steveP911

Primary Issue with Current Suspension Setup

Instability during high speed (threshold) braking (over 100 mph). Car tends to yaw a bit (see video and look for yawing starting at brake marker 4), forcing me to modulate (reduce) braking and counter steer. This affects turn-in point consistency and ability to improve trail braking toward apex. I have been told that this is somewhat normal, but I would like to try and minimize it or get rid of it through suspension improvements. Hoping that there are some here that might offer some tried and true advice. I currently don’t have other handling issues with the car, but if this issue can be resolved while improving the car's cornering performance, all the better.


Current Suspension Status (if not listed, it’s OEM stock)

All

· Ohlins R&T coilovers (standard springs and valving); typically set at 2-5 turns from full stiff

· Apex SM-10 wheels, 18”

· GiroDisc 350mm rotors & Ferodo DS3.12 pads (OEM PCCB calipers)

Front

· Tarett Camber Plates (996FSMT)

· Camber setting approx. neg 3°

· Tarett Drop Links (996FDLNK)

· Toe setting approx. 1’-2’ total (in)

· Tarett Sway Bar (997FSBK-GTS); setting at mid position

· 255/35-18 Falken RT660 tires (basically new); hot pressure approx. 34 psi

Rear

· Camber setting approx. neg 2-1/2°

· Tarett Drop Links (996RDLNK)

· Toe setting approx. 5’-10’ total (in)

· Tarett Sway Bar (997.1RSBK-OEM); setting at soft position

· 315/30-18 Falken RT660 tires (basically new); hot pressure approx. 35 psi

Possible Suspension Improvements – are these piecemeal and not well integrated? What is a better/best value set of mods?

1. Replace any/all stock bushings with new versions or non-elastomer versions?

2. Install GT3 style Lower Control Arms in front (longer version?) with non-elastomer bushings.

3. Install GT3 style Lower Control Arms in rear (standard version?) with non-elastomer bushings.

4. Install adjustable rear Toe Link Arm/Link (I like the package from RSS that includes front LCA’s and Toe Link (https://soulpp.com/product/porsche-9...age-1-package/).

5. Adjustable caster arms (https://www.elephantracing.com/porsc...ive-anti-squat). I have not used any Elephant Racing products, but their product descriptions seem very good.

6. Update Ohlins R&T coilovers with spring rates and valving more appropriate for track.

7. Install GT3 brake master cylinder upgrade (but does this create a PSM fault?).

Thanks very much for your time - hope it's not too long of a read.

Steve

ProCoach

Looks like two separate issues to me.

The instability you are having on brake application is not out of hand at all.

It happens so far before turn in that is shouldn't/won't affect that or any aspect of cornering.

The brake zone is downhill. The road is not perfectly smooth or flat.

Unless you want to tie the suspension down so much that it makes it nearly impossible to drive on the street, you're always going to have some of that.

Does not appear to be an issue preventing the car from going faster.

Veloce Raptor

You might consider one or all of the following: adding just a little more toe in in the rear, lowering rear ride height a little bit, and braking 10% less intensely in order to keep the rear a little more planted. An added benefit of the last one is that you will also get a little more braking power out of the rear tires, which will probably shorten your braking zones

steveP911

Thanks Peter. Yes, I have been told by several folks that it can be considered somewhat "normal". I was getting used to it and then I rode in my instructor's M3 and it was dead steady there, so I am back to wondering if there are things I could do.... You are correct, turn-in at Summit Main T1 is not affected by this. But it is an issue for me at T10 to the point where I have to be pretty much fully off brake when I turn in. I am only driving the car on the street now just to keep it exercised. But you are right, if it is too tight it would be unbearable on these roads.

"Does not appear to be an issue preventing the car from going faster". I think I know what that means.

Cheers,
Steve

steveP911

Thanks VR. I was thinking of the rear toe increase, but I hadn't thought about lowering the rear a little bit more. I can see why that could help some with some lessening of forward weight transfer. I have tried to develop a better initial braking profile to minimize upsetting things but I like your point about the overall intensity reduction. Makes sense. I know that I am getting too much transfer to the front.

Thanks for your time and expertise!

Steve

jgiannone

Removing all of the rubber bushings is good, particularly in the area of a rear-engined 911. With the stock rubber bushings, you get a lot of unwanted toe changes in compression and rebound, magnified by running sticky tires on soft springs. Add the stiffer springs. It helps to control all of the camber and toe changes.Make sure that under rebound (heavy braking) the rear isn't toeing out.

Nickshu

My track modified 996 Carrera and 996 GT3 both did this under braking. Same chassis and suspension design as your 997. I did a few things things to try to improve it on the Carrera and several things on the GT3 and was never able to dial it out. On the GT3 I ran two different spring weights, 3-way JRZs, PMNA RSR suspension arms, and two different differentials (along w/ various alignment tweaks). It was still there and if you hit it right it was rather unsettling. In the end I decided I just needed bigger ***** LOL as the car was going to yaw but it wasn't going to come around on me. I did notice eventually that if I changed my braking style to a much harder initial punch it reduced it dramatically. Try manhandling the initial braking on those high speed entries, for me I was able to "brake through" much of the rear end wiggle by changing my braking style. I think these cars just tend to toe out in the rear under braking.

Frank 993 C4S

To the OP - Do you have a limited slip diff?

ProCoach

Sure like what jgiannone, NickShu and Frank have to say.

One of the major issues with this platform is how well and from what state you begin the pitch motion (change weight distribution from the back to) forward under initial braking.

Many drivers tend to inadvertently breathe off the throttle in anticipation of braking. The idea of “going to have to slow anyway, so let’s start now.”

This creates a very unstable (floaty, unloaded) platform to introduce proper braking application, which should be as quick as you can and biased with the greatest pressure initially, so you can bleed off and have more options deeper in the brake zone and finishing after you bend in to the corner entry.

If a driver accelerates fully to the point where they brake, and there’s very little time between positive thrust forward and negative thrust decelerating, the car likes that. It’s loaded, then loaded. Happy.

The car ALWAYS likes to be TOLD what to do…

OTOH, if you breathe out of the throttle (long g goes to zero or slightly negative, low value equals light coast, higher negative value equals heavy coast, not to exceed -0.3g, beyond which is for sure braking), the car gets floaty or the weight distribution is not loaded anywhere.

Shocks don’t work well, bars, springs and bushings don’t load up well, the car is just not ready to accept a big, forceful weight distribution change.

The unstable state is exacerbated by extended coasting after letting off throttle before applying the brake.

As @Nickshu points out well, his purposeful application helped reduce this feeling of the car having a load in its pants.

This is also the reason why accelerating to a brake point is always better than worrying about just the brake points. For any zone.

mikemessi

I have a base 997.2 with upgraded 997.1 turbo brakes, 997.2 GT3 sway bars, Gt3 lower control arms, bilstein pss9 coilovers and corner balanced. I struggled with rear instability (rear yawing to the right) since the car was new (stock) and persisted through all of the above mods. When I added Tarret rear upper dogbones eliminating the rubber bushings it completely transformed the car. I can't stress enough how much it changed the car. I played with alignment for a year before doing the upper dogbones and am convinced the dynamic changes in rear toe under braking was the root cause of all my problems.

steveP911

No, I do not. My shop guy (who I trust) has been telling me that will help with this issue and others. But I can't manage that $ this winter. Maybe next year. He also said that the OEM LSD's do not really hold up and the only way to go is with a race brand like Guard, Holinger, or Drexler.

Thanks!
Steve

Norge911

Before you do any suspension improvements, install a Guards 60/40 Limited-slip diff. On the 997 there should be allocated space in transmission, ie lower engine and transmission 3-4”, remove cover plate on transmission and install diff. 4-6 hrs of work.

this should improve stability a lot during braking and give you lots more capability when exiting curves.

with new diff, you can turn off all stability control in dry conditions.

Frank 993 C4S

Depending on your type of 997 Guard, Holinger or Drexler do make an LSD for your car. If it's PDK I believe only Wavetrac and maybe OS Giken is a possible solution.

Clarification: since you have a 2005 997.1 Guard seems to make an LSD for that:

Limited Slip Differentials | Guard Transmission (guardtransmissionllc.com)

steveP911

Thanks, Frank! I do have a manual. Thanks for the Guard link.

Frank 993 C4S

This will solve your "problems" (personally, I do not see a major problem in your video), make the car more stable under braking and corner exit.