Off-topic - reducing rear lift

[JulianEdgar]

I know no one here seem to care much about rear lift, but if you're interested in car aero...

Our W212 Mercedes 2.1 diesel twin turbo. Excellent mileage (6.1 litres / 100 km over 40,000 km - that's 39 US mpg - for a large car that's quite quick at the top-end), but crap ride (the suspension has sunk a bit I think, and it's quite wandery in crosswinds and at speed).



Testing showed it has zero front lift – the front ride height does not vary at all with speed. However, at the rear, the ride height increases by over 10mm at high speed, indicating the rear lift is occurring.

Testing was with this:



...and this:



It's an ultrasonic sensor working with an Arduino and software I wrote in XOD. I don't think I have mentioned it here before because I wasn't happy with the accuracy versus direct sensing of suspension ride height - but the results are indicative.

Pressure testing supports the idea of lots of rear lift:



So, what to do about it?

I tried a corfboard rear spoiler at 90mm height:



It increased pressures at one spot on the rear boot (trunk) from zero to +40 Pascals average.

I then tried a smaller spoiler (45mm height)...



...and it dropped the increase in rear decklid pressure increase only marginally - to +35 Pascals average. (All at only 80 km/h - 50 mph).

Now I am thinking of small aftermarket spoilers like this...



...or bigger ones like this...



I haven't done any throttle stop testing yet (or, alternatively, mileage over 1 km at high speed), but I am leaning towards a smaller spoiler.

Unfortunately, my wife (whose car it is) isn't at all interested in contributing $ to a spoiler. So I might have to show a mileage increase as well.

[kach22i]

When I developed the rear spoiler on my S10 pickup truck (see link in signature below) I did it mostly by seat of pants.

However I had a piano hinge connection that started to rust in the process and set up a resistance, and that help set the spoiler angle and height as a result.

I was very concerned with uplift having gone from a short wheelbase Geo Tracker that could spin on black ice to a longer wheelbase but still unsteady mid-sized pickup truck (quote #15 link).

I assumed ownership of the truck while temporarily driving an hour to and an hour from work. This made highway safety in winter conditions paramount.

With my prior vehicle (Geo Tracker - Suzuki Sidekick) I was doing mostly short city drives and walking to work so highway safety wasn't so critical.

Most women respond to safety issues, sell this project as a safety device.

In combination with my other aero-treatments I have a net gain in MPG and Down-Force (safety).

See post #21
http://forums.pelicanparts.com/off-t...eriment-2.html

"a shallow (20 degree) spoiler as long as 60mm is capable of actually reducing the car’s drag while reducing rear lift or creating modest downforce".

Additional threads:

Pickup Truck Aerodynamics - CFD Study Chevy S10
https://ecomodder.com/forum/showthre...s10-35043.html

[aerohead]

What year model is the car?

[JulianEdgar]

Originally Posted by aerohead

What year model is the car?

2010

[COcyclist]

If we can reduce lift and reduce drag simultaneously then I am all for it. I do care about lift. Most of the time I drive below the speed limit for optimum mpg. However, there are times when I drive faster here in the wide open, sparcely populated rural west, and like Kachi said, I also drive this car in terribly slick winter road conditions.

[Ecky]

Does the piece of the rear of your Insight reduce lift, or is it purely for drag reduction?

[JulianEdgar]

Originally Posted by Ecky

Does the piece of the rear of your Insight reduce lift, or is it purely for drag reduction?

The rear spoiler? It reduces lift. Measured pressure change on upper surfaces with spoiler and fins: