Cycle News - Archive Issues - 1980's

th e creation o f th e pow erplant th at would m eet th ese goals . Probst 's ra cin g back g round is unmistakable in the design o f the prod uc tion engi ne; Ihl'SOHC layo u t was an ea rl y casua lty in his qUl'SI for power . With a ll th e ca refu l co nsidera tion spent o n the nature of the new BMW m otorcycle engi ne, it's not surprising tha t th is powerpla nt is th e cen terp iece of the K100. In th e sim p lest term s, the engine is a liq u id- co ol ed , DOH C in line four, m ounted hor izon ra l lv with t h e cyl ind er h ead o n the left side o f th e motorcycle. The cra nkshaft ru n s lon g itud in all y. a nd is mou nted to the righ t of the bik e's centerli ne to ma in ta in overall bal an ce. T he cra n k is forged steel , a nd is su pporte d along its length by five m a in bearings. Seve n of th e cra n k we bs a re fo rged as cou n te rba la nce we ig h IS, wi t h th e eigh th we b m achined as a gear 10 drive th e clutch a nd secondary shaft. U n iq ue to th e KIOO is th at the cra n ksha ft is full y access ible a fter remo vi ng the right side e ng ine co ver. The cra n k, main a nd rod bearings ca n a ll be service d with the engi ne insta lled in th e fra me . Forged steel co n nec tin g rods link th e cra n ksh a ft to th e pistons, running in plain bearings a t both ends. Pi ston bore is 67mm , with a stroke of 70mm to m ake the en g ine slig h tly und er squa re. The relatively sma ll piston bo re was chos en for tw o reaso ns : Fir st , th e undersquare sma ll bo re/ lo ng-stroke engine is inherentl y torquey a t very low rpm - a cha racter istic lo ng associated with BMW. Seco nd, th e sm a ll bore m eans th e engi ne ca n be more co mpact: a n im porta nt cons idera tion wit h t he lo n g itud i na l e ng i ne o rie ntat io n . BMW was serious a bo u t engine dimensions: eve n th e cyl inder wa lls are " Sia m esed" together as a u n i t casti ng with in th e e ngi ne bl o ck. There 's no room for larger pistons wi th this d esign, a nd the K 100 engine in p rod uctio n to day will never grow beyond 1000cc. As with th e recen t Box er engi nes, the KlOOdoes not use cas t iron slee ves i n aluminum cy lin d er s. Instead, the aluminum cy linder walls arc lined with BMW's Scan imet (nickel-silicon ca r bide) coating. Since the KIOO engine was envisioned as a torquey and (rel ativel y) low-revving powerplant, it's not too surp ris ing that BMW opted for a conservative two-valve cylind er h ead. Standard p oppet valves measure 34mm on th e intake side, and 28mm o n the exh a u st. Valves are act ua ted by sh im- adj usted buck et ta ppets rid- in g agains t d ouble o verh ead ca msha fts. Val ve adjust ment sh ims are o n top o f the ta p pets, a nd a specia l 1001 facilitates shi m rem o val without rem oving the ca msha fts. The cam shafts a re chain-dr iven by a sprocke t on the fro m e nd of the cra n ksha ft, a nd th e ro lle r chai n is continually adjusted by a h yd raul ic ten sio ner opera ted by a light spri ng a nd engin e oi l p ressure. In spite o f the simple valve la you t, the KIOO fea tures a co ntem porary cyli nder h ead. Viewed fro m th e side, th e int ak e va lv es are positioned slightly forward of th e ex ha us t valves, p romoting a swi r l to the in ta ke m ixlure for im p rov ed com b ustio n . T he ' combus tio n chamber itself is ro ugh ly D-sha ped, with a ge nerous squis h a rea to prom ot e turb ul en ce d u ring th e co m p ress io n stroke. T he spark plug is located towards the center o f th e co m b us tio n cha mber for optimal flame propagati on. Th e e nd result o f all thi s co m bus tio n engi neer ing is that th e KIOO will di gest crumm y regular-grade gasoline - wh il e running a respectable 10.5:1 co m p res sion ratio. Controlling th e co m b us tio n mixture is the electron ic fuel inj ecti on syste m, a variant o f th e proven Bosch LE-Jetron ic un it used o n m any European autos. An electric fuel pump. located within th e aluminum gas tank, su p p lies fu el a t a constant pressu re to four injecti on nozzl es (o ne p er cy linde r). The vo lume of th e pump act ua lly far exceeds what th e e ng ine ca n usc, with the excess fue l cascaded back into th e gas ta nk. T his co ns ta nt fuel flo w m inimizes heat b u ild-up at th e injecti on nozz les a nd preven ts vapor -loc k w ith i n th e inj ec tio n system. Sensors a re employed to d etermine the tem pe rature o f th e intake a i r, tem perature of th e engi ne, q ua nti ty o f inta ke air, a n d engine speed . T his in fo rmation is fed into th e fu el injection co m p uter, m ounted under th e seat. The computer processes input from th e sensors to det erm in e the timing a n d duration o f the fu el delivery from the inj ection nozzles, which spray into the individual intake mani folds . This same computer is part of th e electro n ic ignition system. featuring electron ic (not mechanical) rpm d erived spark advance. An elec tro n ic vacuum sens o r in th e in ta ke tract is also linked to th e spa rk adva nce. provid inga vacuum -ad vance fun cti on to th e system . In addition, th is co m p u ter p revents th e engine from o ver-revvi ng. If the en g ine speed reach es 8600 rpm, Instrumentation is simple. Speedo and tach are electrically-triggered; other readouts are handled by idiot lights. 2~ Our best quarter-mile time was 12.12:·within a few clicks of BMW's 11-second goal. A grabby clutch made high-rpm launches difficult. th e ignition ti mi ng is re tarded to 60 BTDC. effec tively reducing engine powe r. Shou ld th e rider ignore th e power d rop a n d insist on burying th e tac h, th e fuel injection co m p uter shuts o ff th e injectio n no zzles a t 8750 rpm. Ex ha us t is ro u ted th rough a fourinto-o ne syste m co ns tr uc ted o f sta inless stee l. T he muffler h as a curious rectangular p rofil e. a nd is mounted o n th e left side o f the m ot orcycle. A black steel h eat sh iel d runs th e fu ll length of th e muffl er to prevent burned shoes a nd ankles. Interesting a bo u t the K I00 is that if a turbocharger were mounted just aft of th e cylinder block, th e intake feed could be routed directly to the top of the . en g i ne - resulting in the shortest turbo ex ha us t and intake tr in the act industry. Now yo u don 't su p pose that BMW is ... Naw, couldn 't bel Power output from th e engine is via a dry. single-disc, diaphragmspring clutch mounted in a forged aluminum basket. The front of the cl u tc h sports a spring- loa ded da mper to cu sh io n the eng ine agai nst th e shock of downshifting . As with th e o ld Boxer-bikes, th e cl u tch is ca ble activa ted, progressi ve and ea sy to control, a n d has a very li ght cl u tc h pull. T he transmission o f th e K I00 o wes its heritage to the Box ers th at preceeded it . Both a re three shaft, fivespeed un it s vi rt ua lly in d ent ir al in la yout a nd d esign (a n d in fact were both desi gned for BMW by th e G erman engineerin g firm G etra g ). T o make the K100 as light as possibl e. th e selec to r drum and the sh ift forks a rc both m ade of aluminum alloy . While adeq uate for normal street use, powershifting and other dragstrip antics are bad business with the KIOO. In sp ite of the few ex tra ounces of weight, forged steel m ay have been a better choice for the shift forks. From the transmission, power is routed through a univers al joint to a feature synonymous with the BMW marque: shaft drive. Like th e R80 G /S and th e R80 ST, the KlOO employs a sing le-sid ed swinga rm . (A side note for th e nit-picky: the KlOO has a sw ingarm, wher e most motor- cycles em p loy a swingi ng fork - fork describing a two-pronged suspensio n co m pone nt. This d istin cti on may gain si g n ifica nce with th e advent o f hubcenter steeri ng; a d esi gn th at em p lo ys a swin garm in th e front suspens io n.) U n li ke that o n th e 800c c Box er s, the sw i nga r m o f th e K-bi ke is m ade entirel y of cas t a lu m in u m , with ribs ca st a lo ng its length to res is t torsio n a l loads . The swingarm pivots in tapered roll er bearings m ounted directl y on th e ba ck of the transmissio n housing. Therefore, the drive train of the BMW KIOO - from the front of the engine to the rear wheel - is a rigidly bolted-together assembly made of massive cast-aluminum components. Since th e rear wheel fits onto a swi n g a r m, changing the rear tire couldn't be easier. Four bolts attach the hub directly onto the ring gear of the final drive. More importantly, the rear disc brake is mounted i n boa rd of th e wheel. a n d need not be disturbed for ti re changing. Simply ce n tersta n d the bike. remove four bolts. and roll the wheel out from under the fender. No m ore leaving th e ca liper dang li ng by its hose. or pounding axles o u t wi th gr eas y screwd r ivers. With a n integrated and rigid drive lin e, th e engine acts as a frame co m ponent a nd th e need fo r stre n g th in th e m otorcycl e frame is g reatly reduced . In fact , th e frame need only locate a nd su p port th e steering head, su p port th e sea t, a n d p rovide a mount fo r the gas ta n k. As a result. the stressed -mem ber fram e of th e KlOO is a n abbrev iated four-legged tubular steel structu re th at hunches ov er the top o f the engine - weighing in at less than 25 Ibs. The KIOO's front suspension is littl e changed from the traditional telescopic forks found on the Boxer twins. New for the KIOO is an aluminum upper tri pie clamp (the old Boxers used a sta m ped steel plate to locate the tops of the fork tubes ). and an over size front a xle to minimize flex. Conspicuously absent are any form o f fork bra ce, spring preload or damping adj ustm ents. or any form of a nti-d ive system. BMW claims to have

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