hello ulf,
sorry, but i have to correct your post:
pitch cercle is 98mm, o.k. no prob.
BUT: offset is not an absolut thing, the offset is absolutely depending on the rim width. so you can't say at all that matra is using offset 28mm ... without giving the rim width. my bagheera for example has fitted 7x15 off 13.
the offset ist the distance between the middle of the rim and the mounting face. so if you mount a 7"-rim instead of a 6"-rim with the same offset, you might get some problems as the 7"-rim is placed with its centerline at the same position as the 6"-rim, but so the 7"-rim will be larger half an inch to both sides. and this may be touching suspension parts or even chassis at the inner side.
... this is the classic list of peugeot 205 GTIs rims and you see, offset is not an absolut thing at all.
greetz, kris
Sorry to contradict you Kris but the correct offset IS a fixed amount. It should not be altered with the wheel width. This is a common mis-conception. The mounting face to the centreline of the wheel must stay the same otherwise the wheel bearing will suffer. The whole point about the offset is to line up the centreline of the wheel with the centreline of the wheel bearing so the loading is taken vertically through the bearing. Anything different puts a side torque on the bearing. Wheel offset is not designed to align the wheel within the body or clear the suspension.
You state it yourself above - with the same offset the centreline of the wheel will be in the same place and the wider wheel will have extra but equal amounts inside and outside. This is EXACTLY what you should be aiming for. Anything else puts a side torque on the wheel bearing and they will fail in a shorter time and you may damage other components. Any deviation from standard also alters the suspension geometry and can affect the handling. A larger offset wheel will reduce the track whilst a smaller offset wheel increases the track width.
With low mileages and only a small deviation from the standard figure it will probably be fine, but you should not change it by much. A few mm is all, so somewhere around 26 to 30mm for the Bagheera and Murena. You could use a small spacer to bring a 38m offset Italian wheel (Fiat/Lancia/Alfa) to the right position and use proper longer wheel bolts, although they would not be my choice. These spacers must have the proper centre spigot size as the wheel centralises on them (not the bolt holes).
Agreed, if you use a wider wheel then the inside edge gets closer to the suspension, which is why you have to limit the width of the wheel you fit. You can only go so far above the original. And due to suspension movement and general flex, whatever clearance is there when static, will become reduced under road conditions, so you need to allow extra clearance compared to that visible when stationary.
Roy