Check my mech (please)

[RichardB]

I've just finished a build of a lightweight frame and fitted the rear mech. I've cleaned, overhauled and repaired these in the past, but never fitted a new one. In particular, I have never had to mess with the B tension screw. Photo below is of the mech on the largest sprocket (it's a 14-28 block, single 44T at the front). Does it look right to you? I have the mounting bracket correctly fitted behind the lug on the frame hanger, and from the angle of the pulleys I think I have the chain length about right. But I expected the guide pulley to be a bit closer to the sprocket than this. I have read anything between 5 and 10 mm between the teeth of the pulley and the teeth of the sprocket. The gap here is about 14 mm. I have tried screwing the B tension screw in and out but it seems to make no difference at all to the angle of the pulleys. You can see in the photo that it is fully out (which in my understanding means the pulley should be closest to the sprocket) but screwing it all the way in doesn't move the mech a millimetre either way. H and L limit screws are set correctly and the mech changes up and down as it should on the bike stand, but I know that sometimes what it does on a stand isn't necessarily what it does on the road under pedalling pressure. Weather and roads have been too wet for a road test yet.

Am I missing something obvious (in which case, educate me please) or does it look fine and I can stop overthinking it? Thanks all.

 

[Ming the Merciless]

You want a gap of at least 5-6mm between the guide pulley and the biggest rear cog. You appear have well over a cm of gap. Is the front on the small chain ring as well? Other than that, if it’s shifting and running well don’t stress about the b screw.

 

[ColinJ]

Is the front on the small chain ring as well?

... single 44T at the front...

 

[Ming the Merciless]

So the answer is yes 👍

 

[RichardB]

You want a gap of at least 5-6mm between the guide pulley and the biggest rear cog. You appear have well over a cm of gap. Is the front on the small chain ring as well? Other than that, if it’s shifting and running well don’t stress about the b screw.

As per my OP, it's a single ring on the front, and the gap is about 14 mm. It'll have to wait for a drier day to give it a road test. As I said, it is changing fine with the bike on a stand, but that doesn't always reflect on road behaviour. Still a bit puzzled about why the B screw is doing nothing. If you think it looks OK, that's great, thanks.

 

[Tigerbiten]

How much is it tucked up when it's on the small sprocket ??

If it's not to tucked under to much and you have some spare chain, I'd try adding some links and see if it's the chain tension pulling the mech away from the large sprocket.

 

[RichardB]

So the answer is yes 👍

And simultaneously on the largest as well. Schroedinger's Chainring.

 

[Ming the Merciless]

Is that a wolf link though? Seems not to be necessary for such a cassette. Just mount derailleur directly.

 

[RichardB]

How much is it tucked up when it's on the small sprocket ??

If it's not to tucked under to much and you have some spare chain, I'd try adding some links and see if it's the chain tension pulling the mech away from the large sprocket.

Good suggestion, hadn't thought of that. Photo of it on the smallest sprocket below. What do you think?

 

[RichardB]

Is that a wolf link though? Seems not to be necessary for such a cassette. Just mount derailleur directly.

What's mounted to the hanger on the frame is what came in the box.

 

[Ming the Merciless]

Really? Looks like a steel frame, and that the black thing is mounted where the derailleur is normally mounted.

 

[ColinJ]

Really? Looks like a steel frame, and that the black thing is mounted where the derailleur is normally mounted.

I think you are right!

 

[Sea of vapours]

That does look peculiar and it's odd that the B screw is not doing anything useful. If that is an RD-A070, however (?), then it looks like that in all the photos I've just seen, and since it has a maximum of 28 on the rear, as you have, but is designed for a triple front, perhaps things look more normal with different chainring sizes, other than your single 44.

 

[si_c]

It's a Tourney derailleur so they have that extra swing arm on them - they are designed to cope with a very wide range of sprockets at the back and it helps give the derailleur additional room to clear the larger ones. Mrs C has exactly the same setup on her town bike but that has a 14-34t megarange freewheel on it.

The older steel frames were designed around having a short cage traditional mech fitted, the setup you have there will work without any problems though.

 

[C R]

I think the reason for the b screw not appearing to be having an effect is that the pivot it acts on is too far from where it needs to be.

The mounting is a bit strange. The bit between the derailleur pivot and the hanger would usually be a claw mount, that would attach to the axle slot, with the pivot ending located over the hanger. In this particular case, though, the extension does not seem to have a claw, so the derailleur ends up too far from where it should be.

ETA, it should still work fine as @si_c says.