Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/1451100
P88 FEATURE I STATE OF AMERICAN FLAT TRACK as "moment of inertia" (MOI). This is not a physics textbook nor a Kevin Cameron column, so we'll turn to Wikipedia for the quick and dirty summary: "The moment of inertia, otherwise known as the mass moment of inertia, angular mass, second moment of mass, or most ac- curately, rotational inertia, of a rigid body is a quantity that de- termines the torque needed for a desired angular acceleration about a rotational axis, akin to how mass determines the force needed for a desired accelera- tion. It depends on the body's mass distribution and the axis chosen, with larger moments requiring more torque to change the body's rate of rotation." Its equation (I = mr 2 ) is ex- tremely relevant in flat track because a bike with high MOI allows for more consistent torque delivery, better grip and greater rideability across highly changeable dirt surfaces, which translates into more predictable mid-corner performance and superior launches off exit. There are several ways to en- gineer more MOI into a motorcy- cle—such as the utilization of more massive, greater diameter cranks and flywheels in the engine and/ or using heavier rear wheels. And herein lies the compli- cation. Engineering high MOI numbers into a performance motorcycle is something one would look to avoid in just about any application outside of one: flat track racing. Related to that, the Mission SuperTwins rulebook is split into two parts. One covers "race-on- ly" engines such as the FTR750 and the Harley-Davidson XR750 and Honda RS750 before it. The other covers "production-based" engines, which are derived from street motorcycles and therefore locked to their "low MOI" OEM crankcases as found on the showroom floor. Estenson Racing Director of Operations Tommy Hayden ex- plained, "With a production-based engine—there is almost no reason whatsoever you'd want a bunch of inertia on the street, going stop- light to stoplight. The only place it makes sense is in flat track. We're limited on what we can do based on the crankcase size with our bike in particular. If I had a magic wand, that's the first thing I would change about our motor. It's great on the street. It's great in road racing. It's great for what it was designed for. But that is a huge limitation in flat track racing." Smith said, "There's only really one thing that Indian has done that no other manufacturer has and that's design an engine with a really big crankshaft and massive flywheel, which trans- lates to this big inertia number that directly turns into traction on a dirt track. No manufacturer is going to build a modern-day street bike that's lazy and really Before the FTR750, you could win on production- based motorcycles. Bryan Smith proved it in 2016. "No manufacturer is going to build a modern-day street bike that's lazy and really slow out of the hole. It would be the worst street bike, but that's what you need for flat track." -Bryan Smith