickness of steering versus later-style telescopic forks for compliance of ride and superior resistance to lateral stresses, particularly important for sidecar outfits.

These days, a good Earles fork can be frustatingly difficult and woefully expensive to source. Only the leading link forks from the /2 twins are suitable for conversion bike use; the superficially similar forks from contemporary single-cylinder models like the R27 cannot be used. Most of the units that have been separated from their original frames depart from perfect straightness to some degree; they can be realigned by skilled specialists, but only at additional expense. The suspension units themselves often have to be rebuilt or reassembled from pricey individual reproduction parts. Wheel and brake choices are also limited with the leading link front end, as are handlebar choices, by virtue of the unique Earles fork top plate and bar mounts. (However, there are some handlebar options, such as original or reproduction Hoske clip-on racing bars (which actually bolt on), that will only fit an Earles fork with its unique top plate.)

Even if complete and in good condition, an Earles fork cannot be simply bolted on for a conversion bike project. The Earles fork for /2 twins has a bowed rear brace that in most cases lacks sufficient clearance from the front engine cover when the larger Airhead engine is installed in the twin-loop /2 chassis. Fitting of a /5 front cover or machining of a /6 or later front cover can help create additional clearance, but a minimum clearance of 12mm must be available for safe operation of the motorcycle. Most often the Earles fork brace must be relocated 25-50mm higher on the fork or cut and patched with a welded plate to reduce it's fore-aft dimensions; this extra machining should be done by a skilled professional and adds to the overall cost of the front-end installation.

As a cost-effective alternative, one can fit a /2 US or /5-/7 telescopic fork to a solo conversion bike for a fraction of the cost of a fully functioning swinging fork model, and have the advantages of long travel, quick steering, and even front disc brakes (with a handlebar-mounted master cylinder) if one is willing to sacrifice the Earles' superior strengrth and fabled cloudlike ride. Standard /2 steering head bearings and races are used with special adapter sleeves on any /2-/7 telescopic fork, and the entire steering head can be rebuilt and adapted for less than $100. In addition to saving money and allowing the use of more affordable and available (albeit less sturdy) 19" Airhead wheels up front instead of an 18" /2 model, bolting on a telescopic fork gives you additional choices for braking (see below). The telescopic fork also obviates any clearance problems with the front cover of the larger '70s engine.



Top: Earles fork front end (Brock Downey)
Middle: Detail showing fork clearance (David Makin)
Bottom: Telescopic fork adapters (left), bearings, and races for /2


Rear Suspension (Swingarm) and Final Drive

All 1960-69 /2 twin-loop frames employ a twin-shock swingarm rear suspension; stock /2 rear shocks are typically used to cushion the back end of conversion bikes. However, the conversion builder has several swingarm options available for creating a suitable driveline for the output of the '70s Airhead powerplant within the earlier /2 frame.

The stock /2 swingarm and driveshaft can be retained, permitting the use of a /2 final drive and rear wheel; to accomplish this simplest plan, a spacer adding length to the driveshaft and remapping the driveshaft bolts from the U-joint to the /5-/7 transmission output flange must be machined and installed. If you have a complete /2 rolling chassis, this option is cheap, even with machine work figured in, but the spacer option provides the least sturdy means of linking the Airhead engine with the rear wheel and is suitable only for lower-output (40HP or less) conversion applications.

The second option is to use a /6 driveshaft that has been machined to fit within a stock /2 swingarm; the cost-effective usage of the longer Airhead driveshaft doesn't require an additional separate spacer and can accomodate engine output up to 50HP reliably, while still allowing use of the original /2 final drive and rear wheel.

The final alternative for the rear end is a custom conversion swingarm. A specially machined hybrid unit with an installed /6 driveshaft relocates the rear shock mount and allows the use of the rugged and reliable /5-/7 final drive; although much more expensive than keeping the /2 swingarm ($400-$500), a custom conversion swingarm is by far the most roadworthy, able in conjuction with any Airhead final drive to handle the demanding torque requirements of even the most powerful later engines. The maintenance savings from using the hardy later final drive may actually offset the initial expense of a custom swingarm, making it a more cost-effective choice in the long run. Conversion swingarms are also strongly recommended for sidecar outfits.


/2 (background) and custom conversion swingarms
Photos by Brock Downey


Ignition Coils

Space is at a premium in /2 conversions; the larger Airhead engine not only requires more room for itself, but it also demands that additional bulky components be mounted on a frame not designed for them. The high-voltage coils that power the modern ignition system for the Type 247 Airhead engine are one such space consideration for conversion bikes. The camshaft-driven magneto ignition unit on a stock /2 uses a built-in ignition coil and is housed completely within the diminutive crankcase of the older engine. On /5 and later machines, the ignition coils are separate units, nestled under the fuel tank on either side of the double-wall main frame tube, bolted to purpose-built welded mounts. A /2 conversion project's design challenge is to accommodate the external coils in a substantially more limited undertank volume already bristling with other components.

Clever restorers have experimented with over-under and fore-aft mounting of the Airhead ignition coils on the /2 main frame tube, and more drastic relocation options are feasible. However, perhaps the most elegant solution to adapting a later ignition system to a /2 chassis involves replacing the big Bosch units with a single compatible dual-lead ignition coil from another make of motorcycle. As shown in the photos at right, a dual-lead coil (such as the stock model from a Kawasaki KZ440, pictured above right with Bosch coils for comparison) takes up very little space and provides spark to both cylinders of the Airhead engine.

The lower photograph at right shows a typical dual-lead coil mounted beneath the fuel tank on the /2 frame. Dual-lead coils from various Japanese multi-barrel motorcycles will work for this application, and coils from 440cc and 550cc Kawasakis contemporary to the Airhead bikes are favored; used and in good condition, these coils are ubiquitous and exceeding cheap. Suitable coils should have detachable high-tension leads (i.e., not epoxied in place), as extra-long spark plug wires must be constructed to accommodate the longer span from coil to cylinders on the conversion bike.



Battery

To some extent, a battery is a battery. However, conversion bikes must balance competing damands on their storage batteries to optimize performance within their unique constraints. Higher amp-hour and cold-cranking amp ratings are necessary with the larger displacement and higher compression of the later engines, but the big powerplants in the older frames also make space for the battery very tight. Stock Airhead batteries of standard dimensions cannot be used in the standard location for /2 conversion applications, so builders must get creative as they design their electrics.

All manner of small vehicle batteries in the 14-18Ah range have been used successfully in /2 conversions. Modern sealed, maintenance-free AGM/gel batteries are preferred, since they are easy to care for and do not require any provision for a battery acid vent tube. Twelve-volt units are, of course, the norm, matched to the upgraded electrical system, but even pairs of tiny 6-volt units have been wired in series on some bikes (thereby doubling the voltage) in order to minimize volume. Shorter, wider batteries that are sufficiently thin can be made to fit in the stock battery tray on the /2 frame, but space is tight because the bulky Airhead airbox creates some impingement. Taller batteries with narrower side-to-side dimensions can squeeze between the tubes of the twin frame loops to exploit the area under the seat but are limited fore and aft by the seat mounting plate. More drastic battery relocation is possible, and it is common for conversion sidecar drivers to mount a large, powerful automotive battery in the trunk of the sidecar itself.

There are many viable solutions for mounting an appropriate battery in a conversion bike frame, and careful planning and research will uncover feasible individualized options.


Wheels and Brakes

Swingarm choice dictates the rear wheel used on a conversion bike; any option that retains the /2 swingarm (see above) also retains the /2 final drive and stronger 18" /2 wheel. A custom conversion swingarm will mandate the use of the more robust Airhead final drive and an 18" /5-/7 rear wheel. Unless special machining is attempted, either /2 or /5 drum brakes will be fit to the rear of any conversion bike, appropriate to the wheel and final drive selected.

Up front, Earles fork conversion bikes can use the stock 18" /2 wheel (the same as the rear wheel) and an Earles fork drum brake backing plate with drum brakes, distinguished from the /5 front drum brake unit by its deeper dish, wider shoes, and machined, block-shaped cast alloy brake stop. As an alternative, builders can opt to use the /5 brakes, brake plate, and 18" rear wheel (thanks to identical fore-aft hub castings). The brake plate must be modified with a weldd brake stop machined to the dimensions fo the /2 brake plate's in order to engage the Earles fork, and minor spacing adjustments may be necessary, but the /5 wheel and brake parts may be easier and less expensive to source.

Conversion bikes with telescopic forks can fit the stock /5 fork lowers, 19" front wheel, and drum brakes or upgrade the front end to a later model with either single or dual front disc brakes, using later-model Airhead handlebar control levers and a handlebar-mounted disc brake master cylinder. It is a compromise of vintage aesthetics for the sake of improved performace and practicality.

Most builders of solo telescopic conversion bikes find the inexpensive and easily maintained /5 option with its simple yet powerful twin-leading shoe drum brake to be a nice option that minimizes the impact on the /2's vintage looks as while affording the bike both practicality and roadworthiness. Conversely, many conversion sidecar drivers, already wed to the strength of the Earles front end, match their front wheel and sidecar wheel to the rear wheel on the motorcycle (either /2 or /5 rear) so that a fourth wheel, carried as a spare on the back of the car, can be used to replace any of the three rolling wheels. Drum brakes are used in any matched-wheel application (/2 or /5). Some sidecar owners go a different route, fitting custom single or dual disc brakes to their Earles fork and carrying two spares, front and rear, stacked on the back of the sidecar.


Modified /5 brake plate (top) and matching-wheels (x 4) sidecar outfit
Photos by David Makin


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