Coast down testing, 2000 Honda Insight (gen. 1)

[aerohead]

Originally Posted by RobertSmalls

I was hoping to get the aerodynamics of the tail nailed down in SolidWorks COSMOS FloWorks, which can illustrate streamlines but can't tell you overall drag. I plan to run a few variations on a model of the car to check for flow separation in parallel flow and in gentle crosswinds, then build one a little bit more conservative than what the simulation tells me I can get away with. Here's an old model that's similar to an Insight:


I could make a much better model by importing the side, top, and frontal views of the car, then adding detail where necessary.

For construction, I really have no idea what I'm doing. Maybe an Al frame with the bottom half of the skin coroplast, and the top half acrylic. I'm sure you'll be going with foam and fiberglass. Will that be on a wooden frame?

Since I didn't know any better at the time,for my CRX,I simply built a cardboard and masking-tape mock-up on the car,fiber-glassed it also on the car.When it was built up with multiple laminations and cured,I pried it off the car,soaked the cardboard out from inside,leaving a "slip-fit" tail which fit on like a glove and held with a few fasteners.
Because of the ovoid,compound"eggshell" curvature,no additional reinforcement was required.
A dual trailer type electrical umbilical allowed for all the lighting.
Dirt simple.

[3-Wheeler]

Reviving an old thread here, as I am again attempting coast-downs with the Honda Insight.

This time around, instead of using a GPS to record speed reduction while coasting, an old camera with video mode is being used to record the speed from the digital speedometer.

So far the camera has been used to record the Crr component of coast down testing, from speeds up to 35 mph, all the way down to 2 mph.

So far the results are not as consistent as I would expect, but are presented just as recorded.

Shown below is a graph that shows the 13 coast down runs thus far, and the thicker red line shows the average of this data.

All the runs are done on the same section of road on the way to work and back. The road is reasonably flat over this section according to Google Earth (within several feet). The road typically has almost no traffic in either direction during my use of the road.

It can clearly be seen that the Crr curve does not go to zero at zero velocity, but instead stops at something close to a deceleration of 1.0 meter/second-squared. This was something that I did not expect based on most information that I have seen.



The Crr graph for bicycle coast down testing is from:

The Recumbent Bicycle and Human Powered Vehicle Information Center

It shows that Crr stays to a non-minimum value at zero velocity.



Jim.

[MetroMPG]

Not commenting on the info as shown, but I was thinking recently about using this method (video of the speedometer) to compare aero mods and had this thought:

1) Switch the display to KM/H (for higher resolution), or,

2) If you have a ScanGauge, switch to KM/H and then change the speed correction factor to increase resolution even further. I'm not sure if it will let you *double* speed, but that would be cool & useful.

[3-Wheeler]

Originally Posted by MetroMPG

Not commenting on the info as shown, but I was thinking recently about using this method (video of the speedometer) to compare aero mods and had this thought:

1) Switch the display to KM/H (for higher resolution), or,

2) If you have a ScanGauge, switch to KM/H and then change the speed correction factor to increase resolution even further. I'm not sure if it will let you *double* speed, but that would be cool & useful.

Hi Metro,

I have heard from members of the IC forum that the VSS (speedometer) signal gets five pulses per tire revolution on the digital speedometer of the Insight, so switching to KM/H should show an increase in velocity accuracy.

However, I still get quite the variation in seconds for each drop in 1.0 mph decrements during the coast-downs. The graph below shows the variation in the change in seconds per mile-per-hour drop.

The graph below is very typical of the variation I see for each run. The X-axis is mph and the Y-axis is the change in seconds per mph drop.

In the graph above, I have done quite a bit of smoothing using a custom software program to filter this wide variation.

Now the question is, what is the cause of this wide variation?



Jim.

[MetroMPG]

What were the weather conditions like (any wind)?

[jime57]

How about using some other device to generate the wheel speed data. Maybe you could put a little drive wheel of some type against one on the tires to get a much higher angular rate. I remember those little bicycle sirens that made contact with a tire, and spun very rapidly. A Hall cell crankshaft pickup from a wrecked car would easily generate a wheelrate signal. an alternative would be to simply use a much smaller wheel in contact with the ground, coupled with some sort of Hall cell generator. I'm thinking

[jime57]

Ok, here it is, buy a hundred #10 nuts at the hardware and epoxy them equally spaced to the inside of a rear wheel rim. Make a bracket to hold the Hall cell in close proximity. You will generate a lot of fine resolution data. How to use the data I leave to the electronics experts ;-)

[3-Wheeler]

Originally Posted by MetroMPG

What were the weather conditions like (any wind)?

Hi Metro,

The wind conditions were very low.

I usually look at flags that several people have in their front yards as an indicator of whether I can record a coast-down at that time or not.

Today for example, it was blowing at about 5 mph or so based on the flags, so the end result; no coast-down this morning.

Jim.

[3-Wheeler]

Originally Posted by jimepting

How about using some other device to generate the wheel speed data. Maybe you could put a little drive wheel of some type against one on the tires to get a much higher angular rate. I remember those little bicycle sirens that made contact with a tire, and spun very rapidly. A Hall cell crankshaft pickup from a wrecked car would easily generate a wheelrate signal. an alternative would be to simply use a much smaller wheel in contact with the ground, coupled with some sort of Hall cell generator. I'm thinking

Hi Jim,

Ideally I would use either a single magnet inside the wheel, or multiple ones to get a digital count of the wheel rotation.

Someday I will learn how to program a microcontroller and have that make the "count" and store that information on an SD card for later analysis.

Jim.

[Nicko]

1. A medium cost, accurate and practical way to get coast down data is to connect a $40 Wahoo wireless bike speed sensor to some rotating part of the car and to collect downloadable data with the $80 Wahoo Ant+ Key connected to an iPhone. Any Garmin sports watch or bike computer can also record the wireless speed signal. Downloadable is the magic word here.
2. With proper understanding of the physics involved and a clever protocol/analysis, accurate Crr and CdA can be calculated.
3. GPS data is way too noisy for this.
4. Estimating drag without a power meter: Triathlon Forum: Slowtwitch Forums