Candid Cameron

Candid Cameron

Q Could you explain (or at least speculate) on Honda's rationale for chopping off the tail of its new 8OOcc MotoGP bike? For years I have read that closing the air pocket with an appropriately shaped seat/tail-section is just as important, aerodynamically, as opening the gap with a properly designed fairing. Now we have Kevin Cameron suggesting that HRC wisely bobbed the tail off the new GP bike because, well, hey, they don't really do anything anyway and that no one push-starts their bike anymore. Have the laws of aerodynamics changed or were all those big, bulbous, hump style racing seats just misdirected efforts or even worse.. .just fashion?

Lawrence Somma Stamford, Connecticut

ASeatbacks are in a mostly separated flow regime, so they have no possible aerodynamic function.

I therefore felt upwelling optimism when I saw that Honda had done away with the decorative seatback. Others, such as Erik Buell, claim the seatback can have a useful aero function. Honda apparently disagrees. Riders do on occasion raise their rears on a long straight as a means of gaining perhaps 200-300 rpm. Note also that Honda has made the fairing smaller and smaller.

What’s more, after struggling for several years with the more rearward weight placement of the V-Five engine, Honda has finally done the rational thing: moved the rider ahead to get the front-wheel load needed to keep the bike steering as it accelerates off turns. I’d say the bum-stopper on Nicky’s seat is now about 10 inches ahead of a vertical plane through the rear axle. With the rider this far forward, does it make sense to keep trying to build a roof over the rear tire?

A motorcycle makes essentially no effort to close its own wake; just compare its cutoff shape with that of a salmon or a trout, or with that of Denis Manning’s record-setting streamliner. As a result, a roadrace bike leaves behind a large, energy-rich wake essentially the size of its frontal area. Its drag coefficient is around 0.6, as compared with a modern sedan car at 0.35, a P-51 Mustang at 0.14 or a rigid airship at 0.05-0.07. This is why a) motorcycle performance depends so much on rapid acceleration, and b) small frontal area is so sought after.

-Kevin Cameron