I finally had the chance to take the car to the track last weekend and it was great - except for one thing.

The rear wanders under braking.
It feels like its wants to spin, but it still turns in fine.
None-the-less; super disconcerting since the car doesn't feel planted

Posting here in case this is a known thing with all of the 981/982 cars and because track people understand track problems.

Car:
718 GT4 w/ PDK

Setup:
Alignment (w/ driver weight):
Front: -3.0 w/ 0.9mm out per side
Rear: -2.4 w/ 2.2mm in per side; TPC toe link

Wheel: 19x9, 19x11
Tire: GY SC3R 265/305

Sway:
Middle / Middle

Ride height:
left it "stock" - whatever the factory or dealer set it to; and I'm not sure how to measure it either

Brakes:
Ferodo DS 3.12 front and rear
Brembo Slotted Rotors

Buttons:
all of the center console lights on
PDK Sport - light on
Shocks - light on
ESC Off - light on
ESC + TC Off - light on
Exhaust - light on
Autostart - light on


So far I've been told:

1. its the bad dynamic toe; it goes out in full droop; you should add more static toe in
2. you must have installed the toe links offset bushings backwards
3. car still has some kind of stability enabled and the car is modulating the rear brakes side-to-side
4. pdk car has something different with the diff
5. its the diff; you need to buy a fancy new diff
6. don't brake so much

The last one (#6) is probably true! - but I'm not so sure about the others. Please feel free to refute or add to this list! I'd love to solve this one.

Local folks: ThunderHill East - felt the most braking for 10, 14, and 1

Otherwise the car really impressed me. Amazing track car right out of the box.

Video:



Look at "actual" Green means 'in porsche spec' Red means 'out of porsche spec'



left

right



Edit 1: added brake pads info
Edit 2: added alignment photo, link to videos
Edit 3: added TPC link photos; small side of offset bushing faces forward

 

It is a known problem and usually aftermarket rear toe links plus 2-3 mm rear toe per side fixes it. It's not the diff if you are just starting to track it and if the toe links were reversed you could see this issue. With Tarett links I ran -3 mm rear toe, with TPC I ran -2

 

Also consider going zero toe in the front

 

Known issue with 981 chassis anyway. One recommendation is to go with a slightly less aggressive pad in the rear.

 

Yes, and the 981 version also benefited from a slightly unorthodox braking technique in heavy brake zones

 

I think you are braking too hard

 

We chased this on a 981 CS version. Given what we found would start with different brake compounds between front and rear to effectively change the bias. You likely need less braking in the rear for what you are getting in the front. For us, a bias adjustment and guard diff solved the problem.

 

Assuming the reasons behind this are the same with the 718 as they are with the 981, the DSC Sport controller is the other important piece of the puzzle that compliments the TPC offset rear toe links.

I came to my 981 GT4 from a 991 Carrera S, and found this unsettled behavior under threshold braking on track very unnerving. Part of the difference I felt was due to the fact that the rear-weight biased 911 is so inherently stable under braking, and so I was accustomed to being able to brake late and hard with complete stability with the 911. The GT4 requires a different approach under braking. (As did my first track car, a 2011 E90 M3 sedan, which also danced around in the rear under heavy braking.) Aside from the weight distribution, though, is what you pinpoint--the issue with rear toe changes as the back end unweights under braking in the GT4. The TPC offset rear toe links certainly help to reduce toe changes, but I believe it's the DSC controller that really does the trick in keeping the back end planted under heavy braking. Once I replaced my GT4's Tarett rear toe links with the TPC links and the DSC Sport controller, and I cleaned up my braking technique a bit, the car became much, much more stable under braking.

 

The unsettled rear gets better with the toe links.

Ultimately I started to trust the car and accepted it; it moves a bit but even in severe braking areas like Sebring T7 it still stayed straight

 

Thanks for all of the tips!
I really wish I had my AiM setup working for this event. I'm curious what the rear wheel speed sensors say.. if its a lockup / abs hunting issue then I may be able to see it in the data.
The car really does seem like it wants to kill me in threshold braking; and that instability prevents me from turning in. When I ignore it and just turn, it turns fine.
I'll have data for the next event -- which I haven't scheduled, but I'll see if a Sonoma event is soon.

I'm going to get under the car this weekend and take a look. I'm kinda hoping they are backwards since that's an easy fix.

I can see how it would be less twitchy if I did this. And I had no issues with turn-in understeer - so worth a shot.

Several people making this recommendation.

Tell me more. My technique is to push on the brake pedal - and to do so early enough that I'm not triggering ABS.

You think you are joking .. but this is a fix.

You are now the third person to recommend a diff.

I'll look into the controller.

 

Have you tried 8-10 braking?

Works wonders for me in my 981S and got the rear under control easily by shifting some of the load forward at "8" for that split second before going to "10" on the brake.

 

This is exactly what I was talking about

 

This right here is what has made the biggest difference for me across all cars......and karting.

 

Thank you thank you thank you.....

I will be here all week, matinee on Wednesday.

Please take care of your servers and don't forget to try the veal...

 

Of all the people I've had in my right seat over the years, only 1 coach/instructor told me about 8-10 braking. Guess it's not a widely known or used braking technique. Not sure why.

The reason it works is because when you're coming hot into a hard braking zone and you go to threshold (10) braking right off the bat, all that load is getting thrown from the back of the car all over the place, and the rear end gets loose as it has no idea what direction to throw the weight in.

If you stop at 8 on the brake for a fraction of a second, and I mean a FRACTION of a second, it shifts some of that rear load forward to the center of the car.

Then, when you go to 10 on the brake (threshold), a lot of that weight that was loaded up in the rear is now in the center of the vehicle so the rear end does not get anywhere near as loose.

At 8 on the brake, you've already moved the weight forward, so the rear end stays planted when you hit 10 on the brake.

Works every time for me.

Practice it, but practice it when no one is on your rear bumper and you are safely distanced from any other car on the track, at least when you first try it.

Good luck and please report back on your progress OP.