Chin Spoiler Aerodynamics Stability

vesnyder · vesnyder's Garage

In Wayne Dempsey's book "101 Projects ..." he states "you should not install a rear spoiler w/o mounting a front spoiler as well. ... the cars becomes aerodynamically unstable w/o the front chin spoiler". Anyone ever experienced this? My '83 Cab has the rear spoiler and not the front, should I rush out and buy the chin spoiler?

BlueSkyJaunte · 11-30-2001, 05:05 AM

Spoilers should be added in pairs. I noticed a significant difference in high-speed handling and stability when I added my whale tail (previous owner had installed a chin spoiler but no rear wing). If you never exceed 70 mph, you probably won't notice a difference, though.

Check out this link http://www.pelicanparts.com/techarticles/911_speed_prep/914_speed_prep.htm for a detailed discussion. Search for "STAGE III" on the page to find the section where the info about spoilers begins.

Mrdi · Mrdi's Garage

My 88 coupe seems fine at 115 with just the chin. I don't think I'd want to have just the tail though w/o a front. The chin seems to be rather unobtrusive a large front however would seem to require a rear. Honestly it appears the chin is more of a cosmetic spoiler to cover the "scrape" bar.

11-30-2001, 08:09 AM

In my opinion it's worse to have the rear spoiler and no front spoiler than the opposite. Still having a front spoiler only isn't ideal either. The subject is thoroughly discussed at www.netmotor.de although in German rather than english.

I have the front spoiler only (for cosmetic reasons) and have been up to about 125 mph since I got it. Couldn't feel any difference compared to earlier when I had no spoiler.

911pcars · 11-30-2001, 09:47 AM

The side view of a 911 has the same contour as a ..... wing. When you approach the magic speed, the car gets "light". This causes the chassis height to increase (however slightly) which allows more air under the car making it even lighter. The most pronounced symptom is a lighter feel to the steering and more susceptibility to cross winds. With the right combination of factors at play, you can loose control. Remember the shape of the 935's? Not necessarily pretty, but effective at speed.

Sherwood Lee
http://members.rennlist.org/911pcars

autobonrun · autobonrun's Garage

I agree with the earlier posts, rear without the front is a problem, especially at high speeds. I've been searching for the article, but I recall reading that the turbo style rear spoiler adds up to 400 lbs of downforce. The front spoiler on the other hands scrubs off air from going underneath the car, reducing lift. Together, they keep the car planted. If your rear spoiler is pushing down on the rear without a lip to keep the front down, this could cause problems. The lighter front going into high speed turns will lead to more understeer.

When I first bought my SC, it did not have spoilers. At 130 mph, I could not keep the car in a straight line, it would drift left and right from where I was pointing. After adding the spoilers front and rear, it was rock steady at even greater speeds.

If you only drive the speed limit with no track time, I don't think your present setup would be dangerous; just not optimized.

rscupper · 12-01-2001, 11:07 AM

Sherwood - 935s not necessarily pretty? Boo-hoo!

PanAmeriCan · 12-01-2001, 02:13 PM

I don't have the numbers in front of me, but Paul Frere quotes in his book that the spoilers do not so much create downforce, as reduce lift. The lift (at elevated speed) goes from approximately 265 lbs/ rear, 185 lbs/ front to roughly 10% of those values with the spoilers installed.
Fairly easy to predict adverse situations having only one of the two spoilers fitted.

HTH,

masraum · 12-01-2001, 02:18 PM

I have an '88 that came from the PO with a front spoiler and no rear. My understanding from reading Paul Frere's book "Porsche 911 Story" is that having either without the other results in actually increasing lift at the end without a spoiler. So techinically having my front without the rear results in more lift on the rear axle at speed which makes the car more susceptible to oversteer at high speeds. Not a pleasant thought. The front spoiler keeps air out from under the car and Porsche found that by extending the front spoiler lowere towards the ground it decreased lift and eventually actually made downforce, but it increased lift over the rear so they compromised. I believe the front spoiler will be more effective when I get my car lowered from US height putting the spoiler lower to the ground. It also can't help the US height cars actually have the rearend jacked up more than the front.

All of that being said I have had my car into the 120-130 range quite a few times (in a straight line) with no problems. I have also had the car at 145 with the targa top off (

and LOUD) again a straight line with no problems. And have also had the car at redline in 5th ~150

with an S2000 right behind me. It does feel a little light above 140, but it's not that scary, of course the times that I was at top speed I was running down a highway with tall trees on either side that probably kept most of the winds and cross winds to a minimum. In Houston I run 70-95 almost daily with no problems at all. I want a tail and can't wait to get one, but money is an issue right now, so I am forced to wait.

I wouldn't want a rear without the front. I have mental images of the LMP/GTP cars that have gone airborn. I doubt a 911 would, but I'm thinking at high speeds things could get messy with only the rear and all of that lift on the front.