Aerodynamics

[rebelce]

Thanks Blue, I'm glad you waded in on this with your substantial knowledge.  Looking at your post the crux of it boils down to the statement:

 "A correctly shaped tail with no separation will still have MUCH less drag than any cut off "Kamm" rear end regardless of speed."

I wholeheartedly agree with you but when someone doesn't have a solid knowledge of aerodynamics (and I certainly don't) then it is very easy to attempt a visualization based on what you "think" air does. Finding an optimal shape to enclose  our package under these circumstances can lead you down a path with poor results or worse, a car that is unstable. An incorrectly shaped tail section can be as bad as a KAMM effect tail. This discussion along with observation, trial and error and asking a lot of questions are the only options for many of us and I thank you again for your invaluable input. Another weak link in the fray to be ironed out.  I hope all this hasn't distracted our original question.

[Harold Bettes]

Thanks Blue for calling the Basic Sediment (BS) of the overly simplistic Wikipedia for what it is.

Simply put is that separation (non attached flow) is the bane of most fluids problems be they in ports or on vehicle shapes. As an example the problem often in motorcycle is not the entry losses but the exit losses (from separation) are the more costly.

Staying with logical and sensible applications of the basic laws are the basics upon which great ideas are most often built.

Regards to All,
HB2

[Blue]

... when someone doesn't have a solid knowledge of aerodynamics then it is very easy to attempt a visualization based on what you "think" air does. Finding an optimal shape to enclose our package under these circumstances can lead you down a path with poor results or worse, a car that is unstable. An incorrectly shaped tail section can be as bad as a KAMM effect tail. This discussion along with observation, trial and error and asking a lot of questions are the only options for many of us ...

I agree, aerodynamics (like electronics) is not intuitive to anyone outside of the field.  I can give you some basic guidelines though:
First, test, test, TEST!!!  You are absolutely correct that optimized shapes take trial and error.  What is not commonly known is that we can tuft test at low speed (literally 40 mph) and see much of the separation we will see at 400 mph.  I know Speed Week comes but once a year, and most LSR vehicles are not street legal, but yarn, a video camera, a pickup and a deserted 1/4 mile of pavement is all we need for low speed.  Put yarn under the car too and prepare to be shocked that a flat belly with a skirt is full of reversed flow and drag.  Duct tape the camera under the tail of the car to get this shot.

Second, longer in the back is better than short.  It is not guaranteed that a long tail will not separate, it is always harder to keep flow attached on a short tail.  Just as important, stability in yaw (fishtailing or spinning) is best resisted by a longer, not taller, tail.  Look at Bub-7 and try to avoid those tall verticals too close to the rear axle.  Closing the tail out means covering the chute tubes (or better, pointing them up), and makes a push bar difficult.  IM<HO, it's worth it.

[wobblywalrus]

There are a lot of interesting ideas on this thread.  Especially the ones about reducing turbulence at the back end.  It was time to apply some of them.  A few pix of the Bonneville.  The bike has its short "street" swingarm.  The back of the tire is further aft than the metal with the "salt" swingarm.  Hopefully this will not make the bike slower.  It was a lot of work.   

[wobblywalrus]

Here is the third picture.  One was posted twice by mistake.

[Grandpa Jones]

Hello Wobbly,

I like your tail section, it's got a cool aviation (or Airstream!  ) look to it. Do you
have any more pictures of it? I'd like to see the construction details.

I bought an '01 Bonnie this spring, and I don't regret it one bit. Great bike!

Thanks,

Dave

[wobblywalrus]

Grandpa, I was hesitant to reply until after I carefully looked at the Speed Trials data.  The atmospheric and track conditions were similar to last year's runs.  We used the same gasoline.  The Bonnie ran 118 and 120 last year for a 119 mph average.  This year it ran 127 in still air and 123 against a 4 mph headwind.  This year's bike is 7 or 8 mph faster than last year's.

Most of the changes this year were to the chassis.  No engine work and about 6,000 street miles were added over the summer.  A radial front tire, new for this year, has less rolling resistance.   I ran lower tire pressures this year, too, so the advantage of the radial tire was probably offset by the increased rolling resistance due to lower tire pressures.  The increased speed is almost certainly caused by better aero.

The streamlining alterations this year are lower bars from a 2009 Thruxton so I could tuck down tighter and the tail section.  Tuck and the tail work together.  I try to arch my back when I am in a tuck so the flow crossing over the windshield slithers across my back and down across the tail with as little turbulence as possible.

I am a not-so-flexible precodger and Bonnie is a street bike with a standard frame.  This results in a relatively tall bike with a high center of pressure.  The aluminum I use is a bit on the heavy side compared to thin fiberglass or carbon fiber.  Both of these adversely affect the handling and I have had to modify the chassis to make it run nice and straight without snaky high speed handling or speed wobbles.  See Team Go Dog Go build diary.

What I do is have a lot of fun using backwoods engineering and my transportation bike.  Tom Mellor's grey Triumph triple and the Amo 1000 cc Kawasaki are better examples.  Some construction pix will be attached to a subsequent post. There is a special picture I need to download from my wife's camera.

[wobblywalrus]

The aluminum for the framework are various scraps that I buy from a local fabricator.  I drill holes in them for lightness with hole saws.  The cladding are sheets of 0.020 or 0.025 aluminum from the local Ace hardware.  I cut the panels oversize and beat them into shape with a teardrop shaped plastic mallet on a shot bag.  Then, I beat them into final shape on a football dolly with a concave-faced body hammer.  The beating work-hardens the metal so it is nice and strong.  Occasionally the metal work-hardens before I am done forming it.  A propane or MAP gas blowtorch is used to reanneal the metal so it will be easy to form.  The last step is to trim the panels to shape and rivet them together.

Good quality body tools are essential.  Avoid the el-cheapo starter sets.  They are usually not hardened right and the tool's balance in the hand is wrong.  The shot bag and rubber mallet are from Eastwood www.eastwood.com  Martin Tools makes good hammers.  They are available from Eastwood.  The hickory handles work best for me.  The football dolly can be real hard to find.  Mine is from Harbor Freight.  Amazingly, it is good quality.  I wear ear protection when beating.  The ringy can drive a fellow dingy.

Building this stuff is a lot of work.  My grandchildren, Maximus and Madeline, are too young now but they will be helping soon enough.  Madeline is wearing the dress.

Good luck with the Triumph.  They are a lot of fun.

[Grandpa Jones]

Wobbly,

Thanks for the pictures, I really like your work. The fairing looks great.

Have fun with the grand-kids, you can't start them too young! 

Cheers,

Dave