Digsy Posted August 23, 2005 Author Share Posted August 23, 2005 Interesting. I assume that the figures that it is pre-populated with are just an example? Link to comment Share on other sites More sharing options...
Digsy Posted August 23, 2005 Author Share Posted August 23, 2005 3m2 is a little low an estimate for Jake's frontal area isn't it? -Ian [/cymbals] Link to comment Share on other sites More sharing options...
JohnA Posted August 23, 2005 Share Posted August 23, 2005 Interesting. I assume that the figures that it is pre-populated with are just an example? yeah, you need to enter your own 60-55mph and 15-10mph figures, as well as true total weight. You can measure the decel figures using the accelerometer, making them much more accurate than looking at the speedo and your watch Link to comment Share on other sites More sharing options...
Digsy Posted August 23, 2005 Author Share Posted August 23, 2005 Doesn't that calc imply that the coefficient of rolling resistance is a constant, though? The info I have says that it increases with speed. Link to comment Share on other sites More sharing options...
JohnA Posted August 23, 2005 Share Posted August 23, 2005 Doesn't that calc imply that the coefficient of rolling resistance is a constant, though? I'm not sure, I remember that both calcs use both time measurements. I've got them somewhere, they were in the manual. So it gets an idea of the 'tailing off' that happens as speed increases Link to comment Share on other sites More sharing options...
Digsy Posted August 23, 2005 Author Share Posted August 23, 2005 It should be easy to verify. I think the website calc uses small decels so that it can be pretty sure that the deceleration will be almost constant over the time. If you know the decel and the vehicle mass, you can work out the retarding force. The retarding force will be a sum of the rolling resistance force and the aero drag force. If you assume rolling resistance force is constant, and aero drag force is directly proportional to speed, then if you plot retarding force versus the square of the average speed during the decel, you will get a straight line. The slope of the straight line will be Cd x frontal area x air density / 2 The line intercept on the Y-axis will be Crr x vehicle mass x 9.81 I think. So if you take three decels instead of two and plot retarding force against average speed^2 and they do not fall on a straight line, then the Crr must also vary with speed. At this point I'd like to make it clear that I do have a healthy sex life. Link to comment Share on other sites More sharing options...
JohnA Posted August 23, 2005 Share Posted August 23, 2005 ... aero drag force is directly proportional to speed not speed squared then? At this point I'd like to make it clear that I do have a healthy sex life. ah, but does it involve another person though? Link to comment Share on other sites More sharing options...
Digsy Posted August 23, 2005 Author Share Posted August 23, 2005 not speed squared then?Yes, I originally typed it using just speed and then had to go back and add the squares in - hence the edit. I missed one, but you know what I mean. ...and yes. Link to comment Share on other sites More sharing options...
Digsy Posted August 23, 2005 Author Share Posted August 23, 2005 Double post. Link to comment Share on other sites More sharing options...
Digsy Posted August 23, 2005 Author Share Posted August 23, 2005 Have a look here. It does the calculations I mentioned before. It's the accelerometer I'm using. Here you go: The same calcs but with allowances for air temperature and pressure. Coastdown.zip Link to comment Share on other sites More sharing options...
JohnA Posted August 23, 2005 Share Posted August 23, 2005 I've extracted a frontal area (appoximation) of 3.37, but I've yet to find what it's measured in, heh... My initial measurements were not very accurate, I have doubts as to how 'level' my road is (VERY important for these tests) and it is hard to measure these things even with light traffic around (the 20->10mph measurement was the hardest one, there is always some dumbass appearing from nowhere and trying to 'play') I tried 3 measurements for each, so I could use the average, but the variance was too high. Anyway... I also said 'square' when I should have said 'cube' Link to comment Share on other sites More sharing options...
Digsy Posted August 23, 2005 Author Share Posted August 23, 2005 I was working with around 3. It would help my calcs if it was higher because I'm under-predicting at the moment. Aerodynamic drag force is proportional to speed squared. The power to overcome it is proportional to speed cubed. Link to comment Share on other sites More sharing options...
JohnA Posted August 23, 2005 Share Posted August 23, 2005 Yep, that's it! I knew there was something right about the square, yet power is cubed. lol. Link to comment Share on other sites More sharing options...
Digsy Posted August 23, 2005 Author Share Posted August 23, 2005 I just did a rough-as-a-bear's measurement of the frontal area and it came out at 2.1m^2 Sounds a bit on the low side to me. I'll have to do it again but on paper using a proper front view and some basic measurements for scale. Link to comment Share on other sites More sharing options...
JohnA Posted August 24, 2005 Share Posted August 24, 2005 Mine is also *very* rough, it was derived from Cd*A=1.112, which itself was based on rough measurements on a public road. Link to comment Share on other sites More sharing options...
Digsy Posted August 30, 2005 Author Share Posted August 30, 2005 OK. I just worked this out using a scaled picture overlayed with a 10x10cm grid and I reckon the frontal area of a bog-stock Supe is: 2.07m^2 Which means that if the book figure of 0.33 for the Cd is 0.33, then Cd.A should be no more than 0.69. So summat is up somewhere. Link to comment Share on other sites More sharing options...
JohnA Posted August 30, 2005 Share Posted August 30, 2005 2 m^2 feels about right. Not sure about the active spoiler when extended, or aftermarket wider tyres though... Link to comment Share on other sites More sharing options...
eyefi Posted February 13, 2006 Share Posted February 13, 2006 did anyone get an accurate frontal area? Link to comment Share on other sites More sharing options...
Digsy Posted February 13, 2006 Author Share Posted February 13, 2006 I never did any measurements. Enough people have these powermeter things now that we should be able to get some proper timed coastdowns so we can at least get decent CdA figures. Link to comment Share on other sites More sharing options...
eyefi Posted February 16, 2006 Share Posted February 16, 2006 i've got something that draws power graphs from ecu log files. it seems really good. i just need an accurate frontal area and weight. how will a coastdown give an accurate CdA figure? isn't there too many other variables to make it accurate? Link to comment Share on other sites More sharing options...
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