Bike weight limits, fatter tyre for more weight? I have a touring bike which I use for commuting, I carry about 30 kg of tools with me usually on this pannier type thing I made on the back. And usually a work belt on the front which is 7 kg

 I'm wondering how much more I can safely carry on the bike without stressing it too much. The tyres seem to be under a lot of strain in particular. I'm 75 kg, unloaded with just me the bike tyres look fine, when loaded they're flattening out to an alarming amount.

I just ride on the road, but they're not good roads, I want to add about another 20 kg of tools and I'm wondering if I should put a fatter tyre on the back. I have 29 inch tyres on my mountain bike which look like they might fit. Or whether I'm worrying needlessly?
The flattening is normal, since there is so much weight. Don't really try the MTB wheels. They will probably not fit and you will have to be rolling with different sizes of tyres that may not fit to the frame. Increase the air pressure (or go tubeless) and make sure you change tyres in short periods of time. Don't let them wear totally off.