I have 20 or so tall (75'-90') black locust next to my garage. One snapped off about 10 feet up last summer and I'm nervous about the others. I know topping is bad but dont want to completely cut them down. what will happen if i cut 30' off?
You will effectively ruin your trees. They will sprout a whole bunch of fast growing, poorly attached water sprouts. In 5-8 years you will have nearly the same mass of wood above your garage, but it will be weaker, poorly attached wood. If they are topped, your time/expense in future maintenance will be significant.
Your potential for damage will be higher in wind or ice events.
Your trees will be UGLY.
If you don't want to remove them, but want to feel a higher level of saftey from these trees, get an arborist on site, and have them evaluated. And not just from the ground. Problem trees could be discovered before a failure, and your conifdence in them will almost certainly improve.
They can be crown reduction pruned without having them topped. This will lessen wind loading, and provide less area for ice/snow to adhere to. Safer, still beautiful, natural looking trees.
If you feel there's to much risk to accept, I'd encourage you to take them out AND replant new trees of better placement/species instead of topping. This process could even be done incrementally over a few years, planting replacement trees now, and giving them some time to mature before you begin removing the old ones.
I think many unknowing homeowners have their trees topped because they see other people have it done, with the idea that less tree is less liability, when this simply isn't true. Help break the cycle please.