Poor Man's Ground Effects -- I need some input

[Jack Olsen]

Just for the record, my red skirts (version 1.0) in action:

[Craig 930 RS]

Well whaddya think?

[Jeroen]

so... how did it affect your track times/top speed?

[Jack Olsen]

The car was MUCH more stable (and quick) through high speed sweepers. It was a little less predictable through elevation/camber transitions that involved a lot of lateral loads. I suspect I was getting some odd moments where one side was almost totally blocked off while the other was raised a couple of inches (from body roll).

All in all, I was very happy with them. Of course, I also went off the track, twice, and tore up the front piece, which I'm trying to re-fashion in a stronger way.

Lap times were down close to a full second with the skirts. A competitor who runs very similar lap times to me had his laps remain consistent (and now slower) than mine.

I'm going to test the new verion a little more scientifically, next time, with my data logger, which is now installed.

[Craig 930 RS]

Any chance of a blind test??

Have a friend who can install/uninstall and then you drive er...

[masraum]

Very cool, Jack. Not only is it cool to see you trying things, but it's even better when they do something (positive).

One thing that I've been curious about. I see some cars these days that look like they have a notch in the center of their air dam. I'm not sure if the reason is because the front center is what takes the most abuse or because a fast smooth flow under the center of the car might help with the low pressure under the car.


this first photo has a bonus






Of course, if you look at the GT2, GT3, and CGT, they don't have this same "feature" so nevermind. Forget I said anything.

[lateapex911]

This is the kind of thread i HATE! 6 pages, and I had to read every damn one of 'em! Jeeez..I got 911 work to do!

Jack....please...PLEASE go back and retest with and with out...very interesting stuff, but back to back testing is so important!

[Craig 930 RS]

Jack -

As an aside, what brand/type tires do you run at the track?

[Jack Olsen]

Those are BFGoodrich g-force T/A KD street tires. They scream a lot, but are cheap (one set per year).

[rcwaldo]

Regarding the first picture with the girls, I dont think those were the 'skirts' that Jack was referring to

Yes, the black art of aero tweaks are interesting and sometimes difficult to theorize, much less test.

Jack, I think its great you are exploring the 'other' facet of racecar performance!

Now, just get BB2 to really minimize her roll, and these aero bits will shine even more so..

Chris Walrod

[Yellowbird RS]

Here are some pics. of the CTR underbody.

[Scuba]

[i] I worry about trapping to much air around the headers and exhaust, although I guess they lose most of their heat out the tailpipe rather than through radiating.

I'll admit it, I'm too lazy to read all 3(make that 6) pages of this, so if somebody already addressed it - sorry!

But don't forget, your exhaust system has a constant heat source (exhaust gas) and requires a heat sink (airflow) in order to keep from turning into a giant ball of liquid hot MAG-MA!! So, no, they don't lose most of their heat out the tailpipe, the heat of the exhaust gases is actually mostly dissipated in the first lil' bit of exhaust headers - that's why headers like to get hotter than the sun, and the exhaust coming out your tailpipe is only warm....

[DonDavis]

Awesome. I love it.

I also think the 'blind test' would be neat, if it could be pulled off.

Jack, is that something you would be willing to try? Or would it be too time consuming/logistically improbable to accomplish?

[Jack Olsen]

I'll do A/B tests with it on and off, but I don't see a practical way to do the test blind. I'm not going out for a session on the track without being able to walk around the car, first.

The bottom line is that it's pretty easy to get max speeds through turn 2 (a 90-95-mph sweeper) and it's also fairly straightforward to look at max speeds at the end of the front straight. In fact, with the DL-90 I can look at dozens of segment times from different sessions with all the lateral g goodies and (if we get it hooked up) engine RPMs, as well.

Assuming I have the pieces ready to go again by next weekend, I'll have two days of sessions to run it both ways. And that will produce tons of data to look at.

[banjomike]

Jack,

I began to put these together when this thread first started, and had trouble posting them. I forgot about it until I saw the thread resurface just the other day. I'm glad that your side skirts seemed to have helped.

My idea was to run skirts down the sides of your underbelly tray. Body roll would have less of an effect since they are closer to the center so the "seal" wouldn't be as likely to break.



The way your underbody tray sags a bit (from the measurements in your photo) is very much like the groud effects tunnels the pros make. I was also thinking that a material like (I looked this up in McMaster Carr) "cloth-inserted neoprene rubber sheeting" (basically a material like tire rubber impregnated with fibers) would be good because it would be okay for the stuff to actually drag on the ground (as long as it was mounted well enough) would be a good skirt material. Finally I had some diffuser thoughts. The pictures sum up my original ideas.





A workable maximum angle for the diffusers could be determined using some simple Reynolds number calculations, but I'm supposed to be doing billable work right now...

anyway just my $0.02

[Craig 930 RS]

Do a walk-around for track readiness -- and then have the add-ons removed. No risk and you have a true nearly objective comparison. Si?

[Yellowbird RS]

yes I'm thinking about cpoy the rear diffusers of the VW bettle RSI. with some mods. to fit on a 911.

[Craig 930 RS]

Also remember that there is little effect from the diffusers - and to a lesser extent the front and sides - until 70/80 mph --

Cool project. I would love to see a rear diffuser for a 911 - I will also build one once the ideas come forth......

[Randy Webb]

Jack, do you know what your typical speeds on the track are?

And... any sense of how much time is spent at what speed? Say x minutes between 80-90 mph; y minutes between 90-100mph, etc.?

[Jack Olsen]

Willow Springs is a very fast track, considering there is no banking. If you run a 1:30, your average speed is 100 mph. There is one section of the track where speed drops to just below 60 mph, but the rest of it is generally between 90 and 130.