firemn131

So, Not even sure how to describe this. Or if it was present on the car prior to the wheel change.
I have to admit that I have no idea what my offsets are. I will check tonight.

So, when turning at low speed, when I go a little bit past a 90% degree turn the wheels will automatically increase the degree of turn felt by automatic increase in steering wheel rotation. I have to prevent the steering from going to the stops. It presents in both forward and reverse.

I also hear and feel as if the tire are skipping or chudding/dragging during the turn.

Car drives straight and tracks well under all conditions to include hard braking.

No vibes at all.

I self aligned it and have about 1/8 - 1/4 toe in, thinking that this was the cause.
My other blind guess is that the rims have an offset that is causing it.

I will measure current offset against the factory rims tonight if possible.


I believe this has been present for a long time. Hard to remember because the car was down for an extended period.

Steering works well to return back to straight until it goes a tad beyond a 100% turn.

Thanks

docmirror

Get it looked at very soon. Divergent steering has killed people. Related to camber, wheel offset, and toe settings. If you get in a situation where the steering pulls that hard it might get out of hand.

dr bob

Caster is typically the bit of geometry that causes the steering to self-center. On undamaged 928's though, there's no available adjustment that will cause caster to go that far though.

Meanwhile, the steering is also self-recovering on center so long as the front wheel offset is 55mm or greater. While 50mm offset feels OK on smooth roads going straight ahead, the ackerman effect is exacerbated when the offsets are too low. Many with 50mm offset report symptoms you describe, along with a lot of audible scrubbing in low-speed turns. Just that small difference in offset makes a big difference. If, after you recheck alignment, you still have the symptom, you really need to get to the core of the problem before you drive the car. A random impact with a pavement irregularity risks having the car lurch to one side, and with the benefits of negative scrub radius gone with less than 55mm offset, offers you no help at all in recovering.

Genuine Porsche wheels have the offset number cast into the wheel right next to the wheel size. Less than 55 is serious reason for concern. In my opinion and experience anyway.

FredR

+1 on what Doctor Bob says albeit I believe the magic number for offset is ET60mm.

I can get my front alignment pretty close by eye-balling and feel but that is for the settling down period. In fact just done this over the last week. When I saw the numbers on the laser alignment rig I use I was surprised at just how accurate I managed to get it- I could have driven around on those numbers permanently I suspect. However, there is no substitute for the laser rig and its ability to support small adjustments to get the system evenly balanced both sides.

As to what is causing your issue remains to be seen.

Rgds

Fred

firemn131

Not sure if I found the correct casting information.
But, my original wheels that are not on the car have-
47 on 2 of them and
52 on the other 2.

Condition is most noticeable when backing up.

FredR

Those numbers are typical of aftermarket wheels that are more in line with other Porsche models. I had a set of OEM wheels from whatever Porsche model they came from- with ET52 on the fronts- filled the wheel well nicely but handling wise another matter. A friend of mine purchased them for his 911 that they were a perfect match for.

Be careful - sounds as though you need to avoid reversing at 150 mph!

Rgds

Fred

firemn131

I couldn't find anything else so I measured from the mounting base to the wheel rim.

Current wheels
Part # 996 362 124 front offset is 5 7/8 (5.88)

stock wheels 6 3/16 (6.19).

So I will go back to the original wheels and see if it makes a difference..

I do like the look of the turbos though.

FredR

Those numbers are not offset values - they look more like backspacing figures

firemn131

haha,
totally confused. I thought they were one in the same..

back to educating myself.

thanks

J

FredR

John,

Offset is more a European convention [I think] which is why Porsche use it. I believe you chaps over the Pond are more into backspacing as a defining concept.

Offset= rim width- backspacing.

Thus if you have 8 inch [200mm] front rims, the offset would be 51mm [more likely 52mm allowing for rounding errors].

Rgds

Fred

jcorenman

Here's some info on wheel offsets: https://www.lesschwab.com/blog/post/...s-wheel-offset
Porsche wheels are always positive offset (called "ET"). As mentioned, factory wheels will generally have the offset and width on either side of the valve stem, aftermarket wheels will be marked on the back of one of the spokes.

Those look like aftermarket "Cup" style wheels, made by Mille Miglia in Italy. Fronts are typically ET52, rears 47. Rear offset doesn't matter, front does-- as discussed above. We have a couple sets of those wheels (17" and 18"), and what Dr. Bob said above is exactly correct... depending on nuances with the front alignment they do want to go full-lock, but (in our case) only at large steering angles like hooning through parking lots.

Backing up doesn't count... the caster effect is reversed and the steering is unstable-- it will tend to go to full-lock.
As Fred said, be careful when reversing at high speed...

"Original" is confusing, neither of your wheels are original to the 928. What is the full part#? 996-362-124-00 and -01 are 7x17 ET55. Those are the turbo twists, right? The Cup wheels are ET 52, which would be worse.

But tires make a difference also, especially if not worn uniformly. For example if the alignment is off and the wheels are toed-out, then the inside wears faster which is the same as positive camber even after the alignment is fixed. And positive camber (i.e. not enough negative camber) will also create what you are describing.

It's all connected: wheels, tires, alignment. I would stick with the twists, and if the tires are worn funny then replace them. Then get the alignment checked, and make sure everything is tight-- if the steering rack is floppy for example then front-end alignment is going to be all over the place.

hacker-pschorr

That was my first thought, one of the tires has an odd wear to it. This is very common with snow tires and reason why I try to rotate them mid-season, especially new ones. The snow's for my fathers truck has one tire, that if installed on the front will cause the truck to pull hard left. On the rear...no issues.

I would swap front left / right and see what happens (the directional pattern is more for wet conditions, won't hurt anything over a couple mile test drive).

If the rears will fit within reason for a test drive without damaging the fenders I would try that too.

Unless you have another complete set of "known good" tires on different trims, start there.


As for offset....I understand the engineering behind the concern of sub et55 wheels. But the sheer number of 928's running around on et50 and et52 wheels is not insignificant (even on the track) and they are not dangerously flying off the roads. My track wheels are et52 on the front, going hard into corners at at Road America I've never experienced any unsteadiness compared to stock wheels with et65.
The only noticeable feedback is the self-centering feel of the car is reduced as offset drops. This may bother some people and if so, by all means put the correct wheel on the car. Otherwise, I'm not going to worry about it.

docmirror

I hate to do this, but really it's camber thrust applied through the tire contact with the surface that causes much of divergent steering problems. Caster is the 'solution' that vehicles use to minimize, or correct for this camber thrust. When I say solution, I mean that castering is applied to the geometry of the suspension to resolve the inherent thrust created by the dynamic camber(tilt in or out) angles as the car is moving. The suspension moves as thrust is applied, and without going into the physics of it, the car like any other mass tries to maintain it's direction unless upset or disturbed by an altering force(turning of the wheel in this case).

I went through this many years ago by studying the first road-going front wheel drive cars built by Auburn, Cord, Dusenberg. The Cord L-29 and 810/812 models were FWD back in the 30s and they suffered from serious torque steering, and camber thrust divergence. They wrecked a number of cars due to camber torque thrust before solving it. Of course, in FWD vehicles, there are other forces at work, but the concept of the thrust angle of the front of the car isn't changed that much due to being RWD. In fact, FWD actually helped reduce camber thrust up to certain steering angles, as the thrust tended to apply negative camber(depending on wheel offset), even with accurately offset wheels directly over the thrust plane of the hub. But, that led to an unfortunate condition where there is a 'breakover' angle when the camber thrust overcomes the ability of the driver to counter it, and the wheel will go to the lock under force of the motion of the car, and at that point, the driver is now simply a spectator cuz there's nothing they can do to stop it.

Dr Bob has it right from a practical POV, but the caster angle is the result of or solution for the camber thrust applied to the suspension. I don't believe there is any camber adjustment on the 928 front suspension, but I've never looked close enough. In some cases, McPherson struts have clamps with cam ellipse shank bolts so that the camber can be adjusted a bit to compensate for wear.

Take note of these hubcaps, and see how far they stick out from the tire center thrust line. There is a massive offset on this wheel, and the rim is close inside this chrome hubcap. The hub is way out on the end of the axle, and the nut is directly behind this hubcap with almost no space. Note also the cross leaf spring, with the vertical shaft that supports the huge wishbone truss from the center section out to the hub.

http://silodrome.com/wp-content/uplo...-Phaeton-4.jpg

The Cord front suspension is a swing axle design, and it has massive caster at low loading to counteract camber thrust. Cord actually reversed the engine in one car, and put a Cadillac rear drive on it to test the front suspension and they still found that camber thrust was an issue, notwithstanding the drive axle loads.

Anyway, caster is the solution to camber which is what primarily induces divergent steering.

firemn131

"working from home" today, so several competing priorities.

I changed out one front wheel.

For clarity.
Car drives and tracks great under all conditions to include hard breaking at higher speed. No vibes or uneven wear pattern. Just don't remember the self induced turn geometry when last driven (3 years) before major overhaul.

Turbo Twist that was on car. (Auto Atlanta matches the part # to 911 "98-05- or Boxter "97-04)
Front
Rim Part # 996 362 124 00 Offset 55 Rim 7"
Tire 205 50 ZR17

Rear
Rim Part # ??? Offset 55 Rim 9"
Tire 255 40 ZR17

Previous wheels after market Cup's

Front
BORBET / Made in Germany

Rim Part# ?? ET 52 Rim 7 1/2. shown on back side 7 1/2 5X17H2
Tire 225 45 17

Rear
Rim part ?? ET47 Rim 9'". Shown on back, 9J X 17 H2

So, if I am reading other posts and table correctly.

http://www.landsharkoz.com/forms/928wheels.pdf



The twist should be OK?

The Cups have offsets that are a little light.

But the tires could be interchanged from the twists to the cups?
Are the 225's a bit too wide for the front (within the margin though)?

Sorry for the ramblings, but questions

Are both of these sets of tires and wheels OK to use with no adverse affects?
Should I have a 10mm spacer added on the front?

Table shows these 17's fit a GTS from 92-95
Thanks

Mikebte

Have you looked at all your connecting points? Upper, lower ball joint, tie rods?
The joints can build up grooves and at the right angle cause the tire to move in or out and the joint location. If it moves in or out it will change toe in/out and affect your wheel alignment.

could also be bushings. That is the first place I would start.