[heydowns]

We've always had much better luck with optical encoders than GTSs.

The GTS needs a significant amount of ferrous material, and it needs to be extremely close (as previously posted).

Our rookie year, we made optical encoder sensors and discs. The discs were evenly punched aluminum mounted on our wheel axel. The sensors were like those inside a computer mouse (the ball mice, not the new optical style).

Last year we used similar sensors for our shooter wheels to monitor/adjust speed. Here, instead of a punched disc, we made L-shaped, velcro mounted "trip tabs" from lexan.
This was after we failed to find enough mass around the shaft to trip a GTS.

The other nice thing about the optical sensors is that they are very cheap in comparison to pre-fab GTSs.