Third-Reef

The best and most accurate method to measure the drag on a vehicle is to measure the decrease in speed over time while coasting. Do the math on it and you can come up with a value for the total drag on the vehicle (aero, rolling and mechanical). Needs to be well controlled for comparison runs (same MPH range, location, no wind, aprox same temp). But if you wanted to measure actual drag change between aero setups this would do it accurately.

T10Chris

Not necessarily. Flat floors, diffuser etc.

gophaster

Regarding the sensors I posted, it says on the website that they output a certain voltage depending on what position they are in. So I'm guessing that they would read say 0 volts when fully open and say 5 volts when compressed (then various voltages in between) that is what the logger will interpret. If that is how the sensors are setup then you could maybe use a universal datalogger like the AEM AQ-1 (under $400). It has tons of possible inputs to log, including four 0-5V inputs and four 0-16.5V.

http://www.uspmotorsports.com/Engine...FUVBhgodtzsLbQ

rs10

Interesting! It says it can log shock position, which could be what we are looking for. If it can be hooked up to the ride height sensors on the car and the car's gps antennae, or perhaps the GPS in a smartphone, we could measure downforce for under $400 plus installation. (The GPS is needed to keep track of the speed when hights are measured.)

We just need to figure out how to hook it up to these sensors ... .

rs10

I had a chat with AEM. The first question is whether the sensors communicate via OEM cambus. If so, it won't work.

If no, then we also need the harness ($276) and a GPS sensor. (They think it won't work with the car's GPS sensor - I forgot to ask about using a smart phone for GPS.)

Then the software will display the volts coming from the sensor. (It just might be possible to set up the software to display height in mm or cm - they're not sure.)

However, either way, we'd need to figure out how to translate between volts and height. This would require adding/removing weight from the car and quite precisely measuring the resulting height. And the result might not be linear. Not a problem to estimate a function that fits well - Excel can do it. But if there is any way to have the software display not volts but, e.g. milimeters, plugging such a function into the software might be a problem (don't know, didn't discuus it with them).

The ideal solution would gather height data from the car's senors in mm, not in volts, and would be inexpensive. So this isn't ideal. But if the ideal does not exist, then if it works, it could be good enough.

Anyone have any idea if these sensors communicate using cambus (spelling?)?

rs10

Alternately, one could use the search function to search for this text:
There can't be too many threads that include it.

RngTrtl

i believe this is what you want if you want something that is accurate.

http://www.windshearinc.com/services...s.htm#Services

From what I can tell after scoping around, its about 2k/hr. two hours should suffice id think.

Carlo_Carrera

Even those type of ground effects systems produce a little drag. Not nearly as much as wings, spoilers, dive planes, etc, but a little.

rs10

Not necessarily. In fact, flat floors generally reduce drag. And I believe a good diffuser can do the same. Lip spoilers are another example. The wheel well vents on a 991 GT3RS or a Pagani Zonda (and probably several other street cars and lots of race cars) are another.

Note however that all of them may increase a car's downforce by reducing lift. As I posted previously, you can definitely decrease drag and reduce lift at the same time.

rs10

That's what I want if I want something precise. And if I have a big budget and am not a person, but a race team.

Which is not to say anything bad about them. But since I'm just one person, and would test the car as is, then test it after making a mod several hours later, then test it again after my next mod a month later, etc., I'd quickly blow through 4k!

No, gophaster's post has given me hope that this can be done for under 1k. And once it's done, I can test again whenever I want for nothing. And as a bonus, even if my results aren't precise, I'll know that they are accurate in the real world.

(it seems pretty clear it can be done for 2k plus setup costs - using both the AEM data logger and the height sensors mentioned earlier. With luck, 1k can be saved by using the OEM sensors.)

Carlo_Carrera

A flat floor may reduce drag but it will not by itself produced downforce. Diffusers do produce downforce but also increase drag a little bit. Lip spoiler absolutely produce drag while also increasing downforce. Wheel well vents do reduce lift but that is not the same as producing downforce and there is still drag produced.

Basically anytime you displace air by moving an object through it some drag is produced. Even a rain drop, the prefect natural aerodynamic shape, produces some drag.

In order to produce downforce air speed and pressure must change and with any air speed/pressure change there is some increase in drag. All downforce generating devices produce some drag. Devices like ground effects and diffusers just happen to work in ways that produce a lot less drag than wings and spoilers.

Reducing lift negates the lifting forces of the air flow which allows the gravity to do its job that is not the same as manipulating air pressure to producing downforce.

T10Chris

That's just getting into semantics at that point... the effect at the tire when lift is reduced is the same as if downforce was added. 10#, 20#, 30# etc etc of reduced lift goes back to weight the car puts into the tires.

Dock

^This

Carlo_Carrera

It is not semantics and it is not in any way the same. They are two completely different aspects of physics and fluid dynamics at work.

All reducing lift does is enable gravity to function unencumbered by the air flow and pressure.

Producing downforce uses air flow and pressure to literally push the car into the ground. Sometimes with a force exceeding the car's actual mass. Current F1 and LMP1 cars produce so much downforce they can be driven upside down at speeds above 120mph.

Reducing lift negates airflow and air pressure. Producing downforce utilizes air flow and air pressure to do work.

Carlo_Carrera

Incorrect, as usual.