Join Date: Jul 2015
Vehicle: 2001 SLK320
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There are two approaches I can think of:
Trip the airflow.
How high does one have to raise the level of stagnant air to give cabin air conditions to top of head rather than mixed / fast airflow up there? 1,2,3 inches ?
Trouble is, the jet of air from the new deflector ( say the deflector is an inch high) is the same speed and density as the air above that it is trying to push up, so it’s unlikely to deflect much. Mostly a one inch high deflector might give one inch more quiet cabin air.
But,,,, the lip shape is critical, and air tumbling off the back of the deflector might come down lower than before. Also, is the deflector if big enough, it can have vortices generated at its ends ( like on a wing) that will tumble into the cabin to be felt as turbulence.
I think the deflector might need to be too large ( unsightly) with washout at the ends, to work much and need some testing to prove.
The shape of the top of the windscreen frame top is driven by need to look good, fit snug to roof part, not be sharp edged, seal properly etc. Aerodynamics are always a compromise. The smooth shape in inevitable. But like the smooth curve on the top rear of the bootlid is improved with a small lip spoiler that trips the airflow to create a defined separation point, adding such a device just behind the windscreen top seal might reduce turbulence felt in the cabin. The small lip spoiler on the SLK32 was claimed to increase downforce not because air hitting this small spoiler pushed the boot down, but because it controlled the way the air separated from what was a a very curved edge before, give a defined point.
One quick way to trial is with Duct tape ( copyright Mythbusters) . Fold double layers ( for stiffness) into shape of a T such that top of T is exposed sticky side and vertical part of T is about one half inch. Now stick T inverted about an inch or two in front of rear edge of windscreen frame.
It is stiff enough if you don’t see it flapping about,,,,,,,, if you see what I mean.
Attach some wool tufts 4, 6, 8” long all the way the top edge of the inverted T. Go for a drive. The shorter wool tufts will give a visualisation of airflow. Nice if they are pretty straight. Lots of thrashing around means air is mixing into cabin space.
Try the tape blade in different positions closer to/ further from windscreen and also heights 1/4, 1/2, 3/4 inch. But start by tufting car with no duct tape, for comparison. Test early morning with no wind, so that crosswinds do not come into play.
Carry an observer ( not a short one, 6 foot tall is better) and record their subjective impressions. Video each test. Post the videos here!
Also, wool tuft the top edges of the side windows and rear windblocker device too! Wear a cap, and put short wool tufts on hat too.
Only do testing when road is empty lest you end up on someone elses Youtube video!!!
( or rent a wind tunnel, but the one in my city is upwards of $20,000 per day!)
Final trip blade could be made from 3mm perspex in colour to match car, apply CarBra on paint, use contact glue on top of CarBra film to hold perspex. Chamfer front edge of trip blade for looks.
I suspect this might work somewhat but not perfectly.
There is no such thing as a free lunch.