Aero Experiments

"I-have-to-many-sports-cars-according-to-my-wife" (aliastoyotadude)
02/12/2019 at 14:38 • Filed to: None

Picture of test subjects.

So I have a “backyard engineering” idea, that might not be so “back yard”. If I were to get two string potentiometers and a data logger, could I not connect the potentiometers to my suspension and data logger as well as the TPS and speed sensor to the data logger to accurately measure drag, lift/downforce, and engine load? The system I am looking at would probably cost $400, but should give readings as accurate as about ±5 lbs of lift or down force and a data collection rate of 20 khz, which I feel would be accurate enough. Especially with so many people on the internet that think they know about aerodynamics, but don’t. Getting some real world data on popular modifications that do not have a lot of data could be useful to the community as a whole, as well as making my car better in the process.

As for the actual tests, there are several, relatively smooth roads in the area that all have low traffic and different speed limits. So, in theory, I could drive down each of these roads several times collecting data. I would then average out the data to create a graph and equation for suspension travel vs speed and throttle vs speed. Thus giving real world numbers on how slippery my car is going through the air and a way to show numerical improvement (or lack of) with modifications.

So if there are any engineering types reading this, why would this not work? I feel like this is just too simple of a solution.

DISCUSSION (10)

TheRealBicycleBuck > I-have-to-many-sports-cars-according-to-my-wife
02/12/2019 at 15:07

I think that what you will really learn is how much your suspension moves, even on a really smooth road.

These questions are best answered with a wind tunnel.

gettingoldercarguy > I-have-to-many-sports-cars-according-to-my-wife
02/12/2019 at 15:17

Yes, this works. It's best to find a long tunnel that's very smooth so you won't have as much noise to squelch.

I-have-to-many-sports-cars-according-to-my-wife > TheRealBicycleBuck
02/12/2019 at 15:17

Yes, but I do not have the monies for a wind tunn el. So I am looking to see if there is some way to get less accurate but still usable data for cheaper that a wind tunnel.

I-have-to-many-sports-cars-according-to-my-wife > gettingoldercarguy
02/12/2019 at 15:19

A tunnel would be nice, it would take out the wind variable. I will have to see if there are any around.

SPAMBot - Horse Doctor > I-have-to-many-sports-cars-according-to-my-wife
02/12/2019 at 15:22

You could set up a smoke machine and a leaf blower to make a backyard wind tunnel. Or, you can study the boundary layer with short pieces of string taped to the surface and film with a high speed camera. This can give you decent data if you are just looking to compare changes.

kanadanmajava1 > I-have-to-many-sports-cars-according-to-my-wife
02/12/2019 at 15:33

You might be able to see lift/downforce by measuring the suspension movements but you’ll need quite accurate sensors. A rough calculation. If you would get downforce of 50 kg (not likely but we need some value) and if all of our springs would have a spring ratio of 60 N/mm (value for Opel Calibra’s KW front coil-over set) you would get ~ 2 mm of suspension movement if the downforce would be distributed evenly.

But a regular car is unlike ly to get much downforce or lift within speed limits so you might be trying to measure movements less tha n 1 mm.

You probably cannot measure drag accurately enough by monitoring the engine. But you can do a coastdown test. Let the car get slowed down (use clutch/neu tral ) by the drag and measure the time that it takes to happen. This will include the tires and powertra in friction but you try to split the equation components.

During a calm day on suitably flat and even road you can get quite accurate res ults with this method.

ateamfan42 > SPAMBot - Horse Doctor
02/12/2019 at 15:47

Or, you can study the boundary layer with short pieces of string taped to the surface and film with a high speed camera.

This was not an uncommon practice for even the major car manufacturers decades ago. May not be as sexy as a wind tunnel, but it can yield some useful data.

I-have-to-many-sports-cars-according-to-my-wife > kanadanmajava1
02/12/2019 at 16:16

According to my calculations it will take about 5 lbs to move my rear springs 1 mm and 9 lbs for the front. The string pots accuracy is +or- 1 mm. S o this way I should be able to measure 10lbs of lift/downforce in the rear and 18lbs in the front. The changes should make at least that much difference, but I am not sure if they would make a big enough difference to distinguish it from the normal movement of the suspension.

And thanks for the suggestion, I had forgotten about the coast down test. At least to do that I do not need expensive, easily broken equipment.

TheRealBicycleBuck > I-have-to-many-sports-cars-according-to-my-wife
02/12/2019 at 16:49

A little googling led to the se :

http://www.autospeed.com/cms/article.html?&title=Real-World-Measurement-of-Drag-and-Lift&A=112736

http://www.autospeed.com/cms/article.html?&A=113225

They are using a potentiometer connected to the suspension via an extension arm on the wiper and a multimeter to get the average suspension compression similar to what you are describing.

Learn something new, yada, yada, yada.

I-have-to-many-sports-cars-according-to-my-wife > TheRealBicycleBuck
02/12/2019 at 20:39

A rotary potentiometer with a lever... cheaper and less like ly to brake than a string potentiometer . I like it. Also, I should be able to to put a shorter lever on the pot to achi e ve greater precision.

I appreciate your help.