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Hi Jay, you're pretty much correct on all counts. Offset is measured from wheel center line, so changing from 9" wide ET55 to 10" wide ET55 adds 1/2" to the inner wheel section and 1/2" to the outer. Changing from 9" wide ET55 to 10" wide ET67 would basically add the entire 1" to inner section, etc.
For reference, the 3.6 Speedline wheels come in 18x10 ET61 at the rear but they leave a fair bit of gap between wheel and fender. Raj suggested 8mm spacers which I've tried [effectively ET53] and this still leaves room on the outer + the original 8mm that were borrowed from the inner section, therefore I think 18x10.5 ET55 would pretty much be perfect. I'd be slightly concerned about 18x10.5 ET60 rubbing the inner wheel well but you could add a thin spacer, however it still depends on what size tires you're running. 265 would be too narrow, 285 could rub and I'd want 295 or 305 anyway to take advantage of that extra 1/2". This is the part that I see as the dilemma...
So, when you add the spacer, it effectively reduces the offset?
Also, each inch in width correlates to a value of 16 in offset - is this accurate (based on10.5 with ET of 61 being repositioned wth an 8 mm spacer).
Each inch in width is split evenly on either side of the centerline.
Without have your old car her to go outside and look at, do you know if the space on either side of centerline is evenly spaced to the contact points (inner fender edge and wheel well)?
Yeah, I'm probably making this harder than it has to be, but I've spied a set of rims that I really would like to make fit if possible without contact and without rolling fenders.
Last edited by Jay Wellwood; 01-28-2015 at 10:38 PM.
Reason: bad math
Yea, adding a spacer lowers the offset because it reduces the gap between wheel center line and the wheel mating surface. Remember, increasing the wheel width 1" adds half to inner and outer ASSUMING that the offset stays the same. Adding the 12mm [technically 12.2mm since 25.4mm = 1"] to offset would essentially take the outer 1/2" and move it to inner, so you'd add the entire inch to inner.
I don't know if inner and outer fender are evenly spaced from the hub, that's why I gave the 3.6 Speedline example which is the closest practical one I'm aware of that you can estimate whether or not the 10.5 fits. The biggest issue though is tire size like I mentioned. They will probably work fine if you have a couple thin spacers available OR you go with a narrower tire like 265. Anything beyond that and you're in uncharted territory for 968's my friend, but that makes things more interesting.
Not sure if this will show up well, but I've attached a scaled sketch showing the change in wheel position based on increased wheel width that also takes into account the offset.
In both figures, the right side of the figure is the 'inside' face of the diagram.
You'll also see the original designed size 968 17" (with appropriate offsets) wheels.
To complete this figure, all I'd need is the relative position from the face of the wheel rotor to the inside edge of the fenders as well as relative position between the face of the rotor and the inner wheel well edge. With this information added, I could model just about any wheel/tire combination to determine what the clearances are PRIOR to laying down ca$h and try to install to confirm fitment.
Hope the below helps. What I did do with my 968 Sport was to swap the wheels and tyres for a spare set of 964 Cup 2 wheels I had lying around. Offsets now being;
> Front - 7½J x 17 | wheel offset 52 | tyres - 225/45 ZR 17
> Rear - 9J x 17 | wheel offset 47 | tyres - 255/40 ZR17 98Y
Even with the ride height lower, as per CS, the wheels and tyres fit nicely with no rubbing and no spacers. When out on the track after the change it actually felt a little more 'planted'. But then with the smaller offsets (effectively 'pushing' the wheel outwards) there is a slightly wider track and hence a slightly lower centre of gravity.
Hope the below helps. What I did do with my 968 Sport was to swap the wheels and tyres for a spare set of 964 Cup 2 wheels I had lying around. Offsets now being;
> Front - 7½J x 17 | wheel offset 52 | tyres - 225/45 ZR 17
> Rear - 9J x 17 | wheel offset 47 | tyres - 255/40 ZR17 98Y
Even with the ride height lower, as per CS, the wheels and tyres fit nicely with no rubbing and no spacers. When out on the track after the change it actually felt a little more 'planted'. But then with the smaller offsets (effectively 'pushing' the wheel outwards) there is a slightly wider track and hence a slightly lower centre of gravity.
Great illustration.
Interesting...any idea how much clearance exists between the inner fender well and the inside edge of the tire?
...any idea how much clearance exists between the inner fender well and the inside edge of the tire?
Just checked in limited light conditions from the outside of the fender/wing (unable to see from the inside of the fender). The rear, as it is, has a 12mm (1/2") clearance or gap from a dead vertical outside fender line to the out-most point of the tyre wall rubber.
The front has a slightly greater 20mm (3/4") clearance or gap of the same. Both front and rear tyres will just about clear the fender arch if pulled straight out at rest - the rears with about 5mm ride height clearance before fouling the arch. The fronts do not rub on full turning lock so all good.
The way I try to remember it (it's easy to forget or get confused with offsets) is the bigger the offset the further the wheel has to go in to the wheel arch to reach, and bolt to, the (rotor) hub. The spacers are to return the wheel closer to it's original location.
Smaller offsets make the car track wider - looks meaner too! All so long as you don't start to rub the inside fenders.
What I sometimes wonder is what is the max tyre width I can fit on a 17" 9J wheel. I changed the 225's for 255's quite easily and the grip is loads better. Wonder if there's a calculator, ratio or rule of thumb about min/max tyre widths for a given wheel. Also the tyre widths quoted are the section width and not the actual contact or tread width. Might mean the smaller your profile the more the section width gets closer to the contact/tread width.
The way I try to remember it (it's easy to forget or get confused with offsets) is the bigger the offset the further the wheel has to go in to the wheel arch to reach, and bolt to, the (rotor) hub. The spacers are to return the wheel closer to it's original location.
Smaller offsets make the car track wider - looks meaner too! All so long as you don't start to rub the inside fenders.
What I sometimes wonder is what is the max tyre width I can fit on a 17" 9J wheel. I changed the 225's for 255's quite easily and the grip is loads better. Wonder if there's a calculator, ratio or rule of thumb about min/max tyre widths for a given wheel. Also the tyre widths quoted are the section width and not the actual contact or tread width. Might mean the smaller your profile the more the section width gets closer to the contact/tread width.
Ahhhh yesssss...brings back memories of the contact patch debates on the 928 boards from back in the 00's!
01-28-2015, 10:01 AM
Yet another Wheel Offset Question
