Improving air-flow over cabin - Mercedes Benz SLK Forum

SLK R170 General Discussion SLK 200, SLK 230K, SLK 320, SLK32 AMG

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#1 Old 3 Weeks Ago
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Improving air-flow over cabin

I have made efforts to improve the comfort level in the cabin when the roof is lowered. I've achieved this by the fitment of a high quality, perspex, full-width, wind-deflector and by getting the rear, quarter-windows to also raise with the fronts. It's a pretty cosy environment and the substantial output from the heater adds to that, even on a cool day!
There is now only one area to which I hope to still make improvements and this is it :-
When the roof is lowered, the air-flow over the top of the windscreen can be felt over the top of ones head. O.K., it's not THAT bad, but I feel that it would be better if it was absent. I will wear a lightweight hat and sunglasses for ultimate comfort under those conditions. So I am wondering if any SLK World member has devised anything that clips-on (or hinges) from the top of the windscreen and which, when in position, causes the air-flow to be diverted over the passengers heads? I have though to carry out a few experiments with pieces of semi-ridged plastic, held in place at the inner top of the screen with some tough adhesive tape. The upward angle of those (two) pieces could be adjusted, as could the dimensions of each piece. An ultimate size and position could be found that leads to the air-flow, even at speed, to be diverted smoothly over the occupants heads. Then a permanent version could be fabricated from thin aluminium sheet. A suitable means of location could then be devised that allows the device to be either hinged and permanent or clipped in place and then stored when not in use. I have also considered a full-width cowl that could have been called a "windscreen extender". This would look like a narrow roof section (very narrow) I think that it could look VERY sleek and stunning! It's a pity that I can't do it on photoshop!
So that's it! I want to be sitting in my SLK and feel cosseted, cosy and serene! A beautiful Summers day in beautiful North Wales with no wind or gusts to cause distraction. Any ideas?

David
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#2 Old 3 Weeks Ago
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I don't recall anything for the SLK.

I have a vague recollection of another marque that had a plastic cowling,
similar to the ones you see on side windows.

Have a look at Japanese sports cars (both marque and country).
They tinker more than we do.

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There are two approaches I can think of:
Deflect airflow.
Or
Trip the airflow.

Deflect.
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.

Trip.
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.
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What did you do to get the quarter windows to close with the roof open?
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Quote:
Originally Posted by DAJ 190 View Post
When the roof is lowered, the air-flow over the top of the windscreen can be felt over the top of ones head.

I'm not taking the micky, but, like bikers that want to fit a perspex barn door to stay in a "bubble" of still air, you are missing the point of a convertible... or a motorcycle
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Quote:
Originally Posted by Alex 3 View Post
What did you do to get the quarter windows to close with the roof open?
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This is a topic of great interest to me. If I could raise that stream of air by an inch it would extend my top-down season by at least a month. My wife is short, I'm average height; she has no problem when I have to put on ear muffs. And it's only my right ear that gets cold - left is fine! And yes, my seat is fully lowered. Turning on the heat at maximum fan does raise the air flow, but I just can't get that last inch I need.
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Quote:
Originally Posted by efair View Post
This is a topic of great interest to me. If I could raise that stream of air by an inch it would extend my top-down season by at least a month. My wife is short, I'm average height; she has no problem when I have to put on ear muffs. And it's only my right ear that gets cold - left is fine! And yes, my seat is fully lowered. Turning on the heat at maximum fan does raise the air flow, but I just can't get that last inch I need.
I agree, it’s about extending the season where ever one lives. I am lucky that I can drive year round pretty much except long trips at highway speeds in winter are a bit too stoic.

I am interested about the rear window lift. Do they really make a significant improvement.?
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Quote:
Originally Posted by Pictor View Post
I am interested about the rear window lift. Do they really make a significant improvement.?
Yes, a really massive improvement! AND it looks so much better with continuous glass front to rear. There is one further plus, I.M.H.O. That is to have both front and rear windows lowered with the roof raised! It looks stunning and reminds me of the "pillarless" Mercedes Coupe of the 70's ....

David
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