Caravanserai
from the workshop. For every tradepost you have, the standing is improved by 0.35%. The base standing is 70% for Zebra's. Note that there is the diminishing effect, explained here) . This percentage is the percentage of 'normal' trades for hostile races, such as the Zebra's. A random number between 0 and 100 is generated, and when that number is higher then the race standing, there is no trade at all (and hence no titanium). Higher standing results in more trades, more trades results in more titanium.