Front mech wont work

[e-rider]

I seem to have a fundamental problem with my front mech. It simply wont go 'in' (towards the frame) far enough. I was adjusting the stop screw and nothing was happening so after a bit of investigation, it appears that the mech is bottoming out ie. it wont go in any further. I'm using a Shimano 105 chainset. Does anyone know if its possible to change the chainline slightly and get the chainset a few mm out from the frame; perhaps with a spacer or something between the BB cup on the RH side (hollowtech II )?I think this is called the Q factor or something?

 

[RecordAceFromNew]

There are at least 2 obvious possibilities I think.

The first is wrong spacer placement in the bb. Iirc HT2 comes with 3 off 2.5mm spacers for fitting behind the cups, unless you are using E type front mechs there should be 2 on the drive side and 1 on the non-drive side for 68mm bb shells, and just 1 on the drive side for 73mm shells. If an E type front mech is used the drive side is reduced by 1 spacer.

The second and perhaps more common phenomenon is that the chainset is running on a road chainline while the front mech is designed for mtb chainline. What front mech have you got, what chainline the 105 is giving you, and do you have a 68mm or 73mm bb shell?

 

[e-rider]

There are at least 2 obvious possibilities I think.

The first is wrong spacer placement in the bb. Iirc HT2 comes with 3 off 2.5mm spacers for fitting behind the cups, unless you are using E type front mechs there should be 2 on the drive side and 1 on the non-drive side for 68mm bb shells, and just 1 on the drive side for 73mm shells. If an E type front mech is used the drive side is reduced by 1 spacer.

The second and perhaps more common phenomenon is that the chainset is running on a road chainline while the front mech is designed for mtb chainline. What front mech have you got, what chainline the 105 is giving you, and do you have a 68mm or 73mm bb shell?

The BB doesn't have any spacers - it's a 5600 Shimano 105 BB 68
The front mech is a 5600 105 braze on so definitely not for MTB use.

 

[RecordAceFromNew]

The BB doesn't have any spacers - it's a 5600 Shimano 105 BB 68
The front mech is a 5600 105 braze on so definitely not for MTB use.

As I indicated earlier (confirmed here e.g.) Hollowtech 2 bb's should have 2 x 2.5mm spacers on the drive side for normal non E-type front mechs and 68mm bb shells. If they are truly missing I would have thought the chainline would be accordingly reduced (to perhaps ~40mm?), and might have given rise to the problem you are experiencing.

 

[Zoiders]

Is it a new BB and did you match it with the old one?

The other option apart from the spacers is that the mech needs moving in on it's tab, that's the flange sticking out of the down tube that the front mech is bolted on to.

 

[e-rider]

Is it a new BB and did you match it with the old one?

The other option apart from the spacers is that the mech needs moving in on it's tab, that's the flange sticking out of the down tube that the front mech is bolted on to.

the brazed on (riveted) bit of metal doesn't move. The BB definitely didn't come with any spacers. I've checked the instruction leaflet that came with it and there is also no mention of spacers.

 

[RecordAceFromNew]

I might have misunderstood from your OP that you have a Hollowtech II bb (which comes with 3 spacers)? What bb is it and what is the current chainline (distance between centreline of bike to middle of two chainrings for a double and middle ring for a triple)?

 

[e-rider]

I might have misunderstood from your OP that you have a Hollowtech II bb (which comes with 3 spacers)? What bb is it and what is the current chainline (distance between centreline of bike to middle of two chainrings for a double and middle ring for a triple)?

I have a hollowtech II BB but it has no spacers - I have MTB hollowtech II cranks on two other bikes which both come with 3 spacers - the 105 road one came with no spacers. I don't know what the chainline is, but it's non adjustable.



The MTB HTIIs are made for either 68 or 73 shells (hence the spacers to convert). The road HTIIs are specific to 68 shells.

 

[RecordAceFromNew]

You are right that the road HT2 don't have spacers. Is it possible that the front mech is bent, or not parallel to the chainrings, or braze on positioned in error?

Grasping at straws here...

 

[e-rider]

You are right that the road HT2 don't have spacers. Is it possible that the front mech is bent, or not parallel to the chainrings, or braze on positioned in error?

Grasping at straws here...

it's all new kit. I think the seat tube is so oversized that the mech ends up too close to the chainset by about 2mm which prevents the front mech from being set correctly. However, I've now got it working fairly well - but not perfect

 

[Glover Fan]

What bike do you own? For some reason I'm thinking it was a focus cayo, but my memory isn't great.

 

[e-rider]

What bike do you own? For some reason I'm thinking it was a focus cayo, but my memory isn't great.

Yes, Focus Cayo 105!

 

[Glover Fan]

Mate just take it to a LBS. I have the 2009 model and had issues with the front mech pretty much since I got it.

Turned out the front mech was bent and was aligned at the wrong height and a load of other problems. Took them 20 minutes to fix and it is all sorted now.

 

[e-rider]

Mate just take it to a LBS. I have the 2009 model and had issues with the front mech pretty much since I got it.

Turned out the front mech was bent and was aligned at the wrong height and a load of other problems. Took them 20 minutes to fix and it is all sorted now.

the LBS wont be able to help - this isn't a setup problem, it's a fundamental design/compatiblity problem!

Anyway, it's working fairly well now so that will do for the moment.