My brother has one of these http://m.evanscycles.com/products/specialized/hardrock-sport-disc-2010-mountain-bike-ec019399 Recently his chain has started to slip. He went to the LBS and they said replace the rear cassette and chain. I have changed these and he rode it today and it's still slipping. Is it likely that I need to change the front cogs? Could it be the rear derailed cogs? If it is front cogs can someone link me to a cheap new one?