High Speed Instability
 

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:eek:

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.

.http://forums.pelicanparts.com/uploa...1065377004.jpg

Nice pic, nice car, but a bit short on info:D
Is the spoiler there cause you had a similar problem...or just cause it looks racey? I ain't above slapping sumthin' like that on mine....if it wurks.

JP,

More toe on the rear. I find 2-3 mm per side is what you need. Ride heights should be 12mm higher at rear. It also helps to corner weight it.

JP, those ugly wings add downforce to the rear to help push the back end down at speed. Thats why all those Honda's on the street have them, because they always see speeds of 100+ ;)

Your car's fat ass is catching much air underneath, vent the rear, and go duck tail.

It's a shame you can't just throw another car at track events, I love your car without a tail.


M

JP, is it squirming around under braking, or just "driving", or both?

I just went and looked at a couple recent (I think) pictures of your car, you need a very low airdam, the lowest you can get away with, helps if it flares to the wheel openings too.

If you are going to be really hauling ass, let me suggest the ducktail (factory liked them at speed) or a real 2 stage tail.

You guys track, and autox, I find open roads, and go WOT. :)

...not tonight though, I need a valve adjust. :(

(ftftftftftftftftftftftftftftftfttft)

M

Straight line, WOT....bout 6.5K in 4th. "Squirm" doesn't quite cover it. It's somewhere between "Squirm" & "frog hop" :D

JP,

I know you are quite handy in the woodworking shop. I saw this on a Honduh and thot of yew:

http://forums.pelicanparts.com/uploa...1065410507.jpg

:D ;)

As you just found out the settings for auto-x and raod racing are a little (lot??) different. Usually heavier springs in the rear, larger torsion bars in front, very low ride height with a SLIGHT nose down, some serious toe in in the rear, slight toe in front, solid bushings in the rear so the trailing arms do not move and even some bracing on the inner suspension ears so they do not flex and finally make sure the tire pressures are exactly even using a very accurate gauge! Whew, lots of stuff but the most important is the alignment and a good corner balance with race tires on and you in the driver seat with all race gear on. The others can be done slowly as $$$$ and time permit. Most vintage 914s have some serious suspension work hidden underneath that is not very apparant. A final thing to consider is a way the break up the vacuum behind the rear window and on top of the trunk which trys to suck the rear end up as you go faster!!! Good luck.

John:

How serious is serious.....toe in, that is. I can do a wooden spoiler.. hardwood, even. Brazilian chrerry is gud:D

We use 3/8 inch each side in the rear and 1/4 inch in front but I am running Goodyear slicks. If you use DOT type tires it could be a little less as the grip is less. A spoiler would be great, especially in cherry, I bet it would be heavy as hell???? I'd put one on just to see people's reactions!

JP, my car has been rock solid (so far) on 205 STREET TIRES (just switched to MPSC's) and no air dams, no flares, no wings (110 mph is about tops for my run group).

Don't know what my toe-in's are, but the car was corner balanced over a year ago by a local race shop, before I started doing DE's. Perhaps all that beatin up you do to your car at AX'es "recalibrated" your alignments.

John: That thought had crossed my mind.

I'll prolly be putting some of Meullers needle bearings on the rear
this winter. At that time, I'll need an alignment. More rear toe
seems to be the consenses. I hate to booger up a nice trunk lid
by mounting a spoiler. I don't see a lot of high speed stuff in my future, but I ain't going to the big track without a sure fire fix.
I can probably find a spare lid for a spoiler.

Mr. Rogers, et al:
Other than the one problem, the car was wonderful on the track.
It was like a huge AX track. The car reacted the same, the speeds were just highter....and *lots* of room to slide around and enjoy the car (traffic permitting).

I was in the novice class, this being my first track time. In traffic I rolled thru the twisties without touching the brakes....except to keep from running into the back of or crowding folks. I couldn't pass some of the faster cars (passing zones on the straights)unless I backed off them in the twisties, then got a run on em' on the corners leading onto the straights.....then I could get em' before the nasties set in. I think some of em' didn't like being passed by a 914. I also think some of then could use some AX time.

By the end of the third session, I was exhausted. My neck hurt, wrists and forearms were throbbbing....but I was smiling:D

Yeah, it's a blast seeing the looks when our "lowly" teeners keep up with and pass those steroidified Beetles. Your AX skills are showing if you are brake free in the turns. That blue flag with the yellow stripe ain't just for teeners - keep grinning!

When the twisties were clear of traffic I was using the brakes.
Them "short chutes" are longer than most AX straights. :D
There is where the AX stuff comes into play....hard OTG at every opportunity...hard on the binders & slither thru the corner, take the "racing" line, not the AX line (gotta remember that)....ah, more room...more gas....another corner....jump the curbs, here comes the straight, nobody ahead.....boooring when ya gotta feather foot it.:(

HAHA HARRY, I saw that car on ebay also. The funniest car I've seen in a long time. I like the garbage can on the hood and the removible rear doors

There's something to be said for the airfoil shape of cedar siding....but I'm not sure what.

JP,

Yea, yea, yea... I'm glad you tried a drivers ed.
was it with PCA?
did you run with an instructor?

This is only my $.02, and I know people will disagree.
I base it on 12 years of DE, and 8 years of wheel to wheel in 914's....

At 110mph, you don't need the spoiler. I think at about 130 it will start to become important. I've ran my car (about 120 top speed at our tracks) both with and without a rear spoiler... you can't notice a difference.

The aero is not causing your car to move around on the straight, its much more likely an alignment issue...

I also agree with some of the above comments, its useful to corner balance, and more useful to run with a downward rake to the car. I know you have hard rear bushings... I've been told by Allen Johnson in colorado that he will use different rear toe based upon type of rear bushing... I think with his monoball (similar to muellers) he will run with very little toe. He also fabricates a toe rod on the outside of the long, which is heim jointed and locks in the adjustment...

If memory serves right: He said he was running zero toe with the monoballs and the toe rod in combo...
Point I'm trying to make here is:

1) I doubt that aero is your problem
2) Toe is not always your friend, and you have to set toe based upon how much variation/deflection you expect to occur.

I would almost assume that your alignment settings have changed since they were originally set. I'd check both front and rear... and look for other odd problems that could contribute?

(bearings, rubber joints-even front stuff like the ball joint.)

brant
brant