RedRSA

Interested in experience of others using RSS rear control arms and 18" wheels. After my first track day I swapped tires side to side and upon inspection noticed that there was contact between the inside wheel edge/wheel weight and the bottom rear brake duct. Very surprising and disturbing, but at the same time, educational.

For one thing, the geometry of the rear suspension is remarkable at keeping the tire more parallel to the track surface than on a camber-changing arc of older/less sophisticated designs. The unexpected consequence, however, is that the seemingly fixed relationship between the wheel and control arm changes: the clearance between the inner wheel edge and the control arm/brake duct changes through the suspension travel. Under compression, the gap is narrowed. Easy to see: when the wheel is jacked off the ground there is about 1" of gap, and when on the ground the gap is reduced to under 1/2"; under full compression the gap is reduced further.

I recalled that the RSS control arms are bigger (thicker top to bottom) than the stock parts - I will take and report measurements - which eats up some of the factory clearance, and switching to 18" track wheels reduces the available clearance by another 1/2". The combination results in contact with the brake ducts mounted on the bottom of the control arms.

1. Has anyone else using RSS rear control arms and 18" wheels experienced this?

2. Does it only happen with the PASM Sport button off? I started by not using PASM Sport, but after seeing this used it to limit travel. When activating the Sport button the contact area did not increase, but I don't know if using it from the beginning would have prevented it.

3. On rougher tracks were you need the softer suspension and also absorb more dramatic suspension movement, how do you keep the wheels and weight from wearing through the brake ducts?

4. Has anyone found an effective way to modify the brake ducts, such as making a recessed area by cutting away some if the contact point and molding in some sort of insert to remove contact while retaining full air flow to the brake? Any other effective solutions?

Thank you!

paver

It happened the first time i used 18's...OZ's. That was with stock control arms. Different wheel now but it seems like I was able to get the tire shop to move the weight enough not to hit.
On a side note I put some of those caliper temperature indicator stickers on for Road Atlanta last weekend. Lost a rear brake duct three weeks ago and wasnt able to get a new one on in time. Was surprised to see the temp indicator showed the same on both sides. Maybe they are no good? Looks line they move in twenty ° increments.

RedRSA

Wow; a little surprised there was contact using the stock control arm, too. In my case with the RSS control arms, there is contact with the wheel itself as well as the weight, so the RSS control arm is enough thicker to make contact regardless of weight position.

Interesting to hear that the temp strips didn't show much difference, but a little more investigation is required before assuming that the brake ducts are not effective. For example, not sure how accurate those strips actually read (I use the ones from Alcon) and while heat obviously transfers to the caliper, it would be more telling to compare the rotor temps immediately after the session. The best test would be to measure the air flow through the duct, but that is beyond my capability and (in general) I rely on Porsche engineering to only put things on the car if they have been proven effective.

With all the owners using RSS control arms and 18" wheels, someone has likely discovered the problem and developed an elegant solution. Being efficient (aka lazy) I would prefer not to reinvent...! ;-)

paver

Not sure if I was clear, but I wasn't saying the ducts don't do anything. I was questioning the accuracy of the temp strips. I just looked at them again and I believe they register in 40 degree F increments so it's possible one caliper was several degrees hotter and didn't move the indicator up to the next stop.

RedRSA

Ahhh, gotcha. I found the temp strips to be relatively helpful in comparing readings from caliper to caliper, but put less stock in their reading accuracy.

Spyerx

Interesting, I've a .1 using the .2 rear cooling ducts with stock arms (modified with solid pucks, mono ball links, etc) and have not noticed any rubbing with 18" wheels. I do have some rubbing on the front cooling ducts and have trimmed them to minimize under full lock (it would click on the wheel weights)... no issues with 18" wheels.

Question: did you need to modify the clamp on the rear ducts to get them to fit on the RSS arms? I seem to recall they don't fit exact. On my arms, I have the ducts locked up closer to the inner monoball/pivot, still shooting the air at the lower part of the rotor, but not 'right on the rotor'.

Wondering if moving it inside some will will remove some of the rubbing. Do you have a picture of where it is rubbing on the duct?

RedRSA

The RSS control arms are enough taller/bulkier that the stock brackets will not fit, you need to use RSS-unique brackets. I don't recall having much variability in the mounting position. I took some photos and will post when I can get them transferred.

modoz61

I haven't had the issue my 18" wheels are BBS perhaps there is something in the actual wheel profile?

mooty

no prob with my BBS as well.

you might chk with 5 lug guru, doug. he had narrow body car with forgeline, i do not recall 19 or 18" however

RedRSA

Thanks; standard wheel size and profile, though I'm sure there could be variances between brands. These Finspeed wheels were selected because they provided more rotor clearance than the other wheels I tried.

Thx, JH. The issue should be hub-independent. FYI, per the article I think Doug went with 19" wheels; if so, he wouldn't have the issue.

Terry L

Assuming the brake ducts are the scoops which ride on the control arms, if the car is lowered, the rear wheels will eat the ducts until they are worn away enough. I ran through two sets before giving up. This was true for stock 19" wheels as well as CCW and Fikse 18s. This is a zen thing - you learn to accept such imperfections.

RedRSA

Zen and the art of racing…there must be a book about that somewhere! :-)

Yes, I'm referring to the brake ducts/scoops that mount to the bottom of the control arms. There must be a way to modify them. I wonder if they can be warmed and formed with a heat gun?

mojom3

What he said. I run both CCWand Fikse 18s on a .1 GT3 and both rub with the factory control arms. They haven't worn through yet, but when they do they will still direct most of the air flow to the rotor,assuming you trimmed the backing plate. If memory serves, I saw 30 degree diff between rotors when testing using a laser non-contact pyrometer on the rotor, one side with duct, the other side without. Enough to keep the ducts on.

RedRSA

Good reference point, thank you. What "backing plate" are you referring to needing to be trimmed? I don't recall there being anything that would block the airflow from the either the upper or lower brake ducts.

Spyerx

On .1 cars the thin metal backing plate behind rotor needs to be trimmed a bit for best air flow.