Had the same thing happen to our 1999 R72. We had to take them into the machine shop and added 1_2 inch ring around the inside to give more support. Kurt
Check out www.gemplers.com as they have the reinforcement rings for sale. Don't know all the sizes they have, but used some on tractor wheels and seemed to work well. Stopped the cracks.
Ordered new rims today (ouch),but we didn't want to take a chance on them coming apart. Did you have the shop weld the ring right on the ring or did they cut the middle out and weld a new ring inIJ Thanks for your reply.