ASK THE MXPERTS: ADDING POWER WITH WEIGHT

Dear MXA,
    I just picked up a used 2014 KTM 250SX two-smoker and love it but would like to try a flywheel weight as MXA suggested in the test of this bike. Question is: should I get the 9-ounce or 11-ounce weight?

The simple answer is the 9-ounce weight for motocross and the 11-ounce weight for offroad, but it’s never that simple. Think of a flywheel weight like a yo-yo. Heavier yo-yos handle better than light ones because of the increased moment of inertia. With more moment of inertia, a yo-yo and a motocross bike accelerate slower, power up with more control and, once in motion, stay in motion. Without any flywheel weight, you couldn’t ride your bike. It would have a short powerband, no thrust and a lot of rev. Before KTM chose the stock flywheel weight for your 250SX, they had to consider seven factors:

(1) Overall engine weight: Lighter is always better when it comes to the overall weight of your bike.

(2) AC current: The size of the flywheel must be large enough to house the magnets that are used to produce engine spark but at the same time small enough to make room for the stator windings without creating a bulky engine case.

(3) Engine acceleration: An engine builds speed quicker with a light flywheel and slower with a heavy flywheel.

(4) Inertia: A heavier flywheel carries speed further but is also harder to slow down than a light one.

(5) Flywheel effect: Every rotating component in the engine and chassis creates its own spinning force (clutch basket, gears, chain, wheels, and how the crank counterbalances piston weight and stroke). The manufacturer needs to pick a flywheel weight that best accommodates all the independent gyroscopic forces inside the motorcycle.

(6) Angular momentum: Believe it or not, too much flywheel weight will make a bike hard to move around or corner.

(7) Intended use: The power characteristics and riding styles used on a 60, 80, 125, 250, 300, 500 and 4-stroke all require different flywheel mass.

However, just because a KTM engineer selected a given weight, it doesn’t mean that you can’t be a homegrown engineer and change that weight to suit your needs. And history proves that most factory two-stroke racers, whether on Yamaha YZ250s or KTM 250SXs used flywheel weights on their works bikes.

A two-stroke uses a lighter flywheel than a four-stroke because it fires every stroke, compared to the four-stroke’s every-other firing pattern. Two-strokes don’t have to maintain momentum while waiting for the next explosion, thus two-strokes deliver a snappier and more abrupt style of power. With the dominance of four-strokes, a two-stroke’s rat-a-tat-tat power delivery is often too aggressive for riders bred on slower-revving four-strokes. This is where a flywheel weight comes into play. It can make a two-stroke feel torquier, broader and more hooked up. And, while you might be tempted to think that a heavier flywheel would cost the KTM 250SX horsepower, it is not so. It just makes each gear pull farther, stronger and longer. However, many riders prefer the snappy power and hard hit of a two-stroke and don’t believe there are any benefits to a flywheel weight.