I know it is apples to oranges (sorry for going a little sideways on topic), but I was surprised by our recent experience on the new trailer for our ballista. We wanted the tops of the fenders to be flush with the top of the deck and thought we had plenty of clearance built in with the axle under spring; I could put my fist in between the top of the tire and underside of the fender with thumb up and heel down, so about 4 inches of clearance.
However, after loading the heavy ballista on I could just barely fit my fist in there going the other way; so only about 2-1/2 inches. We checked to make sure there wasn't any scuffing of the paint under the fenders, especially after a few good jarring road "hits", and were relieved to find that the tires never hit.
Again, these were tandem 7k lb axles with about 10k lbs gross, but I was really surprised that it didn't hit. I guess once the leaf springs are squatted down it takes progressively more to move them.
YMMV, just saying.
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