Unstable during braking carbotech xp10/xp8
 

Did my first track day last weekend on a proper race pads. I went with recommended xp10 fronts and xp8 at the rear for track days. Also have front z1 2 piece slotted rotors and stoptech sport rotors at the rear also slotted. Ever since changed to this setup, the car is very unstable during hard braking. I could feel it the first time I tried them during bedding on the street. It almost feels like the rear wants to step out in either direction. The bite is superb but I had to back off during braking because I totally lost confidence that I can keep it under control. For non track days I use nismo pads all around and do not get that sensation under hard braking whatsoever. The car stays and behaves as it should. I tried to search but could not find anyone else with similar issue. Is it possible that the brake bias is too much up front due mixed pads? My next step is to try same compound all around instead going step lower in the rear. Just not sure whether to go with xp10 or xp8. Any feedback is very welcome.
My current setup:
Front brakes air cooling ducts
Stock nismo shocks with swift springs
Front and rear whiteline swaybars
2.5 neg camber with zero toe up front, 1.5 neg camber with slight toe in at the rear
hankook rs3 275/305
os giken rear diff

You could try running XP8's all the way around. And adding a little toe in, in the front. How much toe in do have in the rear?

2nd the motion...going with xp8 all around.

I have, a similar set up with no issues during braking. I am running -2.6 front and -2.0 rear with a touch of toe on for both.

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I have the same "unstable" feeling, moving a little side to side on the hardest braking points. I think it's more to do with the very good pad+stock suspension with a side of abs kicking in. XP8s and XP10s all around didn't change it for me, still on the stock nismo suspension and springs.

Add more weight in the trunk. Keep spare tire and full tank of gas.

Not sure how much toe in at the rear but as far I can remember it wasn't much. It's been awhile since I did an alignment so I will do one this Saturday and see if anything changed. Still running stock toe bolts so maybe something moved. I will start with that before anything else.

Maybe little more camber in the rear would help. I get to do my own alignments so I could play with it with no extra cost. Also been thinking that possible extra brake bite causes more aggressive weight transfer towards the front which stock suspension can't handle. I see you upgraded to coilovers which I am sure could be a factor for more stable braking. I'm planning track day in two weeks so would love to figure this out before I get back on the track.

It could be a contributing factor you spun out during the rain at the thunderbolt. I could definitely feel it more loose during wet sessions. Very scary and no fun. What pads are you running now?

Did my fuel starvation fix so I can run less fuel!

Definitely think its an extreme weight transfer issue. I have the same carbotech set up, and since raising the front a bit to help with nose dive the issue is less apparent. Unrelated to this particular issue, I'm increasing my spring rates soon to a stiffer set up and think it should help by resisting nose dive as well.

That was just plain old going to fast, ran out of talent. Getting XP10/8 combo and trying out FrozenRotors blanks. Used up my current XP10/10 and Z1 slotted rotors last weekend.

I think that is the issue as well. It seems like I am not the only one so it makes me slightly less irritated. Just need to figure it out the proper setup. It seems like it's a nismo thing! :rolleyes:

If i recall correctly Zs have more weight up front than rear, so hard braking, especially with good pads, will do that. I now (after tracking) see why I was recommended 16/9 spring rates for the car back when I ordered my suspension.

Nonetheless conditions were horrible. Are you still planning for another track day this season? Im going most likely the 29th at thunderbolt. Let me know if you're interested. I'll send you the link.

Have you been monitoring tire pressures.

Okay, I am no where near stock on my suspension. I am running the SPL catalog, bushing and and arms. I also have a Wavetrak rear diff. My spring rates 13K front and 11K rear. I am running at the full height of the coil over which make it about an inch lower than stock. I have removed all the stuff out of the trunk and the latest change included detaching the rear sway bar. I have no issues at all with squirm at all even when braking from 130 down to about 40mph.

I assume you have SS brake lines and good fluid as well? I am sure stiffening up your suspension will help. As mentioned above tires and proper inflation will make a difference.

I experienced this feeling as well when running XP8's all the way around. Went to XP10 upfront and XP8 in the rear. I can't say it is any better with XP10/XP8...but it is certainly not worse. I am wondering about the the tire pressure recommendations and keeping the spare tire in the rear. I typically run at 40 PSI all the way around (taken immediately coming off track) and run with out the spare.
What are the recommendations on Front vs Rear Tire pressures...as well as with or without spare?

My alignment settings are as follows:
Front: -2.75 Camber; +.05 Toe with Total Toe = +.12

Rear: -2.5 Camber; +.15 Toe with Total Toe = +.35

What brand of tire are you running? I am running Toyo r888 and Nitto nt01...After asking others at the track and my own driving experience I try to keep my tire temps between 34 and 36psi hot after finishing a session. The r888 felt better to me with all tire temps being the same. On the nt01 I run two less pounds in the rear. I start the air pressures at 27psi and make adjustments from there. As I am getting better I will start measuring the tire temperature to make sure I can stay as close to the recommended tire temperature as possible.

Best thing to do is to keep a note book of all the changes you make. And record what each change feels like.

Tire pressures are very much dependent on alignment and weight balance. If you have more weight in the rear and additional negative camber i would keep the pressures the same front and rear no greater than 39psi hot

More camber and no additional weight - less pressure overall but less in the rear (this will induce a bit of understeer at first, drive accordingly)

Also like Hotrodz said, its dependent on tires too. More so the tread depth, design and sidewall profile.

Extremes or summer sports - higher cold pressures around 35-37psi.

Lower profile tires need less air for the same level of rigidity. Stock or larger profile tires require a bit more.

Also i personally don't recommend running low pressures under 30psi unless you know you will be putting a lot of force into them and have a stiff enough suspension to force pressure on to them like Rodz car.

Yes, I have SS lines with rbf600 fluid. When it comes down to tire pressures, I did try go higher/lower psi and appeared it did not make any difference during braking. I will re-do my alignment and go from there. If nothing changes, I will use my nismo pads for my last event of the season.

My 2c:

-Check to make sure you have at least .15 degrees toe in per wheel in the rear. If not, make that happen.
-Try to ensure that you aren't overdriving the rear tires and getting them greasy(er) than the fronts. Lowering your rear pressure will help on grip and make them a little warmer but its still possible to overdrive them and still get them too hot. if you raise the pressure back there, as long as you aren't overdriving them they should stay a bit cooler, but it will lower their grip a bit and make it easier to overdrive them.
-I don't recommend swapping back to xp8s all around since that pad is not nearly heat tolerant enough for our heavy cars front brakes if you use them near their abilities. Furthermore, a swap to xp8 in the front from xp10s would effectively alter your brake bias more towards the rear. I don't see that as being a good thing for stability under braking if you are already having issues.
-Maybe try being slightly slower on your application and release of the brakes.

All that being said I think all the slush in the rear stock suspension between the rubber bushings and fluid filled bushings contributes to this. Also as the suspension unloads in the rear and goes into droop I believe the suspension geometry starts to generate toe out, causing instability, well that and the fact that the wheels are pulling back pretty hard on all those flexible bushings while braking, adding more toe out. Which may be why having stiffer springs in front seems to mitigate this some. It doesn't change the amount of weight transfer but it does reduce the amount of suspension movement in droop in the rear.

I did the install of a switch that turns off the yaw sensor. This mod (off mod) along with TCS off made a huge improvement in the handling and brake wear on the track.

Yeah. I have gotten a weird lateral "bucking" at higher speeds. I still felt in control, but it was a little squirrely.

I also have XP10's & XP8's