Need some suspension help/advice
 

Hello,

I recently came back from an HPDE at PIR (Portland international Raceway) and it has a back straight that is curved. When on the back straight at over 100MPH my 09 C6Z06 seems to drift to the outside and I feel a hint of body roll or wobillyness. It's enough that I wont shift up to 4th because I'm afraid of the car going sideways. When I talked the instructor about this, he rode with me next session and his reply was, "It's just a Corvette thing and every corvette I've been in does that.".

My question is does anyone else experience what I ]'m describing and can I make the car feel a little more planted in a high speed, curved straight, with shocks or a suspension upgrade. I also don't want to make the car uncomfortable for long distance rides. Thank you

Chris AK

Could be a lot of things.
From what you write it may be an oversteer issue.

So sticker tires might help.
Lower the suspension 1/2 on stock bolts and corner weight the car.
Upgrade a sway bar...not sure which one or both.
Loose suspension component...grap your torque wrench.
Bumpy section of track.
Shock hitting its jouce under full compression.
Adjustible compression and/ or rebound shock (or coil over) purchase.
Adjust your driving line to compensate.
Try trial braking.
Adjust your alignment specs.
Just have fun!

What you are feeling may be normal for a street/stock Vette on the track.

As Kenny says above, you can start with making sure your car is set-up correctly:

Tire pressure (I usually start out at about 27-28 psi cold, and watch my front tire pressure, if it gets above 36 hot, I let some out). I have run both OEM GY and NT05. The NT05 are more sensitive to pressure.

Get a good alignment (go to the Pfadt site and get alignment specks - use the "Performance Street" or "Performance Street - Track Use with Street Tires" settings depending on desired tire wear).

While getting the alignment - have all suspension components checked for looseness.

Next you can start down the road of handling improvement:

I went with DRM valved Bilstein shocks (firmer but less harsh ride) and Johnny O'Connell front and rear sway bars from Pfadt (really reduces roll and with adjustments you can reduce the Z06's oversteer).

You can try different tires - the NT05s are more predictable with a more gradual release of grip. The Michelin SS are suppose to be awesome. OEM are not bad on the track and give you run flat to and from the track.

There is a lot more you can do (coil overs, bushings, spindle lowering, R compound tires, etc) but you will be getting further and further away from a street car.

Jim

Thanks for the advice. The car is pretty much stock except for the Michelin Super Sports (Run Flats) that are on the car. I have only owned the car for three months and it has 23K on the odometer. I will get the alignment check and fixed before I go back out.

I think the track had a "washboard" hump going into the curved back straight that they fixed, but there may be a hint of the hump still being there. The straight is curved enough that I can make a standard 3-to-4 shift, otherwise it will shift into 2nd, so you have to make a movement as if you are shifting into the 6th. In all other corners the car seems to behave itself. I hope it's just as simple as torquing down a couple of nuts and getting an alignment.

Chris AK

Chris-
Sounds like you've gotten some really good advice already,but I'll chime in with my 2.

Sounds like you are describing some body roll, and maybe understeering/oversteering but I'm not 100% sure which one. Is it the back of the car wanting to drift to the outside, or the entire car wanting to?

Here are some suggestions either way:
Ensure all components are tight and track worthy
Lower the ride height slightly on the stock suspension
Get a good track alignment

If you still have an issue the first thing you want to tackle would be sway bars. Sway bars will greatly reduce the body roll you are describing and give you a more planted feel while maintaining factory ride quality. Also, because swaybars like the JOC are adjustable, you can actually tune them to counteract understeer/oversteer situations.

A little info on the JOC products:






Spend some time playing around on the track, and have some fun! We all drive a little different so have a blast and eventually the car will dialed in perfectly for you.*
I definitely wouldn't settle for "Its a corvette thing....". These cars are world-class track cars with a few very minor adjustments. Let me know if I can help with some alignment specs or other info while you get your car dialed in. :flag:


http://www.pfadtracing.com/photos/da...rvette_Sig.jpg

My car has a bit of a nervous back end.. but I've just kinda learned how to deal with it, left foot braking through faster corners and watching my throttle application on corner exit.

My car is on Pfadt coilovers, stock sway bars. I'll be at the track this Saturday on fresh NT05's (that I've not had on this car before) and the additional HP from heads/cam, and a fresh alignment that I'm having done to Pfadt specs (track use, street tires)

I can report back here as to how the alignment works out - from what I've read I'm thinking I might not have enough toe in on the rear..?

It is always a good idea to make sure nothing is wrong, but it sounds like you are simply approaching "the limit". As you get more seat time, the wobbliness is just dealt with by the driver. You could tune the car by doing things like adding rear toe in. That will help. Or you can simply raise the limit by adding performance parts or better tires. You wont really make the car more stable at the limit, you just raise the limit and end up under driving the car. I would suggest making sure nothing is wrong, and then get more seat time. If you get the opportunity to have an experienced driver run your car, DO IT. You may be surprised how much wobbliness is acceptable and can be dealt with by the driver.

Just my 2 cents from a guy who still blames himself for most handling problems after 10+ years in the sport.

:bigears

Thank you for the advice. There is some good info on the PFADT website and I will start there.

I was thinking more about what it felt like and there was a little of both, understeer and oversteer. As I am accelerating through the straight what it feels like to me is the car starts to understeer, then I let out on the throttle just a tad, then it goes into an oversteer, then I get back on the throttle, then it oversteers. The whole time I am also making minor adjustments with the wheel and the car just seems to drift slowly to the outside of the track. None of the understeer or oversteer is drastic and I don't even here the tires, I believe the car can probably hole a tighter radius through the straight and wonder if that section of the track is negatively cambered because all cars were drifting to the outside, just in my car it seems the back and front end were alternating in slip.

The straight is maybe just over a quarter mile and I will be about a car to a car and a half over in width to the outside of the straight at the end.

I will start with a more aggressive alignment and probably go back out in June or July and see if that helps. Thanks again

Chris AK

It definitely sounds like you're just at the limit of your current setup. Play with that alignment (and ride height a little), be safe and have some damn fun! :thumbs:

http://www.pfadtracing.com/photos/da...rvette_Sig.jpg

A lot of excellent comments about setting up the car to get through the turn better but maybe you should look at how you are driving.

First, are you looking far enough ahead? The feeling you are describing sounds like what people feel if they aren't looking far enough down the track. They don't have the proper sight picture and believe they are going to go off track because they are steering the car to abruptly for the turn.

Second, why are you in third gear when you enter the straight/turn? What speed is the car going and can you roll all the way wide open in third gear as you track out onto the straight? If you are in the 80 mph range and can't use all of the throttle available to you shift to 4th and use more throttle. You can generate just as much HP in 4th as you can in 3rd if you aren't able to use all the power available in 3rd. 4th gear gives you much better on and off throttle control and will make it easy to modify your arc through the turn by modulating the throttle.

Bill

I can second what bill has said about using 4th over 3rd. My home track has one bend that I take at about 85mph, and I often go through there in 4th because it has a more controlled feel. Less nervous.

Yeah, I'm far from blaming the car because this is my first HPDE and I don't have the natural talent that I thought I had from my Grand Turismo playing days. I would say at the start of the curved straight, I'm at ~2000RPM in third gear a couple hundred feet I'm at above 4000RPM and this is where I shift into 4th. While I'm shifting, because of the curved straight, I'm pinned to the drivers door and I have to grab the shifter with the palm facing the passenger's side to avoid accidentally shifting into second. I'm a slim framed man and was sore in the hips and thighs because I was sliding from side to side. I digress, The wobiliness usually occurs right after the shift and continues through most of the curved straight.

Could it be the shift mid turn or could I accidentally be using the steering wheel as leverage to shift to 4th and that causing the car to feel a little unstable? Here is a photo of the track:
http://i43.photobucket.com/albums/e3...ps2ab202ff.jpg

I am talking about turn 6 and I shift right before the apex of that turn, usually. Thank you

Chris AK

I drove a student's Z06 for the first time at Monticello in NY. The main straight has a kink in it that I took at 140 MPH and commented to the (experienced) student that the car was on edge. When I got my Z06, driving it home thru the NYC interstates the back end hopped around on the bumpy pavement.

The first mod I did was to install Bilstein Sports and I've not had that nervous feeling from the car since. Now having been on this forum I would get the revalved DRM Bilsteins.

I would suggest the Pfadt street/track alignment (I haven't done that yet) and the shocks as a good starting point and not spend a lot of $ chasing things down.

Also the symptoms you report of understeer/oversteer off/on the throttle is generally a good thing as it allows you to vary your line through the corner, but it needs to be in a way that is comfortable for you to deal with. Our Z's power means you need to be very patient with the throttle and I agree with other forum members that short shifting into 4th gear may help "slow" the dynamic changes you are feeling. Some of that should come with additional seat time at the track (I'm probably over 10,000 track miles and still learning/having fun).

makes me nervouse about taking my car to the nordschleife... well, car is still shipping and i plan on driving down there straight from the port, but still, z06 makes me very scared (i did run my m3 at 7:45).

i agree with people on the 4th vs 3rd gear, at least i did that with my other cars a lot... but from what you saying that you are not even at that high of rpm when you are coming out of the turn... hmm. how is your tire pressure?

It sounds like you have the stock seats/safety belts. On the safety belts, if you pull them all the way out, when they retract tight to your body, they will lock in place and help you stay in the seat better. Before you do that, if you move the seat back just a little, after they are "tight" you can move the seat forward to its normal position and they will be even tighter. If you are getting seat belt pressure on your shoulder you can get seat belt pads that velcro in place and that pads some.

A full 5 or 6 point harness and competition seat solves the problem but are a fair amount of $ for just the occasional HPDE.

Got my alignment touched up this morning and as I suspected, I had no toe-in at the rear. Put in a total toe of .30 and I can already tell it feels more neutral at speed.

I have the same Z as you but I installed the O'Connell Track package 2 years ago and even before that never felt the centrifugal force pull me out as you describe. I do have the Michelin SS but they're not run flats. Are you sure you have SS runflats as I thought those wouldn't be available till the C7 comes out. The SS has a softer side wall and I wonder if that could be the cause. I use a basketball-like knee pad on my left to brace against the door in right turns and also jam an old couch pillow there to stop me from sliding in the seat. What's the tread depth of your tires and what PSI do you run?

Originally Posted by amoahkuc
Thanks for the advice. The car is pretty much stock except for the Michelin Super Sports (Run Flats) that are on the car. I have only owned the car for three months and it has 23K on the odometer. I will get the alignment check and fixed before I go back out.

There are no Pilot Supersports in a runflat yet. Maybe you have Pilot Sports or more likely PS2 runflats.

a lot of great guys gave great info ... take it to heart.. they do it often people..!