I rode with a 12" telescopic for all of my first years of real road cycling. That's absolutely, without a doubt, the most effective, legal form of defense you can have. 4 inches of 4140 striking surface is vicious.

I struck a giant bull mastiff puppy and killed it with a relatively (so I thought) weak strike. I'm not a particularly big guy, either. I was just startled and terrified. I didn't even realize it was a puppy until afterwards. It was 110 lbs and full of run. I just needed to HTFU but I was 20 and stupid.

I decided that day that I'd rather become famous by being the cyclist who was killed and eaten by a dog, then to carry the teleskuller around anymore.

But if you want to destroy stuff, a telescoping baton, with 4140 striker, clipped to the frame...the only thing more psycho would be a chain. Just swinging a chain, Mad Max style, Beyond Thunderdome type ish.