Car Noob
08-01-2010, 02:36 PM
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Car Noob
Hey so I'm trying to figure out what I want for rims but I don't quite know where to start because I don't know much about cars... I need help with the wheel size... Etc what exactly the offset does, and the width of the rim does to the tire.... And how to know what size tire to get, and what tire sizes mean... And how to know if tires are low profile... Lol thanks for the help to whoever tries to teach me
08-01-2010, 03:00 PM
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(Experts, correct me if I am wrong in any of this)
1 - Your user name will generate alot of flames on you on these boards
2 - Rims size format:
18x9 +40 = 18 inch diameter rim that has a width of 9 inches, and the offset is 40mm
3 - Tire size format:
225 40 18 = the tire's tread width is 225 millimeters. The side wall is 40% of that width. It is sized to fit an 18" rim
275 15 22 = 275mm tread width, side wall is 15% of that, and it fits on a 22" rim.
5 - What each does:
Rim diameter - Determines the base size of the tires possible. There is no "better" size. Judging by your avatar, you have the standard OEM 18" rims, which means you have the upgraded brakes that comes with the sport suspension package (standard on most RX-8s, but not all). This also means that you can't go lower than 17" rims, as 16" rims do not clear your brake calipers. Upsizing to 20" rims MIGHT be possible, but probably alot of fender work to make them clear everything.
Rim width - Determines the range of tire widths possible. On the OEM 18x8s, you can do a 205 to a 245 without issue. 255 is possible, but you will get alot of bulge from the tire as it is too wide (reasonably) for the rim, but possible. 225 is ideal for 8inch wide rims. Going higher makes the tire sidewalls bulge, going lower makes the adhering base stretch.
Offset - Specifies the distance from centerline that the wheel hub is. If the wheel hub is exactly down the center line (in to out) of the wheel, this is an offset of 0. Negative offset means the wheel hub is moved towards the inside of the wheel, which has the effect of pushing the wheel farther out from the car (since the hub location is fixed). Positive offset is the reverse, moving the hub closer to the spokes, and the wheel further into the car. You can't have too high of an offset or the wheels/tires will rub against the inside of the wheel well, too low or the wheels/tires stick out from the wheel well. Going flush means that your tires and rims perfectly line up vertically with the fender. Keep in mind that the closer you get to sizes that don't fit, you will run into issues in the front before the back, usually, due to the fact that those wheels/tires have to turn.
Tire tread width - Determines the amount of base tire gripping surface you have on the road. Wider is usually better, but to a point. Wider causes fender issues eventually, and the wider you go the more steering feel you lose. Whether this is worth the traction you gain is subjective to 99% of people. In a purist track mentality, there are arguements for each, to the point that on autocross some people endlessly debate whether 245 or 255 is better, as most feel that they are each faintly on each side of the steering-feel/traction point of perfection. Usually car setup and driver skill matters far more than that 10mm.
Tire sidewall - Expressed as a percentage of the width, the sidewall is the last component that determines the total rotational circumference, and also acts as a spring. A taller sidewall is more flexible, a shorter sidewall is stiffer. Stiffer increases steering response, as the tire flexes less before the steering input is applied to the road, but there is less absorption of bumps, jolts, and hitting potholes and such generally means more likely to have rim damage. More flexible is the opposite of each of these. 35-40 sidewalls are usually best for a combination street/track Low profile is usually 30 or lower.
All of that being said, your OEM size is 18x8 +50 225/40/18. The most popular size to change to ix 18x9 +40-45 245 or 255 / 35 or 40 / 18. Get better traction and, if you are smart about wheel weights and go to a lighter rim, you also reduce unsprung / rotational weight which improves most dymanics of the 8. The prices are also usually similar to OEM sizes, and moving up much from that starts increasing the price drastically.
1 - Your user name will generate alot of flames on you on these boards
2 - Rims size format:
18x9 +40 = 18 inch diameter rim that has a width of 9 inches, and the offset is 40mm
3 - Tire size format:
225 40 18 = the tire's tread width is 225 millimeters. The side wall is 40% of that width. It is sized to fit an 18" rim
275 15 22 = 275mm tread width, side wall is 15% of that, and it fits on a 22" rim.
5 - What each does:
Rim diameter - Determines the base size of the tires possible. There is no "better" size. Judging by your avatar, you have the standard OEM 18" rims, which means you have the upgraded brakes that comes with the sport suspension package (standard on most RX-8s, but not all). This also means that you can't go lower than 17" rims, as 16" rims do not clear your brake calipers. Upsizing to 20" rims MIGHT be possible, but probably alot of fender work to make them clear everything.
Rim width - Determines the range of tire widths possible. On the OEM 18x8s, you can do a 205 to a 245 without issue. 255 is possible, but you will get alot of bulge from the tire as it is too wide (reasonably) for the rim, but possible. 225 is ideal for 8inch wide rims. Going higher makes the tire sidewalls bulge, going lower makes the adhering base stretch.
Offset - Specifies the distance from centerline that the wheel hub is. If the wheel hub is exactly down the center line (in to out) of the wheel, this is an offset of 0. Negative offset means the wheel hub is moved towards the inside of the wheel, which has the effect of pushing the wheel farther out from the car (since the hub location is fixed). Positive offset is the reverse, moving the hub closer to the spokes, and the wheel further into the car. You can't have too high of an offset or the wheels/tires will rub against the inside of the wheel well, too low or the wheels/tires stick out from the wheel well. Going flush means that your tires and rims perfectly line up vertically with the fender. Keep in mind that the closer you get to sizes that don't fit, you will run into issues in the front before the back, usually, due to the fact that those wheels/tires have to turn.
Tire tread width - Determines the amount of base tire gripping surface you have on the road. Wider is usually better, but to a point. Wider causes fender issues eventually, and the wider you go the more steering feel you lose. Whether this is worth the traction you gain is subjective to 99% of people. In a purist track mentality, there are arguements for each, to the point that on autocross some people endlessly debate whether 245 or 255 is better, as most feel that they are each faintly on each side of the steering-feel/traction point of perfection. Usually car setup and driver skill matters far more than that 10mm.
Tire sidewall - Expressed as a percentage of the width, the sidewall is the last component that determines the total rotational circumference, and also acts as a spring. A taller sidewall is more flexible, a shorter sidewall is stiffer. Stiffer increases steering response, as the tire flexes less before the steering input is applied to the road, but there is less absorption of bumps, jolts, and hitting potholes and such generally means more likely to have rim damage. More flexible is the opposite of each of these. 35-40 sidewalls are usually best for a combination street/track Low profile is usually 30 or lower.
All of that being said, your OEM size is 18x8 +50 225/40/18. The most popular size to change to ix 18x9 +40-45 245 or 255 / 35 or 40 / 18. Get better traction and, if you are smart about wheel weights and go to a lighter rim, you also reduce unsprung / rotational weight which improves most dymanics of the 8. The prices are also usually similar to OEM sizes, and moving up much from that starts increasing the price drastically.
Last edited by RIWWP; 08-01-2010 at 03:40 PM.
08-01-2010, 03:28 PM
#5
OEM wheels are 18x8, +50mm offset, 5x114.3 bolt pattern, 67.1mm(?) bore.
OEM tires are 225/45-18.
http://www.miata.net/garage/tirecalc.html -- This tool is your friend if you intend to go with non-OEM size tires. Also, check the "Will it fit?" thread for weird wheel sizes.
OEM tires are 225/45-18.
http://www.miata.net/garage/tirecalc.html -- This tool is your friend if you intend to go with non-OEM size tires. Also, check the "Will it fit?" thread for weird wheel sizes.
Last edited by maskedferret; 08-01-2010 at 03:39 PM.
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