Another stability at speed question - different post...
 

I had an '89 911 Targa for several years. I autocrossed it alot. Never a problem. Never a question about high speed stability (after I got rid of a Turbo Whale tail - without front spoiler).

I now have a '73 Targa and it's a whole different animal at high cruise speed. That surprised me. There are no aero aids. I set this car up to the same specs as the '89 (lowered a bit) and WOW - I found cruising at 80 was a handful - even pulled off the road to see if a tire was coming off. The first thing I did was to back off the little toe out I had in front and it seemed to calm down the tendency to be darty or twitchy. I've been autocrossing it for a few months and all seemed to be getting better - tweeking the torsion and AR bar sizes along the way. I really don't feel the suspension or corner balancing is the issue here.

Today at 80 mph - the darty, twitchy handling is back. Based on how the car feels, I'd like to start with the steering. The feel I get back through the steering wheel is disturbing. For instance: I can have a light positive pressure holding the wheel left and then, no feel - pressure is gone and then it switches to right pressure. While that's happening, the car makes a little dart to the right and I correct - but, it jumps back to the left and I start countersteering to stay in a straight line. That's pretty unsettling.

When I parked it in the garage, I checked to see if there's any slop in the steering. The steering wheel and tires seem to move in unison with no discernable play. I checked the steering column under the bonnet and the u-joints have a tiny bit of play, but not such that I think should be an issue.

Has anybody had a similar issue?... and fixed it?

I think it could possibly be tires (newish "street" Toyo R1Rs front and rear) on 15s. The fronts are 195 / rears 205. I've heard that the composition of these 205s are slightly different than the 195 - i.e. the magic grip rubber is throughout the 195s while the 205s are about 50% magic, then better tread wear rubber under that.... rumor? I wonder if this is a bad mix.... like different brands.

Can anyone share ideas, insights, comments?

thanks,

Tyre pressures? Has a big influence (ask me how I know!!)

When was the car last 4-wheel aligned?

Have you checked the front wheel off the ground with the grabbing at 3 and 9 o'clock for the wheel bearing check and 12 and 6 o'clock for the ball joint/tie rods?

It's a 30 minute check to eliminate them.

Report back!!!

Neil

ball joint, same as above, you may need to jack the wheel with the wheel off the ground.

tie rod ends.

toe, fron and rear. toe out can cause strange things. caster could cause a different feel also. after driving a 930, getting in my 77s, it feels like the front wheels are off the ground, at low speeds.
rear end movement.

you are really talking about 2 different cars when you compare the 87 to 73, even though they may be the same.

At 80 your 73 should be rock steady with no weirdness. At higher speeds the lack of a ducktail and front spoiler can be disconcerting but 80 is too low to feel much of that.
I'm betting alignment or tires. Can you swap tires with someone to see what happens? Or just put your fronts on the back to see if you've changed things. I ran my 73 once with different tires front and rear and it was scary, so the car is very sensitive to this. Hard to imagine that Toyo uses a different compound front and rear but who knows.

... thanks for the feedback.

- the car has been aligned and corner balanced (twice this year).. when I swapped torsions after a few events... currently 21/26 - not radical.

- the car has lots of new stuff also - ball joints, Elephant tie rod ends, rubber bushings everywhere. (interestingly, the tie rod ends bring the rods to horizontal as they should be, but I can still get a tendency for the steering wheel to jump out of my hands going over harsh bumps).

- I'll check the front wheel bearings for slop and adjust if needed.

this leads me to the tires. I have a couple of spare Toyo 195s I can mount on spare 15x6s and continue to search. Also, a friend has offered to swap wheels/tires and see what we get.

I appreciate the comments and input!

thanks,

whats your tire pressure?

how many miles on the existing tires?

what happens heading into a strong wind vs. going with the wind?

is the effect progressive with speed?

what aero thingies on the front & rear?

When you say you removed the "toe out", back to what? Zero? I believe the stock setting is -1/8. I would do that first before I spent any money on other items. Twitch steering, as you have found out, is often caused by insufficient toe in. Either in one or both front wheels.

Like you, I had no aero aids, F or R on my 73.5 targa.

It was solid at 95-100, no tail, no chin spoiler.

Steering felt a little lighter, but not 'darty' or scary.

I had the same tire size all around with Fuchs if I recall correctly... size recommended by Porsche.


Try the other wheels and tires... air pressure is important on those cars... maybe one has a broken cord????

Hi,

I'll try to answer all who took the time to respond....

- just checked pressures (still at AX cold) at 29F/31R (cold)
- tire mileage ~ 3000 (since putting car back together) with 6? autocrosses
- yesterday had a bit of a cross-wind (but, I do need to be more attentive heading into a stiff wind vs. downwind - that seems natural though)
- this occurence is in same spot of road it happened last time
- effect was progressive.... I slowed down at 80
- no aero.. stock targa
- front toe now at "0" - straight ahead
- on both occassions, I was out being frisky on a twisty back road, then home on the frwy...

thanks,

Sounds exactly like my car after I put new Potenza S03s on the rear.

In my case it was mostly a problem on the freeway with the grooves. My handling was scary at times.

If I was you I would borrow another set of tires (and wheels) and see how it drives.

I chased my problem for over a year. Nothing helped until I changed rear tires.

toe it in just slightly & see if that fixes it - do that while you are looking for a set of tires/wheels to swap

also... you've tested this on various different roads, right?

Spent many years racing open wheelers and toe out, while it improves the corner turn in at speed, plays havoc with straight line stability. The car wants to follow every little deviation in the road surface. I would definitely try some Toe-in before wasting too much time and money chasing other items. You have to remember that setting up a car for competition and highway use is always a compromise.

...good suggestions. What still surprises me is how subtlely different the '89 (2900 lbs?) and the '73 (2300 lbs) react to alignment. The '73 seems much more sensitive. I'd forgotten how much - owned a '69 for a brief while - way,way before the '89. While the '89 was a great car, I'm getting more of a kick out of this early car... maybe due to the Euro Carrera 3.0 attached to it? :)

thanks folks,

yes, i like the feel of the older cars.

it would have to be a turbo for me to own anything newer than 83, really 79, but for other reasons.

add toe in as suggested.

how low is the front. any spacers to raise the steering rack.

mine reeks havic when there are big ruts in the road. when i change lanes to get into the turn lane going to work, i know mine is going to move around a bit. but these ruts are pretty bad. 2-3 inches. i dont have the reack spacers to raise the rack and i have very wide tires, 225 un front. i dont think the tire size helps.

Can you be more specific? Did you get a printout?

A little toe in rear and zero in front. A lowered car is fine but if it is too low in rear compared to the front your car will be very unstable.

Tires for a Porsche has to be N-marked.

Found issue with stability at speed...
 

... not what I expected. Went out today and had same handling issue on different freeway... having a slight countersteer to keep it on the road.

I put it up on jack stands today and took off the REAR wheels and started pushing and pulling on everything and found the left rear hub was wobbly. I checked the right rear and the secure castellated nut left about 1/4" of the axle flange showing and had it's cotter pin in place. I looked again at the left and the nut had NO cotter pin and had backed off about 1/4" and was flush with the flange shaft. My wheel was trying to come off. I went ahead and took the axle out to check that the flange wasn't broken or something else was amiss... buttoned it up this afternoon and tightened the nut and installed a cotter pin.

I'll take it out tomorrow and see what I get.

Thanks all for the suggestions - which I think would be spot on in most cases.

....PS - this increases my excuse list for autocross. :D

Buddy's 88 had wobbly bearing all around coupled with low tire pressure.

He was also complaining about the car being unstable on the freeway.

Definitely his wheels had slop.

After changing new back tires(General G-max AS 03) my boge shocks expire, buy a new KYB shoks and this shocks make my car dancing on the back like no shocks are there. Using KYB and a damage shocks are the same now. any other part to check that cause this problem?? Car is very lose on the back and I can run more than 50 because it feel like an old cadillac. And at high speed is scary, Jejeje recomendation?others