The reason you didn't find an 'explicit general' answer... is because
it's relative. The stock set up for the TT and the NA are even different
from each other because of HICAS linkages needing to be cleared in the
rear wheel well. But... even at that.... stock TT rims from what I remember
:
front 16x7.5 45 mm offset
rear 16x8.5 35 mm offset.
Offset, from what I recall, is the measured distance between the center
line of the rim to the mount point of the rim. We have positive offset
on our 90+ rims. The more negative offset, the more of the depth... the
more positive, the more the center piece protrudes out relative to the
centerline of the rim.
If you are JUST considering the rims... you would be making a mistake.
Depending on the + taken on the rims (+1 to get 17", or +2 to get 18"),
do a corresponding drop in the profile to compensate. Depending on how
much compensation with the profile... it's possible to bring the total
diameter back to stock wheel (tire and rim combo) levels.
That's ONE aspect.... and how that affects the 'fitment'... is obviously,
someone upgrading will probably want to increase their tread width., and
treadwidth is NOT the contact patch on the ground as many would believe,
but I believe the distance measured from one sidewall to the other sidewall.
Although in most cases, this is proportional to the tread contact patch...
I'm saying this from a purist stand point. At least, that's what a B.F.
Goodrich speaker once told
me when he gave a little talk on tires, rims, etc...
So, as you know.... if you increase the width of the rim and increase
the width of the tire, you risk a number of things. In front and rear.
You risk :
front
-----
1) tire rubbing against fender lip on the outside
2) tire rubbing against wheel well on the inside
3) rim not clearing the brake calipers.
4) tire rubbing the upper suspension member.
rear
----
1) same as above.
2) same as above.
3) same as above.
4) for a TT, you have to make sure that it clears the HICAS linkages.
Sometimes, even the SHAPE of the rims (not the rim itself, but the 'face'
of the rim, will help or hinder in this fitment process. Some rims kind
of 'bulge' out in their design. So, with the same offset, and the
same width, the design will easily clear a larger brake caliper set-up
then say... stock.
Then again, another design (which is rarer), that has the spokes (or
whatever you want to call the things that join the mount point to the rim)
beveling inwards, would actually be WORSE then stock for clearing the calipers
given the same rim width and offset.
Personally... from a purist stand point... I'm not a big fan of using
spacers to 'make'/'help' a rim fit for clearance, etc.. I know a few who
have used these with much success... and then others who have had vibration
problems, etc...
If you do use spacers, make sure you use the long racing style wheel
studs.
I have seen someone with 285's in the rear with no rubbing, but I have
seen someone with 265's in the rear... w/ rubbing. The offset and rim width
places a pretty tricky roll in getting a proper fittment.
Sorry... there is no panacea. Wish there was.
Andy Woo
With the stock brakes a 45mm offset will allow running of a 285 series
tire on a 10 inch wide rim.
Some light contact on the plastic fuel filler shield on the drivers
side but it is very light and of no consequence.
With 17 inch wheels you are stuck to 9.5 inches and near 30mm of offset
in the rear because the HICAS link will hit the inner wheel lip if you
go wider or more inboard. Certain styles of 17x10" wheels can be run shaving
off part of the HICAS link gives the clearance needed, although it's not
recommended. With 18 inch wheels you can go wider and more inboard because
the larger diameter of the wheel allows the HICAS link to ride inboard
of the wheel.
When choosing the rear offset be aware the HICAS causes the rear wheel
to turn. This makes clearance between the inner rim and lower control arm
near the leading edge of the arm critical. More than 45mm inboard offset
is not possible because it will contact under HICAS operation. Depending
on how much the rim lip sticks out it may contact lightly which can be
easily solved by slightly bending the hemmed flange of the lower control
arm carefully with a
small crescent wrench. It is only a light rub so the wheel won't get
torn off the car.
On non HICAS cars this isn't an issue and 50mm of offset can even be
run.
It sounds terribly complicated but it is really quite simple to run
steam rollers in the back.
Mike Kojima
