On my BX 2200, with I think inch and a quarter spacers, the left gage wheel assembly will just hit the tire when the deck is raised a bit above the 4 setting. I don’t think inch and a half spaces would work on mine at most deck heights.
When you say steep slope, how many degrees is that slope?
I had my four tires foam filled by an industrial tire shop to increase stability. Actually, the front were foam filled first to prevent punctures. Puts a lot of weight down where needed for stability.
My BX with the foam filled tires and the 60 inch MMM hanging down low is quite stable. This is about my limit (pucker factor) for sides slopes though. I did get off on the high side and push on the ROPS hard and it was rock solid. Of course, that was a static test. The dynamics are different when the tractor is moving. Mowing a cross slope only when it is dry is good advice.
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