Hello. I've just bought a Triban RC520 from Decathlon. However, the rear derailleur (Shimano 105) wasn't shifting into either of the lowest two gears (small chainring and biggest two cassette cogs). So having googled the problem, I did the following: Pushed the derailleur with my fingers to establish that the derailleur was physically capable of moving to the right position and able to select the lowest gears. It did this OK so I *think* this ruled out the limit screw as the cause of my problem. I then twisted the rear derailleur cable barrel adjuster anti-clockwise by several turns which I *think* increased the cable tension. This was partially successful as I can now select the second lowest gear via the shifter. However the barrel adjuster will now not move any further out and I still can't select the lowest gear. The chain keeps hopping on and off the biggest cog with each rotation of the cassette. So... my (eventual) questions are: Do I now need to undo the cable bolt and pull the cable tighter to get even more tension? If so, is this something that a mechanically incompetent person can do without special tools or will the cable somehow ping back and the whole transmission implode into a thousand pieces in a comedy fashion? Will adding tension to enable the lowest gear selection have implications elsewhere, i.e. impact higher gear selections in any way? Sorry for the length of the post and my obvious lack of knowledge. I've tried to google it but don't want to risk knackering up my lovely new push iron. Many thanks.