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/125860
...... , , ." - . ... - - ... ~ " • 250 YZ MONOCROSS - . By Jim Gianatsis When Hakan Andersson won the 250cc World Motocross Championship for Yamaha last year, he did it ridi ng a pretty amazing motocross bike . It was basically a Ya maha 250 YZ which had the frame and rear suspension changed to give the bike close to seven inches of rear wheel travel . It was a novel idea. When the bike first appeared on the grand prix circuit the benefits of a long travel rear suspension seemed ob vious, but no one was really sure tha t the new Mo nocross cantilever suspension system wou ld work. It did though, and Hakan's Wo rld Championship was the resu lt. T he ca ntilever mon osh ock motocross suspension was act ua lly the design of Belgian engineer Lucien Tilkens wh o sold the pate nt to Yamaha. Maico experimen ted with t he cantilever sus pe nsion design and last year Werner Sc h utz ro de a Kr am er-sponsor ed l\Iaico (w it h works su pport) that used th ree da mpers m oun ted paral lel under th e sea t. The I 9 i 4 Krarn er -Maico has tw o gas-oil Bilstein d ampers in p lace of the sing le lar ge damper em p loyed by Yam ah a . Since n o two sh ocks , w hic h ac tually wo rk independe n tly of each o the r wh en m o unt ed as rear suspension on a m o t ocr osser, will give the exact same dam ping ac tio n, one big a dvan tage of th e Ya ma ha mo noshock design is immed iatel y obvious. With on ly o ne damper operating, there is no conflict or imb alan ce in dampi ng ra tes . Ya maha realized the su periority of the monoshock over co nventional d ua l rear shocks for long travel suspension system and had the technical ability to foll ow th rough on the design. With on ly one large shock working on a triangu lated pivoting rear fra me the re is no swing arm flex. The monoshock is so m uch larger than a conventional shock tha t it is readily ab le to cope with the stresses involved with a lon g travel rea r suspension in mo t ocross wihout overheating. Yamaha was also ab le to design into th eir Mo n ocross two in novations wh ich conventional shocks co uldn 't duplicate. O ne was progressi ve dampi ng wh ich varies acco rdi ng to im pac t shock at the re ar wheel, rider we ight, the speed at which t he suspension is traveling and ho w far it has moved . T he other is a rub ber m em b ran e , housed in one end of t he shoc k in an alu mi n um cha m ber, which has the shock oil o n one side an d ni trogen gas at pressure on the othe r , which flexes it self to co mpensate fo r t he space taken up by t he damping rod in th e shock cy linder. T his mean s there is n o air whi ch can get mixed in wi th t he o il to a ffect t he dampin g acti o n. Th e di agr am s w ill better sho w how th e Yamaha Mon ocross sus pension sys te m w orks. We were really exc it ed abo u t d o ing a tes t of th e 25 0 Mo n ocross No gu ch i/Y amah a when Dirty Distributing Compan y in form ed us that th ey wo uld be receiving a limit ed n umber 'o f the bikes fro m Yamah a of .Japan , thro ugh Noguchi. Dirty will be p utt ing th e bi kes in th e hands of top local racers to com pe te on, and relay bac k to Yam ah a ho w the bike s hol d up under other t ha n fac to ry racing co ndi tio ns. ' If t he Mon o cross p ro ves reliab le, 250 YZ version s will be ava ilab le fo r sale in the states perhaps as ea rly as t hi~ fal l. T he bike we ro de was t he one wh ich Dirty will be sponsoring Tony Wyn n wi th on the Nationa l a nd In ter ·AM A circuit. Tony is one of the fastest 250 riders in the country . and one of our • l testing sessions was Tony 's fi rst ride on th e Monocross. 28 He liked it so much that he ran it through a whole tank of gas before we co uld ge t hi m o ff of it. .... -, .- ,. L ' ..., • • • .. . - Tony Wynn rocke ts t he 250 Noguchi/Yama ha o u t of a be rmed tu rn . If we could fi nd an y fault w ith t he handl ing at all, it . / • O IL. Th is is what t he Monocross sus pen sio n looks like , with the tank, saddle and side panels remo ved. The d iagram shows how the damper operates. A small jolt on the damping rod keeps t he spring loaded valves closed and damping action is stiff . Oil pressure fr om a large jo lt w ill force open the sp ring valves and the damping action becomes less. The rubber membrane moves ba ck and forth in proportio n t o the amo unt of oil forced in and out by the piston .