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FlatRacer BMW Mancha Negra project

FlatRacer BMW Mancha Negra project

Our FlatRacer Mancha Negra started with another project Edgar got involved with. It centered around an ex-racing BMW R69 from our friend Dr. Rafael Povoa. The original idea for this R69 was to build a mild custom bike, keeping many classic elements in place and subtly altering its looks, the most noticeable being a Vincent fuel tank.

As the BMW R69 Super Schnell build progressed, the idea of another custom bike started to germinate in our minds. Something along the same vein but slightly different.

For this purpose, we've acquired a BMW R50 frame and started to build a tuned R100 RS engine based on an engine block that we had used for many years as a frame mock up for the development of our products.

The engine was built by Edgar and Filipe de Sousa from TwinFlat (www.twinflat.pt) and was tuned to provide superior torque and low engine speed tractability without omitting the top end thanks to the Dell'Orto carbs.

The running gear was certainly a challenge and there were some obstacles to be conquered such as the hybrid rear swing arm, a well know issue when fitting a post 69 running gear on a /2 frame.

Our Mancha Negra features a 1979 BMW R100 RS engine block with 1977 RS internals including the timing chest and finned front cover. The cylinders and pistons are however post 81 so Nicasil coated (reduced oil consumption). The gearbox internals (five speed) are also post 81 (thus allowing for the later and lighter flywheel) fitted with close ratio gears but built into a four speed /5 smooth outer case. The forks were sourced from a 1939 BMW R66, the front wheel from a Honda Black Bomber, the tank from a Suzuki GS400, the mudguards and tail light from classic BSA's just to mention some of the most noticeable parts.

The FlatRacer Mancha Negra has no pretensions of a Bobber, Cafe Racer, Custom or Resto-Mod project. It is a classically shaped motorcycle that we've created from scratch as a break from our day to day Cafe Racer work.

Below you can read our adventures and tribulations building the Mancha Negra, enjoy!

FlatRacer Mancha Negra BMW custom motorcycle LH to

Project progress

Due to our normal working hours and incredibly busy schedule, the FlatRacer Mancha Negra took us over a year to build, which is fast by our standards thanks to the assistance of several specialists. We would like to thank the enormous help of Antonio Pereira dos Santos, Filipe de Sousa, Simao Branco and Fernando Rodrigues from TwinFlat (www.twinflat.pt) and Jorge Rocha for the outstanding paintwork.

Frame & Handling

We've started with a 1963 BMW R50 frame than in order to accommodate the large engine, we had to persuade to open slightly its lower bars with a scissor car jack (our hydraulic jacks wouldn't fit). As were using a larger 12v Odyssey PC545 battery and one of our s/s battery trays for the Monolever models, we had to fabricate new lower mounting plate and side lugs to accept the BMW waisted rubber bobbins. The frame seat mounting plate had to be re-positioned and re- welded to clear the R100RS air filter box. Some damage had to be attended to, such as fabricate from scratch new centre stand mounts. The frame was then hand sanded, powder coated for rust-proofing and re-painted in Spies Hecker 2 pack custom mixed paint.

To mount the tank at the rear and domiciliate the front mounting plate for the modified Biltwell seat, we fabricated a single part for both purposes. It is attached to the frame through four s/s bolts in a similar way to the original seat. Lugs had to be welded to fit the scissor seat springs. This was an involved task as we had to ensure perfect symmetry or the seat would not align with the frame centre or worse, the springs would be askew. We’ve used a simple digital angle finder and a vernier calliper to accurately positon the lugs and tack weld them in place before final wrap around weld taking in attention and keeping in check heat distortion.

As we were installing uprated 12v electrics with relay and fuse boxes of our own design, we replicated a very rare R69S under the tank fitting plate. This allow us to neatly install coils, relays, fuse box, regulator etc.

Front end - Forks, wheel & bars

Originally, due to supply constraints and practicality, were we going to fit a /5 front end, a solution that we were not overly keen (look at the awkward /2 US models). Very luckily, we've managed to track down a 1939 BMW R66 telescopic forks in fairly good condition. We've fully rejuvenated these, including fitting new stanchions intended to fit Russian M72. The steel quality is actually quite good but we had to machine the tops to fit the top yoke and new stainless steel top nuts. As we wanted the headlight to be positioned lower, we've unsoldered the original pegs from the fork upper cowls and re-positioned these lower by brazing them back.

The front wheel is from a 1966 Honda CB450 Black Bomber laced to an 18" flanged alloy rim and s/s spokes to which we’ve fitted a new Metzeler Me11 tyre. After some calculation, we've designed and machined a spindle and spacer to fit the Honda hub to the BMW forks. At the same time, we've fabricated new stay brackets out of 15mm s/s tubing and machined spacers that allowed us to fit one of our BMW R65 fork braces without modifying the BMW R66 fork sliders. We’ve then welded some lugs to the brace in order to accept our new 4.5" s/s mudguard.

The handlebar is made of stainless steel, designed to fit the Vincent twins, fairly easy to fit, it required only drilling a centre hole for the bar end indicators. The new throttle and /5 housings had to be reamed to 7/8", a common problem as BMW used slightly thinner tubing for their bars (typically 21.9-22.00mm) and the industry standard is imperial sized 7/8” so 22.2mm.

Engine & Gearbox

The engine was a mix 'n match of parts from varied sources. We had to forage our stores and luckily found almost all the parts necessary to build the engine albeit we had to cannibalize our project R100 RT from 1983 (the future Mancha Rapida basis) for its Nicasil 1000cc cylinders. The engine block and outer cases were thoroughly cleaned and sand blasted with fine silica where applicable (the result is a good as aqua blasting).

Edgar and Filipe from TwinFlat (www.twinflat.pt) then rebuilt the engine with the most utter care. All the wearable components were renewed, such as piston rings, gudgeon pins, big end shells, main bearings, oil pump, cam chain kit, seals, etc., etc. In order to fit the post 81 Nicasil coated R100 cylinders to the earlier block, Antonio Pereira dos Santos machined a small recess in the cylinders' bases to allow us the fitment on the earlier block, something he does fairly often for the TwinFlat new builds.

The gearbox is an interesting part as it was supplied to us already fully rebuilt with close ratio gears by Steve Scriminger from SED (www.scriminger.co.uk), apparently intended for a racing side-car rig. Although the gearbox is internally based on a post 81 design allowing for the lighter flywheel, the outer shell is an early 4 speed /5 smooth case, ideal for this job.

Carburetion

The early R100 RS engine was originally fitted with Bing CV 40mm carburettors which work absolutely fine, however, we've felt the need to fit something more exotic looking that offered that extra performance and zest.

The installed Dell'Ortos PHM 38 were standard equipment on the BMW R90S and provide that extra rush of acceleration thanks to its pumper design, injecting circa 4cc (adjustable) of extra fuel every time the throttle is opened fully (WOT), offering instant acceleration.

These are fairly easy to fit and we do sell a similar conversion kit that includes not only the brand new Dell Orto PHM 38 carburettors but also the superior stainless steel adaptor stubs, original BMW inlet tracts, rubber sleeves and throttle and choke cables albeit we've used manual choke levers for practical reasons.

We've kept the original RS air box filtration, using a superior and free-flowing K&N air filter. By keeping the original long inlet tracts, we've maintained good venturi effect; promoting good gas flowing and lower engine speeds, making for a more tractable engine. Head ports were gently de-burred and cleaned (although not polished) for a better gas flow and performance.

Exhaust

As we were using a R100 RS engine, we've adopted one of our s/s single balance pipe exhaust manifold systems designed to fit the /7 Series albeit without the kink at their ends, where they meet the silencers. It was fairly easy to adapt and the /7 exhaust mounting clamps attached to the engine studs were retained without issues.

New manifold/cylinder head ring clamps and big rose nuts (commonly found on the R69S and supposedly better at dissipating heat) were fitted without issue.

The rear silencers are a standard BSA Gold Star DBD34 fit that we easily adapted, requiring only from us to fabricate mounting brackets made of stainless steel tubing (heat bent, squashed flat and drilled to bolt to the silencers and frame mounting lugs). The original R50 centre stand needed just a slight nudge to clear the silencers when retracted.

The noise is intoxicating and despite being loud, it is not annoyingly so.

Rear end - Drive train, shock & wheel

Very seldom you find good working original shocks on bikes that are +25 years plus. We've decided to shun the original shock cartridges and fit Hagon replacements. These offer superior performance and excellent value for money. We've then sourced the highly polished stainless steel rear shock turret caps, lower shock shrouds and swing arm pivot caps for longevity and good looks.

Hagon supplied the 18" flanged 2.5" x 18" rim which we laced to the original /7 hub with stainless spokes and nipples. Our choice of tyre fell on the ubiquitous Metzeler Me77 for its excellent performance and looks. The sourced 37/11 bevel box was in excellent condition and required nothing more than good clean and fresh oil. It was however necessary to machine a new stainless steel rear wheel spindle to mate the hub to the LH side of the R50 swing arm.

The modified swing arm proved a challenge to modify and fit with the correct geometry and alignment. We've used a front section from a R50 and a rear RH section from a /7 that had to be spliced and welded in place with great care. We've then fabricated a bracket made of 10mm s/s plate, heat bent, drilled and trimmed to size, to connect the RH shock to the /7 hub. A spacer made of 4340 high tensile steel was made to connect the drive shaft to the gearbox output flange using 12.9 longer HT steel bolts.

Clocks & Electricals

We've used an original and brand new BMW /5 main wiring loom. It was fairly simple to adapt to the /2 frame as we were using /5 handlebar switch gears and the very similar /2 headlight connector board/ignition panel plus /7 engine electrical sub-looms which are quite easy to adapt.

For practical reasons, we've cut the main loom under the seat so out of view and used weatherproof connectors to create a rear sub-loom to ease the removal of the rear mudguard if the need arose. As per our usual practice, we only use weatherproof connectors and properly insulated Lucar terminals (4.8 and 6.35mm) with clear silicon sleeves. No colourful pre-insulated terminals as these are a pet hate to us.

We've fitted a new 450w alternator together with its uprated diode board and a high output voltage regulator (allows full charging from lower engine rpm, as fitted on Police bikes) plus a Boyer Bransden Micro Power electronic ignition kit from our catalogue of Performance parts. Battery is the truly superb Hawker Odyssey PC545, installed in one of our stainless steel tray kits designed to fit the Monolever models. In order to secure the battery, we had to modify original BMW /5 straps (grind the wider ends) to accept the lower battery and the Mono tray.

Bodywork & Paintwork

After some sketches done by Edgar and weighing all options for its design, we've decided in some unusual choices.

The fuel tank is 1978 Suzuki GS400 fuel tank heavily modified with some neat mods such as the Fiat Punto engine brazed in core plugs to for the side logos and the recessed section for the clock . This conversion along with other fabrication work was done by Fernando Rodrigues at TwinFlat (www.twinflat.pt) and Edgar.

The front and rear mudguards are made of stainless steel and intended to be fitted on a classic BSA, as it is the tail light assembly, that required heavy modifications and fabrication to fit. The seat is a heavily modified Biltwell unit, upholstered in dark red leather by CE Moore (www.cemoore.co.uk) in Park Royal, London who are conveniently just 300 yds from us. The forks were sourced from a BMW R66 from 1939, the rebuilt front wheel is from a Honda CB450 Black Bomber and the handlebar from a Vincent in stainless steel.

As we always recommend to our customers, we've assembled and dry built the bike prior to the final paintwork. This allowed us to envisage the best way to fit the parts and carry out minor tweaks without unnecessary and unsightly damages. Yes, time consuming and frustrating at the times but we can assure you that it is time well spent getting the minute details right.

Once we were happy with the bodywork and the way it would be fitted to the frame, it was only left to choose the colour and proceed with paintwork. Edgar and Jorge Rocha, our usual painter, did the extensive preparation work with Jorge gaving it the final lick of 2K paint. Although very toxic and requiring proper breathing apparatus, we prefer 2K (two pack) finish for its toughness, superior finish and longevity.

We've painted not only the tank but also main frame, swing arm, fork shrouds and sliders, fork yokes, headlight shell, number plate, handlebar switch gear housing and levers plus other small parts. The paint colour formulation is Jorge's own concoction, based on Solid Black albeit mixed very subtly and in small quantities of very fine Gold and Rubin Pearl dust. The effect is only visible under direct sunlight, giving the whole bike a very subtle warm dark copper glow. Supplies were made by our long time favourite, Spies Hecker, who also provided their truly superb body fillers to perfect minor issues.

The end result was well worth the effort and we are quite pleased with the Mancha Negra looks, on it flows nicely together and has in our view the correct stance.

If you want to build a similar bike, please do contact us, we can supply many of the parts featured on the Mancha Negra and can offer some build advice based on our experiences.