SMOOTHER, SLIM­MER, BET­TER

Dirt Rider - - Gear Bag - STORY BY PETE PETER­SON PHO­TOS BY PETE PETER­SON AND JEFF ALLEN

The YZ450F was our 2017 Mo­tocross Shootout win­ner (Fe­bru­ary/ March is­sue), so when Yamaha came out with an all-new bike for 2018 we were ex­cited to see if it raised the bar for the class even higher. A pow­er­house since 2010 when the Yamaha en­gi­neers re­versed the en­gine and cre­ated a straighter, down­draft in­take, the new YZ450F gets a slightly straighter (more ver­ti­cal) in­take tract and higher-lift cams. The in­take valves now open ear­lier, open far­ther, and stay open longer, with the aim of in­creas­ing midrange and giv­ing a torquier char­ac­ter. Over­lap, when both valves are open, has been in­creased in an ef­fort to im­prove top-end and over-rev. The throt­tle body is now a Mikuni unit (a switch from Kei­hin), with a push-on cold-start knob that re­leases with a twist of the throt­tle. The pis­ton is now a bridge box style, lighter by 2.3 per­cent and with a com­pres­sion bump (12.8:1 ver­sus 12.5:1), and the new ex­haust header is 12mm longer and nar­rower at the junc­tion (38mm di­am­e­ter ver­sus 41mm). There’s even a new spark plug with a longer elec­trode for im­proved burn ef­fi­ciency and a cap holder on the valve cover. Ac­cord­ing to Yamaha, the changes mean an over­all horse­power boost and claimed 3 more ponies at both 6,000 and 9,000 rpm.

The clutch pres­sure plate is re­designed with more rigid­ity in its outer area and less at its cen­ter, the clutch’s steel plates are pre­ci­sion ground on both sides, and the springs are de­signed to stay straighter dur­ing com­pres­sion for smoother en­gage­ment at high rpm (such as launch­ing off the start­ing gate. Yes, the bike still has the Launch Con­trol Sys­tem). Also sec­ond, third, and fourth gear cogs are 1mm wider (the ra­tios are un­changed) for dura­bil­ity with the stronger en­gine out­put.

Oh, and there’s that magic but­ton on the han­dle­bar. The en­gine is elec­tric-start only (it will start in gear with the clutch out), with a starter that is more com­pact than the one on the FX line, with a claimed 70 per­cent more crank­ing power and draw­ing 20 per­cent less elec­tric­ity. As a bonus you might not nor­mally think about, be­cause the bike now has a bat­tery, Yamaha was able to de­crease the ig­ni­tion’s elec­tri­cal out­put and there­fore de­crease the drag the sys­tem puts on the en­gine, which Yamaha reps claim im­proves low-rpm power and re­sponse.

An all-new frame has main spars that are now straight, and the frame is nar­rower at the rider’s an­kles. The en­gine is held 2 de­grees more ver­ti­cal, and the en­gine hang­ers now at­tach at the rear of the head and are alu­minum rather than steel. The steer­ing stem is 6mm for­ward, the triple clamp off­set changes from 25mm to 22mm, and the han­dle­bar mounts put the han­dle­bar 5mm higher.

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