Recommendations for aero upgrades & stability issues.
 

Hey,

After a couple months of owning the 2011 C63 and upgrading from 18" oem rims to newer ZF01 19' rims the car definitely seems to be a lot less stable even after proper balancing and alignment including camber adjustment. The tires are 255/35/19 in the front and 275/30/19 in the rear, all four are pilot super sports and have phenomenal grip. At around 80/mph the car begins to have a pretty rough vibration in the steering wheel and anything above 120 the wheel is slightly moving back right and left and feels like the car is going to lose all of its grip and just go into a wall even if the pavement is 100% dry. The car is also a lot more sensitive to pot holes and any groves on the highway (Miami doesn't have the best roads). Somebody told me that the car's height increased with the bigger wheel so more air is slipping under the car without and the car has no aero on it except for the very small stock spoiler. I'm wondering if you guys can recommend me things to do the car at a reasonable price. Currently, I'm looking at H&R coilovers but want to know if that's worth it and if that will make the car handle worse or better than stock suspension and if it works with the "Sport Handling" mode the car has. I'm also looking for lighter brake rotors and good brake pads for resonable pricing to lower the unsprung weight in all four corners, I don't mind what the aero pieces look like and I want to make the car as stable as possible. I've seen people put canards on the front and rear bumper behind the wheels? I'll do as much as possible without hurting the top speed of the car as well but I don't think she'll ever go over 160-170 again most 150mph. Just a lot of recommendations on making the car more planted and handle better and of course look more aggressive. Thanks guys.

TL;DR
Car is unstable at speeds exceeding 80/mph and looking to the most aero possible including even canards if that helps. Read the entire thing if you have the time; would really appreciate the help.

Thanks again! :D

First thing I'd check is if the wheels are hubcentric, or if you have the right kind of lug bolts installed (ball vs cone). It sounds like a wheel problem. What else do you have done - did you do springs too?

Countless people have upgraded to 19" rims here, and not even one of them has "aero problems" as a result.

This is not an aero issue.

Like BLKROKT said, you have created a problem and that needs to be addressed, sticking on wings isn't going to make anything better - can stock is stable to nearly 200mph, something is wrong with what you have had done to the car.

did you reuse your stock lug bolts?

Wow.. This is a loaded post

For the wheels: 19" rims aren't your problem. People run 19's all the time on our cars... 19's were were an option. I'd check to make sure the wheels are hubcentric and if they are, make sure they're the right size. If they're too big, you can get an adapter so you won't get any of that vibration... Check to make sure you're running the right lug bolts... Your problems don't seem "aero" related at all. I'd check those things out ASAP because it seems like you like to do a lot of highway pulls... With your wheels in the situation they are, it sounds really f***ing unsafe to drive lol. I'd honestly stick with the stock wheels til you get it figured out.

For the Sport Handling mode: All this is, is traction control... Has nothing to do with handling characteristics or compatibility. Coilovers, in any sense, will improve handling characteristics with the exception of comfort lol

Regarding the wheels.
 

Okay, wasn't sure how accurate it was saying that car has too much air slipping under it. Anyways, all I know is that the wheels came with their own lug nuts since the wheel had a different finished. I don't know if they are or how to check if the wheels are hub-centric. I bought them through a very well regarded wheel shop named WheelsBotique. The wheels are Zito ZF01 (the ones on the liberty walk c63 in the US). I would try on the older stock except for the fact that the rear rims have completely shredded tires on them so I would be spending even more money replacing the tires on those. If anyone can help me know if they're hubcentric that'd be great and if they aren't... is there a way to convert it easily and for a low price? Thanks.

Responding
 

No, I believe the shop used the ones that came with the wheels.

Good, dont want to use factory ball seat on an aftermarket wheel.

To check the hub, take off all the lugs on one wheel and pull it off. you will see a little circle lip on the hub, and that should fit perfectly in the opening on the back of the rim. if the hole in the rim is too big you will need to know the rim hub size and get adapters (they are pretty cheap).

You will be looking for hubcentric adapters 'rim hub size' to 66.6 (66.56)

This is good advice, the wheels on these cars use the hub to center them, the lug bolts just hold it onto the car, but if the hub hole in your wheel is not very tight over the hub, then it can be slightly off center when bolted on. Most aftermarket wheels come with very large hub holes in the center of them and then you buy adapters to size it down to your exact hub size. If the adapters are missing or for another vehicle they may be larger than your hugs causing the wheel to be improperly centered.

Stock tire is 235/40-18 front and 255/35-18 rear.
Your tires are 255/35/19 in the front and 275/30/19 in the rear

https://tiresize.com/tyre-size-calculator/

Tire diameter
Stock front/rear: 25.4”. 25.0”
Yours fromt/rear: 26.0”, 25.5”

Conclusion: The diameter of your new tires is bigger by about a half an inch, which means that you car is sitting ¼” higher. That is unquestionably NOT why your car is vibrating so much.

I’d take it back to your tire shop and ask them to check two things. First, have each of the tires rebalanced. Bad balance will certainly cause the problems you’re experiencing. Then have them check the runout with the wheels and tires mounted on the car. You can check runout at the wheel OD or at the tread surface. I prefer the latter. If the wheels are not properly centered (see centering ring discussion above), you can see the tread surface rise and fall each time they rotate.

tire runout:
wheel runout, starting at 2:40 -
Longer more detailed checks:

When you say the car was properly aligned, was it aligned to stock toe?

Out oif curosity, did you check the tire pressure to make sure the tires were not over inflated?
Ideally, you want them around 40 psi cold.

Right -- if you could post your alignment specs, that'd be helpful.

So I don't want to derail, but I have to ask.

Aftermarket rims require new bolts?

Just got these wheels on, niche misano's with w204 offsets & hub size, bolt size, etc.
As far as I know, it was ok to run oem bolts (my new ones have not come in yet; also locking bolts??) so could anyone kind of clear it up a bit? Car was smooth up to 80mph, havent gotten up any higher as have been busy lately. >>Thanks, and sorry for slight derail.

Check if the Niche wheels require conical or ball seat bolts. OE/OEM are ball seat.

Yep looks like they use ball seat bolts. Thanks!