Building my first bike - choosing the right components

[pigman]

Would a rear derailleur made for 7 speed cassette (specifically a RDA550) have enough lateral movement to cover the width of a 8/9 speed cassette?

not sure.
i had a Deore DX which i used on 10 speed successfully.
Ive got an XT 8 speed that doesnt want to accept any more cogs than 8

 

[Woz]

Something like a STX-RC MC38 would do a fine job all round. Quality, silver and cheap.

 

[Mike-p]

Ive got 46-30 on my old stronglight cranks. Super compact ain't that new!

 

[hamster]

What puts me off standard chainsets is that modern cassettes all start with a 11 or 12 sprocket, so coupled with a 52-or 53, im carrying a couple of gears that never get used. The last time I needed something bigger than 48x12 was descending into Bourg from the Galibier

Totally agree that 53-12 is not needed anywhere short of a bunch sprint - and even the pros can only hold it for 200yds.

 

[Ugo51]

Something like a STX-RC MC38 would do a fine job all round. Quality, silver and cheap.

Yes, that's the other option.
The reason behind the RD-A550 is that I can pair it with a front derailleur of the same series, which is not available for the STX, as it has to be braze on

 

[Woz]

Totally agree that 53-12 is not needed anywhere short of a bunch sprint - and even the pros can only hold it for 200yds.

Yup. Is it any wonder why people started doing the unthinkable of putting MTB parts on a road bike and Gravel bikes taking off?

Shimano have had their head so up their own arse year after year that they could never right size gearing. An intelligent move would have been different gearing options between gruppos.

But no....one size fits all. Racing size. Dura-Ace and Sora essentially on the same gearing. What a joke.