Street Touring Hatchback (STH) discussion/setup

[Mini7]

I’m not hacking up the strut.

Holding out for a slimmer end link

Ordered a set of Power Grids today. Will keep you guys posted when I get them fitted.
Failing that Eurocodes. FKs are decent quality rode ends.

 

[xXDavidCXx]

Quick notes from extra Solo + fun runs.

Low tire pressure bad (RE71R)

Hot tires bad (RE71R)

High pressure, cool tires good (RE71R)

1/8 total rear toe out, not enough, but better

Bested my fastest competition time (low pressures) with 3 passengers and high pressures

Was 10th in raw time

Will run 44/40 psi at future hot days

Today was in the 90s, tomorrow will be 60s

Drove @aroundomaha’s car with similar recommended higher pressures, gained 1.8 seconds over his best time, then with some coaching, he gained 1 second.

 

[bfury5]

Installed the RS3 ducts before today's event, but still struggling with front brake temps. Rotors are basically smoking at the end of 3 runs, but pads + fluid are fine the whole time.

At this point I think it's one (or a combination) of the following things:

1. Brake bias - I knew adding the 986 brakes would bias the brakes slightly more front than OEM due to the smaller piston area compared to the non-pp caliper
2. Brake pad - maybe I'm just running too aggressive of a pad? Will have to look into getting something softer maybe
3. Rotors - I put on some cheap no-name blanks when I installed the 986 brakes and new pads, and maybe the vane structure inside is worse than a name brand centric blank or stoptech slotted?

 

[Mini7]

Rotor mass, curved directional vanes so the rotors act as an air pump and metallurgy all play a part.

I chose 330x32powerbrake rotor to fit under 17” wheels. Smaller than what you are running in terms of a heat sink, but with good vane design and I’m told great metallurgy from a small family owned foundry in Italy. I do not run super aggressive pads (driver defendant). My preference is for easy modulation for trail-braking finesse. According to the temp paint on the rotors, they are seeing 1000+F. My brakes are great. I’ve run a full 60-min session at VIR with no issues. Super consistent no fade. Slicks are going to place an additional heat burden on the brakes and accelerate rotor and pad wear.

What pads are you running?
Get some temp strips on the inner face of the caliper. Rotor paint is stupid expensive but a good tool to dial in your pad selection.

 

[bfury5]

Rotor mass, curved directional vanes so the rotors act as an air pump and metallurgy all play a part.

I chose 330x32powerbrake rotor to fit under 17” wheels. Smaller than what you are running in terms of a heat sink, but with good vane design and I’m told great metallurgy from a small family owned foundry in Italy. I do not run super aggressive pads (driver defendant). My preference is for easy modulation for trail-braking finesse. According to the temp paint on the rotors, they are seeing 1000+F. My brakes are great. I’ve run a full 60-min session at VIR with no issues. Super consistent no fade. Slicks are going to place an additional heat burden on the brakes and accelerate rotor and pad wear.

What pads are you running?
Get some temp strips on the inner face of the caliper. Rotor paint is stupid expensive but a good tool to dial in your pad selection.

The 330x32 would be larger than what I'm running, the 986 calipers work with the non-pp 312mm rotors. I wanted to keep the brakes small to continue to use 17" wheels for track. Stoptech ST-40 would have fit, but cost more than 2x what I spent. The problem is the RS3 ducts barely have enough time around an Autox course to provide air to the brakes.

I'm running Porterfield R4S. Love the pads, even with the rotors as hot as they are I never lost braking ability, just seriously begin to worry about what effects the heat might have.

 

[GTIfan99]

The 330x32 would be larger than what I'm running, the 986 calipers work with the non-pp 312mm rotors. I wanted to keep the brakes small to continue to use 17" wheels for track. Stoptech ST-40 would have fit, but cost more than 2x what I spent. The problem is the RS3 ducts barely have enough time around an Autox course to provide air to the brakes.

I'm running Porterfield R4S. Love the pads, even with the rotors as hot as they are I never lost braking ability, just seriously begin to worry about what effects the heat might have.

Have your turned xds off?

 

[bfury5]

Have your turned xds off?

Yeah, been off for a while (in anticipation of future LSD)

 

[xXDavidCXx]

Good for second place and 9th in raw time

Cold and wet but drying conditions, ran 31 psi front/rear and never had any heat in the tires. Rear shocks at 17 from stiff. Stability control intervened a few times to keep from spinning out.

 

[GTIfan99]

Yeah, been off for a while (in anticipation of future LSD)

Interesting that you're getting that kind of heat autocrossing, especially with xds off. My 2018 S would cook the brakes, tires, wheels, bearing, to the point I was worried about damage. Last event before replacing her with the 19, I ran with xds off (this was GS setup and know it wasn't legal, but was it's last event and wanted to see what kind of difference it made for science). It was night an day.

I ran the 19 in GS for 6 months and didn't have brake heat issues when with xds, so I'm guessing it's the combination of xds and no lsd/vaq that caused the heat.

When was the last time you checked the xds setting?

 

[Mini7]

The 330x32 would be larger than what I'm running, the 986 calipers work with the non-pp 312mm rotors. I wanted to keep the brakes small to continue to use 17" wheels for track. Stoptech ST-40 would have fit, but cost more than 2x what I spent. The problem is the RS3 ducts barely have enough time around an Autox course to provide air to the brakes.

I'm running Porterfield R4S. Love the pads, even with the rotors as hot as they are I never lost braking ability, just seriously begin to worry about what effects the heat might have.

My bad, I assumed a Macan setup when I saw Porsche calipers. Still surprised that Auto-X will cook the brakes. Lower speeds and lack of airflow combined with continuous use and lack of straights to cool down.

 

[bfury5]

Interesting that you're getting that kind of heat autocrossing, especially with xds off. My 2018 S would cook the brakes, tires, wheels, bearing, to the point I was worried about damage. Last event before replacing her with the 19, I ran with xds off (this was GS setup and know it wasn't legal, but was it's last event and wanted to see what kind of difference it made for science). It was night an day.

I ran the 19 in GS for 6 months and didn't have brake heat issues when with xds, so I'm guessing it's the combination of xds and no lsd/vaq that caused the heat.

When was the last time you checked the xds setting?

Checked them yesterday during the even just to make sure. Another thing that's probably attributing a lot is how wide open our courses have been. With events here limited to 50 people, the courses are much more open than usual so workers don't have to run all over the place for cones. I've hit the top of 2nd a few times and most of those stretches were followed by pretty tight corners. I also find I dig really deep into braking zones, so it could be a little my fault as well.

 

[JackRabbitSLIM]

I'm considering higher front pressures to act as more spring, to keep the tire contact patch from being overloaded, in the same way that Jackrabbit suggested that we try higher front spring rates to make the front "happy".

Tire spring rates are really high even at low tire pressures, so "adding spring rate" with tire pressure doesn't really make sense in terms of roll rate. Yes it will change the handling, but not because of the spring rate.

Adding more actual spring rate to the front will keep the front tire from being instantly overloaded. The front tires on these cars are always going to be overloaded not matter what, the trick is to get the timing right. You want the rear to do something before front starts grinding.

 

[bfury5]

With 1000lbf of corner weight, a 255/40/18 tire at 35PSI has a spring rate of 6730 lbf/in.

Just wondering where you got that number from? Everything I've read about tire stiffness indicates that they don't have a linear rate so I'm curious how that springrate was calculated

 

[bfury5]

Correct, it's not linear. I just chose 1000lbf because that's the front corner weight. I don't remember where the info came from but I made a calculator a while back.

(This graph is only for 255/40/18 at 35psi)
View attachment 187214

I'm intrigued as to why that graph is measured in lbf and not deflection, as that's usually a much better indicator of springrate. I'll have to do some research.

 

[bfury5]

This is just one example, but hoosier lists springrate data for their FSAE tires. As you can see, at a given PSI as load increased so does springrate of the tire. Comparing different PSI at the same tire load shows an increase in springrate of the tire, as expected.

https://www.hoosiertire.com/contingency_rates/fsae/