cds72911

I'm starting a fresh thread to further discuss suspension questions that came out of a thread that I started about fender rolling. I made the bad assumption that the tire rubbing was a result of insufficiently rolled fenders, when the real reason probably lies more in the suspension setup chosen by the previous owner for track use, coupled with wider tires.

The other thread turned into a great discussion about suspension changes that result from lowering, specifically caster. I learned a ton in the process, but now I need to sort out the best way to correct it.

The history is that I bought this 2002 996 about a year ago, and the previous two owners were active in PCA driving events, one participating, the other both participating and instructing. The owner that originally set it up for track use installed the X74 suspension (30 mm lower than stock carerra), front GT3 control arms, and had it aligned for track use (it was explained to me as "GT3 alignment"). I haven't checked the alignment, but I did make sure that I readjusted it back to the marks made by that owner (amazing to me, it had moved from the settings the prior owner had marked with a paint pen).

The car handles amazingly, almost like a completely different kind of car than my 2000 with PSS10s, lowered and aligned to GT3 specs. It is nimble, light, tracks well, turns in easily, etc.

But... I recently changed rims to slightly wider rims and introduced (or maybe just exacerbated) rubbing issues in the front tires.

The car we are discussing:



And the 2000, just for comparison sake:



Let me also say that I don't have a local Porsche dealer or any indies that are Porsche racing people. The one independent guy I know of is well reputed, but I don't think he has much experience with oddball track setups (I should probably verify this though).

Here are the symptoms I am trying to dial out - primarily rubbing of the outer edge of the tire on the lip of the fender opening and secondarily contact with the front wheel well liner/metal substructure in full lock situations (low speed parking lot mostly, especially noticable in reverse).

BUT... I don't want to kill the superb handling in the process!!!!

I'd also note that the X74/GT3 rim and PSS9/PSS10 and GT3 rim combination has worked well for others with no reports of rubbing or other ill effects.

Thankfully, a fellow rennlister @The Radium King taught me a bit about the effect of caster changes while lowering, especiallly with GT3 lower control arms. Since the front arm is fixed length, the geometry changes introduced by lengthening the control arm (with the shims used in the GT3 setup) cause the wheel to move forward in the rear wheel well (extra positive caster). The 996 doesn't have adjustable caster, so that makes resolution interesting.

As a point of reference "The optimal range is 6-7 degrees." (of caster for a Carerra). https://rennlist.com/forums/996-foru...te-camber.html

The working theory is that the suspension setup - especially the shimming of the GT3 control arms, in combination with the wider front tires, has made this issue evident.

So, what things could I do:
1) go back to prior rim selection - see if the 10mm narrower width is enough to eliminate this issue...
2) switch the PSS10 suspension over from my other car - raise the ride height to mitigate rubbing on suspension compression
3) adjust the caster - check to see if my GT3 contol arms already have an offset hole (they should!) and install the front arm in a hole more offset to the front
4) reduce the shim pack thickness in the control arms - take out one or more of the spacers

or some combination of the above...

The first thing I should do is remove the front track arm to see if the bushings in the control arms have two holes (one offset) like most of the ones I see online. If they do, I can try to make sure they are on the proper sides of the car to benefit me (offset holes oriented to the front), and move the arm to the forward hole. If not, I need to think about buying caster correction bushings from Elephant, Tarrett, RSS, etc. and installing them to get the same result.





Swapping the PSS10 suspension from the 2000 is straightforward, and would gain height adjustability. I do suspect that the PSS10 is sprung softer than the X74 and I don't know what this would do to my handling. I suspect it would be a downgrade. But, I'd be able to tune the height to perfection.

Also, I think it may be worth looking at the shim pack. This car has three shims (slightly more thickness on the driver's side) - which seems to be a lot compared to what a lot of other Rennlist racers in other forums are using. I think this setup would have gained the PO a lot of additional camber, so maybe perfect for the track?

Supposedly "each 2mm in shims translate to about 0.25 degrees of negative camber" - that was one person's experience (same thread as linked above). Another online source says "10mm provides about 1 degree of additional negative camber."

As it sits, I have 21mm of shims on the driver's side and 17mm of shims on the passenger side. So, if the above is true, then I'm looking at around 2+ degrees of additional camber from the shims (and some unknown amount of caster, since I don't know the geometry of how much that moves the wheel center forward on an arc at the outer end of the control arm, but certainly a lot more than the amount added by the shim since it is a long arm). The top mount is maxed out to the inboard of the slots, so the negative camber from the shims is in addition to whatever camber is dialed in up there.

It seems to me that I could take out some of these spacers and simultaneously reduce caster and camber, maybe helping with the rubbing, but impacting handling adversely by reducing camber (probably fine for my street use). This feels like a good option, but I'm probably missing something...? Maybe this should be my first move, since it would move the entire knuckle position inboard and maybe tuck the tire under the fender more, at the same time as moving the caster so the wheel is more rearward in the opening...





After any of these I'd need to get it aligned. I should get a good baseline anyway, but these options all muck with caster, camber and toe to some degree.

Also probably a good time to replace the hardware, since things are looking a bit corroded down there...

Any and all advice, opinion or thoughts would be greatly appreciated.

cds72911

Also, I am super curious to learn more about other similar setups. If you are running GT3 control arms, what combination do you run?

1) Which shims do you have installed?
2) What suspension do you run? M030, ROW M030, X74, PSS9/10, Ohlins, GT3 oem, Motons, etc.?
3) What car? C2, C4, C4S, GT3, etc.
4a) Do you get any tire rubbing? 4b) Under what conditions? (long sweepers, quick/harsh compression, driveway maneuvers, full lock turns, etc.)?
5a) Are you using anythjng for caster correction? Which product? (Offset hole in stock bushing, aftermarket adjustable control arm bushings, adjustable track arms, etc.)?
6) Which rim size/tire width do you run?

Ratchet1025

Great start to the thread, I am looking forward to the education process. Regarding your options...

"So, what things could I do:
1) go back to prior rim selection - see if the 10mm narrower width is enough to eliminate this issue...
2) switch the PSS10 suspension over from my other car - raise the ride height to mitigate rubbing on suspension compression
3) adjust the caster - check to see if my GT3 contol arms already have an offset hole (they should!) and install the front arm in a hole more offset to the front
4) reduce the shim pack thickness in the control arms - take out one or more of the spacers"

Please remove "1)". That is not a reasonable solution.
Also, "2" does not seem necessary, my C2 is lower than yours (pics later this weekend)

moburki

1) Can't tell you a the moment, but I can tell you it's quite a bit less of a stack than yours (GT3 arms). Maybe the equivalent of one of your shims. But, I have camber top plates to increase camber also. That's an option too, and won't impact caster nearly (if at all) as much as. Camber is -2.5* on front. I'll be going to 3* soon. That also "tucks" the wheel in the fender, and may help with rubbing issues.
2) Ohlins R&T.
3) C2
4) No rubbing at approximately GT3 ride height.
5) RSS thrust arm pucks. Caster approximately 7-8*. I don't think more caster is a bad thing.
6) Stock lightweights at 8"R 10"F. 245F and 275R tires (NT01).
For height reference:

The Radium King

thanks for the props. i'm no suspension expert btw, and hope some chime in. i think the caster impacts are from widening the lcas and not so much from the lowering.

my setup (on a boxster; same as a 996) has non-adjustable lcas, so i am getting almost -3 with just camber plates. i am on 8.5" ET 53 18" wheels running 245/40 tires. w a 7mm spacer i was getting some inner rubbing (which i fixed with a heat gun) but went to 15 mm spacers and have good clearance all around.

i think most of your rubbing is due to the caster. otherwise, note that fitment will depend on your offset, diameters, widths, camber, ride height, suspension type (coilovers vs struts will have different inner tolerances). also think about your setup and how you achieve your camber - pushing the bottom out will widen track (good) but impact outer tolerances (and caster if not adjusted for). pulling the top in will impact inner tolerances. spacers (for looks or to achieve fitment) push the wheel out from the hub and can impact suspension geometries (scrub radius i think it is called) as would an aggressive ET on a wheel (what is an aggressive ET but a built-in spacer?). you can achieve the same effect as spacers by widening your control arms. most gt3 lcas can go out a max of about 16 mm - you look to be there; with your strut tops full in i expect you to have some significant camber currently. if you are spending money you can get an effective camber gauge on ebay for small $.

cds72911

Thanks, all.

Moburki, I'll add camber plates to my list of possible options too. That would allow me to move the top inward to help with clearance and potentially dial out some caster.

The Radium King, I suspect you are dead on that most of the rubbing is caster caused by the lengthened LCAs, but the additional spacing from the shims is probably also adding to rubbing against the outside of the wheel arch (since it pushes the knuckle and everything else outboard by the same amount).

I measured 17 (passenger side) and 21 mm (driver side) of spacers with a good digital caliper, so that tells you how far out I am. That seems to be a crap ton...

I'll take offset out of the picture, since the twists that didn't rub and these rims are the same offset. Whew, one less variable. The twists are narrower though, so the tires are too.

The post about cheap ebay tools got me thinking about quantifying things "as-is", so I can see how to tweak them, and measure the impact. I have tools that I used for race cars in the past. Aluminum toe plates, measuring tapes, home build camber bars and a digital (Wixey) angle gauge (collecting dust somewhere in my shop). I should be able to measure caster (and toe and camber), right? I wonder if it is "close enough" to zero the angle gauge to vertical (either a bubble level or laser), and then measure the angle of the strut by attaching the angle gauge to the side of the strut. I think caster is based on the centerline of the shaft, but the wall of the strut should be roughly parallel (I would think). This might give me a rough idea.???

Who knew it'd be such a multi dimensional puzzle.

Flewis763

Have you rolled your fenders?

cds72911

I don't know if this is a legit way to measure it, but if it is, I am working with roughly 10 degrees of caster.


(And yes, it is filthy)

cds72911

Not yet. Fender roller is on its way from another Rennlister.

Porschetech3

Seems to me the logical solution is to install adjustable "tuning fork" control arms. Lengthening the LCAs with fixed tuning forks causes a pull toward the front increasing caster, using an adjustable tuning fork you can keep the caster (and also the torsion on the inner mount bushing) as it was.

cds72911

Porschetech3, I agree, that would dial out caster, but I may still be dealing with a wheel that is going to hit the fender because the LCAs are so long.

I'm leaning toward removing some LCA shims, and readjusting the toe to see how it feels.

Flewis763

Roll fenders, get some adjustable solid thrust arm bushing. Make your adjustments and be done.
Fyi I run 275/35/18 on the front of mine so you can go pretty big with proper setup

cds72911

Win #1 for the day: the LCA do have offset holes. They are oriented toward the rear, per Porsche spec, but I will move them to opposite sides of the car to gain some caster. Lets see what this gets us.

The Radium King

i think one of the problems with understanding caster is that everyone uses a different name for the arms in question; tuning fork, caster arm, thrust arm, trailing arm, tension/compression link, etc. (porsche calls it a ... control arm). i think the proper name should be caster arm (that's what it adjusts) but the bushing it is attached to is called the thrust arm bushing, so i try to keep it constant and call it the thrust arm.

an adjustable thrust arm is $305 each, so $610 total. an adjustable thrust arm bushing is $100 each, or $200 total. that's a big difference, unless you need the added adjustability that comes with an adjustable control arm (doubt it in this case?). now, the effort to swap out the thrust arm is much less than the bushing i think (original bushing has to be pressed out which requires removing the control arm?) so may be some saving in labour. but again, make sure you don't have some unused adjustments in your current bushing.

and what's with the different shims on either side of the car? set up for a nascar oval?

factory caster specs are 8 degrees +/- 30'

Porschetech3

Yea, Porsche just calls them "control arms", like they are the only control arms on the car???lol.. I actually like calling them "diagonal control arms", but no body else seems to call them that, so they don't know what I'm talking about.

I actually bought some cheap (read heavy) adjustable control arms when my factory ones started to make noise from the joint wearing. I probably won't ever need to adjust them, but they were half the cost of the factory control arms.