Any problems changing medium rear derailleur for long cage?

d3matt

My bike (from new) has a medium cage Sram X9 rear mech and there is a lot of chain slack. So much so that it gets commented on by others. I've checked the chain length (by compressing rear shock and chain around 2 large cogs, plus 1") and it's correct. It seems that a 3x9 bike should have a long cage anyway, but Boardmans use medium.
I was considering swapping to a long cage rear derailleur. Does this have any implications that I'm not aware of?

supersonic

What gears are you getting slack in?

d3matt

All 9 when on the smallest front cog.

supersonic

Hmmm, odd. Can you get some pics? Small front cog and largest rear? The small front and 4 or 5 largest at the rear should be ok with this mech if you have measured the chain as you say.

d3matt

Will do.

madmole

b tension on rear derailer?

but yes, long cage would be better fit

d3matt

Yes the b-screw is the key here I think. Sram tech manuals says the gap should be 6mm (from memory) but on the Boardman it wont go less than about 20mm. But to stop this chain slack, you need it all the way out so there is a massive b-tension gap. I think they've got the hanger design wrong as the rear derailleur isn't in the ideal position. The Boardmans just use a standard Halfords hanger which fits all their other brand bikes. I think they got this wrong as it's not ideal.

madmole

Check the derailer is on properly, and the b tension screw is actually against the tab for it. Its easy to put a derialer on so the srew is against the side of the hanger

cooldad

You think too much. Stick up a pic.

madmole

Will be hard as I'm stuck in a hotel in Dublin being forced to drink beer, and my workshop and bikes are back in London

Normally the smaller the gap the less chain slack and the chain is forced further around the rear cogs. So that huge gap is your problem (and the fact that triples are supposed to be long cage)

cooldad

Not you, d3matt - it's his problem.

d3matt

I've had a play and taken some photos.
If I screw the b-tension screw all the way in, the chain slack goes but then the gap between the top jockey wheel and the largest cog is way out from the SRAM recommended 6mm - but instead about 22mm.

d3matt

But then if I adjust the b-tension screw out and reduce the gap, I get excessive chain slack.
This is on the smallest cogs front & back.


And the b-tension screw isn't out all the way on this example above.

Also as I said before, it's also not possible to get the gap anywhere near to 6mm either. I think the hanger used isn't ideal for the gears. Boardman use this hanger which is a generic one for all their bike brands. What's the consequence of having a b-tension gap of three times larger than the recommended/ideal?

supersonic

You are not using your gears correctly - there is no need to use the small/small combos. if it works fine on the small front and largest 4 or 5 combos then all is fine.

DickBarton

Using the correct gear ratio would certainly help...I've gone from long cage to medium cage as it's much tidier (less dangling down to get caught on stuff) and if you believe the marketing hype of yesyer-year - you will shift quicker (I suspect you do but quicker in fractions of a second and not actually noticeably so).

As much as we have 9 gears at the rear and 3 at the front, you shouldn't need to use every combination...we have so many gears at the rear to allow less of a 'jump' when shifting to allow us to remain smooth when pedalling, but this invariable leaves a large number of gear combos never used.

If it is a problem, I'd removed 2 full links from the chain and never use small cog front and 4 smallest cogs at rear (and also never use largest cog at front and the 3 largest cogs at the rear)...

Chain length looks fine to me when your chain is in a 'proper' gear but when in the 'wrong gear' (smallest - smallest) it looks wrong but putting a longer cage mech on is unlikely to make any difference to that.

The gear combo you have there (smalleest - smallest - can be achieved with the chain in the middle ring and on the larger cogs...and would be a better option - less stress on the chain as well.