Try a hiking gaiter. In the summer I use a Mountain Hardwear Men's Ascent Stretch Air Perm Gaiter because it is much cooler than my winter gaiters.
If the problem is just with the area of your pants below the shine, then you can use a lower gaiter.
Many gaiters designed for winter use have an area of protective cloth on the inside ankle area to protect against tears and abrasions from ice spikes and crampons, so they certainly should hold up to an occasional brush with a bike chain.