2003 Yamaha YZ450F - Dirt Rider Magazine

Bikes are bikes--right? Not even close. For example, take the '03 YZ450F. Deposits ran like water into dealers' cash registers when the general public got wind Yamaha's new 450 valve-pusher matched the "Six Million Dollar Man's" tag line: "Better, stronger, faster." Yamaha took its pride-proven YZ426F and transformed the beast into a lean, mean racing machine. She proved to be quite the rock star, but when the hype started to subside and the dust settled, Honda's CRF450R was left standing atop the podium.How did this happen?Simple, the red bike is built for the masses while the blue YZ-F is a pure race bike and thus not as user-friendly in comparison. It's as if the Yamaha is too good and caters to a smaller group of riders. Don't get us wrong, the YZ450F is an awesome machine--heads above its brother, the 426F--but we thought it could be better. That's why we sought out Pro-Action guru George Quay for a helping hand.Pro-Action states, "In stock form, the '03 YZ450F's fork has poor low-speed pickup and deflects off small bumps. When you come into a corner hard under heavy braking, the front end will dive and blow through the stroke, making cornering very unstable. The shock also suffers from poor low-speed pickup and midstroke harshness."The PA folks set up our bike for a 170- to 180-pound rider. In the fork, the stock 0.46kg/mm springs were retained to suit our rider's weight. Pro-Action installed its K2 Millennium valve, which helps control oil flow and allows a lighter two-stage valve stack. Because of the lighter valving in the first stage, the bike can pick up smaller bumps more efficiently and deflection is decreased. On larger hits, the stiffer second stage of the valving in combination with the better-managed flow of the piston helps control bottoming and hold up the front end under heavy braking.The stock 5.3kg/mm shock spring was also kept for our selected weight range. Pro-Action used one of its anodized-aluminum shock pistons with a three-stage valve. The piston is designed to help control the oil flow while preventing blowby caused by the shock body expanding faster than the stock piston. The swap should eliminate the midstroke harshness when coming out of choppy corners. Well, enough of this techno mumbo jumbo; let's get to what you all have been waiting for. How does it work?On the track, Pro-Action's fork revalve delivers as promised. The modified fork stays up in the stroke, and the deceleration harshness of the stock unit is gone. On fast choppy straights, the YZ-F bar stays in your hands rather than trying violently to leave your palms. You can hit square-edge bumps straight on without getting that twitchy feeling in the front end. On big slap-down landings, the fork does not bottom violently; however, we did get some deflection on hard landings. We adjusted the compression from 12 clicks out to 15 and set the rebound to nine clicks out (versus 10); this helped with the deflection on those hard flat landings.The shock was equally impressive. On slow acceleration bumps, the bike's rear end stayed low in the stroke to get the power to the ground much better. It settled down, which made less work to get it out of corners. Yet fast choppy straights were handled much better. The YZ-F tracked better and did not do anything unexpected. Coming up to big jump faces, the bike felt firmer without the dreaded wallow. Landing off jumps, too, was great. The shock felt as if it had more capability to handle your overjumping that tabletop or double.Even though we covered the DEP system during our massive YZ-F exhaust test last month, we still wanted to complement the newfound suspension with the better power delivery. It produces a great curve and comes on strong, and pulls into a meatier top compared to the stock exhaust. The DEP is for riders who like to carry the rpm from the lower mid to the top. Those who really torque the bike around and short-shift Yamaha's beast may want to shy away from the DEP setup, though all of our testers were very pleased with the system.The target was to get a calmer and better-handling YZ-F. For riders who are at odds with the handling of their YZ450F, Pro-Action has a pair of loaded dice that comes up a winner. The improvements instilled enough confidence in our test crew to hang it out a little further. Where the stock suspension was a little grim, the Pro-Action work helped us relax even in the most appalling conditions.The Pirelli MT450s get the job done when it comes to putting the big power down to the ground.The Works Connection Pro perch takes the place of your stock unit to offer a clutch adjuster and hot-start in one.The DEP headpipe/muffler combo builds smoother power down low with a hard-hitting mid to top-end.SettingsFork compression at 15 clicks out; rebound at 9 clicks out Torco 7-weight oil at 100mm air space 0.46kg/mm springs Fork height 5mm from topShock compression high-speed at 1.25; low-speed at 12 clicks out; rebound at 10 out Static sag at 25mm Torco shock oil