Help! Compact problems one week before Alps

mabeleliza

I've just changed my current DA 39/53 and 12-25 for a R700 34/50 and 12-27 in preparation for a trip to the Alps next week.
Problem is when in 34/27 the jockey wheel of me short cage DA rear mech bumps into the 27t sprocket. I've tried adjusting the B Tension Screw on the mech as tight as possible and although it is better it doesn't solve the problem.

I know this mech has a total capacity of 29 according to Shimano and I'm asking it to cope with 31 but everything I've read suggests this isn't the problem.
Any suggestions gratefully received.

Cheers

binlinus

mabeleliza wrote:

I've tried adjusting the B Tension Screw on the mech as tight as possible and although it is better it doesn't solve the problem.

Cheers

The gear outer cable between the chainstay stop and the rear mech may be too long. Shortening it will pull/roatate the rear mech back slightly and so move the jockey wheel away from the cogs. Then fine tune with the B screw.

Bear in mind, once you cut the outer cable there's no going back unless you have another cable outer handy.

Bin

mabeleliza

Bin, thanks for the suggestion, it does look a bit long now you've mentioned it. Is there a guideline as to how long it should be? Andy

binlinus

mabeleliza wrote:

Bin, thanks for the suggestion, it does look a bit long now you've mentioned it. Is there a guideline as to how long it should be? Andy

Hi Andy

Well, no, but say take 10-0mm off and see how it is. I think it's just a matter of trial and error for each bike.

Bin

John.T

Have you shortened the chain. Your set up should work fine but chain length is critical. You need the chain to be as long as possible but for the rear mech to still have tension on the chain. If the chain is hanging down then it is probably too long. Set correctly the jockey will catch the 27 cog with the bike upside down but is clear when being ridden. The weight of the chain does this.

binlinus

John Tiffany wrote:

Have you shortened the chain. Your set up should work fine but chain length is critical. You need the chain to be as long as possible but for the rear mech to still have tension on the chain. If the chain is hanging down then it is probably too long. Set correctly the jockey will catch the 27 cog with the bike upside down but is clear when being ridden. The weight of the chain does this.

I'm pretty sure chain length is not an issue here. As long as the rear mech can accommodate the chain when its run big chainring to biggest rear cog without jamming or nearly jamming; and run inner ring to samllest rar cog without the rear mech curling back so far that it fouls the chain, then, the length is ok.

It's an issue with the rear mech outer cable length, B-screw tension, and, if used on the older horizontal dropouts, the position of the wheel axle in the dropouts (too far forward and the jockey wheel touches the biggest cog).

Bin

John.T

Bin. With 50/34 and 12/27 one link either way will produce the 2 positions that you mention. This range is pushing the mech to it's limits but when set up right it does work fine. You are right that outer cable length will have an effect but the B screw should be able to acomodate this unless the cable is way out.
Maybelisa. I have just checked my bike and the jockey is actually closer to the cog when on 34/12 but when I simulate a longer chain by easing the chain part way of the front ring when on 34/27 the gap to the cog closes up. I would still say to check chain length. You could well have it a bit long. What Bin says about drop outs is also relevent.

binlinus

Good post John. Think we got all possibilities covered

Bin

John.T

Maybelisa. Please let us know the outcome. That way we all learn something.

nick hanson

I was told that the bike shop in Bourg D'oisans uses a tie wrap to strap the outer cable to the chainstay near the dropout,& this has the effect of throwing the rear mech forwards,& clearing the largest sprocket.
Has anyone tried this :?:

pigman

theres a screw next to where the mech screws into the frame's gear hanger - the screw acrually pushes against the gear hanger. Tightening it up (clockwise) pushes it further away from the sprockets and is what is used for wider ratios. Anticlockwise pushes it closer to the cogs and is used for racing ratios.
Also make sure that the mech is able to pivot on the gear hanger bolt. sometimes the rubber inside shimano ones gets dry and the mech doesn't swing/pivot very well, meaning that the mech sticks and doesn't tension the chain.. This is usually on older/used mechs( but necessarily knackered). I've taken out the rubber seals on my pub bike and it now works well.
good luck

John.T

Pigman. This is the 'B' screw we referred to earlier. Good point about the B pivot though.

Floodcp

Mabeleliza

I had the same problem albeit with a campag setup. I shortened the chain and it worked perfectly ever since.

Only had to take a couple of links out.

Flood

magliaceleste

I know it doesn't really help, but the R700 and a 12-27 cassette with a DA short cage rear mech WILL work perfectly, I ran this combo myself. I reckon one of the tips above should do the trick.

mabeleliza

Hi all, apologies for late response.

Seem to have sorted it out with a combination of all the suggestions given. I shortened the outer cable by about 40mm (risky amount I now realise but got away with it), tightened the B screw all the way and took one link out of the chain.

I may try Nick's cable tie tick as well as I did notice that pulling the outer cable up a bit at its highest point seemed to help too. This way I can loosen the B screw a little - never feels right to have adjusters at their limit does it?!

One last thing, all this fiddling about has screwed up my indexing? I'm a bit ham-fisted - does anyone have the definitive technique for getting the best adjustment every time?

Thanks to all again, really helpful.
Andy

John.T

The rear mech chain tension is ment to work by balancing the tensions of the A and B springs. The outer cable should be long enough to have a minimal effect on this. If the lower run of chain is catching or is very close ( less than 5mm) from the little hook on the cage when in little / little then the chain is still too long.
As for setting indexing. Put the chain onto either the 2nd or 3rd smallest cog and tighten the cable adjuster until the chain just catches the next cog. Then slacken it back until it runs quietly. Check this on both chainrings as you may need to trim it slightly to find the best setting. Unfortunately too tighter bend on the outer cable can upset shifting by increasing cable drag. Your 40mm may have been a bit much.
I have just checked both my bikes which run 50/34 and 12/27 and the B screw is at its slackest setting. That is right out.
Enjoy your trip.

magliaceleste

There's no magic to it.

Basically put the bike into the smallest rear sprocket and the slacken the cable by screwing the barrel adjuster in at the rear mech to almost all the way in (leave half a turn) and do the same on the downtube adjuster.

Now click the rear shifter up one gear, the bike won't change up as there won't be enough cable tension. Keep winding the barrel adjuster on the rear mech out until the bike changes up a gear - each turn increases the tension on the cable (this is obviously a lot easier on a workstand as you will have to keep 'pedalling'). Keep turning until the chain starts grinding and/or changes up another sprocket. Then slacken back the barrel adjuster again by a turn or two and it should stop grinding/drop back to the correct sprocket.

If all is well you should then be able to shift up and down the block. Try riding it as it always works better on the stand than real life! You may need a slight tweak one way or the other on the barrel adjuster if it's slightly out.

mabeleliza

Thanks guys, I'll try that.