I just replaced my old 53 and 42 T chainrings with a smaller more compact set 48 and 38 T.
Will I need a new front deraileur or should my old one work?
I'm having trouble adjusting it but I'm pretty clueless and how to properly adjust it.
It's a late 90's Shimano Ultegra groupo.
Fr der are pretty forgiving of moderate changes in CW size such as you
have done, the main change needed will be a lowering of the FD about
2cm +/- on the seat tube. Clampons move easily, but brazeons have
a more limited range of up down movement. www.parktool.com and clik on the REPAIR button for a stylized pix of
a bike. Mouse over the bike to the FD area and clik will take you
to the FD adjustment section for pretty complete info on howto. You may
need to shorten the chain depending on how well the RD wraps it up.
Is it a clamp on FD? If it is, unclamp it and move it down the seattube to accommodate the new diameter of your large chain ring. You will probably only need to do this 2mm or so, but it might make a difference since your smaller chainring is also smaller. Then retension the cable so it is taut. Adjust the H/L screws if necessary (but, this will probably not be necessary). If you do need to tweak, very slight turns are all you will need. Check shifting with your bike on a stand if possible. Report back.
My arithmetic engine failed, 2cm would be the diametral difference
between the 53 and 48t CW, so half that more or less would be
the proper radial difference and the amount to lower the FD, but
the right amount is determined by its relationship to the CW.