Cycle News - Archive Issues - 1990's

WORKBENCH By Brian Catterson Rake and trail and stuff wo of the most com mon ly rnisunderst ood terms in motorcycling term ino logy ar e rak e an d trail. Yet these are a lso quite easil y exp la ined and, once understood, used to alter yo u r motorcycle's handling characteristics. If you look a t the speci ficat io ns tabl e acco m pa nying a road test, yo u 'll usually . see rak e a nd trail listed together, as though they were one measurem ent, i.e, rake/trail: 25°/4.5 in . And while rake does pl ay a part in determining trail, the two a re distinctly separate enti ties. T o quote from William H . Kosba h's boo k of Motorcycle DictionarylTermino logy, rake is " th e angle of a motorcycle front end It is measured between a vertical line from the top of the front end to the gro und a nd th e cent er line run ning th rough th e fro n t end." Kosbab defines trail as " the distance between a vertical line from the front axle to the ground and a lin e running th rough the center of th e steeri ng head o f a motorcycle front end " (See accom pan ying figure.) Now th at you know how to determine ra ke and trai l figures, you're likely asking what purpose do the y serve? Rake is the single most important factor in determining tra il, Simply p u t, the ~ea ter the rake, the greater the trail, and vice-versa . Trail is the factor which gives the fro nt whee l its sell-cen teri ng ability. Have you · ever taken your hands off the handlebars while riding? If you have, you ' ve no doubt noticed that the bi ke continued to go stra ight. T hi s is du e to trail. T If yo u were to p ut a spo rt bik e next to a cruiser, you'd note a visibl e difference in the angle of the front end. Sport bik es ar e based on road racers, wh ich typi ca lly hav e very little ra ke, thus less trail. With li ttle self-center in g effect to overco me, a spo rt bik e ca n be nicked from side to side with ease. Conversely, cruisers have· C more rak e ar a n d tr ail , producin g a greater se lfcente ring effect. This is wh y cho ppers work so well in a stra igh t line (and so poorly in turns). • However, rak e a nd tra il are not absolute, as th ey vary with front sus pe nsio n mov ement. As the forks co mp ress, ra ke a nd trail ar e decreased, easing steer ing. T his is one of the reasons mot ocrossers will drop from a sta nding position down onto the fro nt o f the seat as they enter a tu rn . (The o ther reason is that it puts more weight o n the fro n t wheel, im proving traction.) To best ill ustrate th e effects of rak e a nd trai l, let's 'di scuss the new U.S. Twin Spons roa d raci ng class for H a rl eyDavidson 883 Sportsters. Here, the racers must overcome the problems associated wit h turning rakish cruisers into road racer s. The problem wi th excessive rake is that it kicks the fro nt end out too far , which makes _fo r h ea vy-feeli n g steering and which puts very little weight on th e front whee l. (T his is al so referred to as havin g a rearward weig ht bias.) The result is that the front end will p ush in turns, a nd may wag vio lently fro m side to 'side or ." tan k slap" at h igh speeds. little self-centering effect, and the bike will again tan k slap. T hi s is where a steering damper co mes in . Connecting the C ork tubes with the chassis, a steeri ng damper is essentia lly a shock absorber without a sp ri n g wh ich m od ul ates lat eral Cork movement. H ow ever , a steering damper shou ld be used as a n a id to improve handling, not as a cu re-a ll for improper setup. If in dou bt, experiment withou t a steering damper, the n add one la ter. Two final factors in determi ni ng trail are the amo unt of triple clamp or ax le offset. Som e fork legs have what are called leading axles, which locate the axl e ahead of the fork tubes, reducing trail. Triple clamp offset is the distance between the steering head and the fork tu bes, measured lo n gitu dina ll y. Here, the greater th e o ffset, the lesser the trail. T he desire to alter th is measu rement has led to the development of variab ly-o ffse t triple clamps. . For the sake o f clarity, we've lim ited o ur. d iscussio n to motorcycles "w i th telesco pic forks - whe ther they be o f th e tra d itional or upside-down / i n verted/ ou ter-u pper tube-type (wha tever they 're calling them thi s week) ma kes no di fference, We've purposely left ou t alternative fro nt en ds suc h as th ose used by Elf in years pas t on their road race bikes, or by the Bimo ta Tesi, the RADD Yamaha, etc. Perhaps we'll discuss th ose in a future issue, To compensate, riders will pull the fork tub es up though th e triple clamps, whi ch mak es th e bike less ra kish a rid whi ch puts m ore weight on th e front wheel. T he result is a machine th at is less likely ' to push th e front end or tank slap, and tha t . is far easier to steer. One consideration in determining how far to pull th e for k tu bes up is the amount o f wh eel cle arance ava ilable. TIle best (a nd sa fest) method to determine how far is too C is to p ut the bike on a stand, remove ar the fork tube caps and fork sp ri ngs, and then fully compress the front suspe ns io n. Only with the C orks bo ttomed ca n you determine wh ere the front wheel lies in relation to the triple clamps, exha ust system, fa iring, etc. Use a ru ler to ensure th a t the fork tubes a re pulled up a n eq ua l amount: som e tuners ha ve been kn own to scribe marks o n the sides of the fork tub es to serve as a visual aid. Ano the r consi dera tio n in p u lli ng the fork tubes up is th e correspo ndi ng reduction in ground clearance. This is why Sportster racers fit longer shocks, h igh p ipes a nd rearse t footpegs. In creasing th e len gth of th e shocks (or the. swingarm, for tha t mat ter) a lso has the add ed benefit o f increasi ng the ra ke, thereby p utting more weight on the fro nt wh eel. If taken to extremes, rak e and trail ca n be reduced to th e po int tha t there is too Wor kbmch ill • ""ft':kly colulIUI lkaJins with m OloKy dr ma inlnSaDa. pnformana .net I!'qll i pmnll . H ... ~ _ f1h ips tha i Wodbmc:h em .ddrna? Drop • liM I<) Wodbndl. C'1C~ NftQ, r.o. 10:1: . . LoRI Bndl. CA 9OlIO l-G4!1ll. Hp ~Jtr 60.0 55.0 SO.O ~. O 40.0 35.0 3 0 .0 25 . 0 , 1990 KAWASAlCI ICX 12 5 PIN SHoor-our .... ~ • •2 ..,. S1OC« U . 1 flItO OltCUlf U .J Nfl 40.0 , Gather your thoughts and read up. Of every top performance pipe advertised on the market today, PSI Performance Pipes deliver more torque, revs, and . horsepower than any other brand available, After all , it's your opinion that . counts. Not ours. . . In comparison after comparison, test after test, The Superflow dyno proves without a doubt that PSI Pipes offer the most performance for your money. By a long shot & a hole shot. So if horsepower is the reason you are shopp ing for a bolt-on replacemen t pipe, PSI is the only brand to consider. Remember. All pipes are NOT created equal. 4 0 .0 3.5. 0 ..... ;:'::::::,,=r"" . "V'..-.;-_ " .. ............ .... ..... :~': 30.0 ............... 2.5.0 _ 7:5.0 JO.O 5.00 _ - . . . . . .. .. ... :::::;:::: . ................. ..._ :::.: ..............• - _.. . .... . ..... - _ .................. -. ...... .... 20.0 . . .. _._ .. . ...... __ .....................•. . .... _ .................... _ _ _.._--. ~ ::= ~ = ::: ::' , :=:;:: : , ; , : ~ , ::,, ::. ~ := - ....,... ",.... 40011 SOli toOD 1000 ' 000 fOOD 10000 D nooo 12000 12000 PSI, INC. • RT2, Box 309 • WILD ROSE, WI 54984 • (414) 787·2430 VlSAlMASTEIlCARD ACCEPTED SAMEDAY III'S. DEALERS IlfELCDME 15

• Cover