When you're choosing a spool, take note of the weight of it. A heavier spool will create more understeer going into a corner compared to a lighter weight spool. The last time I tried a spool it was completely made from steel and I came off the track hating it, later I found out the weight of the spool was probably the culprit for the excess understeer. I'll try a lightweight one the next time I decide to test them.

Remember because a spool is a locked front diff essentially, it will cause some understeer on corner entry as the wheels fight each other. On corner exit they really help you get the power to the ground though.