Quote:
Originally Posted by rkntoy
Chime! Actually you can use either number...
If you are trying to compare clearance to UCA or the frame, backspace is what you want to look at to compare apples to apples between wheels. Offset will change with different wheel widths so you end up having to calculate backspace in order to figure out distance from WMS to back side of wheel.
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section width of the tire and offset of the wheel of the exact size you want to run will determine everything. it's the only way to know for sure. backspace is only for wheel clearance. will the wheels fit? yes they will.
backspace tells you nothing really when it comes to tires. it might be import for clearance on the steering knuckle but... in the 4runners case that is not the issue. I agree there is a key measurement for clearance and each vehicle type/suspension variation is going to be different.
measuring from wheel mounting surface to the obstruction, in this case that is the UCA.
I know this measurement is 5.5" on my truck because I can measure it. at full droop. I prob should go measure it at full compression as well.
the tire I'm looking @ is BFG KM3 285/70 R17
according to BFG this tire has a section width of 11.5" ON 8.5"
wheel is 701 MR70178560500 so... 17x8.5 0 offset and 4.75 backspace
so that actually makes things easy because 0 offset, if we did have an offset we would have to account for it.
11.5" / 2 = 5.75" so this wheel tire combo is going to rub.
but this is good because it tells me how far I need to move things.
11.5" / 2 = 5.75" - 1/4" = 5.5" but that will rub so really 3/8" -10 mm offset would be the min value to shoot @. and you can't go to far unless you want to start hacking fenders off.
- offset moves tire out so you would take the (section width/2) - offset and that gives you the inside protrusion.
0 center line just split the section width
+ offset move the tire in so you would take the (section width/2) + offset and that gives the inside protrusion.
for 16" wheels this is different
If I want to go with 285/75 R16
SECTION WIDTH ON RIM WIDTH 11.3" ON 8"
16x8 0 offset backspace 4.5
11.3" / 2 = 5.65 so closer but still this would need a - offset to make up.
bottom line if you going to run bigger tires than stock... real world measurements are the only way to know for sure and of course once you mount the wheel and tire your out that $ for sure. trial and error.
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eimkeith
yea exactly.
