Best way to perform a coastdown test?

[Fr3AkAzOiD]

Looking to do some coastdown testing and was hoping for some tips on the best way to do it.

I found a decent stretch of road and would most probably be trying to do a 60 - 30 mph test as unless done in the middle of the night a 60mph - 0mph would be difficult.

It's a nice little hill that shallows out to maybe a 1% downgrade and the whole thing was freshly paved at the end of last year so there would be less irregularities in the pavement.

My main issue is with how to measure the distance on the different runs.
I would like to know a good way to measure distance in feet without having to walk the street at night with a measuring wheel.

I apreciate any help.

[cfg83]

Fr3AkAzOiD -

Borrow a GPS unit from a pal? Park the car, tape an "X" where you parked for reference, reset the distance tracker, go "Y" distance, park, and tape another "X" where you stop.

CarloSW2

[Fr3AkAzOiD]

Don't know anyone even though have lived in NC for over a year.
Will have to see if the GPS app on my phone will work.

[cfg83]

Fr3AkAzOiD -

Maybe you can use your trip odometer to do the same thing as the GPS?

- Park the car
- Tape an "X" where you parked for reference
- Reset the trip odometer to 0.0
- Go "Y" distance
- Park the car, being careful to make sure the you stop where the odometer increments, i.e. on the 0.1 mile mark.
- Tape another "X" where you stop.
- Keep going forward solwly in 0.1 mile increments and tape an "X" at each place. You may need some sticks for this to mark them accurately.

The problem with this is that your "resolution" will be 0.1 miles, aka 528 feet. That means you will still need a measuring wheel, at least for the distance *between* the sticks.

CarloSW2

[RobertSmalls]

Pace it out, and report your result in paces. Your gait should be repeatable enough on level ground, especially if you leave a marker near where your car is going to stop, and pace from there.

[SentraSE-R]

Measuring the distance is the easy part. Consistently starting the coast at the same speed at the same starting point is the hard part. The best methodology I've seen is Metrompg's starting from a stop.

[wyatt]

I used the method found on instructables once. It is used to find the coefficient of drag (Cd) of a moving vehicle. The Cd of your car can then be fed back into equations to predict gas mileage differences, like the one in the tools at the top of the Ecomodder page. Basically using the speedo on your car and timing the coast down in 5 mph increments, and running the results through the spreadsheet. I have also done this with my phone's GPS. The GPS is faster for data gathering, but slower for data processing. If you have an android phone, the best program I have found for recording speeds is My Speed. There is another that will calculate the grade of your hills, it is called Speed Pro. Both use the built in GPS, so that is your limiter.

Make sure you do bi-directional runs, since runs in one direction can be biased by wind or a slight incline. Also, make sure to do at least 5 sets, 10 is what ASME recommends, but I never have taken that much time. Make sure to start with a warmed up engine AND drive train. It may be nice to record the outdoor temp, conditions (wind, etc), vehicle configuration (any mods), and tire pressure at the time of the runs (when warmed up).

I would also recommend that if you can do 60-30 coast downs, you may want to do a second set of 30-15 (or whatever) to get the rolling resistence better defined. 60-30 does a poor job of capturing Crr.

[Fr3AkAzOiD]

I may end up splitting it up into a 60 - 30 to get an idea of aero changes and a lower speed one to judge rolling resistance.

Also, if all I'm looking for is a rough estimate to check how mods change the car I could also just use a stopwatch to time speed change from 60 - 30 and then use the car onboard average speed calculator to get avg mph and translate to ft per second to come up with a distance.

For that though I would have to find a flatter place to test. Also decelleration would not be constant.

[AeroModder]

I use speed limit markers for my tests. I come to 45 MPH as I approach a "Speed Limit 45" sign, then let off the gas as my front end comes alongside it. (Shifting to neutral has no effect, since this is an auto trans- I've tested it, and the RPMs don't move, nor does the car. This is at speed, though. I don't measure the coasts to a stop, since I'm on a public road. Obviously, the car crawls on its own power from a stop.) Then I look at my speed as I come to the "Speed Limit 35" sign, and the subsequent signs. Before, I'd hit 35 as I came to the 35 sign, but now I'm going 40 as I pass it, and don't reach 35 until I get to the city limit sign. This is on the road going into my town. Two lanes, level road.