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FAQ - More Knuckle Clearance Detail

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wret

15+ Year Contributor
2,931
81
Jul 3, 2003
Bel Air, Maryland
I still can’t make sense of the knuckle / tire clearance. Can you simplify it?

Not really but this exercise might help. Below is a 2D graphical model of the clearance between the rear steering knuckle and the tire. The baseline is a 205/55/16 Kumho 712 tire on a stock 46mm offset wheel. To make this really accurate, we need to know a little more about the tire. Many manufacturers post specifications for their tires. What you need to know are tread width and diameter (which you will divide by 2 to get the radius). You will also need to convert all measurements to millimeters (25.4 to an inch).

Specs. For Kumho 712 205/55/16:

Tread width: 183mm
Radius: 316mm

These dimensions are pictured on the illustration in the (0,0) position. Now let’s say I am considering a change to a larger tire, Azenis ST-115 245/40/18, and I want to know if it will fit. Dimensions of the ST-115 are available on the Falken web site. They are:

Tread width: 221mm
Radius: 326mm

To find the clearance for the new tire subtract the baseline from the new width. Then divide by 2.

221-183 =38
38 / 2 = 19

19 will be the “X” coordinate.

Next, Subract the baseline radius from the new width

326 – 316 = 10

10 will be the “Y” coordinate.

Place a mark on the chart at (19,10). (19 to the right, 10 up).

Now factor in an offset change by counting back to the left one unit for every millimeter less offset. In my case, offset will go from the baseline 46mm to 38mm, so I will count 8 units to the left. That’s where the closest point of my new tire will be in relation to the knuckle.
 

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  • knuckle grid2.bmp
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Anyone care to try this? It needs a little validation.
 
wret said:
Anyone care to try this? It needs a little validation.

Hey Wret if I'm doing it correctly it seems fairly accurate.If you remember a while back I asked you about the 245/45/17 42 Offset and it actually hit right there as it shows on the chart, and required spacers. Looks accurate, awesome! :thumb:
 

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  • knuckle grid2.bmp
    149.2 KB · Views: 2,086
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