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Old 05-04-2020, 04:34 PM   #8 (permalink)
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Originally Posted by JulianEdgar View Post
When writing the book, I found two SAE papers on coastdown testing and looked carefully at what they had done to get good results. It was all way beyond the technology available to an amateur eg specially calibrated anemometers on probes out the front of the car constantly logging data on things like yaw angles of the oncoming air, and sophisticated computer modeling of driveline inertia, etc, etc.

Frankly, I believe that most of the aerodynamic modifications that I see people undertaking do very little, especially those that attempt to decrease drag. However, I can see some major modifications (full length belly pans / undertrays, large boat tail extensions) as giving results above noise.
When replying I wrote a paragraph filled with questions. I think it would be easier to list them.

1.You said "most of the aerodynamic modifications that I see people undertaking do very little, especially those that attempt to decrease drag." Are you suggesting the mods themselves are ineffective or that our current testing system is not good enough to show small improvements and therefore unable to tell if they are actually helping?

2. Do you think these smaller mods would show up using the throttle stop method and could you give some insight into when you think mods start getting too small to make a measurable difference?

3. Would a higher test speed change the threshold of mods too small to test? E.g your experiments have been around 105 kph (65 mph) if we tested at ~125 kph (78 mph) would smaller changes be more evident?

Can you link those papers? I have been trying to get a small collection of reading to do this summer on aerodynamics (when I have the extra cash to purchase some of the texts anyway). If it seems I am avoiding the other methods of testing it is because I haven't completely grasped how to interpret results from them. This is not a direct request for you, I just haven't done my part to sit down and read up on anything yet to be able to ask informed questions about them.

I do have 3 questions for the magnehelic gauge. The "ideal" car would have very little pressure change over the surface of the body correct? So with the magnehelic gauge you check for high and low pressure zones and try to reduce the size of these zones. I think thats how it works right? Is there a limit to effective hose length? For example, if I have the gauge towards the front of my van and ran a hose (~15-20 ft or 5 meters) to the top edge of the rear would the hose act as a restriction to the changes or is that effect really tiny and therefore wont matter? The hose running along the body line wouldn't be enough to change the results of the test correct? Like if I had a hole in the roof above the driver seat and ran the hose down the length of the roof rather than just poking out the top.
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1973 Fiat 124 Special
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Last edited by M_a_t_t; 05-04-2020 at 04:50 PM..
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