The duct outlet has a hard time fitting in the space between all the linkages and the tire. The reasoning is that the duct causes the air inside the wheel to "churn" and doesn't just stay stagnant, ever increasing in temperature. Gearhead's data and results show that, for whatever reason, they work.
https://i1.wp.com/rooducts.com/wp-co...00%2C225&ssl=1
https://i1.wp.com/rooducts.com/wp-co...91%2C300&ssl=1
https://i0.wp.com/rooducts.com/wp-co...00%2C295&ssl=1
https://i0.wp.com/rooducts.com/wp-co...00%2C146&ssl=1