You are probably referring to axle paddles. Again, normally found on hondas, or similar straight axle atvs.Lol...... Funny but I have actually seen something similar to that. They were on the inside of the rim where the brakes would be. It was on a YouTube video where someone was installing a clutch kit on a honda I think.
They are not for water, they are for mud. With straight axle atvs, they get hung up in the ruts a lot easier than with independent. So, while the axle is hung up on the center of the rut, and the tires are unable to to get traction, the axle paddle is able to pull the atv forward.The only possible benefit I could see (and it's far fetched) is that maybe the paddle in the center might help keep the quad from getting high centered?!? Granted the paddle thingy is sturdy enough to handle the full weight of the vehicle. Straight axle, small tires = less ground clearance? Still kind of ridiculous though. Definitely not a whole lot of benefit in water. Like using a spoon from the kitchen to paddle a Jon boat.
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