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Coastdown tests using Tetz's spreadsheets
Mine's faster! No, MINE's faster! Oh, yeah? Yeah! Your mama!
Here's the idea: Gather a bunch of MARSians together, with a bunch of different bikes, and do some coastdown tests to find out which bikes really are fastest. Date and feasibility TBD.
For these to mean anything, the conditions must be controlled and corrections need to be made for factors like rider weight. One of our members, John Tetz (in collaboration with Joel Sanders of Bell Labs), was involved in the creation of some very useful spreadsheets that do exactly this sort of calculation. He also has a carefully measured "calibrated hill" near his home in Succasunna, NJ where some tests can be run.
The spreadsheets and documentation can be found at the IHPVA website. Click HERE to go to the page.
Below are some observtions from John about the spreadsheet and how to run such a test:
John sez:
......This is not the run of the mill spreadsheet. My friend Joel Sanders from Bell Labs has what looks like a breakthrough in a very unique mathematical model. No one else we know of has this at the moment.
The sheet can measure Crr too. I have a special almostflat (there is no such thing as flat ground) section of road near my house set up for measuring Crr.
What's fascinating about Joel's model is we can measure grades too. Its very hard to measure an almostflat grade such as 0.2%, but this can be done. I also used it to measure my steep test grade of 6.2%.
In our mailing list discussion about the test, Andy D. made the assumption that ballasting the bikes/riders to equalize weight would prove more accurate then relying on a mathematical model. Intuitively, one might think so but it's not true. Adding weight, for example, will change the rolling resistance. It just so happens that the influence of Crr (rolling resistance factor) is fairly low in respect to the large influence of CdA (aerodynamic drag factor) at higher speeds so it is possible to do the test this way. But no need for me to lug up the hill and extra 25 pounds or more to equal a heavier bike/rider.
For our test we will have to measure weight of the rider and the bike. We will need to know the temperature and barometer readings. I have already done the necessary runs on the hill to know the grade. I have carefully measured the distance of the trap by calibrating the front wheel of the bike over the distance of a 100 foot tape measure. We will need some stopwatches to measure the time through the trap. We will not use speedometers because they are way too inaccurate. We will probably have to do this near dawn when the wind is close to zero. We will have to do at least 3 runs and if a car goes by this data will not be used.
Another thing we could do is measure power capability, seeing as we each have to climb the hill. I also have a measured distance on the up hill side: 0.71 mile and the altitude is 220 feet. Again we need stopwatches to measure time. If I push real hard I can do this in around 6.7 min. Average speed is 6.7 mph which turns out to be 0.23 hp. or 174 watts. I can do this 4 times but then I cant hold the speed. I'm running at close to max heart rate during these tests.
You don't have to do the climb at max power but can do it a rate that is comfortable for you and get a value for that. Or put out whatever you want. You will be disqualified if you get a heart attack.
My more comfortable long term power is around 0.14 hp or 100 watts. Thats kind of what I do on a flat century like the Sea Gull. In the Foamshell that gives me something around 21 22 mph average. That would be 17mph on my unfaired FWD lowracer.
For the Crr test we need nowind conditions also. I need to build or buy a timer that will measure initial speed coming into the trap. The trap is 100 feet and the speed coming in is about 8 mph. But we need to accurately know the speed. No guessing or assuming here. I am able to control my speed fairly well but it is a chore. To improve accuracy in the reading I use 2 magnet pickups. And I do around 10 runs both up and down the grade.
But this won't do for a group. So i need to find a timer. Timing will be done over a carefully measured distance of say 3 feet. The timer will be turned on by the vehicle, do its counting and be turned off at the end of the 3 feet. From this we can calculate the initial speed. Timing for the 100 foot trap section can be done by the rider with a hand held stopwatch. You will be coasting from around 8 mph down to around 4 mph.Any lower generally means the rider will have a difficult time prevent wander which will screw up the readings.
We (Joel and I) have spent going on 3 years on this project working out the bugs and learning the intricities of onroad measurements. You have to very careful with all the many variables that can screw the data up.
Because of time and weather constraints we might do is only the CdA first and estimate Crr. Then do a separate test for Crr and adjust CdA accordingly.
Hope this helps every one understand what is possible.
