Rear diffuser & other aero questions

[///Mtechnik88]

- I don't recall a lot of discussion on rear diffusers for E30's in the past. Understanding what it's purpose is for, is it pretty much pointless on E30's? If not (for the moddified) has anyone developed any designs for one?

- Example: By adding an Infinity Rieger front spoiler and a DTM lip to that, will it create enough downforce to a appreciably affect handling at freeway speeds? Also, will the added downforce prematurely wear out the front tires (even with reg. rotations)

- Roof spoilers; Aside from channeling the air current more effectively to the rear deaklid spoiler for some touring cars, can a rear roof spoiler assist in added downforce for normal driving?

Rear roof spoiler on CSL

[nando]

I don't think a rieger lip and splitter is going to do anything at freeway speeds, much less wear out your tires faster on the street.. it might help on the track speeds though. I know that my Ryan G splitter does basically nothing, everyone is always saying how you need to bolt down your IS lip or it will fall off at high speeds... yeah. :roll: I've had my car up to 110 on the freeway a couple times to test that theory, and I still have my lip :)

[Bavarian3]

Aero parts such as splitters, lower spoilers, etc do create a good amount of down force but the average person does not need this for highway use.

[e30Matt]

I don't think a rieger lip and splitter is going to do anything at freeway speeds, much less wear out your tires faster on the street.. it might help on the track speeds though. I know that my Ryan G splitter does basically nothing, everyone is always saying how you need to bolt down your IS lip or it will fall off at high speeds... yeah. :roll: I've had my car up to 110 on the freeway a couple times to test that theory, and I still have my lip :)

How far out do you have it mounted?

[Jordan]

- By adding an Infinity Rieger front spoiler and a DTM lip to that, will it create enough downforce to a appreciably affect handling at freeway speeds?

Driving with my Infinity lip/splitter and without it on the highway is a complete night and day experience. At standstill the distance between the front fender lip, and the front tire is 1/2 inch. At 90MPH I have closed that gap, and begin to tuck tire. Take from that what you will.

will the added downforce prematurely wear out the front tires (even with reg. rotations)?

Hard to say, but either way its not something I myself worry about. Tires are disposable wear items.

Roof spoilers; Aside from channeling the air current more effectively to the rear deaklid spoiler for some touring cars, can a rear roof spoiler assist in added downforce for normal driving?

A roof spoiler redirects clean(er) air to a wing on the deck lid, but in doing so it also generates drag. A roof spoiler like the CSL one shown above will not generate any downforce of its own, and if anything it will actually produce some lift.

I don't think a rieger lip and splitter is going to do anything at freeway speeds.

It makes a night and day difference on my car, with and without the lip/splitter. As said above the front of my car drops over 1/2 inch at 90MPH from its standstill ride height. One of the back roads I like to drive has a 2 mile long straight stretch of road. More then enough to attain maximum speed if you so desire. I took the car up to ~130MPH before I backed off the throttle. I was nervous I was going to whack the oil pan. At that speed my lip/splitter were scraping on bumps in the road.

[randomtask37]

jordan, it sounds like that if u were ever at track speeds with your car and u went to turn u would be garunteed to rip up your fenders when the wheels turned.

has that ever concerned you?

[Jordan]

I've caught the fender lips a couple times. They are rolled now, haven't caught them since.

[nando]

90mph isn't a legal freeway speed anywhere I know of.. ;)

I don't doubt that it has an effect at that speed.. but for 99% of people, on the street, it makes no difference.

my splitter is mounted as far forward as it will go (about 1.5"). but it's made of flimsy plastic, and the lip itself isn't exactly the most sturdy thing ever. it sure looks neat though ;)

[Skafrog]

Jordan, thats pretty damn scary. If your tires really are tucking at interstate speed, you need stiffer springs or something.
At 130, my car has no dip as a result of my splitter, and it's 3" out.

[Ryan Stewart]

I have an incredible difference in my car with the Alpina spoiler at cruising speed (~90). SO much so that I didnt remove it before taking the car down for the swap. The car is quieter and the suspension rides softer at speed with it on.

[MattM20]

i dont doubt that you can feel a difference with a front splitter and low ground clearance, but imo putting money into aerodynamic pieces in search of downforce on a e30 is not very rewarding in terms of "bang for your buck." even a ferrari 355, which spent a good amount of development in the wind tunnel and has a unique underbody designed to generate downforce, will only produce around 15kg of downforce at highway speeds. so what hope is there with a e30 that has a high(er) drag coefficient and wasnt designed for any kind of aerodynamic purpose? even a modern stock M3 or M5 is not expected to impress the public with their downforce figures; however, the e60 M5 does have an effective diffuser and a underbody serving aerodynamic purposes, not for downforce, but to cool the differential.

front splitters and rear wings can be effective on properly set up cars, but i would think that, on a e30, a front splitter under a front lip under a stock valance would not be very efficent in terms of downforce gained vs. increased drag due to the increased area on the front of the car and the somewhat flat shape. if your seriously looking for some front downforce there are several e30 325's in the bmwcca clubracing series that use M3 front bumpers, reason being is that one of the builders ive talked to claimed the m3 front is more aerodynamically efficent which allows them to use more aggresive splitter settings.

jordan, do you think you could 3d model your cars front end (everything below the bumper)? or maybe even the mt2 rear wing? i know someone who, if he agrees to it, can test the peices in a CFD program to determine just how efficient they are and how much downforce they generate.

[Ryan Stewart]

Maybe someone could test the system. I know that there are pamphlets showing the Alpina dam actually lowered the CD of the car and I wouldnt doubt the Mtec1/2 might also have the same effect since BMW had the RD money to develop them. Hartge uses the Mtec1 on the HM3. Of course the Evo splitter has to have an incredible effect.

If you can lower the front of the car like a reiger lip it will have an effect on the CD of a car because the air under the car encounters a massive number of obstacles under our underpaneless cars. Limiting the amount of air going under the car will definately lower the CD. That is what the Alpina pamplet credited the lower CD to.

THe big beef with the stock dam though is it leaves the tire exposed.

[nando]

yeah.. my front tires are a good 2-3" past the front valance. it looks mean though :)