[Jay V]

An article in Sport Rider many years ago on aerodynamics showed a larger rider actually filled the low pressure void behind the windscreen better than a smaller rider, which improved top speed.

I would suspect your reduction in top speed was due to the difference in acceleration during the length of the straight, even though your speed entering the straight was higher.

 

[Endurance Todd]

An article in Sport Rider many years ago on aerodynamics showed a larger rider actually filled the low pressure void behind the windscreen better than a smaller rider, which improved top speed.

I would suspect your reduction in top speed was due to the difference in acceleration during the length of the straight, even though your speed entering the straight was higher.

In general I agree with what your saying. In the case of a bigger more fully faired bike, where even a taller rider can tuck both knees and elbows in and get fully below the windscreen, I would agree. However the fairing on our race bike is farily small (see avatar pic) and doesn't have side fairings. The SV is not a large bike with much leg room. I can either tuck my elbows in and have my knees sticking out, or tuck my knees close to the tank and have my elbows sticking out. I can't do both LOL! Either way I'm not even close to looking through the windscreen!

Also the fuel tank on our endurance bike has been modified to carry more fuel so it is a full 1.5" taller than a stock tank, so any difference about "filling a void" behind a windscreen is diminished even more.

Although, I agree that reduced acceleration due to the weight difference is a significant portion of the difference in top speed, when looking at the speed trace it's obvious that aero is playing a signifiant roll. My teammate actually lets off the gas sooner than I and you can see he is still accelerating at 119 mph just before he lets off. On the other hand, even though I am on the gas longer, you can clearly see that my speed has come pretty close to topping out and I'm barely accelerating even though I'm still on the gas.

 

[RecoilRob]

Many years ago I watched a video about an endurance team doing wind tunnel testing with their bike and riders. Pretty interesting and one thing that really struck me was them playing around with each riders positioning to get the lowest drag. We all can tuck as tightly as possible to streamline ourselves but they found that their riders needed to slide back a bit then raise their rumps up while leaving their chins on the tank. I've played around with this and you can feel the airstream trying to lift you when you get up high enough in the back. The most streamlined point is just before you actually feel this lifting and a word of caution....if you want to experiment with this...do it slowly and carefully!! If you raise your rump too high too quickly it'll try to suck you up and off the bike....which would be bad. The team in the video improved their lap times with each rider learning his most efficient position and committing it to their muscle memory. For sure aero is what you're fighting at higher speeds on bikes...which even with fairings is pretty dismal when compared to even passenger cars these days.

 

[Endurance Todd]

We all can tuck as tightly as possible to streamline ourselves but they found that their riders needed to slide back a bit then raise their rumps up while leaving their chins on the tank. I've played around with this and you can feel the airstream trying to lift you when you get up high enough in the back. The most streamlined point is just before you actually feel this lifting and a word of caution....if you want to experiment with this...do it slowly and carefully!!

Being 6'3" and 225+ lbs there is no sliding back for me. I already use every inch of space available on the bike LOL! Getting the chin on the tank just isn't feasible for me, especially when I'm doing 30-45 minute stints at a time.