unless you use a bracket/frame behind the wheel which turns with the wheel, for which you don´t have enough room in the case of the R18, you need to/should mount the sensors in line with the steering axis, otherwise you won´t measure at the same point, when the tire turns --> e.g. the outer or inner sensor would "see" outside the tire thread.
Add to this, that you need a minimum distance for the sensor to work, and you maybe have your answer.
The R18 does not has inner wheel arches (as the DTM car had/has), therefore there are some limitations, where you can mount the sensors.
if you have a look at this photo in HiRes, you see the holes for the sensors in the front bonnet, but closed with alu tape, the car in the background has the sensors mounted
http://farm6.static.flickr.com/5012/540 ... d9e1_o.jpg
BTW, a possible design for the R18 ??
