Back bumper gap fill?

dan_b

dan_b

A2OC Donor
I've seen mention on the site an easy mod to improve fuel efficiency - to fill a gap in the back bumper?

Is this true? If so, what does it involve, what gap? How?! Pictures?!

My modding mood for the forseeable future is going to be all about improving efficiency - is this a "too good to be true" or an "easy win no brainer"?
 
There's a bit more to it - but not much.

The science is pretty simple. If the gap is <20cm, then the air will simply scoot over the top. There will be turbulence, leading to less downforce, but it won't get trapped.
If the size of the object is over the 20, then it will scoop air in. This is one of those.

The main efforts need to be concentrated on:
- smoothing surfaces and thus airflow
- guiding air where necessary to smooth airflow
- block off areas which might otherwise swallow air, causing direct resistance.

The ridges on the side of the rear lights smooth airflow. The ones on the roof probably do, too, I haven't seen numbers to prove that.
The hubcaps on the 3l smooth airflow. That's also why they have narrower wheelwells.

There's a listing somewhere over on the German site of what did what. However, to summarise it:
- thinnest possible tyres
- removal of the rear tyre "bulge"
- longer, higher spoiler (see the FSI in comparison to the petrol)
- smaller radiator air opening.

There's also quote I've just seen where a guy claims a 0.35l drop - from 5.2 to 4.85l/100 just by filling in the rear bumper and other underbody sections. There is a lot of optimisation room here.


for Adrian, as I know he can get it translated, here a quote. I'll translate the most important parts shortly.

Hier sind die wichtigsten Aussagen:
- Die Reifenbreite und die darauf abgestimmten Radläufe haben einen nachhaltigen Einfluss auf die Umströmung.
- Die Kühlluft verschlechtert den cw-Wert um ca. 0,015, die beiden Außenspiegel um 0,015 und alle Fugen und Spalten um 0,010. Wer also alle Fugen mit Klebeband abklebt, verbessert den cw-Wert um 0,010 und kann nicht mehr einsteigen. :D
- Sehr wirksam ist der Heckspoiler (Das gilt aber auch für die 1.4.) und das Abtragen der Radläufe, das aber in der Praxis nicht zulässig ist. Es wurde hinterher ein guter Kompromis für die breite der Radläufe gefunden.
- Alle anderen Modifikationen brachten geringe Verbesserungen. Erst die Summe aller Details brachte den Erfolg.
- Spoiler vor den Vorderrädern erwiesen sich als wirksam. Ich denke, hier sind die kleinen Serienspoiler gemeint. Die Radkastenspoiler sind auf den Bildern auf jeden Fall nicht zu sehen.
- Ein glatter Unterboden bis zur Hinterachse bringt eine Verbesserung um 0,050. Davon konnten in der Realität 0,024 umgesetzt werden. Ein Teil der Optimierungen wird auch in den 1.4 eingeflossen sein.
- Der Auftrieb an der Hinterachse reduziert sich ebenfalls mit dem glatten Unterboden.
- Es sind viele Ergebnisse von Meßreihen angegeben, wobei nicht genau erklärt wird, was geändert wurde. Außerdem sind die Messungen unter unrealistischen Bedingungen gemacht worden, d.h. die Absolutwerte müssen nicht stimmen.
- Die Höhe und die Länge des Heckspoilers ist variiert worden. Welcher für den 3L und welcher für den 1.4 verwendet worden ist, ist nicht eindeutig zu erkennen. Dazu müßte ich die genauen Abmessungen vom 3L haben. Dann kann ich auch sagen, wieviel besser dieser ist.
- Ein Spoiler vor den Hinterachslagern und ein längerer Heckdiffusor sind negativ.
- 10 mm breitere Reifen verschlechtern den cw-Wert um jeweils 0,0066. Ich denke, das stimmt nur, wenn man die Radläufe entsprechend mitändert. Das wird aber nicht genauer erläutert. Wenn ich also 205er Reifen statt der 175 aufziehe, müßte der cw-Wert von 0,28 auf 0,306 steigen.
- Eine optimal geformte Reifenflanke ohne Beschriftung bringt eine Verbesserung um 0,007.
- Reduziert man die Kühllufteintrittsfläche um 1dm² bedeutet das -0,0021. Ich habe bei mir aktuell 4 dm² abdeckt, das bedeutet -0,0084.
- Die senkrechten Kanten an den Rückleuchten reduzieren den cw-Wert um 0,003.
Click to expand...


Bret
 
the width of the tyres and the resulting size of the wheelarches has a serious influence on airflow
the cooling air ensures that the Cd value deteriorates by 0.015, the mirrors a further 0.015 and the panel gaps another 0.01.
the spoiler is very effective (I can see this on mine, with the snow...) and the selected wheelarch size is a usable compromise.
Spoilers in front of the front wheels are effective.
A flat floor to the rear of the car could bring another 0,05. Reality says that only 0.024 was completed.
10mm wider tyres appear to increase the Cd by 0,006 each, but there is no indication of precisely how this was measured and if the wheelarch was changed at the same time.
Optimal tyres without letters would bring 0,007 and the sides of the lights bring 0,003.
Reducing the intake size for cooling air by 1dm2 (10cm x 10cm) would reduce Cd by another 0,0021.

Reduction of 0,01 - so from 0,29 to 0.28 - it's about the same as reducing weight by 100kg.

Please ignore the combination of , and . as decimal points, they mean the same thing ;)
 
Are we talking a custom panel job for filling the rear bumper or is that also an Audi part? Ive ordered the under body spoiler tunnel cover and front wheel arch spoilers from Audi should I change the rear window spoiler too for the larger one?
 
No, it's a custom part job Dan - there's no Audi part to fill the gaps.

I'm currently trying to source some of that waffle-pressed aluminium to see if I can't make the panels myself (when I get Tank back), but am having trouble finding any.

Cheers,

Mike
 
Hi Mike let me know if you do I would be interested if you were to get some and could make up an extra panel! What about the larger rear window spoiler is that an easy fit with Audi parts?
 
Okey dokey Dan.

The rear spoiler is a new screen jobbie unfortunately - it's bonded to the glass you see.

Not sure that it'll make a huge difference, certainly not enough to warrant spending £400 on a screen!

Cheers,

Mike
 
Hi Mike - hmm yes, that's a total non-starter then £400 for a new spoiler (with window attached), no way!
 
hi all the stuff in the pictures is called correx sheeting(house for sale signs) this is very easy to work with and surprisingly rigid. i would really not be happy attaching alloy sheeting to the bottom of my car--just imagine the damage it could do if it ever came off!!! i was talking to Ian(driveforward)at Adi show about making up alloy sheeting for his seat arosa ring special--i think he is now looking at correx sheeting for the safety issues.
i look forward to checking cars at the curry night for the evidence of "for sale" signs attached to rear bumpers--lol
also correx sheeting is available on ebay so is easy to get hold of
cheers mike
 
you'll also find it used by 'togs for gridspots. If you shine the flash through a bunch of this (or some drinking straws) then it "pipes" the light. I'll have to check what it gets called. Coroplast, I think.

Bret
 
Hmmmm, I'd not be happy with the corrugated plastic stuff - it is stiff, but only when you supposrt it fully along the edge.

I've found some ally sheeting called stucco embossed aluminium:

http://imgs.tootoo.com/7a/ce/7acedb5ab90a549001ffd23e21d0b6eb.jpg

As long as the extra pieces are fastened securely, there's no chance of it coming off and it's very thin stuff (around 0.8mm). It'll be in keeping with the car too, being aluminium.

Cheers

Mike
 
Thats thin! What if it did fall off though a big sheet of ally in the road would be more of a hazard than plastic? Just a thought. Let me know how you proceed though as Im interested!
 
dan_b said:
Thats thin! What if it did fall off though a big sheet of ally in the road would be more of a hazard than plastic? Just a thought. Let me know how you proceed though as Im interested!
Click to expand...

The dimpled alloy sheet Mike found looks similar to the factory fit heat shields and for me, its much more suitable for the job than corrugated plastic.
The plastic cannot be bent to shape for optimum fit. It's likely to crush under load so the mounting fasteners may loose their clamping torque and vibrate free. If it did come loose its more likely to become airborne - not good for following vehicles. I suspect the alloy sheet would just lie flat on the road - still a hazzard but probably less so.

Cheers Spike
 
I've been taking a brief look at materials...

* Aluminium composite panels - strong, looks great, only rated up to 80c (at least the example I found)
* Aluminium sheet - very heavy compared to the others & not as strong (by weight), looks better, handles very high temps, noisy
* Dimpled aluminium foil - no strength so would need to be supported by something else, but light & handles high temps, perhaps quieter compared to the alu sheet
* Polycarbonate twinwall - fire resistant, very strong for it's weight, softens @ 140c, looks rubbish
* Coroplast / Correx - fire resistant available, very strong for it's weight, 350c ignition point (softens @ 150c), looks rubbish, silent


For practicality purposes I understand why everyone ends up using correx for this type of project, but it's just a pity that it looks so amateurish.

In terms of road hazard, I'd have thought the correx & the alu foil would be the safest, because they'd just deform if you took a bike over them, whereas the others are rigid & might spill the rider.

I'd be worried about heat buildup if too much of the underneath of the car was covered.
 
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Wow, this is a zombie thread brought back to life!
I was toying with constructing some rear wheel arch lip spoilers but that went nowhere. Let me know if you progress any plans with the rear area floor covers!
 
I accidentally ripped out the 1.2L exhaust tunnel cover when I was leaving the last social (skidpan training, absolutely great + highly recommended), so I either have to get another one, or put in something more ambitious. Hence I'm figuring out the options :) Whatever the final choice, it has to look professional from the underneath or it's not going on my car.

With regard to attaching the cover, something along the lines of what Tony (Hotstuff) was looking at seems the best option (the riv-nuts epoxied into the floor).
 
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Aah, I did wonder what you did to your car when you left the skid pan! (incidentally yes that was a fantastic day - am considering doing it again soon!).

It seems to me the area around the edge of the back bumper is where there is the greatest potential for drag, most of the rest of the car is pretty flat (once the tunnel cover is included)?
 
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