[moldcad]

My M235i's mods are mainly functional and performance oriented (the MP LSD, MP Lowering kit) - but I do have several cosmetics add-ons, as well (like the MP-rear diffuser and exhaust tips - but with the factory exhaust itself). Have been thinking about adding some more, and have a related question to you guys:

- the front MP lip is said in the BMW MP catalog to only be installed along with the larger rear spoiler (not just the mini one which comes from factory), the rationale being that the front spoiler/lip will create enough down-force at higher speeds to make a larger rear piece a must for the rear down-force "balance" better.

Of course, this logic is rather obvious but providing any of those two MP parts really do produce substantial down-force, i.e. are indeed functional and more than just cosmetic add-ons. Frankly - I rather doubt it but my be wrong of course - what is the general consensus here?

Asking because while I do like the look of the front piece (especially if pained white, on white 235i body) - I rather don't care much for the looks of the large rear spoiler (the factory discrete, thin lip looks IMHO much better on the 2-series trunk). So much so that if it really is important to mount both pieces together - I'd rather give up the whole idea...

On the other hand - if my skepticism about any real and substantial down-forces is well based, i.e. each of those add-ons are just cosmetics - I'd go for the white painted front lip... Please advise,

Piotr

[Ron Jeffries]

in a word, no, other than at speeds you shouldn’t be going on public roads, if even then.

[Cumberlandjames]

Additionally, they are not listed as parts for the convertible because the antenna is mounted on the trunk and I guess would interfere with the spoiler mounting. Obviously the engineers at BMW feel there are some aerodynamic impacts of having one without the other but as there are many users who have installed only one item the impacts would appear to be insignificant during normal operation.
On a track at very high speeds, one might feel the impact of having a car that becomes nose or tail heavy at speed. There are cars that have extendable front lips and spoilers, triggered at certain speeds, to increase the down-force and add stability but unless you're going to run your can on the track constantly I'm confident the negative impacts of having only one piece of the pair installed would be negligible.
I would and am more concerned about spending a bundle of money on the front lip and destroying it on speed bumps and uneven roads as evidenced in the many posts by owners.

[230iZTR]

I was under the impression that a rear spoiler reduces drag and lift. As opposed to a wing, which increases drag and makes negative lift, aka downforce.

Similar case with the front lip. It may be functioning as an air dam, reducing drag and lift. As opposed to a splitter which would increase drag and make negative lift, or downforce.

I’d assume the m-perf spoiler is definitely functional. Your coefficient of drag will probably drop a few percentage points. If it’s a 3% improvement, that’s enough to knock the coefficient of drag from 0.33 to 0.32, which is worth (ballparking) the equivalent of about 2-hp at 100 mph and 5-hp at 150 mph.

I’m a little skeptical of the front lip though...it doesn’t seem to hang down much further than the stock bodywork to function as a legit air dam, it doesn’t extend far enough back towards the rear wheels to really smooth out the undercarriage airflow, and the shelf doesn’t extend far enough forward of the bodywork to make me think it’s got enough surface area to really make any downforce nor does it have enough bracing to properly apply it to the chassis if it did.

Bottom line is that I can only see these being functional if you spend a lot of your time in the car at triple digit mph. If you’re going half or full mile drag racing, ripping down the rural autobahn everyday, or entering the 24 hours of Nurburgring...go for it. But at regular driving speeds they’re purely cosmetic. So get them if you think they look nice.

And just for the record, I have the m-perf spoiler because it looks cool. And I don’t/can’t have the m-perf front lip because I don’t have the m-sport package. My car drives absolutely fine with just the spoiler, I haven’t gotten a chance to take it onto a high speed track yet, and I haven’t noticed any instability when really cooking it to pass on rural Nevada highways.

[capt_and]

As someone who tracks the care frequently, hits triple digit speeds while on track, and has tried different configurations, I can say that the effects are minimal. The front lip doesn't so much if anything. It does not have an air dam any greater than stock, the lip doesn't extend out far enough to "split" the air, and because there is a gaping hole in the middle the air just passes right through. As for the spoiler, there is an academic argument that it does improve down force. I'll also say the the diffuser doesn't do anything as well.

Just for everybody's benefit, here are the differences/functions of the spoiler, wing, splitter, and diffuser.

Wing: It creates a great amount of down force. The wing on the car is basically an upside down airplane wing (with a difference airfoil profile). Where there airplane wing creates lift, an upside down wing creates negative lift. This is a highly efficient way of creating downforce with minimal drag because there is no high pressure pockets as with a spoiler.

Spoiler: Changes the air flow over the car to create downforce but also creates a tremendous amount of drag due to a high pressure pocket of air between the trunk lid and spoiler. The high pressure pocket is a force acting the in opposite direction that the car is travelling therefore creating drag. This is in my opinion, a less effective way to create downforce.

Splitters: These act to separate the air in the front of the car. The splitter will reduce the amount of air going under the car by blocking off air that would otherwise hit the front bumper cover and slide under. An air dam will create a high pressure pocket of air that will increase downforce but will also increase drag. The further that the splitter extends beyond the leading edge of the car, the more downforce it can create (there is a diminishing gains effect on how far it sticks out). If the air is allowed to pass through the top of the splitter and travel underneath the car, there will negligible benefits.

Diffuser: This uses the venturi effect to create downforce and is a very effective tool. Without getting too academic, the diffuser basically creates downforce where the transition from the under body to the diffuser. The change in angle dictates how much downforce there will be up to a certain point. If there is too much angle change, then it will not be effective. In order for the diffuser to work, the car should be low and underbody should be flat. The diffuser works best whenever the air can move cleanly under the car without turbulence generated by air pockets and obstructions.

Side skirts: Keeps the air going around the car from entering the under body of the car sinilar to the splitter. This improves the effectiveness of the splitter/diffuser combo.

How does each one affect the car?

Wing: Downforce at the rear, improves rear traction, increases understeer, improves braking and exit stability.

Spoiler: Same as wing but to a lesser extent and greater drag compared to wing.

Splitter with air dam: Downforce up front, more grip up front, reduces understeer/promotes oversteer.

Diffuser: Generates downforce anywhere on the car depending on where the transition from flat underbody to angled diffuser throat is.

Side skirts: Improves the effectiveness of the splitter, diffuser, and flat bottom.

[230iZTR]

Originally Posted by capt_and

Spoiler: Changes the air flow over the car to create downforce but also creates a tremendous amount of drag due to a high pressure pocket of air between the trunk lid and spoiler. The high pressure pocket is a force acting the in opposite direction that the car is travelling therefore creating drag. This is in my opinion, a less effective way to create downforce.

[...]

How does each one affect the car?

Spoiler: Same as wing but to a lesser extent and greater drag compared to wing.

Do you have any sources on that one? Spoilers increasing drag goes counter to everything I’ve ever read about them. They use that pocket of higher pressure air over the trunk to force the laminar flow away from the drag-inducing inefficiencies of the rear silhouette so it won’t create as much drag as a trunk lid without one.

Similar for the wing. They induce a bunch of drag to create a bunch of downforce.

Here’s a reasonable layman’s explanation that skips all the engineering math

I fully accept that spoilers are less efficient at creating downforce, but that’s not their job. It’s to reduce drag. So get a spoiler if you want less drag with the upside of a little less lift, and get a wing if you want less lift (or downforce) with the downside of a lot of drag.

[capt_and]

Here's a video made by the folks over at MIT.

I also studied Aeronautical and Astronautical Engineering for my bachelor's

It doesn't have to be correct to be posted online!

[230iZTR]

My background is engineering also, but this still isn’t adding up for me. I get that MIT is reputable but that video seemed pretty hacky. Are you sure that it’s not the one we should disbelieve just because it’s on the Internet? They were using model cars and the illustrative case for a spoiler was popping the engine cover for crying out loud. They also didn’t show an CFD models.

Further, it doesn’t pass the common sense sniff test. If wings are superior in every way (more downforce, less drag) then why don’t we see them on every car like we do spoilers? If spoilers induce more drag than wings, why does nascar run spoilers at high speed courses? Why can the 911R hit 201 mph as opposed to the GT3’s 193 mph top speed if the GT3 has a wing and the R just has a spoiler? Shouldn’t this CFD analysis show more red behind the spoilered car than the winged car if the wing was generating less drag?

[aerobod]

Typically a lip spoiler is most beneficial in stabilising and controlling the vortex shedding in the low pressure area behind the car, in conjunction with boattailing, some drag reduction can be achieved by reducing the energy in the counter rotating vortices. Overall a spoiler will work to modify the shape drag characteristics, but won't generally have a significant effect in modifying induced drag. A wing with significant downforce will also add to the overall drag force due to the induced drag from the lift (-ve) produced, but will generally have less of an effect on the vehicles shape drag, as it will have it's own shape drag characteristics, as it is most effective in clean air above the vehicle body drag effects.

[wntrsnowg]

Originally Posted by 230iZTR

My background is engineering also, but this still isn’t adding up for me. I get that MIT is reputable but that video seemed pretty hacky. Are you sure that it’s not the one we should disbelieve just because it’s on the Internet? They were using model cars and the illustrative case for a spoiler was popping the engine cover for crying out loud. They also didn’t show an CFD models.

Further, it doesn’t pass the common sense sniff test. If wings are superior in every way (more downforce, less drag) then why don’t we see them on every car like we do spoilers? If spoilers induce more drag than wings, why does nascar run spoilers at high speed courses? Why can the 911R hit 201 mph as opposed to the GT3’s 193 mph top speed if the GT3 has a wing and the R just has a spoiler? Shouldn’t this CFD analysis show more red behind the spoilered car than the winged car if the wing was generating less drag?

Another mechanical engineer going to weigh in here.

Don't discount the results of the tests shown in the video on the grounds that they did not show any CFD models. Real tests are always going to be more accurate than a CFD model, and are ultimately what matters. Why do people bother running computational models than? Because they are cheaper than a real test. No matter how good your computer or data is, a simulation is only a mathematical approximation.

Wings are not superior in every way. We don't see them on every consumer car because cars with massive wings won't sell. Think fast and furious. In the 90's it was cool but now its a joke. The average consumer does not even know what aerodynamics, drag and lift are but they know it sucks to get made fun of so they'll buy the car without the wing.

Nascar teams know what they are doing, more so than any of us in this thread (most likely). A spoiler causes more drag BUT it also could create more downforce. There is a trade off. Another guess is that the aero package is strictly governed by Nascar rules, as it is in almost every professional racing series.

We don't know where you found those simulation pictures so we can't verify the legitimacy of them. Also we don't know what angles and shapes of the spoiler/wing are being used. We also don't know the speed of the air used for that simulation. If you look closely between the two there is a complete aero package difference between the cars, not just the spoiler/wing difference. So we can't draw any conclusions from those pictures.

Point I'm trying to make here is don't reinvent the wheel. So much research has already been done in this field. Stick to generalized truths.

[capt_and]

^all true here.


I don’t blame 230iZTR at all for his position. Car magazines, online articles, etc spew out that information he is using. For aurgument’s, let’s take your 911R example. Having owned a few 991.1s myself, I do have direct experience with this. The spoiler on a 911R is active, as is on any of the wingless models and the turbos. The GT3 wing is fixed, so it has limitations the 911R does not. Plus, it’s not an even comparison, the R uses the 4.0 engine to the GT3 3.8L.

Street cars don’t have big wings because have you every tried to look out of the back window with a big wing?! Plus, I don’t believe the public really benefits from big aero on their car do you?

I’ll close on the “trust the source” debate. I personally would trust the #1 engineering school and #1 overall school in the world than just about anyone else that posts online.

[moldcad]

Wow - as the OP, I'm very grateful to all of those participating in my thread for their input. And you know what? With all this input, and being an engineer myself (with over 30 years experience with Moldflow, the plastics injection molding CAE world de facto standard) - I feels ashamed about the naivety of my question

The more so that when I was installing the M-P diffusor to my M235i, I saw myself how many air pockets are created between, under and above the many under-body components... In such "environment", no diffusor can possibly be functional!

What was I thinking when asking the silly question about the other M-P toys?

Piotr

[Dylan86]

Great thread!

[BEM-S4]

If it makes you happy when you look at it - it's functioning as it was designed

[Dylan86]

Originally Posted by BEM-S4

If it makes you happy when you look at it - it's functioning as it was designed

I do remember a video when the MP parts saloon was released, about the MP spoiler creating some additional downforce at speeds nearing 200kp/h, lol so not exactly "usable" in any way, aside from track or autobahn, which you'd be hard pressed to "feel" the effect, at all..

Definitely a cosmetic approach, to real downforce, and it looks cool

[NISFAN]

Just adding to this thread.....

I can see both references shown being correct. The mistake that you can make is by applying a rule to all. For example, "a wing is a low drag device". Sure it is in the case of an aircraft wing in its cruising configuration, but come landing time with slats and flaps deployed and it becomes very high drag (high lift too). Same with a car wing, add a low angle of attack on an efficient airfoil design and you get lift (-ve) with very little drag. However, as these wings are relatively small and used at relatively low speeds, the lift they generate is not great. Therefore racing car wings are usually the opposite of this efficient aerodynamic configuration. They have high angles of attack, a curved shape and even have gurney flaps (spoiler on a wing, how fetching for this thread ) to achieve the highest possible levels of meaningful downforce. A wing does generate true downforce, and lives in clean airstream (as much as possible) to achieve its goal.

A spoiler is exactly that. Generally speaking it spoils the inherent lift generated by most car shapes. So the downforce it provides is not manufactured by the device, it merely reduces lift on the shape it's attached to. Spoilers (and splitters) can radically change the airflow at each end of the car. To determine whether a boot mounted spoiler increases or decreases drag, you would have to evaluate it on a case by case basis, it's not a hard and fast answer.

In the case of the bmw boot spoiler, it is probably sitting in the turbulent wash from the rear window, and therefore doing very little (spoiling lift or adding drag). Add some vortex generators to the roofline and it might be more effective.

IMO, the bmw kit is form over function.

[msej449]

A serious question: Isn't turbulence from the exposed underbody going to massively outweigh any advantages from splitters, spoilers or diffusers?

I used to own a Lotus Elise and that had completely contained, perfectly flat underbody panels. The current Evora has the same.

By comparison, my M235i is a complete mess: if you wanted to get improved downforce wouldn't it be better to do it under the car? Or is it too messy to make it worthwhile?

As an owner I got a copy of the Lotus book on the development of the Elise and the engineers said it was a real pain to lift lots of component high enough so they could fit a flat base to the body (while still keeping the suspension the same height). And then they had to deal with the extra heat from the exhaust since it was now contained. It still had a couple of open slots in the panels, which I assumed was for airflow around the exhaust. And I recall they said tht after all that effort, you really needed a good flat road surface, like on a race track, before you'd notice the difference against an open underbody. Even so, they obviously still thought it worthwhile as the Evora is the same.

[moldcad]

Originally Posted by msej449

By comparison, my M235i is a complete mess: if you wanted to get improved downforce wouldn't it be better to do it under the car? Or is it too messy to make it worthwhile?.

Exactly my own thoughts and doubts when installing the diffusor On the other hand - with two chrome tail pipes - it looks fine, black on the white car...

Frankly, it was the original "black chrome" tail pipes (which turned out to be just a black paint) which corroded 1 year into my ownership which made me do this mod. I wanted the contrasty look, so gave up on the carbon fibre ones... BTW, in my previous BMW (the F10 528xi) the very same thing happened (my dealer replaced the corroded part twice, and I wanted to avoid having to do the same with my M235i)...

Piotr

[Sportstick]

This seemed related enough to be of interest...

https://www.openpr.com/news/1172322/...RG-Global.html

[tennisfreak]

I'm not an engineer but I noticed a difference in rear end grip and stability when I added the larger BMW M carbon spoiler on the trunk of my car.

[morphomeman]

Apparent but non-measurable improvements are often directly proportional to the cost of the add-on. It's all in the pocket of the beholder.

[DaleHeinz]

Originally Posted by tennisfreak

I'm not an engineer but I noticed a difference in rear end grip and stability when I added the larger BMW M carbon spoiler on the trunk of my car.

I find it difficult to believe anything affixed with 3M tape would create a noticeable difference.