I plan on installing 20 F and 21 R custom wheels on my C8, what's the Pros and Con of this mod, or should I keep the original size and just change the Rims

 

Cons: You need to buy new tires that fit, becomes a more expensive consumable to replace regularly. Thinner sidewall reduces comfort and increases chance of impact break (hit a bump or leave a driveway too briskly and the sidewall can get pinched enough to bust a hole due to smaller air cushion--happened to me before). Heavier vs 19/20 setup.

Pros: Thinner sidewall widens contact patch for more grip.

Side opinion: More intricate, aggressive wheel faces look awkward as 19/20 but natural as 20/21. But need to lower somewhat aggressively (0.75" or more) for 20/21 to look good. If stock height or moderately lowered 0.5", then 19/20 looks better.

 

Thank You

 

Heavier also means slower acceleration and worse ride.

A thinner sidewall doesn't make the contact patch larger. A wider rim and tire makes the contact patch wider. The only real benefit is styling, if you prefer the look. Functionally it'll be worse in every respect. Most race cars use 18" wheel and tires, not 21", or 22" or ....

 

Unless you consistently drive on a giant pool-table do not go with even less sidewalls! You asked. All the best.

 

Chet at Aerolarri is an expert on fitment. I went 20--21's and love the look. Used his recommendation on everything.

 

what size tire did you go with and is there any rubbing, did you lower the vehicle.


Thank you

 

If you only change wheel diameter and tire size and keep outer diameter, tire model, and tire width the same, the shorter sidewall bows inwards less at the shoulder forcing the shoulder/sidewall to stand more upright or perpendicular to the ground. The result is a more squared shoulder (rather than rounded like donuts) and increased lateral contact patch, even if the tire's width spec remains unchanged (ex: going from 305/30R20 to 305/25R21). Think of it like a thin rubber band being more flat against the ground than a thick rubber o-ring. They have the same outer diameter, but the o-ring has a smaller inner diameter (fits a 20" wheel) vs the rubber band (fits a 21" wheel).

Not only that, a larger diameter also increases longitudinal contact patch (rubber touches more ground front to rear, along the wheel's tangential plane). Z06 engineers in multiple interviews explained that effect when choosing 1" larger diameter wheels for the Z06 (didn't need to discuss increasing the width because that's obvious and goes without saying). An exaggerated illustration: Pretend you have an dough roller (3"x11" wheel) vs the C8's 20"x11" stock rear wheel. Exact same 11" width between the two touches the ground but the C8 wheel will flatten more dough even if both aren't rolling. Likewise, bump that 20" wheel up to 21" will flatten slightly more dough (or in this case, flatten more tire rubber against the ground forming more front-to-back contact patch).

So changing to a larger diameter without even changing width still increases contact patch in both length (longitudinally) and width (laterally).

 

20/21 really make the car look super fast - exotic, but that is expensive in more than one way. The cost of those low profile tires is really high, and the ride will certainly be harder.
In theory a shorter sidewall should make the car handle better, but the OEM suspension is tuned for a 19/20 combination, and that is about as good as it is going to get.
You may notice a difference in the steering with less sidewall flex, but the ride will probably be harder. Overall it is an expensive change that will look cool, but any gains in performance come at a cost of ride quality.

I agree with DSO, most race cars don't use bigger wheels because the tires are so expensive, and it makes the suspension harder to tune. Shocks and springs have to be tuned to match.

 

The difference is going to be trivial, unless you've made a huge jump in wheel size, like 18" to 21".

Not if you go to lower profile tires and keep the overall diameter the same. The contact patch only changes if the outer diameter of the tire is larger, and lower profile tires don't.

 

If you feel a non-trivial difference in comfort, then there's a non-trivial difference in contact patch because more road shock transmits from more contact patch vertically back up a stiffer sidewall (sidewall acts as a shock transmitter). A smaller diameter wheel's more rounded shoulder deflects force away from vertical and more into the horizontal resulting in sidewall flex (sidewall acts as a shock absorber). A stiffer sidewall transmits more force from ground back up into chassis. That can only come from normal (vertical) force. Normal force can only possibly come from more contact patch, nothing else. If you notice a difference in comfort going from 20" to 21", then you're feeling more normal force transmitted through increased contact patch. If difference in comfort is noticeable, then difference in grip is not trivial.

1" is enough for Z06 engineers to make a point of it multiple times. A rapidly turning wheel concentrates centrifugal force longitudinally, flattening more rubber towards the front of the car (not only the sides) over sustained duration. A 1" larger 21" wheel stretches the tire patch to an even longer length (not only width). If you cover your tires in paint and dab your car on the road like a paintbrush so it's fully parked under its weight, it would paint a not just wider but taller "11" on the road just parked with 21" wheels vs 20" wheels despite both having the same outer diameter. Because a same outer diameter tire flexes differently when wrapped around 20" vs 21" (the 21" flattens more of it against the ground like a taller, not wider, dough roller). Found the clip of Aaron Link talking to this at 5:38:
"The taller wheel or the longer tire patch is a huge enabler that gives the [contact patch] shape instead of a thin oval, it makes it more stretched out from a fore-aft standpoint. So it gives you more ability to put power down . . . more so than the width."