Barracuda Streamliner  Human Powered Vehicle design part 2

Barracuda Streamliner Design, Part 2

By Warren Beauchamp

I have been racing the Barracuda streamliner for over a year, and though it's a fast and versatile design which allows me to cruise at close to 40MPH, I STILL want to go faster! 

This page will detail the design process for a "state of the art" streamliner, using information that I have gathered from the HPV community, and other sources. I have learned a lot about low speed aerodynamics and laminar flow since my first streamliner design, and I hope to pass as much of this information on to potential HPV builders as I work my way through this project.

6/1/01 - Basic Design goals and initial thoughts  
This new design will utilize a light weight monocoque body design, with full suspension and front wheel drive. The body will have about 5 inches of ground clearance to minimize ground effects and allow greater leaning angles in the corners. It will be designed to allow the use of fatter tires to provide more traction in the corners. I will attempt to use 26" wheels front and rear for lower rolling resistance.
The new body design will attempt to keep the airflow more laminar than the previous design, by borrowing the ideas of some of the proven high speed HPV designs such as those of the Edge series bikes,  designed by Matt Weaver, the Varna Mephisto, and Varna Diablo, designed by George Georgiev. 

This graphic shows the top view of Cheetah streamliner from the Cheetah Java applet. Airfoils similar to this can be created using the Profoil 4 segments design tool.
Older record breaking streamliners like the the Cheetah, designed by Michael Selig, used NACA airfoil wing sections in their design. Current thinking is that since the streamliner shape is not a lifting body (and you don't want it to lift!), it should not use wing sections in the design.

This new "speedbike" will be raced mostly in HPRA events, which have relatively short tracks, sharp corners, and small bumps. Traditional streamliners (is there such a thing?) don't utilize a suspension, and have skinny racing tires. While this works great for straight line sprinting and bigger tracks, I have found that to corner faster, a streamliner needs at least a front suspension and adequate ground clearance for 45 degree leaning in the corners. Wider tires such as the Comp Pools or Avocet Freestyles and a full suspension will be needed when short track speeds are expected to be above 40MPH. Low bike weight does help on hills and in sprints. My goal is to make the new streamliner lighter than the 60 pounds the current streamliner weighs.

One of many design studies for new streamliner

The basic idea for this iteration of the body design will be a laminar shape, with a fairly sharp nose, and the body widening all the way to the shoulder area to keep the boundary layer laminar as long as possible. The body will then transition into a mild concave shape, and end with a sharp tail. The tail section on the existing Barracuda streamliner uses a convex tail section, which allows the air flowing down the sides of the bike to crash back together at the trailing edge, causing turbulence (or at least that's the picture in my head...). The concave design is supposed to allow the air to flow straight off the tail and "re-accelerate" the air at the back of the streamliner. The caveat to a sharper nose section is that it will tend to make the bike more sensitive to the wind direction and shifts in wind direction, which can make the bike a bit of a handful in high wind conditions. Also a shaper nose requires a custom low-Q BB and cranks that are both short and low-Q.

The new side view design will allow more air under the streamliner than the existing streamliner, this should be ok as it will have more ground clearance. The net effect will be a rounder front, which should spread the high pressure area at the front of the fairing more evenly. Because I'll be sitting in the bottom of the streamliner body, instead of on seat mounted on a bike frame, and I will be using front wheel drive, the streamliner height will potentially be 3 inches less than that of the existing Barracuda streamliner, which should punch a much smaller hole in the air. As the total frontal area of the streamliner is the biggest factor in it's eventual speed characteristics, I want to keep it as small as possible. 

Body Composition
This time around I'm going to try to utilize higher-tech materials. Ideally I'll use a Nomex honeycomb core sandwiched between single layers of carbon fiber and Kevlar weave material. This construction will be quite rigid and able to withstand a lot of punishment. 

It may look something like this photo-retouched version of the existing 'Cuda streamliner...

Shrunken 'Cuda streamliner

Going back to my friend the pressure distribution java applet, and taking into account that the transition to the tail section should be held off for as long as possible to keep the air laminar for as long as possible, and then adjusting the plot to get approximately the right aspect ratio for a shape 9.5 feet long and 17 inches wide gives the result to the right. Notice that the red pressure distribution line remains relatively flat and low, and then drops off more before going high at the end of the shape.  

After redrawing the above "wing" with a concave tail obtain a better shape, and then much messing about, I finally ended up with the shape below. This new shape looks just like Matt Weaver's Virtual Edge, only bigger.  After some measuring, it looks like if I use a minimum Q-factor crank, I should be able to cram myself into it. 

On  my previous streamliner design page, I reasoned that any air directed under the fairing would slow the bike down, and designed the fairing to avoid that problem to the extreme. Unfortunately, I created a rather high pressure area on top of the the large down turned nose, which may have overcome any gain I would have gotten by eliminating the high pressure zone under the bike. 

Here is a side view of design exercise #2, which considers dual 26 inch wheels, 150mm cranks and reshaped nose. The old fairing was about 30 inches from bottom to top at it's maximum height, this one should be 27 inches, with a 5 inch ground clearance. This picture demonstrates that a person with a 36" inseam can use a 26" front wheel, with short cranks and chain ring overlap.

Design prototype 2 - FWD, full susp, 26" front and rear.

Currently I'm  working on refining the body shape and determining that I'm going to fit properly.  Also I'm gathering parts to build a FWD lowracer mule out of spare low grade components and muffler pipe using a 24 inch front wheel and a 700C rear wheel. This bike will be used to troubleshoot any geometry and handling issues that may arise. I'm planning on using the Varna style of FWD, as it is relatively simple and fairly bulletproof.

After drawing and redrawing the fairing shape too many times to think about, and measuring and re-measuring pedal box and body clearances, I have decided to create a body that is completely symmetrical from the side view. This way I'll only need to sculpt half of the body. From the male half plug I'll create a female mold. I'll be able to use this same mold for both the right and left halves of the body. This will ensure that the body is actually symmetrical, which is very hard to do with a full plug, and that it is actually straight. My original 'Cuda mold is slightly asymmetrical, and has a slight twist. Also, using a female mold means that the body will actually be smooth on the outside, making finishing the surface much less of a chore. 

Above are finalized top and side views of the fairing. The side view comes fairly rapidly to pedal box height, which will be 22" for 155mm cranks, plus a couple inches to allow for fairing curvature. In an attempt to keep the boundary layer attached, the fairing height continues to grow slightly until the shoulder area is reached and then is cut off fairly rapidly.  

The fairing plug will be constructed from these drawings by building the foam up onto a plywood base for the half mold. The plywood base will provide a fairly sturdy template to ensure that the two halves line up properly when I try to put them together.

Using the side view plans above, and applying head and wheel fairings,  will give me something like the scale rendition above. 

As the fairing shape and size are not dependant on the wheel choices, I decided to go ahead and start work on it. I'm pretty sure now that I'll do a more traditional 406 front and 700c rear. If I could have the cranks and chain ring overlap the front wheel like the Varna does, I'd go with a bigger front wheel, but with the super narrow Q this bike will have, and my requirement to be able to use fat tires for cornering, I'll need the extra wheel clearance. Also I'm now fairly certain that the amount of rolling resistance a larger front wheel would save is negated by the ergonomic problems it introduces. I'm skipping the mule low racer, as I will be using approximately the same front end geometry that the current 'Cuda enjoys. While this bike is designed to be useable in the HPRA races, it is mostly designed to be a high speed Battle Mountain racer. I'm not sure at this point if it will replace or merely augment the current 'Cuda streamliner.

Over the past two years that this project has been in development, I have had plenty of time to review the design. Fortunately, it still looks like this will be a super fast bike! Below is a profile view comparison between the new Cuda - W and the current crop of fastest bikes in the world. 

Scale - Barracuda X is 9.5 ft long

The things I noticed were:

  • The Diablo is way tiny! No way am I going to get that small, but the Cuda-W should be more laminar.

  • It appears that the total frontal area for the Cuda-W should be less than the VE and KE bikes.

  • The wheelbase is also longer than all the other bikes. A longer wheelbase should help the 'Cuda better handle the slight winds that can knock a streamliner over at 70+ MPH by moving the center of pressure further behind the front wheel. 

How a bike handles at high speeds has been a real concern in the past year. As speeds reach and exceed the 80MPH mark, the bikes become increasingly susceptible to slight gusts of wind, so much so that some racing teams are building 3 wheeled speedbikes to go fast without balance concerns.  It has been demonstrated that the further behind the front wheel that a pressure is applied, the less effect it has on the vehicle. This means if the center of pressure is in front of the wheel, the bike will be pushed around by side winds, if it's closer to the center of the bike, the side winds won't be as noticeable. You can try this yourself by having someone push on you while you are riding. Try (on a recumbent) having someone push your foot, then your leg, then your seat and observe the effects.

Now that the fairing design has been completed, It's time to move on to:

Silly drawing

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