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Tire Fitment

Thanks to Tim Wright and Steve Reynolds:

I got out a Yokohama book today and opened Corel Draw to do some sketching..
From the spec book for AVS Sports. (both tires on 7.5" wide 45 mm offset rims)
 Tire size  Meas Rim Appr rim Sec width Dia  Tread rev / mi
225/55ZR16 7 6 - 8 9.1 25.7 7.9 808
 235/45ZR17 8 7.5 - 9 9.4 25.2 8.3 824

Now you adjust section width 2/10 for every .5" of rim width deviation from meas rim width so my 235 should be on a 8" but are on a 7.5 so 9.4 less 2/10= 9.2" sec width.
225 are measured on a 7" so add 2/10 to 9.1= 9.3" sec width.
Then as installed:
Tire size adj sec w
225/55ZR16 9.3 
235/45ZR17 9.2

So the 225 is a hair wider (.05" per side) than the 235 on the same 7.5" rim and the widest spot is about 1/2 higher.
If we assume max sec width is 1/2 way between the bead and tread and plot X section to scale.

The strut is parallel to the side wall and only a tenth or two from the 235/45 - 17.  You can see how it could rub on the blue line but not the grape color line.

If we put a compass level on the strut and clay (and section the clay) we could plot the strut. To get some idea of scale the rim "drops" a 1/2 inch between 17" to 16" rims.
A 1/2" less rim width is a lot and should move things over 1/4" an inch which is a also lot, or using spyders with 42 mm offset would move things over 1/8"  which may not be enough. One thing is true is the 235 on 17" rims must be run at 34 psi and don't move around or lay over much but the 225 with less air (30 psi) and taller side wall require more clearance in use. So 235/45-17 like mine can be run a lot closer to the struts.
If I can clay/section the strut-tire fit and get an angle on the strut this weekend I will, so I can draw it in.
I hope this starts to explain why a "narrow" tire on a 16 rims does not fit but a "wide" tire on a 17" rim does.

Steve Reynolds adds:

This isn't SHO specific but on several other cars I've seen/own, when the tire to strut clearance is less than 1/2" and especially under 1/4" there is evidence of the tire hitting the strut.  When cornering the tire is pulled over and it can rub the strut.  As Tim states the wider lower profile tires often will not rub as bad because the shorter sidewall doesn't allow the tire to move as far.

Sometimes you will have a minor rubbing problem and not even realize it.  Look for a  clean spot on the strut when you remove your tires.  If the tire is lightly rubbing a small spot on the strut may look clean.  In worse cases the paint will be worn away.  

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