i hada crazy idea, let me know what you think

[Cepheuz]

the biggest problem with fuel economy is aerodynamics(as far as I've read) so i though of a way to test the car air resistance and top speed

your going to need a co-pilot for this one

accelerate until you reach you car's top speed mark it
let the engine die coast on neutral (mine also benefits from the clutch being press)

now every time car goes down 5km/h say mark, have your co-pilot write down the time (a clock with a lap time or a cel phone like a sony ericsson that has lap times could come in handy)

make a chart with it X would be velocity Y would be time
if you graph it a curve is going to emerge the steeper the curve, the more air resistance in that speed, until the curve will become a straight increasing (or decreasing line) more than a curb, that is you ideal speed band for you car, as long as you don't go over it, you'll be saving fuel

also if you measure distance and time from when the curb becomes more of a line than a curb, i think you can figure out the mass of the vehicle and with that the amount of power needed to accelerate it or keep a constant speed and that data can help you out on mods (i think :think

what do you think? could this be a good way to find out the best top speed for a specific vehicle whilst increasing fuel economy ?

[Christ]

Might work - but don't leave your foot depressing the clutch while you're coasting, just use neutral.

If you leave your foot on the clutch, you're overheating the clutch release bearing - it wasn't meant to be constantly engaged, only to spin very fast for short periods of time. It will fail.

Also, with many cars, if you hold the clutch in, and it's not properly adjusted (non-hydraulic systems) the clutch disc itself can overheat and the friction material can disintegrate, leaving you unable to move the vehicle under it's own power.

[Bicycle Bob]

From the shape of that curve, a mathematician can sort out the wind resistance and rolling resistance figures at any speed. However, slower always takes less power - the best economy comes at the point where the motor is so under-used that it's excess capacity eats up more than the savings. Also, it is not necessary to start a coast-down test from top speed, although it can improve the data and the fun factor.

[NachtRitter]

Can also use a video camera on a tripod (all securely fastened within the car) pointed at the speedometer in place of the copilot... then you can time it at your leisure (as you're watching the video at home)...

[Christ]

Which would also be safer, most likely, since you wouldn't necessarily have to pay attention to your speedo as much as the road and things going on around you while you're speeding beyond the territory of instant jail time at your "top speed" for aero testing

I really don't think the ocifer is going to like it when you tell him that you're just performing a simple aerodynamic test at 120 MPH on a public road. Just me though - maybe you can get away w/ it LOL.

I would think that testing up to 80-85 MPH should cover just about any possible daily-attained situation. Anything beyond that is a moot data point that is only considered as a non-skew factor for the rest of the data, to show linearity.

Will you be taking readings on how long it takes to drop per increment of speed, or how many MPH dropped in increments of time? (Check speed every 5 seconds or check time every 5mph?)

It might be interesting to make both graphs and compare the plot point densities of each graph. They most certainly won't line up between each other, but it would help create an "average" line between the two graphs, which should be more linear than either of the initial tests.

This is where having a video camera comes in handy - you can draw up both data points while only having made one run. Don't forget though, you'll want to make this run 3-4 times, to properly report the results, then average all the runs into the same plot of data points, creating an even more linear average.

/ramble

OK, I'm done LOL.

Seriously though - if you're going to do something like this - keep it safe.

[almightybmw]

ya know, top speed for some rigs is 80-85mph, so in that case... :P

[Cepheuz]

you are right, you could do it an run until the top speed of the car, or do it until a specific speed (like for example 80mph)

i was thinking of how long it takes to drop a set speed, the resolution of the graph is you choice bu i think 5km/h is good enought

when accelerating from 0 the obstacle to beat is vehicles mass, but that work in favor of you when your doing P&G so the higher speed, the higher mass, the longer the glide will be, and finding out the max speed you can accelerate without working the engine to hard to get to it would make the P&G more efficient

also i heard of a driving style that focuses on just maintaining a constant speed, well i saw a chart where a slower speed (40mp/h) meant more fuel economy than the next step up (60mp/h) and the increase speed means increased momentum, so you can just ease off the accelerator and keep it on a very light load, and when a incline comes, you'll have more than enough kinetic energy to go up the hill with minimum loss (meaning you'll gain it back on the downhill)

[winkosmosis]

I was thinking about this, but a much easier and more accurate- Use GPS. Any handheld reciever that can export a tracklog should work. Each point in time (usually 1 per second) in the log would have a timestamp and speed.

You could also hook up an OBDII reader to a computer and log the speedometer reading.

Apparently that's how coast down testing works
Measure the drag coefficient of your car

[Cepheuz]

swear to god i never saw that site, but good, means i'm not crazy after all,

after reading the website, i can concord that the steepest part of the graph would be the aerodynamic drag in function with speed, the less steeper would be the Drag coefficient and rolling resistance so a good graph can actually provide a lot of info if you analyze it correctly, the good thing is that finding the weight of your car is not so difficult, most have it on the spec and there are a lot of scales you can use (if you know the right person) speed is measured by the speedometer, so you have another data, avilable, so you can find out everything else by that