Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/127268
WORKBENCH By Brian Catterson Why rake? ack in the May 30 issue, we made the "mistake" of discussing rake and trail in this column. Not a mistak e in th e sense of getting our facts wrong - we just ope ned up a big can of worms. In that colu m n we discussed how to measure rake a nd trail, and wh at effects altering th ose numbers would ach ieve. However , th at's no t enoug h. Some of yo u wan! to kn owwhy ra ke wo rks. So here goes. (Where but in 'Workbench can you learn how to change fork o il one week , and get physi cs lessons the next ?) To answer th is question we consulted' Motorcycle Ch assis Design : T he Theory and Practi ce by Tony Foale a nd Vic Willoughby. The purpose of trai l is to make the mot orcycle wan! to ho ld a straight line. Ever hit a bump and felt the tire deflect o rr to one side, then return to cen ter? Probabl y th o ug ht yo u m ade a great save. huh? ope - it had nothing to do with you. This ph enom en on occu rs because of trail. r In essence, because th e co ntac t pat ches of both tires ar c behind th e steering axi s, a self-cent ering effect known as cas to r is crea ted. And because th e fork s ar e a ngled, the fro n t wheel is a ng led to its direction of slip travel so that wh en th e front whee l is turned. th e contact patch is shifted forward. Thus, wh en the front whee l is turned , it uses that force as levera ge to return itself to a straigh t positio n. B "Ever hit a bump and felt the tire deflect off to one side; then return to center?" 1 ~ Go t that? 1£ yo u' ve ever leaned your bik e into It turn with th e throttl e o ff (and the brak es on? ) and felt th e front tire turn in and push , yo u 've felt the effects of trail. But yo u can push th e front end ht the exit o f [urns, too. Here, with th e throt tle on har d you may have felt the front end ge t light a nd the bik e straighte n its path somewha t. despi te the fact that you 've still got th e fro nt tire turned and it's still more or less in co ntact with th e ground . You'll feel thi s more if yo ur b ike is too ra kish (be it th rough steer ing geo metry o r too much front spr ing prel oad ). Motorcycles ha ve what is ca lled po sit ive tra il - negati ve trail (p utt in g the co n tact pat ch in [ront o f th e steering axis) wou ld mak e th e tire do the opposi te, that is deflect to maximu m steering lock, rather than go stra igh t. But trai l does not by and o f itself ma ke your bik e go straight. Anoth er factor is someth ing called gyroscopic pr ecession (no t processio n, which wo u ld eq ua te to a parad e of spi n ni ng children's toys.) 'l Gyroscopic pr ecession is th e selfCente ring factor crea ted by a spi n n ing wh eel. If you were to hol d a bicycle wheel by its axle a nd spi n it , th en turned th e wh eel. you' d feel th is as Of a son of tu g back to center. I $600,000 PLUS Run on Metzeler Tires and Win METZELER 's 1990 Roadrace Contingency Progra m has begun w it h trem endou s success. W it h contingency results ju st through May, M ETZELER Racers have taken 139 1 st Place f in ishes. W it h an excit ing 358 Top 3 Place fini she s t he METZE LER Conti ngency Program has cau ght fire , and is HOTI The Cont ingency Program is nati on wide and racers are w inning coast to coast. M ETZ ELER is post ing $600,000 plus' in t he fo llowing Nat iona l and Club events National Eve nts WERA Sprint WERA Endurance Suzuki Nat ional Cup A MA CCS SI S Nat iona l AHRMA Vintage Nat ion al Club Even ts A RRA LRRS AFM North CRA AMA CCS S.W . W M RRA USBA OM RRA MCRA HRRA Race METZELER tires and w in . Be a part of the contingency program that puts money in your pocket and tires on your bike. See your local Dealer or Road Racing Club fo r more information . ~ M ETZ E LE R ~ LO D N. N UO H 1st · DOUG CHANDLER, Superbike 1st -JAMIE JAMES, P Twins I ro 2nd· JIMMY ADAMO, P win I roT , 4th- DOUG BRAUNECK Pro Twins I 1si- DOUG BRAUNECK 250 GP , 4th - AL SALAVERRIA, 250 GP 1st - FABIAN COR P T TEZ, ro winsII 1st · DOUG C HANDLER S ,750 upersport 1st - NIGEL GALE, U TwinSports .S. ASubscriptionis aboredomprescription I 45 2 0 - 10 7 t hSt .S .W. . Everett. WA 98204 (20 6) 34 8- 40 0 0 For more information. including an Arai Dual DecaVLiterature Pak. with our Full -color brochure, send $2.00 in cash or stamps ($3.00 . ouls ,d. U.S.)10: ARA HELMETS. LTD.• Dept. 64. I P.O. BO 9485. Daytona. FL 3212Q.94B5. X SAN JOSE SUPERCROSS 1st -JEFF STANTON, 250cc 3rd- MIKE KIEDROWSKI. 250cc YUG SLAVIAN G O P 2nd - KEVIN SCH WANTZ, 500cc 3rd - NIALL MACKENZIE, 500cc 4th- MICHAEL DOOHAN, 500cc W R NDURAN H TT, O E AE CE. ALLE K 1st -TEAM SUZU KI - -- ...- -_.--. ===- _ ••- - LA. 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