Click on images to enlarge

WHAT IS IT? The Steahly Yamaha YZ450F flywheel weight comes in 7-ounce and 9-ounce versions. The 9-ounce version is designed to harness the power of current-generation 2014 to 2017 YZ450Fs. It is a bonded-on weight, so it can’t come loose.

WHAT’S IT COST? $139.95 (flywheel weight kit), $149.95 (installed on your rotor if you send it in); $299.95 (flywheel weight mounted on a Steahly-supplied stock YZ450F rotor); $150.00 refund (if you order the flywheel mounted on a rotor, but send your old rotor to them within 30 days); $19.95 (YZ450F flywheel puller).

CONTACT? or (800) 800-2363.

WHAT STANDS OUT? Here’s a list of things that stand out with Steahly’s YZ450F flywheel weight.

(1) Construction. The Steahly weight is a CNC-milled-from-scratch flywheel weight that is added to the stock Yamaha flywheel rotor. Steahly cuts a 9-ounce ring of steel on a lathe. The ring is machined to close tolerances for an exact fit on the stock flywheel rotor. Additionally, Steahly cuts a groove in the ring to hold the special epoxy that cements the two pieces in place. Nine ounces wasn’t a magic number that Steahly discovered through testing a wide variety of weights. It was simply the most weight that they could fit into the space available.

(2) Installation. You can install the Steahly flywheel weight yourself (by following the instructions). Or, you can order the flywheel weight already installed on a YZ450F rotor, send in your rotor to have the weight installed by Steahly or order the flywheel installed on a Steahly-supplied rotor, and then send them your stock rotor (in good condition) for a refund on the price. The only other cost is for a YZ450F flywheel puller, regardless of which option you choose. Both Steahly and Motion Pro sell inexpensive Yamaha flywheel pullers.

(3) Do-it-yourself. These are the steps you need to take to install the Steahly flywheel weight yourself: (1) Order the Steahly flywheel weight. (2) Pull off your stock YZ450F rotor. (3) With supplied sandpaper and your own contact cleaner, thoroughly sand and clean the flywheel where the new Steahly weight mounts. (4) With the supplied two-part epoxy, carefully apply the Steahly weight to the flywheel. The epoxy dries rapidly, so immediately wipe away any epoxy that seeps out. (5) Tighten the four set screws in a cross pattern for equal pressure. Use the excess epoxy to fill the screw holes, so the screws can never back out. (6) Allow your newly modified flywheel to cure for 24 hours. (7) Once the epoxy has cured, bake in your oven on a cookie sheet for two hours at 250 degrees. It can be done with a toaster oven. (8) Reinstall on your bike and prepare to roost.

(4) Acceleration. In stock trim, the fuel-injected YZ450F is very aggressive at first crack of the throttle. This leads to unwanted wheelspin on slick surfaces and, conversely, jumpiness in tacky situations. Deep sand and fast Pros may prefer that hit, but for most tracks the MXA test riders wanted more throttle control. After racing with and without the Steahly-modified flywheel, every test rider was generous in his praise of the benefits of the heavier flywheel. The YZ450F pulled more smoothly and gained traction in places where it would previously spin. The power was more manageable and usable. Flywheel weights do not cost horsepower; they just change the way the power is delivered by torquing up the low end and carrying it further on the top end.

(5) Braking. The extra inertia of a heavier flywheel not only resists speeding up, it also resists slowing down. We thought this might be a negative, but the heavier flywheel helped under braking and transitioning through corners. When chopping the throttle with the heavier flywheel, engine rpm doesn’t drop as suddenly. This made for a smoother transition from full speed to full stop, and it reduced the chance of flame-out.

(6) Options. You can choose between 7-ounce and 9-ounce flywheels, but MXA has always preferred the 9-ounce weight. Given the YZ450F’s abrupt powerband, the heavier, the better.

WHAT’S THE SQUAWK? We would pay the extra money to have Steahly do the work of installing the YZ450F flywheel weight on the rotor and save the oven for baking birthday cakes.

MXA RATING: Performance-wise, the Steahly 9-ounce flywheel weight is exactly what the 2017 Yamaha YZ450F needs. Not only does it help the power delivery, it calms the YZ450F chassis by keeping the power hooked up.

2017 yz450fmotion promotocrossmotocross actionmxasteahly flywheel weightyamaha yz450f