The transmitters (pickup sensors) have a very short range, so closer to the rim will also be closer to the "computer". I'd experiment with the position, but realize that as the battery runs weaker, the "transmitter" range will be shorter and you'll be buying your battery more often. Before it all goes really dead, the operation will be intermittent and you'll most likely wonder what is going on. The sensor is just an encapsulated reed relay that gets operated by the passing magnet mounted on the spoke, in all the systems I've seen. There could be other methods, but this is best for low electrical interference sensitivity and low cost. The distance between the pickup and the magnet mounted on the wheel is more critical than the time to engage. If the time to engage the magnet by being at the magnetic field longer is so sensitive, they are either using inapropriate relay or the system is so marginal, it'll be unreliable and intermittent anyway 9sometimes not engaging, sometimes chattering). I'd primarily concentrate on reducing the distance between the magnet and the pickup. Some of the magnets maybe pretty big and heavy, and depending on how light and well balanced your wheels are, having the magnet towards the rim will exhibit much higher inertia and therefore inballance of the wheel. That would be much bigger argument against being by the rim with the sensor, than the time for the magnet to be engaging the reed relay. So I'd get as tight between the sensor and the magnet as possible and then as close as possible to the hub while still having a reliable operation with weaker batteries.