Front Mech Grief

thegrayson

Firstly, hello all. Sorry my first post is help related. I have just started doing my own work on my bikes given the the lbs has either gone to hell or is to busy to help. I recently replaced the cassette, bb, chain and crankset on my 8 year old mtb. Tuning in the gears nearly drove me crazy. However I now have decent shifting thorugh the full range at the rear and lower and middle ring on the front. I cannot get a clean shift on to the large chainring. It seem that the derailleur is crushing the chain against the chainring. No amount of playing with the cable tension or limit screws seems to be helping. Any thoughts? I haven't moved the derailleur and the new chainrings are the same ratio as the old ones.

Replacing the cables and both mechs is on my to do list but the money isn't there yet so sorting this out would be great.

chedabob

Is the angle and height of the mech correct?

thegrayson

I didn't adjust the height at all. I will take another look at the angle though in case I did inadvertently shift it. Starting to regret that I didn't go for two chainrings and a bash guard, its not as if the large ring gets used that often.

Chunkers1980

If you've put a new crankset on then even if you've not moved the mech the relative position might have changed.

lasty

Sure the new ring is the same size as the old one - maybe a 44t has replaced a 42t and that would foul the mech ??
Just a thought ....

thegrayson

Both new and old are 42t and the alignment looks okay to me.

cooldad

If the mech is pushing the chain against the ring, it's too low. Move it.

konadawg

cooldad wrote:

If the mech is pushing the chain against the ring, it's too low. Move it.

This would be difficult. The correct position is what, 3mm clearance off the big ring? Any seriously lower to cause problems would mean that the FD is actually clashing with the big ring so it would not really be a chain clash at all.

Actually I'm thinking - mr OP, do you actually get shifting to the big ring at all or is it simply a noise thing?

Does the FD actually have enough movement available to shift to the big ring, are the limit screws actually doing anything at all - does the FD stop outwards against the limiter or otherwise? Check for full travel without the chain by dropping it outwards.

Thinking on the lines that maybe your new or removed/replaced crankset is sitting too far out on the BB axle and the FD can't stretch that far.

Hoping you tightend the cranks in sufficiently - they take a LOT of torque to seat well.

Other than that shifting at the front is almost foolproof, minor rubbing issues or something ludicrously obviously wrong aside.

thegrayson

I'll double check the crankbolts. I actually used a torque wrench so that shouldn't be the issue. I can shift to the larger ring if I persevere. Watching the derailleur as I shift once it pushes the chain against the large ring rather the lift it.

thegrayson

The crankbolts seem fine. I was looking at front mech on chainreactioncycles on the off chance that I would need to replace. I see one glaring problem, they all seem to specify a chainstay angle up 69 degrees. According to Kona, mine is 74 degress. What do I do with this?

doggegg

konadawg wrote:

This would be difficult. The correct position is what, 3mm clearance off the big ring?

1mm

thegrayson

Do I need to worry much about the chainstay/ seat tube angle when selecting a new mech? The angle seems to be listed everywhere for Shimano mechs but not so much for Sram.

The Rookie

Not unless you have a bizarre seat tube angle, most fall within a normal range the mechs will cope with.

Simon

Chunkers1980

doggegg wrote:

konadawg wrote:

This would be difficult. The correct position is what, 3mm clearance off the big ring?

1mm

IIRC on the Shimano mech there is a little plastic tab that has a pitcure of some teeth and says 2mm. A 2 pence piece works well...

doggegg

Split the difference