AlexSafarian

A recent problem has shown up with my 07' c6m6 with about 11k. I was on a wide open highway going home late at night going a little fast(120mph), when I resalized the car would begin swaying left and right randomly. I would slow down and no problem but as soon as I would speed up it would start again. It was not windy and I tried it on different highways and the same problem ocuurs now frequently especially when I change lanes it seems like If i got any faster I will lose control and spin out.

Anyone have any idea what it could be from or has experienced similar issues.

I also changed my run flats to Michelin PS2's recently on stock rims I am running 255/35 in the front and 295/30 rear. Could this maybe attribute to the problem?

If anyone could help me figure this out or give me some advice besides telling me to slow down, I would apprecitate it.

talon90

Did you have an alignment done after your tire change? What pressure are you running in your tires?

After the obvious two questions are out of the way next, as you are aware, you've changed the OEM recommended tire size from 245/40 to 255/30 and from 285/35 to 295/30. This could be having an impact on the calibration of the Magnasteer system.

One thing to consider is that the amount of "assist" from the Magnasteer is supposed to reduce at higher speed to allow for more precise input and stability. The input that the system receives is from the ABS wheel speed sensor and various other sensors. The tire change may be influencing the decisions the EBCM is making to provide the wrong amount of assist to the steering for the actual speed being traveled. If the wrong amount of assist is being introduced steering inputs could be magnified and creating a sort of "pilot induced oscillation."

moolys

I was up to 145 and no swaying on the Z51 nice and smooth what a rush

jojog

Could be ruts in the road - the kind you will only see when they puddle up after a rain. I hit a few of those at cruising speed and was surprised as in my C5 I only experienced it at slow speeds. I attribute the diiference to the steering and other changes. Just a guess, though - you might have other issues. Good luck finding the solution.

joec

Probably ruts in the road.. Really weird when I got my first C5 that I had to deal with them... Make sure you know where your driving as they can be a bit to deal with at high speeds..

/jc

Ampcass

Doubt if it is ruts in the road as he said he had the same problem on different highways.

AlexSafarian

Thats what I thought at first so I tried it on a few freeways.

It never happened with my original tires as I have opened 175 going to the track.

My car has been lowered and I have changed the sway bars. I am wondering if maybe the sway bar has gotten loose or something. I will have to check and tighten all of those to further diagnose the problem.

All my tire pressures are normal. I will have an alignment and see if that solves anything.

That steering assist sounds like it might be a logical problem but then again people put rims and different tires on all the time with no similar problems.

PowerLabs

Very unlikely considering his new tires change the actual speed of the car vs the speed the wheel sensors think the car is going at by maybe 1%. Even if you put tiny wheels on the car, all it would do would be make the magnasteer system tighten up the steering at a somewhat higher speed than usual; it wouldn't make the car lose.


Since the OP installed wider, grippier tires, he is experiencing a lot more "tramlining". My Z06 did it a lot; its annoying but something you live with on crappy roads. I would also get an alignment.

talon90

The only reason I raised the magnasteer possibility is that the change to the front tires is more drastic than the change to the rear.

The fronts now have a 6.6% reduction in circumference resulting in a revolutions per mile increase of 56. The rear tires are only at a 3.3% decrease in circumference resulting in a 26 revolution per mile increase.

In stock trim the car has a 33 revolution per mile difference between the front and the rears with the new setup it is 63. I wasn't sure what impact that delta between old and the new would have on the systems. If you did this same thing to the rear instead the car could perceive wheel spin. I'm skeptical about it being tramlining or at least exclusively tramlining but having not felt it it is impossible for me to say.

It could be alignment related. I also suspect given the sizes that the new tires are not runflats. The softer sidewall could be contributing to the "feel" as they are more compliant than the OEM runflat tires and this flex could contribute more than what the owner is used to.

Paul

RushThruLife

Could just be the road your on.

AU N EGL

Roads are crowned too allow water to run off, and I bet the car needs to have its alignment checked. also check your air pressure. at those speeds even a 1 psi difference is noticeable

tjl5709

Asphault roads? These groove real easy from truck traffic.

Check on a concrete slab.

FortMorganAl

Let's see if I understand this. You've changed the suspension components and the tires and now the car doesn't handle as well as it used to and you want to know if we have any ideas why? Does that about sum it up?

AORoads

I might also look very closely into having all 4 wheels re-torqued onto the hubs. 120 mph is no speed to be having anything wrong with either what I wrote or talon90's post.

I'd contact tirerack and ask them if this tire/wheel setup would have an adverse effect on high speed handling.

It is possible that with the wider tires, you may want to, or have to run closer to racing spec alignment. Not sure on that one, but reading talon's second post makes me wonder a bit.

Wayne O

It's difficult to assess the degree of sway you're talking about but at high speed even minute changes in the road surface can affect 'handling' or exemplify inconsistencies with the car's setup.

Certainly check tire pressures and physically inspect your tires; have the alignment checked; and, have your wheel/tires road-force balanced. You could have something out-of-round.

TCKT B8

I agree with Talon. Seems logical that if it was fine before you changed tires, while it may be an alignment issue, its most likely a tire-related problem. Either the different tire size is an issue or one or the tires may have an issue at high speed. I'd check the sway bar, but that's not likely to be the problem. I have found through the years that changing tire brands, sizes, or types often significantly changes handling and ride characteristics.

garye

messin with the OEM setup gets you in more trouble than it's worth half the time. That's why my Z51 set up is staying where it is at. Can't blame any issue on handling with mods.

shopdog

ChargedC6

hmmm ive had my 05 to 180+ and was strait as a bullit. but i do notice the car is very sloppy on the freeway alot of body roll front end seems to float. my srt-8 charger "feels" better on the freeway i defnitly need sway bars.

HawaiiC6

I would highly recommend checking your tire pressure which could cause the car to mishandle.