This weekend's race pics

[cstreit]

Had a great weekend at BlackHawk farms... Here are some pics... My wife is getting good with that zoom lens!

Hanging it out a bit


Ready to go!


Porsche's in the lead!


Chasing an Alfa

[carnut169]

Sweeeet ride Chris- looks like you had fun!

[304065]

Chris- looks like a BLAST!

You coming to Gingerman? I am going to haul all the way out there from NYC- it was the first track I drove and I have a sentimental attachment. . .

[cstreit]

John, VSCDA or PCA?

If VSCDA, you bet! Gingerman is one of my favorites too, and my first track. A great drivers track and very safe to test yourself! Be sure to stop by and say hello!

[304065]

Chris,

I am coming out for the PCA club race at the end of the month. My secret weapon is an old college roommate who instructs for CGI and has over 4000 miles on Gingerman. Heh heh heh.

By the way, see my meticulously crafted analysis of the 2.2 v. 2.4 dilemma elsewhere on the board!

[Jack Olsen]

Great pictures, Chris.

Was this a race or a track day?

[cstreit]

Jack,

Race... Took first place! (I seem due only one 1st overall per year, so I got it out of the way early...)

John,

No, but I will find it.... Thanks!

[Jack Olsen]

Congratulations.

Let me elbow in on the picture show, though. Here's one up at Thunderhill, viewed from behind!



(Think I could fit any more rubber in there?)

And here's the 90 mph backward spin the same day, where the air pressure coming in the side windows blew out the windshield.

[Doug E]

Chris - I assume you took care of that Alfa pretty quick, no? Looks like a great day and nice event.

Jack - that backwards spin must have been something. Great shot with the RF tire lifting.

[URY914]

Hey I can lift a wheel too.

[cstreit]

You guys!

If I wanted a three wheeler, I woulda bought a trike, har har har!

My wheel never gets up that high, I think maybe I'm not driving hard enough, or is it all the weight of that 3.6 back there Jack?!

[jluetjen]

Chris and Jack,
What sort of rear suspensions are you running? I believe that it has a lot to do with how much if any the front wheel is lifting. Front sway bars also have an affect, but often the front bar choice may be the result of the set-up of the rear. BTW - it may not be a bad thing for the front wheel to lift as long as the car is balanced.

Which T-Bars?
Which rear sway bars?
Turbo or non-turbo trailing arms?

BTW Jack, Do you feel that your car has a slight push? Loose? Or does it push or go loose depending on how you drive it?

- John

[Jim Richards]

URY914, it looks like you're only hanging onto the ground w/ 2 wheels!

Jack, w/o the windshield gives new meaning to the advertising phrase "kills bugs fast!" I guess you were spitting them out the whole way to the glass shop.

Great pictures Chris!

Jim
1995 Black 993 Coupe

[Jack Olsen]

It's an interesting question, John, why some cars lift a wheel and others don't.

Mine did it with its orginal stock suspension, and now does it with:

21/30mm torsion bars
22/22mm adjustable swaybars (front is set about midway, rear is stiffer)
Non-Turbo trailing arms, aluminum from a 74.
I also have an 80% (torque-sensing) limited slip.

It's overall balance is great, especially since we added drop links to the tie rods and dialed in a lot more negative camber front and rear and maxed out the front caster with some custom modifications to the strut tower mounts. (That, plus the close-ratio tranny, and the 30mm wider wheels and tires all around, made me six full seconds faster at Willow Springs.)

So I'm not particularly concerned about the lifting wheel, especially since one of my favorite 911 experts says:

The bottom line, lifting a wheel (or even two on occasion) is not a problem! Slow lap times are a problem. If you are getting fast times while you lift a wheel -- that's great! [...] I'd keep talking out loud about lifting the inside front wheel while doing nothing about it. If you are lucky, the other drivers will all be out softening their front end trying to catch up with you!

It even happens when I'm way off the line, and sliding.

[jluetjen]

Chris - Are you running "Turbo" or RSR trailing arms in the back? By the way your car looks really good in the pictures. It looks like you hardly have to dial in any steering lock to get the car around the corners!

Anyway, my suspicion is that the rear trailing arm geometry is what makes the difference. The updated geometry keeps the rear end from "falling over" by raising the real roll center. Or at least that is the theory.

Jack - have you ever taken a pyrometer to your tires? As far as your mods, it sounds like you did what I would do to keep the tire's camber in the "sweet zone". But the pictures confirm that your camber looks pretty good in front, with maybe a touch of positive camber in the back which seems to be tough to dial out given the geometry. In spite of all that you might still have a little understeer. In almost every picture the front tires seem to be turned in further then the car's path. Tire temperatures would confirm or deny if you are understeering. It doesn't look like much and many drivers (Alain Prost for one) like a touch of understeer.

It's always fun to keep fiddlin to see what can be learned.

[masraum]

I'm far less knowledgeable about this stuff than most of the rest of you, but I think the front sway bar has a lot to do with lifting the front tires. When the front end rolls and one tire compresses the sway bar twists and tries to lift the opposite tire. If it can out "arm wrestle" the spring and shock on that side then it will lift that tire.

Just my understarding of things, and other factors will definitely come into play.

[Jack Olsen]

John, the Laguna Seca pictures (with the signs in the bacground) are with the old, almost stock settings, with a little added in front. By the time of the OTC pictures (the rear-shot), I was running 2 1/4 rear and 1 3/4 front. Since then, I think we've got it much better, something like 2 3/4 or 3 rear and 2 1/4 or so in front. I'm looking forward to checking out the difference at Thunderhill in July.

Here's where it was for the OTC:



Also, at Laguna, the car was pushing like a mofo in that turn, which is the really tight final turn before the front straight. Part of the problem is my limited slip, which pushes in those kinds of turns. Part of the problem is the driver, who was coming in to that turn much too hot, and then scrubbing speed foolishly to try and save things.

[URY914]

I thought something was different when on the cool down lap the corner workers gave me waves and thumbs-ups. This picture was taken at turn 17 at Sebring. I have seen several 911's carry thier inside front wheel all the way around the corner. Back in the 80's there was a 935 that I have shot of that was pulling the tire off the ground. It did look cool...

[cstreit]

John,

The trailing arms are aluminum, but I'm really not sure where they came from... How could I tell?

[jluetjen]

How could I tell?

If you have a stock trailing arm near by for comparison, a Turbo arm will be shorter. If you don't have an arm to compare it to, try checking the bracket where the inside of the semi-trailing arm mounts to the torque tube. The mounts for Turbo arms are further back and higher then the stock mounts. There is a picture on page 200 of BA's Porsche 911 Performance Handbook. My understanding is that usually an additional bracket is welded onto the existing mounts and so you would see both sets of holes there.

What's the big deal with "Turbo" trailing arms? As a result of the change in the suspension geometry the rear roll center is raised and the effective swing arm is shortened. So what?

1) Raising the rear roll center means that it is closer to the rear's center of gravity. As a result the roll-couple (the leverage as the center of gravity rolls around the roll center) is shorter. So the rear end will have less of a tendency to roll-over in turns. As a result the rear suspension settings (springs and >>>front T-bars<<< ) can be softened since the geometry is providing more stiffness in roll. Since the front bar can be run softer, I suspect that cars with Turbo rear ends will be less likely to lift the inside front wheel as a side affect.

2) Shortening the effective swing arm in the back means that the tire will gain more negative camber in roll. This is important with wide tires where the tire's grip provides higher cornering forces which forces the rear end to roll more. Non-turbo rear suspensions can cause the outside rear tire to have positive camber when rolling during turns. In extreme situations this can lift the inside of the rear tire and cause a sudden loss of grip at the limit! The shorter swing arm tends to alleviate this.

Why didn't Porsche just do this in the first place?

Short swing arms are not necessarily a one size fits all solution. A downside is that the shorter arm causes the roll center to move around a lot more during bump or jounce. If extreme, this can unpredictable handling on bumpy corners. Long swing arms makes for a more predictable car in most situations. It also means that the car could get too much negative camber when squating which results in the outside edge of the tire being lifted up and a loss of traction. This is why RS's, RSR's and Turbo's also have more anti-droop designed into the front end which helps to prevent the front end from lifting under acceleration and as a result reduces weight transfer to the rear. But massive horsepower and rear grip (like the 935 pictured) can still overwhelm the geometry changes that Porsche put in place.