105 rear derailleur missing screw..?

[Mr. Cow]

Hello

I had an off and on inspecting the bike I noticed there appears to be a screw missing from the rear derailleur?



This is on the underside near the cage section.. can anyone tell me what this screw was for, and if it might stop me nursing the bike home? Upside down the bike appears to shift up/down ok..

Thanks....

 

[Vantage]

That looks to be where the B-ajustment screw was. When screwed in it moves the top jockey wheel away from hitting the sprockets.
Pull back on the derailleur and let it snap back into place then run through the gears. Watch to make sure the jockey wheel doesn't foul the sprockets and if not, it'll be fine. Otherwise your local bike shop will likely have a spare screw lurking somewhere. If not, an M3 screw (I think) of around 15mm length will do.

 

[Mr. Cow]

Thanks, do you mean the b limit screw? If so i think i still have that on the rear with the a screw. Here's another pic, hopefully it's a bit more clear of where the missing bolt is

 

[Vantage]

Got my terminology buggered up
I mean the B tension screw. You have the upper and lower limit screws which are parked close to each other and then you have the B tension one which sits on its own. That'll be the one that's gone AWOL.

 

[rogerzilla]

Nerd fact: the B-tension screw came about because of vertical dropouts. Before that, all road bikes had horizontal dropouts so you moved the wheel until the top jockey was close to the sprockets, then used the dropout adjustment screws - you know, those long M3 screws that always rust into position - so the wheel would always go back in the same place after fixing a puncture.

 

[Mr. Cow]

Thanks for your input chaps, and the quite interesting history of the b tension screw! managed to nurse the bike and myself home without issue.
My off was caused by the chain dropping off the front big ring and ending up between the small ring and the frame. I was powering up a slight incline on perhaps the 3rd or 4th smallest cog, wasn't changing gear at the time.
Could a missing b tension screw perhaps cause something like this? I don't know if the screws always been missing or if it somehow went during the crash. I know a multitude of things could cause the chain to drop but the rear mech, casette, big chainring and chain were replaced new about 100miles ago as they were worn. The only thing different this time is a KMC chain with quicklink was fitted rather than the usual shimano HG job.

 

[Vantage]

I was powering up a slight incline on perhaps the 3rd or 4th smallest cog, wasn't changing gear at the time.
Could a missing b tension screw perhaps cause something like this?

Do you mean you were trying to change gear and it wouldn't? Or you weren't trying to change gear?
If the former and the screw was already missing then yep, I suppose it could.
It sounds like the screw had already been working its way out though long before your off...which is odd because shimano coat those screws with threadlock.

 

[I like Skol]

I think you are barking up the wrong tree here.

The B-tension screw doesn't live in the hole on the underside of the parallelogram assembly. It is at the back of the mech where it mounts to the frame. In fact I have no recollection of ever seeing a screw, adjustable or otherwise, in the location you are showing so just went to look at some of my current bikes.

11spd Ultegra B-tension screw works against the tab/fang at the bottom of the hanger...

Poor quality image of underside of Ultegra, no hole in sight.

Compared to 10spd Tiagra which does have the hole, but it appears to be empty unless there is a deeply embedded grub screw (which I suspect there is).

It looks like the 'empty' screw hole in the underside of your rear mech is a red herring and actually your gears just need the alignment checking and adjusting properly?

 

[Deleted member 23692]

There's a 2.5mm grub screw deep in the hole... or there should be.

Without striping down my mech to determine its function, I assume it's got something to do with the cage pivot.

 

[Mr. Cow]

This is exactly what I am seeing, thanks for confirming it. Mine is a 105 11speed, they must be very similar. I will not worry too much about it then

Compared to 10spd Tiagra which does have the hole, but it appears to be empty unless there is a deeply embedded grub screw (which I suspect there is).
View attachment 465625

It looks like the 'empty' screw hole in the underside of your rear mech is a red herring and actually your gears just need the alignment checking and adjusting properly?

 

[Smokin Joe]

I think you are barking up the wrong tree here.

The B-tension screw doesn't live in the hole on the underside of the parallelogram assembly. It is at the back of the mech where it mounts to the frame. In fact I have no recollection of ever seeing a screw, adjustable or otherwise, in the location you are showing so just went to look at some of my current bikes.

11spd Ultegra B-tension screw works against the tab/fang at the bottom of the hanger...
View attachment 465623

Poor quality image of underside of Ultegra, no hole in sight.
View attachment 465624

Compared to 10spd Tiagra which does have the hole, but it appears to be empty unless there is a deeply embedded grub screw (which I suspect there is).
View attachment 465625

It looks like the 'empty' screw hole in the underside of your rear mech is a red herring and actually your gears just need the alignment checking and adjusting properly?

Correct, in fact the B tension screw is visible in the OP's picture if you look closely.