Weird 911 Handling Problem, Gurus needed!
 

Gurus,

Wierd 911 Handling Problem:
Changing lanes or lines at highway speeds the car starts to weave back and forth. It is borderline unsafe at highway speeds while driving normal. But it is a bit scary at high speed driving. This problem started after making several suspension changes at once.

The following changes were made at the same time in a Porsche shop:
1. Lowered the car.
2. Replaced 23mm rear torsion bars with used 25mm torsion bars.
3. Replace stock non-adjustable spring plates with worn out bushings with 911SC adjustable springe plates with new elephant racing rubber bushings.
4. Installed new Bilstein HD's all around (rear right was blown).
5. Installed bump steer spacers on steering rack.
6. Replaced rear 195/55/15 Yokohama S. Drive tires on 7X15 wheels with new 205/55/15 Yokohama S. Drive tires.
7. Aligned car and added a little more rake.
8. I set tire presures set at 37 psi front and 32 psi rear.

Additional technical relating to the suspension data:
1. The car is a 1976 911S with 169K miles, very well maintained.
2. The front wheels are 6X15 with medium wear 195/55/15 Yokohama S. Drives
3. The steering rack was replaced a year or two back with a low mileage one in excellent condition.
4. The car has turbo tie rods.
5. 60 lbs have been removed from rear of the car.
6. Maybe 350 miles on new rear tires.
7. Previously have run these yoks 195s on 6x15 wheels all around, no problems. Was running 195s all around on 6x15 front and 7x15 rear wheel before suspension upgrades without the problem.
8. Sway bars - 20mm front/18mm rear
9. Front torsion bar is stock 19mm.

Correction action/analysis to date:
1. Took car back to Porsche shop for planned alignment check and problem analysis.
2. Rechecked alignment - it was ok.
3. Checked tire pressure. Changed to 41psi front, 44psi rear.
4. Inspected all bushings. All ok. Only sway bar bushings had small cracks.
4. Took road test, shop owner driving, yep, still weaving.
5. Replaced rear wheels and tires with 205/55/15 Goodyear F1s on Fuchs off the front of a 911 Turbo (they looked like 7x15s).
6. Took another road test, we both drove, the weaving was gone!?!
7. If I hadn't seen it and felt it, I would not have believed it was my new rear tires!

So it has to be the new rear tires, right? But why?
a. Bad tire(s)?
b. Sidewall stiffness change from 195 to 205 in rear?
c. These yoks don't play well with staggered sizes?
d. These yoks don't like the suspension set up?

What is the best course of corrective action? What is the best tire/wheel size set up? I use this car for the street, in town and road trips. I do not intend to race again (well maybe a little racing lite). I have been setting up the car for livable, but high-performace, street driving. Of course I do not want to waste money.
Some options:
1. Go back to 195/55/15s on 7x15 in the rear and 195/55/15 on 6x15 front(low$)
2. Go to 195/55/15 on 7x15 all around(low$).
3. Fight with tire store for replacement of 205/55/15 rear yoks(no$, maybe).
3. Go with different 205/55/15 tires on 7x15s all around (bigger$).
4. Buy 7x16 Fuchs, 195/50/16 front and rear tires (biggest$).
5. Buy 7x16 Fuchs, 195/50/16 front and 205/50/16 rear tires (biggest$).
6. Buy 7x16 Fuchs and 205/50/16 all around (biggest$).

I have 2 each extra 6x15 and 7x15 wheels and 2 195/55/15 yoks availble. I was going for the staggered look/feel of a Carrera 3.0 and the best handling set up. I am willing to go to 7x15 all around but I was concerned about reducing understeer and making the car less neutral and posibably too much oversteer for aggresive street driving with a small margin of safety. I do not want to go lower profile than 195/55/15 or 205/55/15 as highway rpms are as high as I am willing to accept.

I would appreciate the thoughts and experiences of you 911 suspension gurus out there. I am a long time car guy with a little racing experience but reletively new to rear engine handling 911's. But I have never heard of this problem before or a new name brand high performance tire being bad.

Thanks, Bob

I have experienced squirrely handling for two reasons: low tire inflation and improper alignment. You seem to have covered both of these areas so I am stumped. It sounds like you have poured a lot of money into your suspension upgrades. I would still suspect alignment. Perhaps your alignment shop did not compensate for your suspension upgrades or compensated for something they should have not.

The tires that solved the problem had some wear on them, right?

New tires have deeper tread = more tread squirm. With your worn fronts, combined with the extra rear roll stiffness you added with the stiffer rear torsion bars, the new rears will make the rear end loose. When they have some miles on them and they get "scrubbed in"- that is, the mold release wears off and the tread starts to get down to the good rubber and shorter tread, the handling should improve.

This is why it is best to replace tires as a set when handling matters.

Thanks for the input Flieger.

I have experienced "tread squirm" on sports cars after 4 new tires before. But never the weaving. I may have all ways replaced all 4 tires though. These new rears have a couple hundred miles on them already. Plus they were the same mfg/model. I had thought about new tire "tread squirm", but did not think it would have such a profound effect (still my first 911 though).

Do you think new tires up front (195 or 205) would eliminate the weaving? Or are you suggesting my 911 will be unsafe at high speeds with this set up after every set of new tires?

I originally wanted 24mm rears so the set up would be the same as an 78 SC but they were not readily available used. The current set up is the same as an 84-89 Carrera. But I know my car is few hundred pounds lighter.

I do like the thought of not have to spend more money. I think I will go out and do a little rear tire spinning today and tommorrow to see if the weaving gets less. I have plenty of torque with the 3.0 and smaller tires :)

I was thinking about trying to get a Yokohama manufacturers representative involved.

Thanks for the input Targalid.

Alignment was my first thought. The weaving reminded me of when I was younger and autocrossed an 86 MR2. I would kick the camber to full negative on on four corners the night before a race. I would drive it like that to the event and it would not want to drive dead straigh on the freeway for twitching/weaving! But it was a blast at the track!

But remember the weaving went away with different rear tires. Flieger may be on to something. If so, this 911 set up is VERY sensative!

Well, trying to diagnose the cause of "weaving" with only that word and not driving the car is hard to do. ;)

I was theorizing that the weaving was just you experiencing the differential of tread squirm between the front and rear tires. If you put new tires on all corners, the car would likely feel a bit like driving in rain but not exhibit so much yaw instability- just a more vague connection to the road.

About the 911 being especially sensitive to setup;

like I said, changing the roll stiffness distribution towards the rear will only amplify any other yaw instability.

Why is your tire pressure set so high ?
I see you started at 37 psi front and 32 psi rear then you went to 41 psi front and 44 psi rear ...

Normal settings are 29-30 front and 34-35 rear depending on your preference.
I start at 32 psi front and 34 rear and aim for 36 front and 38 rear max during my track sessions and bleed accordingly depending on the conditions,anything above that and you lose traction !
For the street i run at 29-34 in order to get even tire wear.

Cheers!
Phil

What alignment specs are you using? I also question the high tire pressures. It appears your tires are same brand .Has the internal construction of the tires been upgraded or changed ie different belt fabric or number of belts between the time the fronts were manufactured and the new rears? I have seen similar yaw when using tires from different manufacturers F/R but not so much from used /new same type.

I agree tire pressures too high.

A low degree of Castor will cause a car to weave, check the castor, don't know of hand what it should be but has to be over 5-6* to stop weave.

I had a problem similar to what you described after some suspension mods and going to Bridgstone S03s in the rear.

I chased my problem for a year or so. As the tires wore in it improved but did not totally go away. It was mainly evident on the freeway with GROOVED pavement.

I got a second set of tires and wheels with Toyo RA-1s and NO more problem. My S03s are for sale.

I had a problem similar to yours after replacing the rear suspension. Turns out that even after tightening the axel nuts to 200ft/lbs and installing a cotter pin, the axle nut loosened up just enough to cause handling problems like yours. Easy enough to check. A fellow PCAer has a wrench just special for that nut. It's about 4' long.

I like about 5 pounds more tire pressure in rear than in front.

If I were to guess I think you need a little more toe in on the rear. It wears tires more quickly but acts to keep the rear planted. Would like to know the front rear alignment settings to offer an opinion re alignment instead of a guess. Also I prefer tire pressure around 32F and 36R cold for street driven normal loads.

same as wildcats ????

why is your pressure so high ??? i run my front 225 17 at 28 and my 245 17 rear at 31

yours are set almost as high as my pickups tires ...

your rear should be higher psi than the front ... your weight is in the butt area ...

Just because it says max psi on the side wall does not mean you pump them to the max ...

I'd try lowering that a bit first ........

Steve

Definitely sounds like tire pressure is contributing to this sensation. After buying my car and driving it for a few weeks, I decided that I should check my pressures. All 4 were at 51 PSI (!!!), the max indicated on my tires. Lowering them to recommended pressure helped handling a ton and made it drive so much more comfortably on uneven pavement.

Out of all the cars I've driven with NEW tires,....the 911 has terrrible sqirm,..and will require hundreds of miles before she locks down. I hope that's all this is,..in the end.

I stay with 29/F and 36/R (COLD)

Doyle

Bob,

PM me your email address and i'll forward you my alignment spec sheet which was done by a Porsche race shop a few weeks ago.It's a compromise between street and track and it really made a difference to my car handling wise ...
I have about a dozen track days lined up this season and this setup works well for me !

Cheers !
Phil

Thanks everybody! That is a lot of good feedback.

I used to run 29psi front and 34psi or 30/35 (whichever is in the owners manual) before the new rear tires and suspension mods and I had no weave. I just realized I raised them per the shop recommendation when the car was lowered and the new rear tires added. I can't remember if they checked the tire pressure on the Goodyears that did not weave. I will lower them before I drive home on the freeway today. I will post the results.

Wow. Tire squirm differential front to rear causing instability on the freeway. I have a lot more to learn about 911's.