The back spacing measurement is critical in the fitment of the wheel (and tire) to the vehicle. Since the suspension, brake, steering and drive systems are typically located behind the wheel, the back spacing is used to define a volume behind the wheel where these items can exist.
* A related term is known as offset, which relates the hub mounting surface to the centerline of the rim. A 0 offset indicates the hub mounting surface is at the exact centerline of the rim. In this case, the back spacing would then be equal to 1/2 the rim width
(plus the thickness of the bead lip on the rim - see below).
* Offset is measured such that positive offsets mean the inner lip of the rim is closer to the vehicle and negative offsets move the rim away from the vehicle, (see image 2 above).
o Offset is usually measured in millimeters (mm) and often has the designation "ET" prepended to the offset, so a 19mm offset may be listed as ET19.
* Note that offset and backspacing are related but measured at slightly different points.
o For example, I have a set of 15x8 alloy rims, with a -19mm (3/4") offset and the backspacing measures 3.75". So using the centerline at 4", subtracting 3/4" from that gives 3.25" for the rim mounting surface, which is measured off the inside face of the inside bead. So where doe the extra 1/2" come from (3.75" - 3.25")? Its from the thickness of the lip of the rim, in fact a tape measure shows the distance from the inside face of the bead to the outside of the rim lip to be 1/2".
* Backspacing and offset are two different measurements of essentially the same thing, however they are opposite in sign
o So more backspacing means the wheel sits in closer to the axle and that less of the wheel's width will appear outside of the wheel moutning flange, giving a narrower track.
o More offset means the wheel mounting flange is closer to the inside of the wheel some consequently more of the wheel's width is to the outside, giving a wider track.
Hope that clears up the wheel stuff. For more info check out this link (
http://www.4crawler.com/Diesel/Tires.shtml#WHEELS) Great stuff on that site.
Rubbing.. All depends on wheel specification and tire type (A/T vs M/T). With A/T's the rubbing will be more limited with proper wheel specification vs M/T's. Upon full compression expect rubbing regardless whenever you upgrade tires to a significant size increase. Rubbing at the upper control arms can be overcome by upgrading the uca's or changing wheel specification.
16x7 comes w/ 4 7/8" BS I believe. Not sure exactly but it's something like that from what I can recall (please double check tough as I could be wrong). The 17x7.5's I have no clue since I don't own a set.
285's are possible but just trim accordingly. It's not the end of the world and you don't need to tear your 4runner apart. Most of it will rub along the inner fender (plastic pieces) and the end of the front bumper trim. You'll want to cut a good section of it. Most of the rubbing occurs duing full compression at full lock (turns).
Good luck
mtnrac
er said:
Correct me if I am wrong as I am new at this wheel stuff: How can you have an 8 inch wide rim with 4.5 inch backspacing and 0 offset? If the wheel is 8" and backspace is 4.5", wouldn't offset be 0.5"? Not sure if this would be positive or negative backspace though.
Check out this site for details on this:
http://www.rsracing.com/tech-wheel.html#backspace
Good Times, I know your rig is totally custom and you are running 315s. Do you know if the 285s will rub on the front suspension knuckle? My 265s are already really close to the knuckle. Will the backspacing on the wheel help with that?
Does anyone know what the stock Toyota backspacing and offset are on standard toyota 16" or 17" wheels?