# Forums

 Rank: Driver's Permit #1 Posted : Friday, April 17, 2020 6:46:37 PM(UTC) I've heard different responses to this, with some people saying it only changes weight, while others say it makes it more grippy or responsive. So what does it actually do?
 Rank: Racing Permit  1 user liked this post. #2 Posted : Monday, April 20, 2020 6:13:49 AM(UTC) You can change the Rim style - which influences its weight and hence the unsprung mass of the car.Or you can change when wheel size - represented like this: 350/45R19In this case this means the tire has a width of 350mm, the sidewall is 45% the aspect of the width so 157.5mm. R19 means the rim diameter is 19", or 482.6mm. This makes the total tire diameter 797.6mm, and consequently a circumference of 2506mm - when optimally inflated of course.Now, given that the Forza simulation actually does take all these things into account, these numbers, the weight of the car on each wheel influence the size of the contact patch and hence traction the wheel can deliver. The circumference of the wheel determines how far forward the car moves per rotation of the wheel, and the wheel rotations are determined by the ratio setup in the gear box and the rpm of the engine.So the selection of tire size effects acceleration and top speed.Now for average drivers, its probably true that "only the weight" changes. In the middle of the performance band a number of effects come into play that ensure that most of the effects of changing tire size are unnoticed: For example: we know that, at rest, the weight of the car is the static mass on each wheel. And a mathematically rigid tire would have zero area touching the tar as only a line would have contact: to have a 2d contact patch the tire must deform. And pascals law says that pressure is proportional to the area: i.e. for a deformable structure like the tire it will deform such that the contact patch is supporting the pressure: If you inflate a tire to 22PSI then it matters not how big or small the tire is: it will be deformed such that 1 square inch touches the road for each 22 pounds of supported weight. So it's only by driving a car consistently on the edge of its performance envelope - because this is going to mazimize the dynamic forces - aero & friction - that you would be capable of noticing the difference. And notice the difference you will. People who say tire size doesn't make a difference simply are revealing they are not capable of making it make a difference.Edited by user Monday, April 20, 2020 6:16:26 AM(UTC)  | Reason: Not specified
 Rank: B-Class Racing License  2 users liked this post. #3 Posted : Wednesday, April 29, 2020 7:58:33 AM(UTC) Originally Posted by: DREAMS4LAD I've heard different responses to this, with some people saying it only changes weight, while others say it makes it more grippy or responsive. So what does it actually do?If larger rims and smaller sidewalls were always better for performance, LeMans cars would all be donks rolling on rubber bands. Larger rims with smaller sidewalls offer better responsiveness (less sidewall to flex), but it costs you weight, shock absorption, and contact patch size. Weight is the killer here, as a wheel of any material weighs more than a tiny strip of tire compound and the air to fill the additional space. Go with too-wide of a tire and you get a sloppy-handling car, even if it is faster in all other respects.For "normal" race applications, the sweet spot appears to be ~18". Yes, a car like the GT3 RS runs 20/21 staggered rims, but it's built for bankers who prefer a certain look. The GT3 Cup car built for racing uses 18" rims. The LMP1s run 18" wheels when they could run anywhere between 14" and 28".F1 has been using 13-inch rims for years, but that's a very specialized application (they're moving to 18s). Offroad cars use tiny rims, because the benefits of deformation far outweigh the cost in responsiveness in that setting, plus the tires are usually thicker to handle additional abuse.Anyway, drive what you like. If you like that donk look, have fun with it. If you like lap times, go with whatever works best for you in terms of weight vs responsiveness balance.