Cycle News - Archive Issues - 1990's

FIRST LOOK 1 9 9 8Yamaha rzs (Left) The YZ400 looks like the other VZs because that's just what Yamaha wants it to be: just another motocrosser to choose from. It Just happens to be a four-stroke. (Below) Siltin g on the VZ400 feels just li ke sitting on the others, tho ugh the rear side panels are wider to accommodate th e larger exhau st silencer. By Mark Hoyer Photos by Paul Carruthers his is not a trail bike. This is a closed-course racing rn otorcvcle, mad e for mo tocross . . . As far Yamaha hi concerned, it' s just another YZ to choo se from. We heard it ru n: This is not a trail bike. Th is thi ng sna ps. And yo u can buy it. It is the YZ4ooF, Yamaha's version of Doug H enry ' s w o r ks fo u r -s trok e motocrosser, but ma ss-produced for the common man. Wh ile it may not be carved from billet or be sporti ng exotic, titani um fabri cations an d a chrome-moly frame, the con cept behind the YZM400F and the production YZ400F are the same: all-out ~ hard-core mot ocross / su percross perfor.- ~ mance equ al to or exceeding the bes t two-stroke 250. Yamaha naturally chose it s o w n YZ 250 a s the perfo rmance bench mark. And, accord ing to Yam aha and its facto ry test er, Dou g Duba ch , Dubach was actually able to run faster lap times on th e fo u r-s troke tha n on th e new YZ250 tw o-stroke. No t only was it faster (by some three seconds per lapat Carlsbad's high-speed, hard-packed, sq ua reedge-rid dled trac k, and by four seconds at the sand y Los Angeles County Race- as A fa ~ 6 on rear shocks. The main benefit is that an elastomer isn't speed-sensitive, so it works better under a greater variety of cond itio ns . The spring rate is unchanged at .43 kg /m m for th e 250 a nd .41 fo r th e 125. Despite the alterations, Yamaha says the o vera ll weight of the fork has been dec reased, but without any pena lty in terms of a r~ uction in rigidi ty. The 400 has stiffer springs than the 250 because it weighs more, tho ugh the valving is very similar. At the rear, racing has improved the breed: The '98 YZ400F, 250 and 125 are eq u ip ped w ith the same link th a t th e race team has been using all this season on its works bikes. While overall wheel tr avel remains the s ame, the s ho ck stroke is now 6mm longer (J 32mm total) to reduce the operating temperature by lowering th e shaft speed . The sp r ing rate is decreased to match the rear leverage ra tio, though , effectively speakin g, the rear su spensio n isn' t ac tua lly any softer than las t year's. The sw ing arm has a larger-diameter pivo t for increased sti ffness, and the pivot point for the rear s ho ck (a nd i ts n ew , h igher-q ual ity upper mo u n t) ha s been moved 6mm closer to the centerline of the bike on all three models. Internally, the piston sea l now ha s two O-ri ng s underneath it to provide a better sea l to the body and th ereby decrease the chance of blowby. Much of the plastic also comes from the new 250 a nd 125, though th e ta nk a nd radiator s hro u d s are ap pare ntl y specific to the YZ400F, as are the rear side panels. The front number plate and fend ers ate'used on all three bikes. It's no secre t that Hond a's CRs have co ns istently had th e best bra kes of a ll the mo tocro ssers for ma ny yea rs, an d w ay in Palmd ale), bu t it was also less taxing and easier to ride at those speed s. With the YZ250 as the tar get bike for performance, it w as natural (and (,05 t - effecti ve) fo r Yama ha to use as m a ny parts from the 250 as it coul d for the 400. The bike we saw was a third-generation test unit , mo s tly finalize d for p rod uc tion, though the testing department had m a d e ne arl y 20 fin al r eq u es ts fo r changes. How ever , w hat you see here is basically wha t wi ll en ter prod uction in Oc to ber and hi t d ea lers in Dec ember (tentatively, of course). The YZ4ooF's frame is fundamentally th e sa me geo m et ricall y a s the 1998 YZ25O's. Wha t this means is that it gets the '98 YZ250's new fra me geom etry t hat sees the rake brought in a half degree to 27.2 and the trail shortened to 118mm from 122 - changes that w er e made in an effort to quicken the steering respo nse on the '98 bikes vs. the '97s. As a resu lt of th e change in geometry, th e h a n d le ba r s have been mo ve d 4m m higher and 10m m fo rward, giving the bike a s lightly roomier layou t. Structural1y, however, th e frame is significa n tly di fferen t on the 400 . Fo r one thing, o n the four- str oke, the fram e has to ho ld the oil fo r th e en gine. The 400cc engin e and its five-speed tra n ~: , mis sion are also somew hat large r than the 250's, and the fram e obviously mu st be made to accommoda te this. In addition , the loca tion of the carburetor on the four-stroke is mu ch higher (u p o n the cylinder head vs, near the cases on the two-stroke), an d so alters where the rear tubes can be placed. The YZ400F will also benefit from the cha nges tha t were made to the YZ25O's front and rear suspension, changes that are also' found on the 125. The 46mm fork is all new, featuring an increase in di am eter fo r th e da mpi ng ca r tr id ge from 28mm to 32, as well as benefiting from a new outer-tube shape. With the increase in cartridge size, there is a simi lar increase in piston siz e, and the valving is al tered to ma tch th ese changes. The ne t resu lts are said to be more-consistent da mpi ng fo rce and less botto ming. Sho u ld you happen to botto m th e s uspension a n ywa y , la s t yea r's hyd ra ul ic-type bottoming system has been replaced by an elas tomer bumper, simi lar to wha t has bee n used for year s this is a fa ct tha t hasn 't escaped the attention of the eng ineers over at Yamaha . To ad d ress this, tbe YZ250 and 125 ge t new brakes front and rear. The main foc us of the braking changes were to enhance the fee li ng o f co ntrol. A "Honda"-type Nissin cal iper body and s u pp o rt bra cke t a re m u ch m ore rig id than those used last yea r, and the tw opin pad-retaining system used last year has been replaced by a single pi n that is sa id to gi ve better feel a nd long-term d urability. The rear b ra kes on th e 250 and 125 ge t conceptually the same cha nges as the front, as w ell as a larger-diameter disc (240mm from 220). Because of the larger di sc, th e ca liper ha s been moved forward to clear the silencer when the suspension is fully comp ressed. Four-stroke eng ine braking wa s cited as the reason tha t the YZ400 stu ck with last-year' s disc and caliper, tho ug h if clearance was a p roblem on the two-st ro ke, the la rger

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