Meausring lift

[aerohead]

Originally Posted by JulianEdgar

I haven't done any measurement but I would think the boundary layer is so thick back there that the gap basically acts as if it isn't there. Here are some pics of a local car after driving on wet dirt roads that seems to support that idea of no / very low flow:








I have done that extensively on my Insight (and all the data on this has already been published) but the damping and springs on the Impreza are too firm to easily do this statically.

I could do it dynamically (with weights in the boot) but to be honest, I can't be bothered.

The big wing provides downforce, and that's good enough for Alexander and me. All he wants to do now is finished sanding it back and painting it!

Basically, I hope this shows for all those people who spend all their time sitting behind a computer and arguing and guessing, how easy and cheap it is to measure real aero data...

Literally, about a day's work and under US$75...

(...and of course that measuring gear can be fitted to almost any car - you can use it for the rest of your life....)

Other investigators applied measure loads directly over both axles, while simultaneously calibrating the recording equipment as a function of suspension travel.
NOTE: As some rear spoilers produced zero rear downforce, while simultaneously producing unacceptable levels of front lift, this is important enough to take the trouble of measuring.

[JulianEdgar]

Originally Posted by aerohead

Other investigators applied measure loads directly over both axles, while simultaneously calibrating the recording equipment as a function of suspension travel.
NOTE: As some rear spoilers produced zero rear downforce, while simultaneously producing unacceptable levels of front lift, this is important enough to take the trouble of measuring.

Yep, have frequently done both.

[JulianEdgar]

The video.

[California98Civic]

That is a clean and convincing test. Super cool.

[JulianEdgar]

I thought I'd dig out my scope and look in more detail at the action of the smoothing circuit on the suspension height sensor.

The circuit:



Cost - just a few dollars. Note the adjustment pot (1 meg) that allows the smoothing time to be set. For this test I wired into place an additional pot so that I could alter the input voltage just by turning a knob.

On the scope pics the blue trace shows the input signal and the red trace, the output signal. The vertical scale is 2V per division and the horizontal scale, 5 seconds per division

Scope pic 1.



You can see that at the 5 second mark I abruptly decreased the input voltage. You can then see it took about 7 seconds for the output to settle at the new level ie this pot position gave about a 7 second smoothing time. You can also see how the quick fluctuations are completely damped out.

Scope pic 2



In this one I have reduced the smoothing time. Note how now it takes a shorter time for the output to follow the input (about 3-4 seconds) and how the quick fluctuations are now not completely smoothed.

The circuit values can give a smoothing time from 0 to about 15 seconds. In use you set the smoothing pot so that at a constant speed, flat and straight road, the values settles to consistent and readable value within a reasonable time.

[aerohead]

That's good work Julian. Thanks!
The only thing I'd like to mention, refers to a ' SHOWDOWN FOR SPOILERS', which was published long ago, in CAR LIFE Magazine, June, 1969.
A flip-up rear spoiler was fitted to a Chevrolet Camaro Z/ 28, and tested in an un-disclosed stretch of highway near the California / Nevada state line, USA.
At 115-mph, the spoiler created a pitching moment, lifting the nose with 400-pounds ( 180-kg ) force, while providing 125- pounds ( 56.5-kg ) rear downforce.

[JulianEdgar]

Originally Posted by aerohead

That's good work Julian. Thanks!

Thanks.

The only thing I'd like to mention, refers to a ' SHOWDOWN FOR SPOILERS', which was published long ago, in CAR LIFE Magazine, June, 1969.
A flip-up rear spoiler was fitted to a Chevrolet Camaro Z/ 28, and tested in an un-disclosed stretch of highway near the California / Nevada state line, USA.
At 115-mph, the spoiler created a pitching moment, lifting the nose with 400-pounds ( 180-kg ) force, while providing 125- pounds ( 56.5-kg ) rear downforce.

Obviously something wrong with those figures, isn't there? You don't get a greater force on the other end of a lever when the fulcrum is near you. I'd imagine that they were the total front and rear figures, not the front lift caused by the rear spoiler, as you imply.

[aerohead]

Originally Posted by JulianEdgar

Thanks.



Obviously something wrong with those figures, isn't there? You don't get a greater force on the other end of a lever when the fulcrum is near you. I'd imagine that they were the total front and rear figures, not the front lift caused by the rear spoiler, as you imply.

They did their measurements exactly as yours are done. And setup by the expert at the time, a consulting engineer in Southern California, who specialized in this sort of thing.
They tested both airdams and rear spoilers, at 65, 85, and 115-mph.
--------------------------------------------------------------------------------------
For the Camaro, the rear spoiler by itself generated 125-pounds rear downforce, plus 400-pounds of front lift.
The most stable condition was with just an airdam.
-------------------------------------------------------------------------------------
The Pontiac GTO 'JUDGE', with tail-mounted, elevated spoiler, added front lift, with zero rear downforce.
--------------------------------------------------------------------------------------
The American Motors Javelin's roof-mounted spoiler produced no measurable effect at any speed.
----------------------------------------------------------------------------------
This is why I mentioned maybe doing front axle loads on your sons car. It's hard to predict what will happen on any given vehicle.

[JulianEdgar]

Originally Posted by aerohead

They did their measurements exactly as yours are done. And setup by the expert at the time, a consulting engineer in Southern California, who specialized in this sort of thing.
They tested both airdams and rear spoilers, at 65, 85, and 115-mph.
--------------------------------------------------------------------------------------
For the Camaro, the rear spoiler by itself generated 125-pounds rear downforce, plus 400-pounds of front lift.
The most stable condition was with just an airdam.
-------------------------------------------------------------------------------------
The Pontiac GTO 'JUDGE', with tail-mounted, elevated spoiler, added front lift, with zero rear downforce.
--------------------------------------------------------------------------------------
The American Motors Javelin's roof-mounted spoiler produced no measurable effect at any speed.
----------------------------------------------------------------------------------
This is why I mentioned maybe doing front axle loads on your sons car. It's hard to predict what will happen on any given vehicle.

It might be the case that with cars of that shape, odd things (in today's context) happened. I guess for example if the rear spoilers added a lot of drag, front lift would rise. I don't know of any more recent examples where front lift rose with no rear downforce - but I know of plenty of examples where rear downforce causes some front lift.

[aerohead]

Originally Posted by JulianEdgar

It might be the case that with cars of that shape, odd things (in today's context) happened. I guess for example if the rear spoilers added a lot of drag, front lift would rise. I don't know of any more recent examples where front lift rose with no rear downforce - but I know of plenty of examples where rear downforce causes some front lift.

Yes, even Hucho's example of Schenkel's rear spoiler is a bit odd.
Originally the car had no front lift.
With 17.8mm of spoiler height they achieved the drag minimum, while adding some rear downforce, however it also introduced front lift. With increasing spoiler height the downforce continues to climb, however with increasing drag and front lift, up to a point.