Originally Posted by
prathmann
One of my bike bells had a hemispherical compass on top so it was unaffected by mounting angle. Didn't work well on my Bike Friday with a steel stem, but was fine on my touring bike. I only bought it for the bell since my GPS shows my direction.
I had the same problem using a compass bell on my touring bike, aluminum stem but the steel bolts that held the handlebar to the stem caused excessive error on the compass. I suspect that the stem cap compass would have a problem with a steel steerer tube and the steel star nut.
If I was going to put any kind of "instrument" on a stem cap, it would be a thermometer. I toured with someone that had one and we often looked at it in the morning.