Bicycle Mechanics - Rear derailleur's range too small?

Hi, I've been building up an old CAAD4 frame I bought on the cheap and found a problem with the rear mech that I just can't fathom. The bike came with a Shimano RD-5500 9 speed derailleur that I've kept, but I've had to fit new wheels and a new cassette. The cassette's a Shimano HG50-9, so it all should work together but the derailleur doesn't travel far enough to engage either the bottom or top cogs. I've let the H/L screws as far out as they go but no luck there - it only chatters slightly when trying to move into bottom gear and doesn't come close to moving into top gear. I've also disengaged the gear cable and moved the mech by hand - it just doesn't seem to move far enough in either direction to cover the 9-speed range.

Originally I thought the hanger could be bent, but it doesn't look to be and I figured that would only effect one extreme, not both.

So does anybody have any ideas? Or have I just made a massive/embarrassing compatibility error?

Thanks for any help

First let's make sure it's the mounting, not the RD. Loosen both limits all the way and hand spring it across the range of the cassette. If it can't reach low, but has excess travel at high, it might be that the hanger tab is a bit thicker than normal, or that the cassette is more inboard than ideal.

If it comes up short at both ends, something internal on the RD is limiting it's travel. Disconnect the cable and take the RD off the bike. Flex it in your hands and observe what exactly is limiting it's range of motion.

There's a bit of a mystery here, but if you're methodical, you'll figure it out.

With cable disconnected from derailleur: does chain land on smallest cog; can you push derailleur inward with your hand while turning cranks to get the chain to settle on largest cog? If so, the shifter may not have been set correctly when cable was attached; make sure the shifter is set on highest gear (i.e. click the inner lever umtil you run out of clickks) before attaching cable to the derailleur.

I've disconnected the cable from the derailleur, the chain refuses to land on smallest cog and lands on second smallest, and I can't push it to settle on the largest one. When I do it lands on the second largest, but chatters as if it's just about to shift. Definitely coming up short at high and low.

I'll try taking the RD off the bike to see if there's anything limiting the movement inside, cheers for the replies.

Just took the RD off the bike, moved it in my hands, cleaned it fully and made sure nothing was fouling on the inside and limiting movement. Everything seemed fine, so I've fitted it back onto the bike and it's still coming up short on both cogs. I really don't get it.

Any other ideas?

Can you shift in manually through the range? Also try attaching the cable and shifting by drawing the wire from the downtube like a long bow.

If it's can't work the range manually, something is wrong with the RD itself, but I couldn't begin to guess without having it in my hands.

If it can work the range, than it's either a shifter issue, or a shifter mismatch issue, ie Shimano shifters and Sram RD.

I've managed to fix it, turned out the cable clamp had been bent in slightly (I'm guessing from a fall) and was fouling the main body and limiting its range. It just needed prising out and now it's as good as new.

Thanks for all the replies, mystery solved.

Interesting... I did quick look around and found your derailleur accepts max cog of 27 tooth... And HG-50's I thought are 32 cog and 34 cog max...

Nah, I'm running a 12-25 on the back. All fits nicely.