I always start on the medium axle.
It’s hard to say what will happen because you say it’s stuck but you also say it’s oversteering, which are opposite issues.
It’s also hard to generalize what changes will offer what affect when talking about “grip”. Think about it in terms of “inside rear wheel lift”, which is what we are really affecting when we make chassis adjustments.
A narrower rear track will give you a more reactive turn-in, helping the kart tip easier and transfer weight to the outside tire more easily. This will give you more lift and generally free the kart up. However, if you already have too much lift, you’ll only be hurting yourself further by narrowing the rear track because you’ll only free the rear up more. Widening the rear track will slow down the lifting action of the inside rear wheel and make the kart more stable because it isn’t hiking the inside rear so harshly. But slow down that lift action too much and the inside rear may never get off the ground and the kart will understeer and be flat off the corner.
If the kart is “stuck”, it’s because the inside rear wheel isn’t up. The kart isn’t rotating. If it’s oversteering, it’s because you’re lifting the inside rear too harshly.
It’s possible on new tires that the increased grip has made the kart more front grip biased, causing your initial turn-in to lift the rear more harshly and give you a flat kart or oversteer when the outside tire breaks traction.
I think the issue here is your 30mm axle is too soft, causing the kart to lift, but provide no “spring” or be able to maintain the load put on the outside rear tire in the corner. This will cause the kart to either break traction and slide on exit, or cause the kart to be flat on exit when there’s not enough power/corner speed to slide the rear.
I always try to get drivers to break the corner down to 3 points; entry, apex, and exit. If they can tell me what the kart is doing at each point, it’s easier to figure out what is causing the issue.
I almost never tune with hubs because they not only change the stiffness but they change the rear track width too, which can cause contradictory results. I run medium hubs exclusively.
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