Ultima ZT1 50 Spring #587613301 Update: I did get a great bit of improvement out of this spring modification. But I thought the ride would get better from a softer spring. I searched at local mower places and tractor supply and found nothing. Checked the internet but nobody gives the diameter of the spring strand. Fortunately, i was reading comments on this post and Scott Hamilton mentioned his tires were drastically over inflated. My rears were somewhat over inflated. They call for 10psi max and had 12.5 psi. I set them at 7.5 and this is much better. So check your tire pressure BEFORE doing this modification. Be very careful with the front tires; the air goes out much faster. Mine have a max of 22 psi and was at 12.5. When i tried to bring them to 7.5, it basically came of the rim at about 10 psi. I had to remove it from mower and fight for about 40 minutes to remount to the rim. Not sure whether Cub Cadet or Home Depot overinflated the tires. I thought mine was fine since they have such tall sidewalls and are not low profile which definitely have a stiff ride. Thanks for the tip Scott.