I've been tracking my C8 Z51 with stock PS4S tires frequently over the past few months and I'm considering doing some track orientated upgrades. The car doesn't have MSRC and has a front lift which is needed for my driveway. It already has the GM track prep (fluids and track alignment etc), is lowered .75 inch front and rear, and has 20mm wheel spacers (done for aesthetic reasons). The car is driven about 60% on track and 40% on roads. My local track is Harris Hill Raceway which is a fairly bumpy technical track where the maximum top speed is about 120 mph. Even with the GM track alignment I am getting some understeer especially in slower corners. What is the best way to address this? I was considering adding wider wheels and tires (proportionally wider in the front) to help reduce the understeer. And recommendations on tire/wheel sizes and brands?

I've also read about upgrading the suspension to MCS 2WNR coilover kit ,however, this would delete my front lift. The KW Variant 5 coilovers work with a front lift, however, they are $10k and from my understanding they use a similar spring rate as OEM and are not as good for track use as the MCS ones. Would it be better just to try changing my spring rate on the stock coilovers? Or are there some other options on improving the suspension?

Any advice on the wheels/tires and suspension or anything else is appreciated.

 

Consider driving style shift.

https://driver61.com/uni/trail-braki...20the%20corner.



https://www.roadandtrack.com/new-car...ing-explained/

 

Sway bars help a lot. I installed a front sway bar to counter the understeer. It made a huge difference! AFE bar set on the lightest setting and running bridgestone RE-71RS size 265 rails corners. The car now likes to oversteer a bit, but that's way more fun! If I wasn't restricted by the SCCA street category rules I would also install the rear sway bar to further dial in the car.

AFE Sway Bars

 

Regarding trail braking and driving style: I went to Ron Fellow's Level 1 last month were they did teach us how to trail brake the car properly. I ended up the 2nd fastest lap time of the group of 18 so I'm guessing I was doing something right. I feel like my C8 understeers a bit more than the car I drove at Ron Fellow's. Maybe because my suspension is softer due not having MSRC? My local instructor also drove my car and commented how much understeer it had. I would like to dial some of this out if possible and improve overall handling.

 

Adding a stiffer rear swaybar helps counter understeer. A stiffer front one will make it worse.

 

That's not what I have experienced.

 

You can get the car to handle dramatically differently experimenting with toe and camber settings. I'd spend 90 percent of my time on that. If you toe out the rear end you can get it to be as scary as a 1980s 911.

After that you are absolutely right changing the front to rear tread width ratios will have the next most dramatic impact.

A motorsports suspension upgrade is icing. Helps everywhere but it's also another level of pita too.

 

AGREED! The posted info flies in the face of known physics. I'd be interested in an explanation of how stiffening the front-end reduces understeer.

 

This guy explains it better than I can..

"Sway bars are way more complicated than people give them credit for.Big front bar for the win.

A stiffer front bar not only helps you put down power, but also helps turn in.

This is my experience on a miata, a 240sx, and a foxbody. Also, I find that even when putting power down wasnt a problem I still would go full throttle earlier with a bigger front bar, as in full throttle mid corner.

How can a stiffer front bar improve turn in on the front axle while improving putting power down on the rear? I dont know but I have a theory.

A big front bar transfers more weight (to resist roll) on the front axle and less on the rear vs small front bar. At the limit this means understeer, at all points between the limit and static this means better rear traction, but typically we arent at the front limit during turn in. I think turn in is often only using 40-60% of available grip. (How often do you understeer before mid corner?) I think the bigger sway bar pre-loads the outside tire faster and this tire in a compressed state gives more feedback to the driver. i.e. the tires 'spring rate' is compressed and it gives more lateral force with less slip angle. Also this all happens with less vehicle roll. The two combine to make the car "feel responsive".

Even if you do take a hit on ultimate front grip, you can throw the car into a corner with more rotational inertia, then stand on the throttle earlier to power out. Im a believer."

 

A slightly stiffer bar can help with the feel of turn in but not the breakaway understeer. Two different things.

If you have a car that has a floppy front end then the upgraded sway bar will really help feel firm that up but it doesn't help the breakaway characteristics which is understeer.

If the car is kind of floppy and it's not the type of car that I'm going to put a motorsport suspension on then I'll add let's say a 75% stiffer rear bar and a 50% stiffer front bar so that I get better turn in but also don't increase the understeer.

 

295/30/18 fronts
335/30/18 rears

try this setup

also. I upgraded to the MCS suspension and I’m a huge fan. I don’t experience much understeer until I’m really pushing the fronts

 

I currently have 0.1 degree toe out front and rear. Someone at my track recommended adding some toe out the front wheels. Any idea how much would be a good starting point?

 

Thanks for the advice on tire sizes. I'm assuming those are on 10" front and 12" wide rear wheels? What tires do you recommend that come in those sizes?

How much better did you car handle with the wider tires?

 

I got to chime in. Love the sway bar link. Thanks. Haven't got that far yet as my first track day in C8 is coming shortly. Many years tracking cars, but first time in C8. Got a few autocrosses in thus far....which has been great for tight corners and learning the cars traits at lower speeds. I have to say, using my 25 years driving experience, my c8 came to me with great handling. I was dreading the understeer I heard about, but never had it. On stock everything, I wasn't having any issues. So, when I got my 9.5 fronts and 12" wide rears, I did a conservative adjustment adding more negative camber....but not as aggressive as the GM track alignment recommendations. Thus far, it has proven again to be tracking true with very minimal if any understeer.......at autocross tight radius and hairpin and slalom sections. Our autocross set up is 3rd & 4th gear in sections....not the SCCA tighter style. I wonder if I have a Unicorn C8 b/c it's tracking so well. LOL! Big track test will tell. Maybe I should do sway bars sooner than later? Probably will get one track day under my belt so I can have a baseline before changing a lot of things. I always factor in oem bushings as part of my suspension upgrades when the time comes.

 

If anyone is interested, I have MCS 2way dampers for sale. They came off my C8 that participated in a weenie roast and now I have a ZO6. They are basically brand new.

 

Most understeer is worsened by driving style.

There are several things that can be adjusted or changed to decrease understeer. To improve balance, you always want to improve traction where it is needed. I would start with a bit more outward toe. If you're at 0 go out .125" (1/8"). This is best for tighter low speed tracks.

A larger rear sway bar will increase the inside front wheel's contribution. I prefer more of an increased in spring rate than a larger bar, but it depends on how big the bars are relative to the spring rate. Increasing the rear spring rate by 20% might be a better option for this application since the C8 is under sprung relative to the bars and weight of the vehicle.

 

I've been doing lots of research on track setup over the past few months.

Some random thoughts on tracking/my experience

• I think the sway-bar will be my next move, but i'm dreading it as it's a pain to install. Doing this not for more grip but for stability in high speed. Switch backs, slaloms, etc i can't put the power down fast enough as i have to wait for the car to settle.
• Brakes. You will hit the braking limit of this car REALLY quickly. i was deluded into thinking it wasn't much of an issue because they flog the cars in ron fellows, so how can it be. Well, let's not forget most drivers are not going to be going as hard for as long. personally, i found that i was wayyy below pace for like 75% of my track sessions because of the lead follow nature of instruction.
• Turns out the experience of those without and with MR is vastly different. Coilovers seem to do significantly well without it, and not much (~1s) with MR.
• Oh god is it expensive. Brakes (rotors, pads) and tires is 5k. Want more mods? seats 2.6k, lighter/stronger wheels 4k

 

Sway bars always make a difference, but I'd highly recommend stiffening both ends to reduce twist. In my case, since I have much stiffer (4.5-5x) coilovers, I didn't feel the need for stiffer sway bars.

Regarding difference being much smaller if switching from MR shocks to coilovers, I have a feeling that the feedback you've read or had yourself is likely using closer to OEM size and/or lower traction (like OEM) tires. I've not had experience with MR on C8s, but on C7s, the calibration went way off as you increased traction, much reducing effectiveness of MR shocks, and making them a hindrance. The only MR shock OEM calibration good for high traction tires was OEM Z07 calibration...

Brakes get more expensive when they're used close to limit, since they wear down prematurely. I'd normally just recommend upgrading to C8 Z06 brakes for cheap ($1900), but those brakes almost force users to 19" minimum wheels, which limit choices. As such, I've gone with AP Brakes this time...

 

Front swaybar seems to be working! But hey, what do I know?

 

I also went with a larger front size based on allblackeverything's advice (thanks!). 285/645 up front significantly helped but I also noticed further improvement when I ran in Race 2 instead of Race 1 or Sport.