I wasn't really happy with the wire pick-ups on the O&K, they didn't seem flexible enough to make good contact without applying too much drag to the wheels. So at Brighton I picked up some phospher-bronze strip, which is much more springy.
The strip was easily soldered to the PCB on each side, and with a little bending the pick-ups could be arranged onto the backs of the wheels without shorting on the block. The result makes much better contact than the wire, with far less friction, and from the test run I think it runs better too. Plus in my experience pick-ups on the backs of wheels suffer less from dirt build-up in use than those acting on the treads.