Trail Braking and Rotation Questions
 

So just finished first two autox.
First weekend in FRS (RS3) and second in S2000. (RS3v2)

Totally different cars but one thing that bothers me is while the FRS is far more stable in the rear and putting power down is rarely an issue, I cannot get the car to trail brake with or without pedal dance

In terms of autox I'd say it critical.
So without changing much what are my options?

• Firm up rear rebound of shocks?
• Less rear toe?
• Rear sway bar?
• Brake Pad choices?

(If you had any questions about switching to the Sony Action Cam the S2000 video should convince you.)



S2000 Run:
[ame="https://www.youtube.com/watch?v=mnhplGM2F38"]S2000 Autox Test Hankook RS3 v2 | In Car Only - YouTube[/ame]





FRS Run:
[ame="https://www.youtube.com/watch?v=aDWA8YeknWY"]Supercharged FRS BRZ Autox Test | In Car Only - YouTube[/ame]

Do you mean the car is still actively messing with your braking or?

Also I see you are running RS v2, noticed any difference relative to v1 so far?

I don't feel the car interfering, it just feels like there is no rear brake bias or the rear won't unload enough to trail the ass around. Should I soften the rear?

The v2s got up to temp fast and did not have that rubbery feel to them like the old tires did when cold. But once the temps rose the tire pressures jumped almost 8PSI. (50F to 65F Outside) I had to keep the pressures low to keep them from feeling slippery.

Since I am still near full tread depth and its only my first autox on them, it's hard to make any conclusions. They are still a fast tire but for me they felt more slippery in the heat. Thats not scientific and I dont have track time on them yet.

Running same pads F/R?

What year AP2? Stock suspension?

S2k's have a slight rear bias :)

Sure does look good. That image stabilization is remarkable.

Actually no and thats one thing I am going to change before this weekend to test. Currently it runs HawkHP Front (supposedly worse than stock) and OEM rear.

For this weekend going to 1521 squared.

Completely stock car, aside from modified alignment, RS3 tires 225/255s. Carbo1521s Squared from you and old Centric Cyro Blanks Front.
2005 MY. (Pre DBW and TCS non-sense.)

If you saw how much the camera was vibrating on the mount its crazy how well it works. It does cut into the sharpness of the image but for car videos I will take loss of that over having stable footage.

Let's see you have wider tires in the rear than the front and you have more brake pad up front but oem in the rear. I have no idea why it won't rotate. ;)

You should have stuffed the 255s up front :)

That was the S2000 tire specs.

FRS has 235 squared.

interesting....I find my FRS quite loose.....almost too much really....but the stock tires don't help either.... If you need more rotation you can adjust tire pressures by increasing the end you want less traction, but that could hurt putting power down too

You could try aligment....a touch more toe out up front should help rotate the car...or less toe in out back

Take off the front sway bar. If you're trying to induce rotation with trail braking and it's not happening then you can make the front work better or the rear work worse. It's RWD so don't make the rear work worse. Take off front sway which should allow the front to work its contact patch better. Also make sure you have enough camber to maximize front contact patch. The S2k has a pretty aggressive dynamic camber gain up front, the FRS has almost none so it will probably need more static camber to do the same thing.


Without going into a technical dissertation on shocks and springs I will just say, set your shock valving to accommodate spring rate, after that leave them alone. Do your setup aydjustments with ride height, spring rate, sways and alignment.

What swaybars are on the car now? What are the spring rates on the Ohlins? Defiantly use same pads front and back.

Is your FRS loose, your steering input too rough, or are you just too aggressive with the gas?

The FRS understeers at terminal cornering speeds, regardless of tires. If *just* a tire change magically eliminates your oversteering, then the oversteer was driver induced; you don't change the balance or dynamics of the car with just a tire change.

I will attempt to disconnect it, if going with square pads don't do anything.
I feel like the damper settings are very good. I have a tone of grip and stability. I attached my alignment, with my driver weight and gear on.

I went back to stock sway bars because Ohlins did all their design with that in mind.

Spring rates are 343lbs all corners.

You don't have to remove it, just disconnect one side. Nice thing is you don't have to monkey with alignment with this one. I guess the same applies to pads.

I'm just curious if you find the same issues when you are on a road track versus autoX ...

I haven't done any AutoX, only NASA road courses. I wonder if the lower speeds and sharper corners in AutoX magnify these issues.

The car is more lively on a road course. Car tends to get more unstable in the rear under hard braking. And the rear of the car rotates without under-steer in most corners. But in higher speed sweepers you get understeer before oversteer.

In autox, only thing I can say is in the case of the S2000, your limit before snap oversteer is a fine line, that limit is hard to read and sometimes it comes on by a simple pavement change and not throttle.

The FRS you can be almost go flat and when the rear starts to go its does not just go, it starts almost bucking a bit so you can drive through it, or back off.

Hearing this, I like the idea of removing the front sway bar on the FR-S ... Give the front a bit more ability to work dynamically versus being possibly too stiff ...

I'm not an expert by any means, but it seems like a good suggestion that was made.

I wouldn't worry about the fact that they were designed around the stock bars. Increasing stiffness in roll evenly front and rear leaving room for adjustment to dial in for personal preference is a better route in my opinion. Spring rates are set for the desired ride frequency they were looking for. With those rates I would guess is around 1.9-2.0 Hz, someone correct me if I am wrong. Not a terribly aggressive number. Proper bar size depends on a lot of things but you basically want to stiffen the car to give enough roll at maximum grip to give you a proper contact patch depending on the amount of initial camber you are running while keeping everything in balance. A production car is going to need roll stiffness increased to get it right as you would get to a point where your springs would cause you to skate over bumps if they were stiff enough to counter roll well enough.

Don't unhook the front bar. The roll in the front would be horrible.

Watching that video again, that course is horribly tight. I noticed at least one point where it looked like you had to much steering input. The car was pushing and bit as you exited and let out steering input. It is really hard to get a good feeling for setup on a course like that. We have local events where the courses are like that and I don't put any thought into setup from them.

I agree with this. I mean, it's easy enough to give it a shot, but consider that you would be softer in roll in the front with the Ohlins without a front bar than you would be stock. The stock front bar contributes ~262 lb/in to wheel rate in roll, in a stock FRS that is 70% of the total front roll resistance. With your spring rates the majority of the front roll rate is still coming from the bar, ~57%. I would also imagine it would make the car quite tail happy.

Looking at the ratio of the natural frequencies of the front and rear during roll (bars and springs contributing) can give a reasonable indication of the under/oversteer balance of the car - a stock BRZ has a front/rear ratio of ~1.24, a stock FRS comes in about ~1.15 (looser), the most successful STX car last year was ~1.50 (tighter), the most successful CS RTR car was ~1.55 (tighter)... your 343 front rear with no front bar would be at ~0.92 - I would think that would be bordering on very difficult to drive but on a lot as small and tight as that video maybe it wouldn't be as difficult. But I also would think it would do bad things for putting power down.

Your first line says first two autocross. If you are still new to autocross, trailbraking should be the last of the things you should be thinking about. Learn hard straight line braking first. Trailbraking is a skill you can learn later.

From what I understand, having two different brake compounds front and rear , especially when the rear is stock makes the front brakes work super hard. Maybe explains why the lack of trail braking for you.

I ran HP+ on a dba slotted rotor in the front and stock pads/rotors on the rear and a drive on a road course turned my fronts blue.

I'm pretty sure he means the first 2 autocross sessions of the season, not ever.

Just a couple thoughts/guesses...

1) You might need more front camber relative to the rear, have you been monitoring temps across the tires, or checking for wear onto the shoulders?

2) Maybe try playing with pressures?

3) Are you just entering too hot? If you're still carrying too much speed the car will just plow instead of rotating, the fronts need grip for the car to rotate.

@Dezoris

More front negative camber. Most autocross setups I see on the twins, including mine, and cars at nationals target -3.0F / -1.5R as a good all-around setup for the twins. From there you fine-tune to your driving style and tire, and add the desired toe out in the front and toe in in the rear. The added camber in the front will make the car more willing to rotate by increasing lateral grip in the front during corner entry and mid corner.

Also, have you checked how close your tires are wearing to the traction markers? It might give you a clue as to what camber adjustments you need. Also, I think you mentioned running 235 wide tires, what are they mounted on? Mounting too big a tire on a skinny wheel may not be letting the tire use enough of it's shoulder to maximize your grip.

Also, have you played with pressures? If the car feels like it's got too much rear grip when compared to the front, increase pressure in the rear or decrease in the front, depending on what your particular tires like.

May also want to look into the possibility of having lowered the car a bit too far/not enough spring rate/some combination thereof. Whacking the bumptstops does a pretty good job of turning the nose of the car into a plowing mess.

First two autos this season. Done it and track days countless times.

Going to swap pads Friday to test this weekend see if that's the issue.

Wear markers on front are perfect, however maybe could run 1psi more front. The rear I had to lower not enough roll. But something I will play with this weekend. Car does not plow really feels neutral even under heavy braking. It maybe that I am wanting a less balanced car and its too neutral but still fast.

I have hesitated going overboard with front camber but it seems like I am fighting against the truth of front struts on this car.

Running 17x8 wheels 45 offset. 235 should be good. My front traction marks are almost perfect. Rears not rolling enough because I raised pressures to see if it would help.


Car is only lowered 10mm, Ohlins suggested 25mm which is way too much for a street car in my area.

More camber up front. Definitely more camber up front. Especially since the rates you're running aren't terribly stiff.

Even pads all around as you mentioned you were planning on doing.

Do you feel like you're actually losing a significant amount of time? A little stability can be a good thing too. At least we're all talking about solutions that don't involve reducing rear grip.

- Andy

I think I can get about -.5 more out of the plates before having to lower more but not much more than that.

Let me just say the car is quick, very stable. This was my first time out with the Ohlins in autox and managed a top 10 finish and 2nd in class with some seasoned drivers. But where I am losing time is where I can't get the car to rotate through these tight sections under braking. Something that other RWD cars I have driven do well. S2000 obviously is a different animal however it's fast because you can trail brake with zero effort.

I can come in hot under braking and the car will start to rotate around the corner without having to steer my way through it.

So for the FRS I don't want to make changes that will de-stabilize the car however I was looking for ways to be able to dial in more rotation when I need it thats why I posted for feedback.

I would add more front camber, and drop the pressure a bit (unless you're already really low) up front.

I would start with the camber and pads.

A click or 2 firmer on the rear shocks would be fun and free to try.

- Andy

The sony cams picture quality doesn't look any better than a GoPro. Or am I missing something?

What tire pressures were you running the RS3's at?

drop rear tire pressure, increase rear dampening. I started noticing trail braking after this and it was sufficient enough to play around with.

dosen't even have to be a lot of stiffening in the rear. I realized it and it help with my times.

also my rear toe was set to zero and camber was -1.5

Image stabilization is leaps and bounds better. Can't really judge image quality on Youtube too well.

Found the best pressures were usually 33F HOT and 34R HOT

Something else to try.

So I think the issue is front Camber. (I have resisted)
Will -2.7Front be ok, because thats the max I can go before lowering more.

I swapped in the 1521s at all four corners and bedded them.

Lately I am annoyed with the Sprint Kit because the pedal is so firm with so little travel it makes it hard to modulate. I dont know if its because the pads are so damn tight with no piston travel or what. Any feedback?

My first two runs I had no brakes, it was absurd. Then they came on but never got any rotation. ZERO trail braking.


@SubiePig @Racecomp Engineering @wparsons @CSG Mike @rice_classic




Here is the video:

[ame="https://www.youtube.com/watch?v=Y9nxQyZRc9g"]Supercharged FRS Passenger AutoX Runs | In Car Only - YouTube[/ame]

I don't know if 2.7 will be enough with out a temp gauge and measuring but considering the Mac struts and what other's have mentioned, you should probably be going more aggressive than that.

At the 56 second mark (right hander), you are doing a combo; entering a bit too fast and then turning the wheel too much which accentuates the understeer already happening but as the car slows from scrubbing it then hooks up. Try disconnecting the sway. While the front end work better without maximizing contact patch (probably more camber), remove the front sway will allow more front end squat and may unload your inside rear enough to help rotation. Like I said, it's an easy thing to disconnect/reconnect. An easy thing to test.

As for the brakes. Having a rock hard brake pedal is a driver's dream. Having a brake pedal that uses a longer stroke, such as a smaller bore MC, is more difficult (for me) to modulate because the modulation is more of a result of pedal distance or movement vs pedal pressure. It's easier for my foot to modulate pressure when less pedal distance is involved.

Until that front end is working better, don't overcook your entry speed.


p.s. if you want to get loco up in hurrrr. Make your rear tires slightly toe out. That will definitely help rotation. ;)

@Dezoris your car is understeering quite a bit. I'd recommend you get QUITE a bit more camber. Start at -3.2, and don't lower the car too much.

As for the brakes, try altering your seating position to be 1 or 2 notches closer to the pedals and steering wheel.

Watching your recent AutoX videos make me SO thankful for the sites my region has available in NC/SC. We have 3 sites within a couple hours of each other that your supercharger would love. The lot you compete on seems very frustrating; especially if you have a supercharger. Do you have any sites that are larger and more open?

Anyway, back on topic. I compete with -2.7 degrees of camber upfront and -2.2 in the rear and I still get a fair amount of understeer myself. I have a Whiteline 20mm front sway bar that I initially set to stiff, but I am going to move it to the soft setting and see how that works before I alter my alignment. It seems that disconnecting the front sway completely would be a bit drastic for AutoX, but it would be an easy adjustment to make for experimental purposes.

Also, I definitely agree with riceclassic on the amount of steering input you're giving the car in the sweepers at times. Our brains naturally think, "hey, the car is not turning, so I must turn more!" and this definitely makes understeer worse and will put a lot of strain on your outside front tire. I suffer from this habit and it usually means I'm entering corners too hot. Entering corners a little easier and reducing your steering input should help, but I think more front camber and some shock adjustments will be necessary as well.