Monday, December 1, 2014

125CC Wind Tunnel Test 2011

After we returned from Bonneville in 2010 I needed to know what the Coefficient of drag was on our bike. We poured in 400 hours or so with design, carving, sanding etc. and wanted to quantify the hard work and begin applying some science rather than just gut instinct to the project. Remember, the goal is to be the first 125cc streamliner to top 200MPH. So along with testing the current bike i planned on a test of our 200mph streamliner as well. This would be achieved using the current bike and covering the rider area with cardboard and tape to finish out the shape. The bike's cross section is nearly identical to my streamliner design. Even though its about 38 inches shorter it should give us a rough idea of where we are at. The added length of the actual streamliner will ultimately make it even more slippery.
An acquaintance from Texas who had already done some car testing in the tunnel brought in some experience which turned out to be very helpful in terms of process and recording the data.
It turns out matte board and tape go a long way in the tunnel allowing for quick changes yet provide enough detail to record change, for the better or worse. The following photos and video show the bike in different test modes and I will do my best here to point out what is happening.

First test is the Full Streamliner mockup:

We record a coefficient of drag of .078 which is really low but not entirely realistic as there are no vents for air intake for cooling and the pressurized air-box but nonetheless we find it encouraging. The BUBS streamliner in the same tunnel recorded a .080 C of D! Amazing considering how much larger it is and its a complete functioning motorcycle unlike our humble mock up. Hats off to you Mr. Manning.

After the full streamliner test we start peeling away pieces of cardboard section by section and running the tunnel each time we remove something. Remove one item at a time and you can clearly see if it works or not. We peel away until we get down to the bike as it was run in 2010.
This is what i was most interested in; what is the C of D on the current bike? Lets look at the infamous smoke test as the visual it quite entertaining. After all the testing recorded on the tunnels scale they slow the wind speed down and allow the smoke to tell a visual story which makes it all a bit less abstract. I don't have the patience to edit the video so feel free to bail at any point during the 5 minutes of wind and smoke.
As you can see in the smoke test we do fairly well with laminar flow. Yes seperation is happening after the riders body but the smoke is still visible unlike most partial streamliners where it dissapears entirely. The bike as we ran it in 2010 ends up with a Cof D near .34. With further tweaks like the cardboard on the front fairing by the helmet and shoulders, we get it down to .312. The tunnel operator casually mentioned this was the lowest figure ever recorded for a partial streamliner motorcycle in the A2 tunnel. We also find out airbox pressure is outstanding with gains to be made with a radiused entry lip. Clay was very helpfull in modeling the test lip on the airbox entry.

If you look closely in the video you can see the black and white clay used to fair in the edge of the windscreen. The plastic windscreen has always been screwed to the outer surface of the front fairing leaving an edge .062 tall exposed to the oncoming atmosphere. I agree...horrendous. But fairing in the windscreen to be flush with the fairing is a great deal of work and i wanted to be sure it was worth doing.
Crikey! .011 decrease with the clay which correlates to .5hp. On a 125cc motor you can't afford to give away any horsepower as they are too hard to come by and by the time we are done there will literally be no ponies left to buy. Fast forward to a photo one year later of the faired edge for the windscreen prior to paint. A word from the wise man is sufficient.
The most important lesson to be gained from this blog entry; Spend money at the tunnel before spending it on your motor. Way more bang for the buck!
Next time we will cover the applied bodywork modifications gleaned from the tunnel test as we prep the bike for our 2013 effort along with some mechanical changes as we look to simplify the process of running the bike at Bonneville.

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