Cycle News - Archive Issues - 1970's

PART IV: THE MACHINE FACTOR THE THEORY c ~ ..... '" ~ w Z w - -' U >U b y Lan e Campb ell Up to n ow we've n ib bled at the th e ori e s of fast co rne ring witho u t t ou ching very he avil y on th e m otorcycle its elf. So fa r I've tried to sho w wh at t ir es will do for y o u o r agai ns t yo u , then gone into th e b asics of racin g line. No w its time to throw in the m ach ine which ay or m ay not all ow you t o dev elop th e m ost in co rn erin g p ower. L e t ' s l o o k a minut e at the motorcycle as a fre e body and how it rea cts t o the forces around it. To do this we pi cture the motor cycle as a sin gle isol ated object b eginning at rest. The weight of th e m achine for all practical purposes ca n be pictured acting as a force straigh t down thro ugh the b ike 's CG , If th e m achine is restin g on the ground i ts wei ght presses d own with a total fo rce o f " \I' '' th rough the ti res : but we ca n also say th e gro u n d presses UP on the tires wi th a n eq ual and op posite fo rce. Whee ! Newto n 's T h ird Law an ' all th at. So in o u r pictu re if we ta ke away the ground an d lo ok only at the mo torcy cl e we can replace the ground 's influ ence b y tw o fo rc es " F" an d " R" w ho se t otal is " \1'," O K, now let 's talk ab out a typica l 2 50. If it weighs 2 6 0 lb. a nd a 16 0 rider m o un t s it t he gross weig ht of m a n /m ach ine is 4 2 0 lb. We 'll call the weigh t di stribut io n 50 / 50 to m a ke th in gs easy, so at res t each wheel sup ports 2 10 lb . This will also b e true a t cons ta nt speed , strai gh t up . Now , get re ady for so me m en tal ex ercise a nd c heck o ut Fig . I to fix t he size, wei gh t a nd CG o f -o ur im agin ary 250 in you r mi nd . . W e 'r e g oi n g to ta l k a bout accele ration an d a co up le o f th ings wh ich wo rk fo r a nd against you . If we c o n t i n ue to take o ur tire friction co efficien t to b e o ne, whic h m a kes thin gs easier. then sta rting from rest o r f ro m e q u i libr ium , a m or e precise e n g ince ring term wh ich also co vers constan t speed ,strai gh t up , the max thrust ava ilab le at th e rear wheel is eq ual to t he we ight bearing o n it , na m ely 210 lb . So in the init ial instant of accel eration th e m ax acc ele rat io n will be thrust (2 10 lb .) divi de d by we igh t (4 2 0 lb .) or 0. 5 g's . Ar e t hese prac tical figu res ? Le t' s go a little d eep er into this 2 10 lb . thrust. Fo r ease of figu ri ng we 11 say wh eel di amete r is 2 ft. so 21 0 lb . th rus t at the tire co n tact p atch is th e same as 2 I 0 ft. -lb . o f to rque at th e rea r wheel h ub . Sound lik e a lot? Rem ember, in low gear th e ov erall ra ti o t h rough final dr ive and gea rbox is often we ll over 20 to I , Th is m ean s if an engine puts o u t 20 ft ,·lb. at the cra n k, a good ro un d figu re fo r a 250 , there 's a goodly 30 0 ft. -lb . to b e ha d at th e rear wh eel -I F you ca n use it . In ou r exam p le yo u ca n ' t use b ut 2 I 0 lb.-ft. of it -oat least not a t firs t..w hi ch explains why sim p ly windin g it up a nd dumpi ng th e cl utc h is n ot th e fas test way off the lin e , Here's wh ere weight tr an sfer en ter s in. Ch eck o u t Fig . 2, It 's a free b ody dia gram , similar to Fig. I. It illu strat es th e forces whi ch pIa y o n th e m ach in e th e ins tan t the rider drops the hammer. In st ead o f letting it out with a bang our rid er is feathering the t h ro ttle and cl u tc h to deli ver just 2 10 ft .vlb , to th e rear wh eel , th e m ost it will take at tha t instant with ou t whe elspin. Do n't you wish you were that go o d? As soon as the thrust co mes on a n equal an d opposi te rea ctio n takes p lace ac ting through th e bike 's CG, lik e in th e d rawi n g. Beca use the 2 I 0 lb . thrust T and th e 2 10 lb . reac tion act 2 0 " apart (th e CG heigh t ) th is creat es a " co uple " or a torqu e react io n which tries to fl ip the m o t or cy cle o ver b ac kwards. It wo n't of co urse o r at least not in this exa m p le because th e gro u nd sup ports both wheels to ho ld the bi ke level, To ma ke the sy s te m balan ce o u t (w hic h it mu st ) R must get larger than F to se t up a "coun ter to rque : ' In effec t this can be view ed as we ight tra nsf er fro m F t o R an d tha t 's j ust wha t it's called. Now as the num bers work o ur .in tha t firs t in stant that t h rust t akes hol d.7 5 lb . g e ts tr ansferred to the rea r wh eel , ma kin g th e wheel loadin g 285 lb . Fron t wh eel load decreases to 13 5 lb . Crazy . No w the rea r tire will tran sm it 285 lb . thrust so the rider can screw it o n marc and ease the clu tch to t hrow 28 5 ft. -lb. to it. The increased thrust in turn gene ra tes in cr eas ed we igh t t ran sf er , b u t n o t as mu ch this time . The re's a dec reasin g returns lim it to this : 1 16 lb . w e ight tr a nsfer fo r t h e w eight, wheelbase and CG place m ent of thi s par t icular m ach ine . \I'ith typical sp rin gs an d suspens io n th is limit is usu ally reached in less tha n a sec ond . The softn ess or hard ness of th e susp en sion con tro ls the delay in we igh t transfer. This is a mo st critical second : one in whi ch th e rider 's intimate feel fo r hi s engine and his mach ine can ma ke the d iffer en ce b etween a bo bbled start and a clea n o ne. No w let's bear down o n this limit - - FOR CAFE ROAD RACERS a u t omatically coordinate hand a nd foo t wi th the wei gh t tr a nsfer c harac te ristics of his o w n bi ke . Agai n o ne of these charac ter ist ics is weigh t tra nsfer de lay and it has a lo t to d o with the safe feel o f a given mach ine. . , Tw i~e I've men ti oned t hi s d el ay ; II s dir e ctly rel ated to susp ens ion st if fness. Here 's a rul e o f thumb : the so fte r th e suspen sion, th e longer th e d el ay . T oo so f t a nd the b ike's re actions to thrott le and brake a re sluggish . T oo h ard and the sudde n ne ss o f the b ike's r e s pon s e s w ill t end to pr ov oke wh eelsp in o r wh eel lo cking. T ail oring the stiff ness of th e suspe ns io n for safe han dlin g an d good ro ad feel is o ne o f t h e mos t de lica te ta sk s facing th e designer and chassis tun er. It's an art a nd th a t's all I'll say abou t it n ow. O K. So much for all this fore and aft we've reach ed . For th is one m oto rcy c le, 50/5 0 balan ce , 56 " wheelbase, 2 0 " CG height, the m ax wheel load ing due to weigh t tra nsfe r an d also the m ax thrust is 3 2 6 lb. (2 10 p lus 1 16). Th is th rust d ivided b y wei gh t (4 2 0 ) gives 0. 77 g max acc eleratio n. Th at 's all. :\0 more. Any m o re torque th an 326 ft. -Ib.. even if the en gine and gear tr ain ca n d el iver i t. is wasted in w hce lsp in . Thi s i ~ U N LE S S th e rid er d ocs som et hing extra. Lo ok aga in at Fig. 2 and th e do tted lin e co n nec ting the CG with the tir e co n t ac t p at ch . It so happens o u r 0 ,7 7 g ac ce ler a ti o n figure also eq uals th e tan gent (a little tri gon ometry th er e) o f the a ngl e I 've shown as " t he ta, ,. Hmmm , I won't prove it h ere but fo r AN Y m an /machin e y ou draw a lin e from th e co n tac t patch th rough th e c o m b i n e d CG a n d yo u r lim it in g acc e le r a t io n will always eq ual th e tang en t o f th e angle th a t line m a kes with t he ground . So we have tires wh ich supposed ly perm it o ne g. The t ange nt o f 4 5 d egrees is also one. T hen if- our rid er eithe r , sta nd s up o n the footpegs o r h u nkers backward or both he 'll be usin g his body weigh t to sh ift th e co m bine d ma n /m ach ine CG up a nd back to wa rd th at 45 degree lin e. Cra zy . \l' ha t if he pulls th e CG ab ove t hat lin e? Un less th ere 's a b ig ape ha ndy t o grab th e fron t end a nd ho ld it d own, a wheel ie o f course. Thi s weigh t tranfe r biz also w orks o n bra kin g and helps exp la in wh y good r o a d b ik es sho uld have BIG front brakes. Weigh t tra ns fer also lies beh ind the seasone d rid er's advice on how to use the front and rear brakes to geth er. Any ding-di ng in a panic situation can lo ck up the fr on t whe el and d rop the b ike from a sudd en gra b for the fro n t br ak e le ver . 'The ex p erienced rider kn ows weigh t tra ns fe r b y t he scat o f his pants whethe r he 's been to ld abo u t it or not. So h e 'll use a tap a t th e rea r b rake t o se t up weigh t tran sfer (illustrate d in F i g . 3) th en in one co or d ina te d hand /foot m ovemen t he ' ll ease up on rear brake and squeeze o n fran t brake until he 's achieved maxim um wei gh 1 tr an sfer a nd se tt led in to a stab le bra kin g m o de. Th is is sh ow n in Fig. 4. giving th e "second ins tant" and limit co ndition s for o u r example 2 50 : A t the lim it balanced , stab le . 1 g braking a t the maxim um the tires ,....i B allo w, It ca n o nly be d one by a rid er who, agai n , has in tim ate kn o wled ge o f h is m achi ne , it s brakes, tires and suspensi on ; wh o can b usiness. lI o w abo ut for ces in the lat eral pl an e? ' ''he n we lo ok a t the factors wh ich co me in to pl ay w he n th e bi ke is lai d over w e rea lly ge t int o the n i t t y -g rit ty of co rn erin g beh avio ur. Lo o k a t Fig. 5 and reme mber wh at I sai d about fo rce resolu ti o n d iagrams wa y back when I w as talki ng ab ou t tires. It so happens th a t th e weight \I' an d th e centrifugal fo rce F reso lve in to a res ultant R wh ich passes th rough the tire co n tac t area. I've also sho wn the equal a nd opposite reaction of the gro u n d pr e s s in g back against th e mo to rcycl e for equilib ri um. Ka te fo r all p racti cal purp o ses that m o torcycl e is "h in ged " o n the tire contact l ine and ro ta tes abou t it as an ax is wh en ban ked o ver or pic ke d up . No w if that resolved forc e R do es no t pass th ro ugh the contact po in t it th en be c om es a to rqu e wh ich tends to rota te t he b ike /rid er un it o ne way o r the o th e r. The rider, th rough weigh t shift and o ther mean s. co ntro Is this to rqu e to s teer th e ma c hine. O K h o w abou t that upward p o inting react io n? In stab le c o rne ring with the rid er all neatly tucked in it poin ts righ t up a t the motorcycle's cen te rline. Th is wo uld m ak e you bel ieve then tha t a m o torcycl e fra m e need on ly he des igne d to resist straight up forces . T his is true u n t il yo u hit a hu m p . An y bum p . (See Fig. 6.) Hittin g a ny bump in th e road will te nd to kick th e whe el s traight up fro m the gro u nd rega rd less of ho w th e bike is heeled over. If the m ac hine is full y c ra n ked o ver near 45 degrees, one g. a sha rp bump can pu t quite a tw isting impac t into the frame, noL to m e n tio n pl ay in g hell with the riders pulse and resp iratio n rate. I .... W y Fi g. 1: Static case: a ty pical 250cc with rider at rest. Th is is what engineers call a "free body diagram : " a pictur e which help s isolate in th e min d th e motorcycle as a f ree unit and th e fo rces play ing on it. A t rest we have o nly th e weight " 11' '' acting d ownward thr ough th e CG an d tw o forces represen ting th e upward thrust of th e ground against th e wh eels. ' . 420'" Fig. 2 : Th e instant acceleratio n co mes on weight transf er occurs. Th e m ore weigh t transf er, the m ore available thrust at the.rear wheel w hich gene rates still m ore weight transf er--UP TO A LIMIT, Th e lim it is det ermined b}' th e po sition of th e CG relat ive to th e rear tire co n tac t pat ch. Note th e angle mar ked with th e little Greek letter th eta. More about tha t in th e article 's text.

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