I think we have 10-15mm of clearance on Ensign Wedgeley Crusher (125mm wheels with 100mm of chassis). There are some extra bits that stick out underneath that can get him beached, but 18mm is way more than you need at the wheels so you've plenty of room to play there. From memory I feel like some feathers run with not much more than 5mm all round.

Kinda depends on where the weight sits relative to the wheels and other contact points as well though, so YMMV...