In an earlier contribution to this discussion,
Tim McNamara said:
A friend of mine wants to get rid of a set of four alloy wheels and
tires off an 850- by having me take them for my car. I've got a 240
and while the stud spacing measures correctly, eyeballing them it
looks like they are too wide (by a good 2" if not 3") and the
diameter looks too large. Am I right? If so I'll probably just put
them up on Craigslist for him.
There are three considerations in addition to whether the stud holes are in
the right place:
1. Rolling Radius
You can calculate this reasonably accurately by adding the rim radius to the
tyre width times the aspect ratio. This needs to be within a few percent
from old to new tyre, otherwise the gearing and speedometer/odometer
readings will be wrong. You can sometimes get away with a larger rim and a
lower aspect ratio tyre.
2. Tyre Width
If the new tyres are substantially wider than the old ones, there's a good
chance that they'll foul the bodywork on full lock or sever bumps - it
depends on how much clearance there is to start with.
3. Offset
This is the horizontal distance between the plane through the centre of the
rim and the face which bolts to the hub. The wheels on front wheel drive
cars invariably have a larger offset than those on rear wheel drive cars to
cater for the constant velocity joint. If you fit FWD wheels on a RWD car,
the centreline of the tyre will be too far inboard - reducing the track and
increasing the likelihood of body fouling. You can sometimes compensate for
this by using spacers.
--
Cheers,
Roger
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