Ultegra 6800 rear derailleur, B screw fallen out

[rliu]

Hi all, I wanted to double check for opinions on how much of a problem it is if the B screw on my Ultegra 6800 rear derailleur has fallen out? I had two incidences of rear derailleur issues in past six months, the first time the hanger snapped whilst shifting, which I then replaced and carried on riding probably near 2000 miles. Then, earlier this week, the rear mech got mangled again and this time bent the hanger and snapped the cage, leading to one of the jockey wheels flying off and going missing. I had intended to repair this again by just buying new jockey wheels and an inner plate, however today whilst trying to change this I noticed that the B screw had fallen out. From experience I was never really sure what this screw did and from Youtube videos it seems its impact was always quite minimal. Shall I carry on just replacing the derailleur cage and jockey wheels and carrying on using this RD?

 

[chris-suffolk]

If it works without, then you will probably be ok - depends on the size of your largest spoket in the main - but may not run as smooth as it should. Else, you can buy them - about £2.00 on ebay.

 

[fossyant]

Get a new mech if you've bent it TWICE.

 

[SheilaH]

In 35 years I've never had anything like this happen. I'm wondering if there is something else going on, chain length wrong etc.

 

[Ming the Merciless]

Photos please

 

[fossyant]

In 35 years I've never had anything like this happen. I'm wondering if there is something else going on, chain length wrong etc.

Same here but I can set gears up properly. Still on original Dura Ace and Untegra mech that are 30 plus years old. Never broken any rear mech.

 

[Ajax Bay]

Think it would be useful (to the OP) and relevant to further advice if the OP was able to say WHY "the hanger snapped whilst shifting" and WHY, "earlier this week, the rear mech got mangled again and this time bent the hanger". This doesn';t happen without a cause.
Chance of the rear mech being correctly aligned are slim (>> nil). Were the spokes involved in either damage?
OP - buy two new hangers anyway: one for now and one for the next time.

 

[rliu]

Photos please

This is the photo of the screw and plate next to the derailleur without the jockey wheel and inner plate on

 

[weareHKR]

The B screw is there to control the gap between the pulley, the upper pulley on the derailleur, and the bottom of the cogs. The gap is easily seen and measured by the relationship between the derailleur and the largest cog on the cassette & this is where you should adjust it. The gap is subjective, some say 5mm or less & some 7or 8mm, I usually use a 5mm Alen key to check it, as long as it's not making a noise it should be ok.
PS.
It’s also recommended that if you have a front derailleur, it should be shifted into the smallest chainring before adjusting!

 

[Darius_Jedburgh]

Long shot coming.....As mech plate and jockey wheels have been removed, where they in the right position beforhand? Many years ago i foolishly put the plate back on upside down. I got lots of chain jammimg. Just a thought, I may be totally wrong

 

[rliu]

Long shot coming.....As mech plate and jockey wheels have been removed, where they in the right position beforhand? Many years ago i foolishly put the plate back on upside down. I got lots of chain jammimg. Just a thought, I may be totally wrong

Just to explain further, the first time it happened it was a pure case of having bent the hanger too much (e.g. correcting it back after the bike falling on its side) and it just stress-fractured. I actually had the bike serviced with a local shop at the start of January so it won't have been a rookie error or mis-indexing that snapped it this time round - basically I was riding along and had shifted to small-small combo without realising, the chain was skipping round a bit, I stopped to readjust the barrel adjuster, start pedalling off when the mech must have still been coiled up on itself and it snapped the inner-plate of the cage. Having changed the outer plate this morning it was gunked up with years of mud and grit and realised the plate wasn't actually freely rotating by itself from axle, which I suspect didn't help in this situation as there was no flex if the mech was folded up on itself on the small-small.

 

[Ajax Bay]

1) 'B' screw - see below
2) https://si.shimano.com/pdfs/dm/DM-RD0003-09-ENG.pdf
3) Given latest incident, the current hanger may well be OK, but worth checking it's aligned, nevertheless. Quite possible the LBS did not do this, specifically.

 

[Chislenko]

In a couple of days time when I get round to it I will be putting a 6800 11 speed rear mech in the for sale section.

It is a short cage which I swapped out for the longer one to facilitate bigger cassette. Very little mileage on it.

Just out of interest, can't tell from picture, is that a short cage and if so what gearing are you running?

 

[rliu]

I've got a new derailleur cage and fitted with new inner gear cable but now see some slack in the chain, would this be down to the sheared off B screw or is it a case of gear cable adjustment?

 

[Jenkins]

You've not threaded the chain round the bottom wheel on the derailleur!