PINS? - What a brainless and cruel suggestion! So you are chased by dogs and you want to cripple them and hurt them in such a horrible way? I am pretty sure some dogs mean no harm and it is part of their nature just run after bikes. To take one step further: what if kids step on it?
The original idea of pepper spray is actually workable and you can make it yourself. Get some ground pepper and mix it with water. Then put the mixture into the bottle with spray head. Shake it and spray!
Or just put some juicy hot chilli sauce into the water ......there are many ways of doing it.