what causes a inntertube to be cracked all along the sides of it like it has been rubbing something? what is the best tube to buy? are the heavy duty ones worth the exta $?

Age and under inflation causes the cracking as the rubber hardens the same as a tyre, tubes 'should' be changed with the tyre i suppose. I go with every other tyre myself but everybody has a preference.

A tube is a tube, isn't really a specific one, the tyre does the hard work, i have whatever the tyre fitter took of the shelf when he did the tyres. And no the heavy duty ones make no difference, like i said the tyre does the hard work.

My DT has been through most every extreem a tyre can expect on normall off the shelf tubes.

HTH

New tubes everytime with new tyres along with rim tapes. Money well spent !