I need help with getting air into the bum stop on my motorcycle.

[makr]

I have two small radiators under the tail section. They somewhat straddle the rear tire. I have a couple of inches on the outside of the radiator between them, and the seat. I can put two 250 CFM computer fans on the outside of each one.

So the question I have for you fine folks is; How do I vent this? Can I NACA duct the center to create high pressure in the middle and suck through to the sides and vent through the bodywork?. The other way? Low pressure in the center and dump the heat out the back?

I ran it today, and I think I may barley have enough time to make a pass with it all inclosed before it is all hotter then Hades. So I just leave it and cross my fingers?

[hotrod]

The problem with using a NACA duct is that they work best in a body surface that has attached airflow, behind the rider on the tail cone like that you are far from ideal conditions to use a NACA duct and expect it to develop any pressure at all. The fans sound like a good place to start. You might open the very rear end of the cone and duct it to suck air through the cores assisted by the fans pushing air through the cores.

Or replace the air to water radiators with a tank full of chipped ice and water, with a core submerged in the water.

[makr]

I was afraid of that.

So duct it from the front/side and out the center/end?


I wished I had the room for a tank.

[manta22]

"250 CFM" computer fans are probably not going to do the lob. First, those fans' specs are probably pulled out of thin air (no pun intended) by marketing dweebs. Even if that rating was true, it is 250 CFM with zero back pressure-- not a realistic situation. Try to find some 12V industrial fans of similar size; they are much more likely to have the air-moving capacity that you need.

Regards, Neil  Tucson, AZ

[hotrod]

This windtunnel video might help you visualize some options:

http://www.youtube.com/watch?v=G7eSiIOQ02g

http://www.youtube.com/watch?v=LOH570HyAVw  (motorcycle about 6:25)



http://a2wt.com/images/A2web/Mag%20SRider%2006.JPG    <--- motorcycle windtunnel shoot out magazine article

http://www.youtube.com/watch?v=rEBS0Tpja9Y   <--- snowmobile same rider rear air flow issue as a bike


Looking at these videos you can see your dead air pocket of turbulent detached flow behind the driver is much much bigger than your tail cone, so short of putting a snorkel on it, you would be hard pressed to get clean inlet air flow to the radiators.

Larry

[kiwi belly tank]

To a streamliner guy, that looks like a lot of room. Just glass the tail up & turn it into an ice water tank with the radiators inside.
  Sid.

[salt27]

To a streamliner guy, that looks like a lot of room. Just glass the tail up & turn it into an ice water tank with the radiators inside.
  Sid.

Good idea, that is going to be on my to do list.

You old guys sure are smart.

  Don

[hotrod]

The only thing I can think of that might be a negative on the tail mounted water tank is an issue with weight and balance and center of gravity screwing up the bikes handling. Might want to run some tests with some weight up high back there to see if it will cause issues.

[wobblywalrus]

Maybe you could put one or both of your radiators in water under the engine like Anders did with his fuel tank on the his jet bike.

[kiwi belly tank]

The only thing I can think of that might be a negative on the tail mounted water tank is an issue with weight and balance and center of gravity screwing up the bikes handling. Might want to run some tests with some weight up high back there to see if it will cause issues.

No worse than having a heavier rider & it's not going to do traction any harm either. Might be wise to have it full so it can't slosh around.
  Sid.

[generatorshovel]

The only thing I can think of that might be a negative on the tail mounted water tank is an issue with weight and balance and center of gravity screwing up the bikes handling. Might want to run some tests with some weight up high back there to see if it will cause issues.

No worse than having a heavier rider & it's not going to do traction any harm either. Might be wise to have it full so it can't slosh around.
  Sid.

I use a tail fuel tank (1.6 gallon), no handling issues as yet


Tiny

[fastroadster]

As a Ol Radiatorman. The air has to go through the radiators, so I suggest a scoop on the sides to make sure air goes through radiators and exits out the back or bottom of bike. The fans only are needed when you are standing still. You won't need them as long as you're moving.  Just a suggetion!!  Good Luck

[Crackerman]

Can you package radiators in a vee open to the front, scoop air into the top of the box "plenum",  thriughthe rads, back down the sides of the box and out the tail, filling the void with warm air above the tire... Can also use a motorcycle rad fan in the back to pull air through the coolers.

Or scoop in sides, vee arrangement closed at front and fan in back still...

[makr]

"250 CFM" computer fans are probably not going to do the lob. First, those fans' specs are probably pulled out of thin air (no pun intended) by marketing dweebs. Even if that rating was true, it is 250 CFM with zero back pressure-- not a realistic situation. Try to find some 12V industrial fans of similar size; they are much more likely to have the air-moving capacity that you need.

Regards, Neil  Tucson, AZ

You are right. I asked a computer buddy today, who told me the same thing. They are not positive pressure fans.

Hotrod, Thanks for the vids. The guy in the video is even more out of the air than I am, but even with that twig on the bike it seems futile to draw air in.

To a streamliner guy, that looks like a lot of room. Just glass the tail up & turn it into an ice water tank with the radiators inside.
  Sid.

Seems pretty sparse to me, but I think this is my answer. I drew a quick mock up. I think I can only get about a gallon of water and ice in the reservoir. I will do some research, but any idea if that would be enough?

The only thing I can think of that might be a negative on the tail mounted water tank is an issue with weight and balance and center of gravity screwing up the bikes handling. Might want to run some tests with some weight up high back there to see if it will cause issues.

I have been running a water tank back there to spray the air cooled engine. The tank held water, battery, and pump. I never noticed the difference.


Thank you guys for the replies. Splaying the radiators, or using a scoop might do the trick, but I think getting any air there is ultimately going to be the downfall.

[maj]

If you can fit it anywhere in front, a small radiator in front will give much better cooling than most options in the rear