Hi-I will be replacing my derailleur cables for the first time ever (wish me luck) and I need your help.

From what I understand I can buy as complete shift kit or buy some of the parts separately (cables & housing) and use some of the parts already on the bike.

Is using the parts already on the bike (end caps, connectors etc) and replacing the cable w/ new one a recommended practice?

My bike (Spec Roubaix SL4) has internal cable routing. Do I need to use any housing inside the frame or is the liner that come with the kit used for that purpose? My understanding is you use liner to push the old cable thru the frame back out and do the reverse when installing the new one. Is that correct or does the liner stay inside the frame?

As you can tell I honestly have no idea how cables travel thru the frame. This will be a good learning experience.

I ride about 6000miles/year in mostly dry conditions in the Northeast US. I race maybe a dozen times. Could you guys recommend a decent brand? Shop installed Jagwire the last time but I am open to new ideas. Thank you in advance!!!!!