Weird 911 Handling Problem, Gurus needed!

[Steve@Rennsport]

JMHO,.....

Based on a few years of driving 911's, I've learned (through the same experience) that I always replace tires in sets, never just a pair.

With very rare exception for me, installing new tires at one end or the other results in the same, somewhat frightening experiences so its always a four tire change.

Remember, starting tire pressures mirror the weight distribution of the car so one should always have staggered pressures with the higher pressures in the rear.

[DanielDudley]

My wife's Saab just came out of the shop, and it was all over the road. Tires at 42 PSI. Dropped them to 35, and all is well.

The tires were topped up by a Saab technician. It says 35 F 35 R right in the door.

[wildcat077]

Here's a good base alignment setup for the guys who PM'd me for the specs ... You could go more agressive but if you drive your car on the street your tires won't last long ...

Cheers!
Phil

[rusnak]

I kinda skipped through this thread, sorry. Did anyone say to check the rear wheel bearing wear/ adjustment? Push sideways on the rear of the car with your shin while watching the wheel rim for movement.

[RSTarga]

go back to 29/34 tire pressure. The settings you have used are both high and should always be 5 or 6 pounds higher in the rear, except on 964's and later. I have, at times bought new rears and left the original fronts. It may take a few hundred miles for the rear sidewalls to loosen up a bit, in the meantime the car may get that twitchy feeling especially under braking. One of the reasons you should not put different brand or type of tires on the car.

[crashmy911]

You need to run alot less pressure in the front. 37 pounds is way too much.

[Flieger]

I would also agree to lower the tire pressures, but I would think so long as the percent difference stayed the same (higher in rear) the handling would be similar as far as yaw stability, just with reduced grip. More of a 4 wheel drift than swapping ends.

I run something like 28 front and 32 rear warm, they are all less cold but the rear also gets closer to the front pressure, meaning the rears heat up more than the fronts.

[moneymanager]

I'm with Steve. Running different tires front and rear was a disaster for me the only time I tried it. Car felt like it had a gigantic case of oversteer... you'd move the steering wheel an inch and the back end would step out sharply. Scary. Borrow four identical tires/wheels from someone, inflate them correctly, and I think your problem will go away.

[jpnovak]

Was the car corner balanced after replacing the rear torsion bar and spring plates?

[AZAirCooled]

Thanks to everyone for the feedback! My 911 keeps getting better! I love the Pelican Parts forums!

Last evening on the way home I changed the air pressure to approximately(gas station stick gauge) 29/34. WOW! The weird "weaving" went away almost completely! I can barely feel the "weave" but it is no longer scary/unsafe. Today I will check/set the tire pressures completely cold with my digital gauge to 29psi front and 34psi rear. Man I feel dumb as this is what I used to keep the 195s at (per the owners manual).

Based on the great feedback from my fellow Pelican enthusiasts, the small remaining effect may be the "tire squirm" differential between the broke in front 195/55/15 Yokohama S Drives and the new rear 205/55/15 Yokohama S Drives. So I think I will just drive it for a while to see if it goes away as the new rear tires break in.

There has been such a great response to my problem post that I would like to ask those following this thread to weigh in on what you feel would be my best my final wheel/tire set up.
As stated, the car now has broke in 195/55/15s Yokohama S Drives on 6x15 Fuchs up front and new Yokohama S Drive 205/55/15s on 7x15 Fake Fuchs on the rear. This gives me a 1976 Carrera 3.0 tribute sans the rear flares, 185s front/215s on the rear, a little weight, and 20hp (coming with SSIs). Plus I thought 195s up front would give a little less oversteer for additional safety on the street. I was hoping with upgrades, updates, and backdates to match or exceed the performance envelope of a Carrera 3.0.

In the garage I have 2 additional 6x15 wheels, 7x15 wheels, and 195/55/15 matching Yoks.
With minimal money/effort I could change to:
1. 205/55/15 Yoks on 7x15 all around.
2. 195/55/15 Yoks on 7x15 all around.
3. 195//55/15 Yoks on 7x15 front and 205/55/15 Yoks on 7x15 rear.

Long term options are:
1. 195/50/16s on 7x16 all around.
2. 195/50/16s on 6x16 front and 205/50/16 on 7x16 rear.
3. 205/50/16 on 7x16s all around.

Specifically, I would like feedback on staggered tire/wheel combos verses the same sizes all around on 911's. It seems that smaller tires up front would provide some understeer on a car that wants to oversteer. This is my first rear engined car. I have always driven hard on occasion and am accustomed to a predictable, controllable, 4 wheel drift at a cars limit of adhesion. I have heard stories of 911s handling great until thier limit of adhesion when the rear would suddenly come around. I would imagine this to be uncool on a mountain road if true.

P.S. The car received a four wheel alignment but not corner balancing. Rear fenders are rolled. Photos are before lowering.

Thanks again,
Bob

[RoninLB]

I think you're scrambling a narrow body by seeking what it looks like over what should be done


use 4 7x15 Fuchs and 4 205/55x15 tires [roll fender lips] [7s should give you a stiffer sidewall]

set ride height to so called "Euro Height" with rake set at 1 deg

install Carrera front spoiler [worry about tail later]

air psi 29/34 or 29/35


it should stick to the road like glue at 75 mph and be just high enough to not mash your spoiler going into gas stations etc. It should be easy and predictable to drive


------ that's just my routine

[1QuickS]

I'd look closely at rear toe settings. When the rear suspension arm rises (bump or compression due to weight change during lane change) the wheel will toe-out. This toe-out is a steering input at the rear of the car which if the toe setting isn't toed-in enough will cause the rear to step out. When you correct the swerve the opposite happens to the other side due to weight transfer which will set up a divergent oscillation, to a point.

Trailing-throttle oversteer happens when the rubber bushes deflect causing the loaded outside rear wheel to toe-out PLUS the toe-out from suspension deflection and on the early cars with small tires all the weigh transfer would overload the tire. Larger tires, stiffer bushings and large toe-in settings tamed this issue somewhat.

[SAH]

Yes tire pressures are to high. If your problem went away with the change to the 205/55 assemblies than I would strongly recommend looking at what the rear camber is. Had an 86 911 with A008's lowered and alignied and drove great. Customer went and installed Dunlop D40 tires and it was scary in the rear like it had rear steering! Ended up adjusting camber back to .50 to .75 neg and problem solved. Not all tires are created equal when it comes to alignment specs.

[AZAirCooled]

Ok, I think the weird handling described above is almost completely gone! I set the front tires to 29psi and the rears to 34psi, like the owners manual instructs. I can't believe the difference in tire pressure made the car unsafe in a straight line at high speed.

There is a very small sway left changing lanes/lines abrubtly at high speed. Barely noticable. I will let the rear tires wear in more before I make any other changes.