Allignment Question
 

I'm hoping someone can provide some perspective for my 1980 911 SC.

I had new tires mounted in the rear last week and got an allignment. My car handled fine before the allignment and tires. It tracked well on center and was very linear and predictable on turn in and rotation. Right rear tire wear was slightly accelerated compared to left rear. Fronts tire wear is balanced. After the rear tires were mounted and the allignment was changed, I noticed a significant change - not good.

On center, the car hunts. Every road surface imperfection cause the car to move left and right - requiring a minor correction with the wheel. It doesn't feel stable. Turn in is instantaneous and exagerated. As soon as the steering wheel is turned, the car darts that direction and the back rotates a LOT harder than it did in the past. Instead of a progressive rotation, I can feel the tires and suspension load up instantly and the car becomes a lot more sensitive to throttle steer. I noticed that with a full tank of gas, the problem is less pronounced than with an empty tank.

Here are some details to work with:
Stock setup with 190K miles
Front strut bar
Turbo Tie Rods
Closer to Euro than US ride height
Front tires are Toyo Proxis 4 with good tread depth @ 30 lbs
Rear tires are new Toyo Proxix 4 with 34 lbs
Front Left Caster went from 6.4 to 6.6
Front Right Caster went from 6.4 to 6.8
Front Left Camber went from -0.7 to -0.8
Front Right Camber went from -0.5 to -0.6
Front Left Toe went from -0.10 to 0.15 (big change)
Front Right Toe went from 0.10 to 0.15
Rear Left Camber is unadjustable at -0.07
Rear Right Camber is unadjustable at -1.4 (must be that curb that launched over)
Rear Left toe was unchanged at 0.20
Rear Right toe was unchanged at -0.05

Is there a better allignment option? Is there something else to consider?

How many miles on new tires? I run 34lbs front 40lbs rear after going to 17" Rotas with just below euro height, factory tbars and oversized sways.

Scott

Did a Porsche shop do your alignment, or at least someone who's intimately familiar with old 911's?

Also, food for thought...every time I've replaced the rears only, my car has become nervous. I talked to Tyson Schmidt about it (a well known Porsche guru who used to frequent this board) and he mentioned that pairing old tires with new will almost always produce a nervous car because the rubber compounds and traction characteristics don't really match anymore (aged vs. new), sidewall flex changes, etc. He said that he always bites the bullet and replaces all four when his rears wear out. I do the same now and the difference is like night and day.

Mixing new tires with old will do this. The new pair in the rear need to scrub in and heat cycle before the sidewall stiffness and treadblock squirm begin to match the fronts.

So right now the rear is steering the front, and will feel like the tail is wagging the dog until the rear tires stabilize. It's one of the downsides to rear engined cars. They are very tire-sensitive.

That's great advice from everyone. Thanks. I will get a new front set next week based on that information. The rear tires have just several hundred miles on them (one week of daily driving) so they should be well matched when new fronts are added.

Do the allignment specs look right?

And no - it was a tire shop that works on all kinds of cars and doesn't specialize in Porsches. I am tempted to take it to a Porsche experienced shop to set up all four corners with a competitive ride height and a competitive allignment set-up, but I was waiting to add some new bushings and maybe some stiffer torsion bars first.

Due to the adjustability of our cars, corner balance can contribute to either ill or excellent handling. Consider checking/adjusting when your tire issues are resolved.

Sherwood

Rear camber is NOT "unadjustable". It is easily adjusted. Camber of -1.4deg one side and -0.1 deg other side is not desirable. Rear toe-out is also not good for stability - should be slight (15') toe-in.

+1 on replacing all 4 tires at a time. I recall reading in a UpFixin' article (referring to early non-staggered tire cars) that Porsche required new tires to go on the front if only 2 tires are replaced.

I agree, take it back to the alignment shop and show them how to adjust the rear camber before you buy new tires.

The self-alignment book used to be available on the dorkiphus site, start there.

Toe-in also helps keep the car directionally stable. Toe-out makes for quick turn-in but can be "twitchy" on the freeway.

Before you take it back to Joe's Tire, consider either aligning it yourself or taking the car to a shop that knows something about 911's. I would not let some random shop put my car on a hoist, let alone start adjusting the rear trailing arm. It's not that difficult to do, but there is opportunity for a clueless hurried greasemonkey to screw things up. YMMV

front
I like all the caster you can get, 6.5* is great
race cars can use close to zero toe, eevn some toe out but this leads to a darty car, spec is 0 pressed but a little toe in will quiet the car down a lot.
30' to 1* neg cambver is good for a street car

rear
a little more camber than in front 1* to 1* 30' is a good street setting
toe as in the front

OK. I'm thinking now that I need to get the car to a shop who can set up a Porsche properly - especially the rear camber. I'll start there and then add front tires. Thanks everyone for your insight. What a great resource.

have you put 200 miles on the new tires??

About 300 probably. Why?

The tires should be scrubbed-in after 300 miles. I think the twitchy handling is from the alignment rather than the tires.

Ronin,
What years 911 does "pressing" apply to? I just tried to have an alignment done and the shop did nothing of the sorts. Mines an '88. Where did this snippet of info come from please? (sorry to interrupt) Thanks

I can only infer up to 1989


front factory specs when pressed

there is a normal tendency for fronts to toe out

aka: adding some toe in compensates for not pressing

agree - & that's why I asked -- it was one (cheap to fix) thing that could have caused your problem, before getting into other more spendy things to do, as per above

Please keep your threads together :)

http://forums.pelicanparts.com/porsche-911-technical-forum/484581-allignment-question.html?posted=1#post4771554