my unstable 914.

A reprint:

I could use some input from those who spend some time at 100+
in their 914s. At my first ever track day I found the car really started moving around (I mean REALLY) at these speeds. The back end would just move over a foot or so in both directions

Toe? Aero? I'm clueless. My AX set-up slew em' in the twistys, but gave it back when I had to stop gassing it on the straights.

-2 camber at both ends, 1/16 toe in both ends, 5 deg caster at front, LE spoiler in front, nuthin' in the rear, big stickeys at both ends.


__________________

OK, git to it.

Hi JP,

I did two DE's this year and never noticed to rear end moving around on my car. Yes I got to those speeds at Tremblant.

I have 205/55 up front on 6's and 225/50 on 7's at the rear.

Front is lower than the rear. Front has a 'snowplow' spoiler and nothing at the back.

I have zero toe up front and 1/8 toe in in the rear.

Front camber is 1.5 and rear is near 2.0.

Stock underbody swaybar from a 9eleben up front, no swaybar in the rear. Stock torsions up front (from 77 9eleben) with red koni's (inserts will be replaced this winter). Rear has 180 springs and koni sports.

My front end will need to be stiffened up a bit as the car has alot of body roll.

Marc-André

JP...when sorting out the Rocket....I used a big air dam up front with stiff rubber bottom curtain and a duck tail on the rear....it was pretty stable at 140 mph on the straights at Willow Springs.....

I also had four Konis adjusted to HARD, large torsion bars and sway bar on the front only and an LSD in the trans....

we really need to hear from the racer guys, i've been holding off posting until they checked in - but it's been 12 hours so nevermind ...

i used a snowplow front spoiler, nothing in the back. with and without rear antiroll bar, didn't seem to affect straightline stability.

it would help a bit to know the road surface - it might sound a bit like you're oversprung and underdamped, especially in back. (if this is happening over a smooth surface, nevermind...) maybe a bit more rear toe-in. more caster would help your straightline stability too - try for 6 but my guess is 5 is all you've got available to you or you'd be running more without my telling you this ...

could be a few things, but what it -really- sounds like is something's loose. i ran DE's in both my cars at tracks with l-o-n-g straights - like the old 1+ mile CanAm back straight at Riverside, f'rinstance. solid 130+ there, close to that at Holtville with its 4500' straight, bumpy as the dickens besides. never encountered anything i could call twitchy or darty. takes more HP than i ever had to run 140+ where the rear end -might- get light - but go have a look at the 914.6 GT's on the Mulsanne at Le Mans with their 210-HP 906 engines -- they never ran with a decklid spoiler...

a bigger - more aggressive - deeper front spoiler/air dam might get you more planted...

let me pull out the 'For Sporting Purposes' white paper and see their recommended alignment specs - understanding they'll be written for bias-ply racing slicks...

oh yeah - how's your tire pressure ? not enough in the pack and you'll be wandering ...

best i can do off the top ...

JP; what was the pavement like? Are you sure it wasn't rough spots in the pavement moving you around? Could your rear bumper be trapping enough air to make you light? You could stiffen your front suspension to plant the rear better but that should show up in the corners also as oversteer as you know. A wing would probably mask the problem, not fix it.

Hi JP,

I believe that a rear spoiler on the back of the car helps. I've got a small one (about 5" high) fiberglass job and with the spoiler and the rear valence off, it really helps settle the car down on the tracks. I run Thunderhill, Laguna Seca and Buttonwillow mostly.

The small air spoiler in back really doesn't add much downforce, what it does is help the air that is circulating under the car break up, thereby keeping the car from lifting or floating back there. The same goes for taking off the valence.

I know it made a difference for me. And with a built 2.7-6 in my car, I can get her going decent down the straights.....

Just my experience...

Bill

I have some same kind of experiences like JP, but I have noticed that the car came unstable when I lifted the throtle pedal. As long as I kept the throtle pedal down and the car was accelerating it was very stable.

I am sorry I dont remember my toe in readings, but I have some toe in in front and rear. Do you think I should give it more or less toe in in the rear ???

BTW, I just a couple of days ago tested it 200 km/h (about 125 mph) in speedometer with 205/55/16 tires. It was fun as long as I lift of my foot from the pedal....

Front spoilers have the biggest impact. Deeper of course, the more of an effect.
I never really had it dart from side to side.

Paul

JP,

I'm going to stick with my guess that there is an alignment issue or something changing/loose/worn.

Your car is not a 4x4... I mean to say that the pictures look like its lowered a bit, which is really the only Aero needed for stability at that speed.

I just don't think its a rear spoiler issue.
....................


P.S. your going to get hooked on the big track stuff
brant

He already is. I saw the grin!!

JP,

I forgot to mention in my previous posts.. (here and pelican)...

I run the limited edition front on a slightly lower car.

I add this to reinforce that a ground rubbing front spoiler is not absolutely required for aero stability below 130.

brant

here are "The Specs" from the factory publication "Information Regarding PORSCHE Vehicles Used for Sports Purposes - 911S 914/6"

camber front - 20' to 30'
camber rear - 30' to 40'

toe-in front 0 +/- 10'
toe-in rear 0 +/- 10'

castor 6 deg to 7 deg

reminder - especially for the toe and camber - these specs are for bias-ply racing slicks and road racing conditions. and probably - considering that toe number - for professional drivers! the teeniest bit of toe necessary to establish an initial slip angle is definitely the fast way 'round the circuit - not necessarily the most stable ...

I didn't take the opportunity to walk out on the track at lunch time...maybe I shoulda.

The problems on the back straight were worse. There may have been ruts from the CART cars braking, but I didn't notice a lot of roughness at lower speeds. I also doubt I was accelerating where they were braking The car does wander quite a bit on freeway ruts, but I can feel that thru the steering wheel...not so with this.

I'm thinking that my 4-5 year old plastic bushings (the squeaky SOBs) may be shot. The car was aligned this last spring, but AXing on these rough surfaces may have knocked it out. The HD Bilsteins may not up to the task of dealing with 200 lb springs.
Tire temps show I have too much negaitve camber in the rear.

Oh well, 2 more AXs and ulf goes up on blocks for rectal surgery. I detect the formation of a plan....
Some Mueller bearings, Koni sports with collars, and some more toe, me and my die grinder can get more front caster...may as well get another trunk lid to fool with also.

We here near PIR have lots of track days, the Lotus, Alfa, Shelby, Porsche, all have agreements for sharing.
I hear the Vipers get out there and don't use the chicane...more room for the Nitrous to kick in...I'll pass on that !

Could it be that your suspension is bottoming out?

Rear swaybar? How much suspension travel do you have?

It felt more like lift....bear with me here cause I'm just guessing, my experience here is very limited.

It seems that the back end would hop to one side, then the other....and I was gettin' big eyed trying to catch up with it. I only went there a couple times.
My co-driver did it a bunch....but he's nuts (and it ain't his car). His discriptions matched my experience.

Suspension travel is pretty limited. I know it all gets used up at our AXs. The track is like a billiard table by comparison. Hopping the curbs at around 70-80 didn't upset the car at all....well, maybe at 80

JP,
Check your trailing arms and see if they show marks of binding at the top in the chassis, near the inner mount. I added the reinforcement plates to mine and they binded at low ride height. Snap oversteer is the term. Very unsettling at speed on the track like Mid-Ohio. Raised the ride height and the problem went away.
Van

What track ?? what corner/s ??


B

Dood... you were gone and I went to a PCA DE day last Friday at PIR.......I had this little problem and the BBS was down. ....dunno which was worser

Straight line, not corners, not braking. When I hit bout 100, the backend started moving around....side to side...more than a little bit.

I can only think of two area's at PIR that you could get 100mph. Where they running the chicane ??

There is a big bump that upsets ALL cars on the backside before you come into the carrousel. I forget the corner numbers.

Its sounds like a simple small spoiler would cure this and not upset the AutoX setup. I find the little 2.0 4 cyl cars get a little light in the ass over 100mph.

I run the 3 inch Sheridan rear spoiler or the real cheap looking one that people think are factory.


B

Yes, the chicane was in use. Bout 6500 in 4th was just below the nasties set it. I had 200/300 yards and another gear to fool with on the front straight....only did that once

Same same on the back straight...it was worser there.

Spoiler, eh? I'm gonna need another trunk lid.....could go with FG, I guess.

This is exactly how we do it. We run two decklids. SCCA doesnt allow a rear spoiler in our ITA class.. but the similar class for PCA allows a small spoiler. Two decklids. Switch them out when needed.


B

You may also need to go to zero toe in the rear. As the car squats.. it gets more toe in. Not good over 80mph. If you think you will be on the big track more often, set it with a smidgeon of toe out.. so under full throttle it goes to zero.


B

I don't have any class problems, just don't wanna fuck up a gud rear lid.

The track deal was fun. After I do some work on the car (yeah, what's new) this winter, I'll *have* to go to another....just to see if it works right

The car ran like a train..bout the same gas milage, too.
5 ea 30 minutes sessions (my co-driver did 2) and the temps never got over 85C. These events aren't timed, but there are stopwatches. A buddy did us both.
Chuck's best was 1:41 mine was 1:44. A comp coupe Viper did 1:28. A Z 06 did the front straight walls....DNF.
He was OK, I think, but the car was toast....these are a bit more "serious" than AXs. Dunno if this is my forte', but there was ALL THAT ROOM.

I was one tired fella when it was over....neck hurt, wrists & forearms all stiff. Pulled off bout 5 minutes early on the last session...I wuz gettin sloppy.

DE's & TT's will wear you out. You wouldn't think that driving for 30 minutes at a time would do that.
I'm glad you enjoyed it, it's a bigger adrenaline rush than you might expect.

Humm..... at those speeds, the car is not pulling all that hard.....nothing like 2nd or 3rd where I know it squats. It "felt" like lift...or the back end getting light.....but as I said, I didn't do it more than twice. Once on each straight.

I was thinkin more rear toe rather than less.

It will lift under hard braking....then where am I?

Sorry. AFTER the spoiler. Correct. It is lifting at the rear.


B