Axle length can definitely make a difference for the same track width. If you are running at maximum track width, and want to lose a little bit of rear grip without changing track width, a shorter axle can make a difference. But then so can lots of other things that are quicker and cheaper to change.
Grades of axle is a can of worms. Can't see it myself from a scientific/engineering perspective, as the bending stiffness with all grades of steel is very much the same. However they do feel a tiny bit different from each other when I've tried changing from medium to hard. When I say a tiny bit, nothing like as significant a difference as changing the track width a few mm or loosening the seat stays.
Can't help with the seat setup person, but can say that although it was frustrating at first, and I turned a few seats into Swiss cheese, it's really useful to spend a day or 2 trying different seat positions. It will also help your son learn to give you feedback on what's different about the way the kart handles as the seat placement is changed, and to get the best from the kart as the way it behaves in corners changes. Think I probably found 1.5 seconds of lap time working on seat position and kart setup, but more importantly than that I learnt a lot about kart setup and driving. Feed yourself for a day, or learn to fish?
You'll get mixed replies here as well, especially about axle grades!
Liam
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