Interference drag
 

Yesterday I was riding my bicycle on the road. I was in a headwind. Riding 12-13 mph. A spray vehicle passed me. These things are like big insects. They sit high to get around in a field with mature corn in it. The sprayer was like the one in the photo but didn't have as big of a spray head as in the photo Sprayers - John Deere

The bottom of the sprayer is about 6 feet off the ground the tires are fairly narrow to fit between corn rows.

Anyway as soon as it passed at 25-30 mph I got a terrific draft and got up to 21 mph for a couple of hundred yards.

The point is that there is a huge amount of interference drag below the vehicle. It was almost as if the sprayer was filling up the entire space to the ground. As if it was semi truck.

Very interesting. Something to think about!

As mentioned on a pickup truck thread axles limit ground clearance even with high body lift. Also, outrigger spray apparatus may snag a lot of air and drag a substantial wake obviating any potential gain from above average ground plane separation. It's a interesting anecdote but I'm having trouble ringing practical value from it regarding regarding our cars, trucks and bikes. If someone "gets it" much better, please educate! :D

Hi Kamper Bob
Not sure what I can add except to say that 90% of vehicle was above 6'.

By what I know interference drag is the drag produced by two bodies near each other. Examples might be: where a wing intersects the body of an aircraft. It might be generalized by having two bodies with definite airspace between them but at some speed they act like a solid.


I felt that there was a subtantial blockage of wind under 6' where I was and where the sprayer generally was not.

Here is one with 90' of width on the sprayer. My car could drive under it easy. It is hard to visualize how big they are until you get passed by one on your bicycle. It looks like that air should move almost unimpeded under it.

http://www.usagnet.com/dealers/114/0012.jpg

Those massive wheels should displace enough air by themselves to give a bicycle a nice tow.