APS tail

[sofadriver]

  Hi all!
New guy here with a question.
I'm building the body for a small displacement APS bike. The shape of the body will end much further back than the SCTA rules allow so I'll be chopping it off at 10" behind the wheel. This will leave a horseshoe-ish shaped opening at the rear. I've seen lots of APS bikes with that opening closed off but that just doesn't make sense to me.

So, open or closed?

Any input will be appreciated.
Thanks.

[joea]

depends on what your doing with the rest of the bike...

there isnt a one answer fits all in any of this....

[RidgeRunner]

     You could try to build it with an easily removable panel.  Try a couple runs with and a couple runs without, the great white dyno should give you a good indication.

     'Course with areo what works at some speeds doesn't always work the same at others, keeps everything interesting........

                       Ed

[sofadriver]

     You could try to build it with an easily removable panel.  Try a couple runs with and a couple runs without, the great white dyno should give you a good indication.

Wow - I can't believe I didn't think of that!

Thank you!

[wobblywalrus]

What is the wheelbase of the bike?  Do you have any pictures from the side when you are on it?

[1212FBGS]

closed..... absolutely no doubt about it

[sabat]

depends on what your doing with the rest of the bike...

there isnt a one answer fits all in any of this....

closed..... absolutely no doubt about it

Choose wisely.

[fredvance]

Bill Warner's bike and Larry Forstall's Guppy both had panels made to block off the opening in the back. I don't think either ever ran with the panel in. The Guppy ran a 254 NA and Bill 311. I think Ralph Hudson opened his up some but not sure.

[sofadriver]

Choose wisely.
[/quote]

Well, that's always a problem, isn't it. 

I've been searching pics of record setting bikes and I'm seeing more open than closed. I'm planning for the rear of my leg openings (only) to be open. That's going to allow a small amount of  air to enter and I'm thinking that will help relieve some of the low pressure behind the bike. The only other openings will be curved slots below the fork trees to allow the forks to rotate.
Sound like a plan?

BTW, wheelbase is 56". Total width is 18". Height is 37" (42 including the windshield fairing). Total length is about 97". Tail opening is about 20 tall x 11 wide. (that's what I got when I chopped off the "perfect" shape at 10")
Can't seem to load any pics.

[maj]

depends which perfect shape
got mine down to 8" wide  with a 65" wheelbase and 8" overhang (FIM spec)
so far enclosed at the rear, and bottom edge is a tight fit to the tire and swingarm
and an adaptive piece on the bottom to switch from SCTA to FIM

been carrying it with me for 2 yrs now without using ,

last yr broke the turbo before realy finding the limits of the oversize std style tail piece , (i want to know just how much difference it makes) 
this yr rained out
 

[wobblywalrus]

That is a short wheelbase bike and the 10-inch long tail is a significant portion of the bike length.  That means proper tail design is extra important for stability.

The tail on mine is open to avoid the low pressure area behind a flat tail and to minimize the effects of a von Karman vortex street.  The bike has been steady and stable up to 140 mph in all sorts of winds.  That is as fast as it has gone.   

[Jon]

Internal drag will cost you exactly the same as external drag.
I would be trying a closed tail if possible and playing with it only if you hard handling problems.

[sofadriver]

The tail on mine is open to avoid the low pressure area behind a flat tail and to minimize the effects of a von Karman vortex street.     

(oh, man - I started some sh*t now!  )

OK, I vaguely remember studying those vortexes. I know that very tall buildings are tapered at the top to help  avoid the problems. Now I'm wondering if cutting the tail slightly diagonally would lessen the chance of that happening.

[sofadriver]

Internal drag will cost you exactly the same as external drag.

That's true but doesn't it depend on where the air enters? The body of this bike will have almost no air entering ahead of my knees. Wouldn't some air flowing out from inside the tail be a good thing?

[JimL]

The rear tire is a big fan (so is the front, but no help allowed).  The tire drags air around with it and tries to throw it into the oncoming airstream below the belly pan.  Your job is to:

1- cut/limit the available air for the tire to move, which means you may not want to feed it from the rear end of the tail.  A closed tailpiece, pretty tight to the rear tire (around its lower opening) will help.
2- cut/limit the available oncoming air, because the tire is going to drag some air around anyway.  That will require the spat ahead of the rear tire on the bottom of the belly pan.

This tire cutout (below) is wrong because it was cut when the tailpiece was narrower for rear radiators, which pinched it close to the tire.  Also, the white spat seen below front of tire is too short.  I will fix these problems when the bike gets longer and much heavier, before it runs again.

For reference, this is a 650cc pushrod twin that went 163.8 return run this year.  Carbureted on gas, and only went 135 with streamlining removed.  Eight records in last four years with only 650cc or 680cc.  This streamlining aint bad for a starting point; AirTech off the shelf stuff.  Bottom of that tail is a piece of Kents electric bike tailpiece, flipped over.