Drafting

[backinthesaddleagain]

I normally don't ride the interstate, and when I do I try to stay back aways from big trucks, out of fear of not seeing an object or getting hit by a retread.

However, does this observation of drafting make sense?
From far behind the truck the bike pushes the wind normally as if no truck was there.

From way to close the truck pushes all the wind and the bike rides in the slip stream (even being "pulled" somewhat by the vacuum.

[backinthesaddleagain]

Hit enter key by accident - sorry.

There seems to be a distance where turbulence is highest, not so close that truck is pushing the wind and bike is in the slipstream, but not so far that the truck has no effect on it. That seems to be an area where bike and rider are really whipped by the air coming "back together" a distance behind the truck.

Does that make sense?

[rockNroll]

Yes.

[Alex]

Sounds about right. Drafting works, for bikes, cars, and anything else where air resistance needs to be overcome. A few years ago Car & Driver had a mileage contest where they were testing some highly-rated gas-saving type cars. The winning editor got incredible mileage by having a friend in a Suburban drive 2 feet in front of him, with the back doors on the truck propped open. They ran with headset radios to make it at least a little safer to run with such little distance between the bumpers. Not practical in real life, has tremendous safety implications that are easily understandable, but yeah, cutting air resistance helps mileage and performance, sure.

[backinthesaddleagain]

Alex, as the Ninja enters or exits the back of the draft are do you feel the ninja get a little twitchy? If so how does that compare to your ZX10 or Beemer?

[kkim]

http://www.cd-adapco.com/press_room/...sport_CFD.html

[Alex]

I don't notice any significant draft when approaching an 18-wheeler from behind, no matter what bike I'm on. But I never get up that close to them anyway. Once I'm within 3 or 4 car lengths, I'm already moving to another lane. I'm paranoid that something will come under the truck that I won't see in time, and I'm paranoid that one of those tires is going to have a blowout or tread-separation while I'm in the line of fire.

I do notice a large wind effect once I pass the front of the rig. 18-wheelers cut such a large hole through the air that it seems you're in the draft effect as you're next to them, all the way up until you pass by their front bumper, and only then do you quickly feel the blast of the full-force of the air resistance that the truck was shielding you from as you were moving up beside it. Also, if there happens to be strong crosswinds in the area blowing across the highway (from the direction of the truck), once you pass by the truck the wind can feel even more significant, as you're now getting hit by those crosswinds as well as the new lack of draft protection all at the same time.

[backinthesaddleagain]

Originally Posted by kkim

http://www.cd-adapco.com/press_room/...sport_CFD.html

Thanks Kelly that summed it up as follows:
If the second car is in the correct position, it has the effect of increasing the pressure at the rear of the lead car, reducing its overall drag. However, the effects are highly dependant on the car positions. At times, the drag on the second car is reduced as the first car deflects the air over it. In other configurations the drag force on the rear car is actually greater than that on the lead car as it sits in the dirty, highly turbulent wake. Both cars handling are also affected as the front and rear lift on each varies with the car positions and strong yaw forces occur on the rear car when in the offset position.

[kkim]

hiper milers (cars) draft all the time to increase their gas mileage on public roads, but it's a dangerous practice depending on how close you need to get to reap the benefits. Other drivers tend to hate hiper milers.

[backinthesaddleagain]

Thanks Kelly. I wasnt thinking about saving gas. I had just noticed that there was point of higher turbulance as I moved closer to / farther behind a truck and back and forth while on the highway. A friend of mine said he was once catching up to a friend of his on the highway (both on bikes) at a very illegal speed, and a few feet behind his friend he caught some wicked air and headshake til he went around him.

[Broom]

if you spend any time on the track you'll find that you'll gain a lot of speed drafting other bikes

[Nickds7]

Just be careful doing this out on the road. A cop will pull you over in an instant if your tailing a big rig

[backinthesaddleagain]

on the news tonight CBR900 rider rear ends trailer truck on interstate, truck driver had no idea, and rider not as bad as could have been.