This last weekend I was able to reach 160mph a few times (according to the speedometer) on my stock 2006 track. When I reached that speed, I got on the brakes quite lightly and the car got rather loose, with the back end seeming to sway from side to side.. Is this type of behavior normal when braking from these speeds or is something amiss.

When braking from around 120-130 however, things felt solid and firm.

I have the OEM brembos with stoptech stage II upgrade in the front. I still have the OEM rotors and I think OEM pads in the rear.

 

I’m not expert BUT I can speak from my experiences...If I feel im in no danger, cops, other traffic, etc I assume just coast to normal speeds. If I feel I need to slow down I get on the brakes lightly applying more pressure if need be when I get down to slower speeds.

Were you turning slightly, or going over any dividers bumps while braking? Seems to me at that speed slowing down somewhat rapidly shifts the weight of the car, and going around a slight turn, etc will exaggerate that. Anyhoot cruising at 160mph I would assume doing something with suspension for less body roll would be a good call.

 

I was on a straight stretch of road while braking. I was wary of upsetting the car, so I got on the brakes rather lightly. When I felt the car get squirmy, I quickly got off the brakes.

BTW, is the 160 indicated speed correct. I thought and was told there was a 150mph limiter.

 

Let's see...
-Front weight transfer in part,
-Tire deformation from working close to there maximum speed limit plus the "natural" tire lift (smaller contact patch) for an other part.
-Maybe not enough rear toe-in for the situation also (doesn't mean you need more) .
-Relative lack of damping for the road/speed frequency
-probably many other thing a chassis engineer could think of.

Bottom line, normal high speed behavior.

 

Yeah, at 160mph do you really think the brakes shouldn't have any deviation from normal driving ?????

 

After taking the G over 90mph, I will never do it again with stock suspension, hell, even when I had my STi and I went over 90, I could swear the Grim Reaper was in the back seat saying "stock suspension?". I think once the car has stiffer springs much of the weight transfer (when braking hard) will be avoided, to allow for a more even contact patch on all 4 wheels (compared to stock).

 

You're confusing front end dive and weight transfer.

You can limit the front end dive and thus, potentially maximise your front end grip since it limits the suspension geometry change, but the weight transfer will be pretty much the same for a given decceleration.

In any case, when driving fast, it's never a good idea to jump on the brakes or be jerky with the steering wheel.

I push my Z to +120 mph on a regular basis at the track, and even before the suspension mods I now have, the car felt like that under heavy braking.

The rear end instability isn't an issue and it is more often than not a result of tire squirm. The 1/2 inch of thread itself is deforming and makes the car feel weird. You're still generating plenty of brake torque. It's something we have to be comfortable with. The tire's self centering characteristics will keep the car straight.

All that of course is assuming the alignement is good and no suspension bits are loose or broken...

 

more downforce in the rear?

you got big ***** to hit 160 on a public road...

 

Kolia, your right, I should have said front end dive instead. But, front end dive is caused by the weight transfer, correct? If the car has stiffer springs in the front - with correct spring rates, the front end dive will be lessened by more upward force from the spring. Since the front of the car will not be able to dive down (as much as with stock springs), the back of the car will not be able to lift up as much (again, compared to stock).

 

The rear end lift is not a (real) problem either. The tire load will still transfer to the front.

Only a fraction of the weight transfer comes from the car actually tipping forward. The majority of it comes from the car's CG generating a torque moment (proper english nomenclature escapes me now...) around the (mostly) front contact patch. The car is basically trying to stand on its front axle. Suspension compression doesn't change anything to that.

A better suspension and alignement does help with stability and brake performance. But only to a degree.

 

Sure normal braking wouldn't be expected at those speeds, but I was not heavy on the brakes. Instead I was rather light on the brakes and the car still got quite squirmy.

For comparison, I hit an electronically limited 155mph in a big old 5-series and at those speeds, the brakes felt much more secure than that of the Z.

 

In anticipation of running high speeds, I adjusted my tire pressure to I believe 39psi cold all around. The tire pressures were well into the 40psi+ range during the drive. It was also a rather hot day with temps in the 90s to 100s.

At those speeds, would I have been better off running a lower tire pressure?

 

when I was a noob at HSDE's, I nervously lifted off the throttle, no brake what-so-ever, on the slight bend on the front stretch at VIR. I was going about 110mph. It felt like my entire car was floating.
At higher speeds, any change will throw the car out of balance.
IE: small steering input, lifting the throttle, braking, heck another car going past you will really upset your car. And yes it's scary as he!!

be safe

(ps: i'm still a beginner)

 

+1 on what Kolia said

 

Everything you need to know about suspension you can learn from cartoons. Imagine an old cartoon where the car is stopping really fast. What does it do? The rear wheels come up in the air. End of story.

 

Unless you drive an old Q45 [91-94] with active suspension which maintains body level even from a 150 mph panic stop . {a few others even with semi active [air shocks] can reduce the dive and lift....halfway into the braking}.....depends on cycle time.

As suspension dives and rises expect camber changes of 0.8 degrees per inch of body change from normal oem specs.

On lowered cars [excess negative camber] this can lead to serious braking problems as the tires are optimal at - 0.5 degrees.

A 2.5" change ~~ 2.0 camber change from static.

Modern rear suspensions have deceleration toe in control [toe bushings get squished in increasing rear toe under slowing - the faster the slowing the more the rear toes in.

Important to have the toes [rear and front] PERFECTLY in middle of oem range for this to function properly.