Why don't motorcycles have tire deflectors?
 

We all know that a properly designed and placed tire deflector(spat) can reduce drag on cars, why don't motorcycles use them too? (on rear wheels) I have been looking at aero focused production motorcycles like the BMW K1000 and the Suzuki Hayabusa and have not noticed any sort of tire deflectors. It seems like an easy drag reduction mod to me, is there something i'm missing?

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By spats you mean enclosing the rear wheel as on a Vincent Black Prince?

http://www.wired.com/wp-content/uplo...ack-Prince.jpg

You're missing the overall goal, which is styling that makes things look powerful.

No, I mean a air deflector to push air down or to the sides like cars have. I drew a picture of one here.
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Do bikes need a spat with their domed tires? Car tires present a flat surface with a lot of tread to the leading air, but bike tires are round with little tread...could the benefit of a spat be so little that it's not worth adding to the vehicle?

The ground clearance on a motorcycle is roughly half that of a car. The wheelbase is much shorter also. So the cost effectiveness may not be much. Do the A-B-A test and find out. What gets measured gets improved.

I'm thinking this is the reason. Not much need for a spat if the tire is already relatively streamlined. Perhaps enclosing the tire completely at the sides would make more sense, to remove turbulence.

From a purely aero perspective, the wheels are probably the least dirty part of a bike.

Have you ever rode a bike with solid wheels? Side winds blow you all over the road. Enclosed tires on a motorcycle is only a good idea if the body is streamlined.

Fuel efficiency is far far down the list of importance for the industry.

Tire deflectors make sense on wider tires.
Motorcycle tires are round, thus 'cut' better through the wind. Makes no sense to equip them with it, because they will only add drag.

I don't think that it would add drag so much as get in the way.

My bike has 6.5" of clearance, and 5.5" of suspension. The inch that's left over is pretty minimal and can be taken up with tire deformation on a big hit. I don't things hanging underneath to get torn off.

The short wheel base and round tires, combined with minimal clearance all contribute to making rear wheel spats unnecessary.

I've thought that spats on cars didn't necessarily help by deflecting air away from the tires, but directing air away from the wheel wells.

Bigger improvements can be had at the front wheel where relative wheel speed is higher; LSR fenders have shown 2-3 mph top speed improvement on big bikes. This may not sound like much, but given the high speeds it is significant.

http://www.airtech-streamlining.com/...SX13LSR4~1.jpg