High speed instability issue

[NORTY]

So there I was yesterday, on a familiar road that's straight as an arrow, some very minor dips and a moderate crown. I usually get to run it up to about 120mph and hold it there until I have had enough fun. (About 3 miles or so.) When doing 120, I don't have to feed any steeringwheel input to keep it straight. Anyway, I was feeling energetic and ran it up past 160mph.
At 145mph, the car started to move slightly, at 155mph, the car was moving about quite a bit. @ 160+mph, it took all the concentration I possess as the car was "darting" about. I've driven many vehicles past 185 so I counted out "nerves" or fear. It still could have been just that as minimal input can cause the car to move about @ those speeds, so I guess I can't rule that out, but just seems odd as I used to be quite comfortable @ 185mph. Maybe I'm gettin' old! I checked the tires (as I always do) before my drive (30PSI front & rear) Michelin PS2s. They have lots of treadwear left (90%+.) I always inspect underneath the car to check fasteners, so I've ruled that out (Although I just replaced padlets with Hawk HP+ and found 2 calipers loose (Both on the pass side oddly.)
Anyway, the tire pressure had grown to 33PSI front and 34PSI rear as I checked just after the run. I coasted down to about 90mph and the car was perfect again. Anything I should look for specifically, or should I look in the mirror and say "You're too old so stop your foolishness!"

Obviously, the instability is very subtle, or I wouldn't be here to type this! I'd just like to eliminate it if I could.

The only difference is tires. The Micheys are not runflat but the 3 previous sets were Goodyear OEM runflats. Think it was the sidewall flexion?

The road was clean and dry conditionwise.

Anyone?

Almost forgot:

2006 C6Z06 with 44,000 miles.

[motorman]

Originally Posted by NORTY

So there I was yesterday, on a familiar road that's straight as an arrow, some very minor dips and a moderate crown. I usually get to run it up to about 120mph and hold it there until I have had enough fun. (About 3 miles or so.) When doing 120, I don't have to feed any steeringwheel input to keep it straight. Anyway, I was feeling energetic and ran it up past 160mph.
At 145mph, the car started to move slightly, at 155mph, the car was moving about quite a bit. @ 160+mph, it took all the concentration I possess as the car was "darting" about. I've driven many vehicles past 185 so I counted out "nerves" or fear. It still could have been just that as minimal input can cause the car to move about @ those speeds, so I guess I can't rule that out, but just seems odd as I used to be quite comfortable @ 185mph. Maybe I'm gettin' old! I checked the tires (as I always do) before my drive (30PSI front & rear) Michelin PS2s. They have lots of treadwear left (90%+.) I always inspect underneath the car to check fasteners, so I've ruled that out (Although I just replaced padlets with Hawk HP+ and found 2 calipers loose (Both on the pass side oddly.)
Anyway, the tire pressure had grown to 33PSI front and 34PSI rear as I checked just after the run. I coasted down to about 90mph and the car was perfect again. Anything I should look for specifically, or should I look in the mirror and say "You're too old so stop your foolishness!"

Obviously, the instability is very subtle, or I wouldn't be here to type this! I'd just like to eliminate it if I could.

The only difference is tires. The Micheys are not runflat but the 3 previous sets were Goodyear OEM runflats. Think it was the sidewall flexion?

The road was clean and dry conditionwise.

Anyone?

Almost forgot:

2006 C6Z06 with 44,000 miles.

at 44000 miles i would be suspect of the shocks. also steering toe setting is very important at those speeds

[LLC5]

Alignment and shocks are the first to come to mind.

I would also want a solid reason for the caliper bolts to be loose, and a permanent fix before driving much more, especially at those speeds. Good luck with it.

[NORTY]

Originally Posted by motorman

at 44000 miles i would be suspect of the shocks. also steering toe setting is very important at those speeds

Checking my last alignment (Jan. 2008), I've got -0.01deg for total front toe. I don't recall hitting anything that could knock it out, but you never know until it's measured. Maybe time to visit my alignment guy again.

Rear toe is/was 0.03deg.

I never even considered the shocks. Thanks, motorman!

[NORTY]

Originally Posted by LLC5

Alignment and shocks are the first to come to mind.

I would also want a solid reason for the caliper bolts to be loose, and a permanent fix before driving much more, especially at those speeds. Good luck with it.

Thanks LLC5 for aiming me towards the shocks/alignment possibility. This I'm going to investigate more thoroughly.

I have no idea why the caliper mounting bolts were 20-30ft.lbs to break loose, but they were. Odd to me was that they were both on the passenger side. I've never removed them (nor has anyone else) as this was the first time I did brake padlets on the car. I'm the original owner and no one has worked on the wheels/brakes except me. I brought the bolts to 130ft. lbs. The car/brake calipers were stone cold when I did the brakes.

There was blue locktight on the threads of the bolts.

[rc135_tx]

First, I want to say that I love this site. There is no other place to get this kind of direct, quick, & accurate response from real experts. Thanks for being here.

Then, from a duffer non-expert I just wanted to say that my C5 was rock-solid ('comfortable' as you said) at those speeds with a good alignment from a local shop on PS2's & that my new-to-me C6 scared the cr@p out of me at anything above 60 on my first drive on a secondary road. "Darty" wouldn't cover it. An alignment & new tires (GY's this time) took care of it, too.

Chuck

[LLC5]

Originally Posted by NORTY

Thanks LLC5 for aiming me towards the shocks/alignment possibility. This I'm going to investigate more thoroughly.

I have no idea why the caliper mounting bolts were 20-30ft.lbs to break loose, but they were. Odd to me was that they were both on the passenger side. I've never removed them (nor has anyone else) as this was the first time I did brake padlets on the car. I'm the original owner and no one has worked on the wheels/brakes except me. I brought the bolts to 130ft. lbs. The car/brake calipers were stone cold when I did the brakes.

There was blue locktight on the threads of the bolts.

GM does recommend replacing the caliper bracket bolts after removing/loosening. At the speeds you are traveling, it may be something to consider.

[froggy47]

I would venture alignment before shocks & also at those speeds the crown of the road & crosswinds.

Need to do a 2 way run & avg the speeds like Salt Flats.

:)

[motorman]

Originally Posted by LLC5

GM does recommend replacing the caliper bracket bolts after removing/loosening. At the speeds you are traveling, it may something to consider.

i think GM is in a CYA mode because they know most people will not use loctite when they reinstall the used bolts and the new GM bolts come with loctite on them.

[NORTY]

Originally Posted by froggy47

I would venture alignment before shocks & also at those speeds the crown of the road & crosswinds.

Need to do a 2 way run & avg the speeds like Salt Flats.

:)

froggy, thank you for reminding me. The winds were absolutely dead still. I forgot to mention this in my earier post. (I remembered to take notice of the winds because that was a possibility I thought.)