Dec 20

Dec 20, 2004

Coast down tests

I was ready to take the Rocket for it's first maiden cross country voyage today - a 100 km cruise to Banff (web cam view from top of Banff Gondola here), but it snowed last night and I wasn't sure about the road conditions. So, I decided to postpone the journey until tomorrow or later this week. Instead I did a series of coast down tests.

Let me explain what a coast down test is for. First, there are two main factors that slow you down when you are riding a bicycle. One is aerodynamic drag and the other is rolling resistance. Mechanical efficiency is a third, but it's minor compared with Crr (rolling resistance) and CdA (drag). If I were riding my Rocket ('my' Rocket, not 'a' rocket) in space where there is no resistance at all, then my 150 watts of input power would eventually propel me to near the speed of light (It's theoretically impossible to actually reach the speed of light. You can go faster, but that's another subject, and another adventure...). Anyhow, on Earth (or most other planets or their moons in our solar system), you have a couple of things that make attaining the speed of light a bit of a challenge. One is aerodynamic drag - how much the air pushes back at you as you try to punch a hole through it, and rolling resistance - the friction of your tires on the road and the friction in the bearings.

So if I can go 37 kph with 150 watts of input power, then how do I know how much of the resistance to me going faster than 37 kph is due to aerodynamic drag and how much is due to the rolling resistance of my wheels? Easy - at slow speeds, rolling resistance is the dominant factor and at high speeds, aerodynamic drag is the dominant factor. So, If I did a test where I restrict my speed to very slow speeds, then I can measure mostly rolling resistance.

And that is what a coast down test is all about. You basically coast for an exact distance and record how much time it takes you. If you combine the time with the speed you started at and plug those values into a complicated spread sheet, you can come up with a pretty good estimate of your rolling resistance. Once that is known, you can plug that into John Tetz's cool power calculator and figure out your CdA (Aerodynamic drag), and then devise all kinds of nifty strategies for winning the Dempsey Macready prize of $24,000 clams for going 90 km in one hour or the Decimach prize of $18,000 for the first human powered vehicle to exceed 75 MPH).

It is possible to use published rolling resistance data for various tire brands, but unfortunately that over simplifies things quite a bit. They don't take into consideration the temperature, atmospheric pressure, road surface, alignment of your wheels, etc. When you consider those factors, your rolling resistance can vary by a huge amount over the published values.

I carefully measured out a 150 feet of flat road in front of my house, spray painted some permanent lines, then swept off the snow and gravel. A coast down run consists simply of entering the 150 foot trap at a target speed (around 14 kph) and recording the exact speed at the start of the trap then timing yourself until you reach the end of the trap. You do this BOTH directions to negate any road grade or wind, then do it as many times as you can. I completed 11 two-way runs. Then you enter all the data into the TcoastF3.xls spreadsheet and derive an average Crr. The data is in the "speed data" section of this site (button up at the top of the page).

I ended up calculating a Crr of .011 which is very poor. That's good news and bad news - in order for my to reach 37 kph average speed with 150 watts of input power and against rolling resistance of .011, my aerodynamic drag would have to be fairly low - like around CdA .4 to .5 (square feet). That CdA is probably way too low to be real, but it just so happens to agree with Ben's CDF software simulation:

With 'normal' rolling resistance of .006 and a CdA of .5 (sq ft), my estimated power requirement to reach 40 kph is only 115 watts!!

The problem is, why is my rr so high, and what can I do to bring it down to more normal levels?

OK - onto other things.

Here is a shot of the Coroplast wheel well I built. During my somewhat wet coast down test, there was no splashing at all inside the fairing and only a bit of a puddle on the floor when I was done, so it seems to work ok. What I would like to do now is use the coroplast form as a male mold and pull a fiberglass part off of it for a more permanent wheel well that would be easier to remove.

This shot shows the new larger diameter vent hose. It seems to work ok - The window was fogging up during my coast down runs, but there was always a small area just above the vent outlet that stayed clear.

Did I say I was happy with the steering yet? Oh I did? OK, well let me say it one more time in case you missed it. I finally have the steering perfect and I won't change it any more. I promise. It's awesome.


1. Strut slot sliders - Simplify to a folding cover
2. Canopy Bubble - make a sliding convertible top
3. Front wheel well - Make glass version
4. Wingnuts for fairing mounts
5. Electrical - rechargable battery with a panel with switches for rear strobe and front headlight
10. Add a second front caliper brake
6. Make a portable wind trainer using the (mini-rollers)
7. Look into painting the fairing
8. Find a helmet that fits in the bubble
9. Add second brake

TOTAL distance on TCR1
846 km

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