Constantly adjusting front derailleur

vermin

It's a new bike and I've only done about 200 miles on it so far, so I expect some cable stretch but ...

My Cannondale has 10 speed Tiagra gears and an FSA Omega chainset. It's pretty basic, but I would expect it to do its job well.

However, I have found that I am constantly, and I mean constantly as I ride and change up and down the rear cassette, having to manually adjust the front derailleur to stop chain rub. In high gears I have to twist the adjustser to push the derailleur out to avoid rub on the outside of the chain and then, as soon as I change 3 or 4 gears up the cassette, I have to wind the adjuster back to stop the inside of the chain rubbing. What could be causing this problem? Is it a badly aligned derailleur, too much flex in the chainset/axle, or something else (have I just set it up badly perhaps)?

Thanks

Ben6899

I would set up from scratch, paying particular attention to:

1. Longitudinal alignment of the mech (make sure the cage is perfectly parallel with the chain rings);

2. Height of the mech cage in relation to the chainrings (2mm* above the large chainring); and

3. The lateral postion of the mech. In small chainring/big cassette cog – with the cable detached – wind in the low limit screw until the inner plate of the mech cage is 1mm* from the chain. In big chainring/small cassette cog – with the cable now attached (nice and taut) – wind in the high limit screw until the outer plate of the mech cage is 1mm* from the chain.

This should give you good shifting and mech cage clearance in all but the cross-chaining combinations (big/1,2 and small/9,10).

*double check these measurements in the Shimano tech docs, but I think they’re correct.

Let us know how you get on.

vermin

Super answer, thanks Ben. Will give it a go.

Ben6899

You're welcome!

I should add that, depending on frame geometry etc, you may get derailleur rub in only big/1 or small/10 or it might extend to big/1,2,3 or small/8,9,10. It definitely varies on my bikes.

keef66

It sounds to me that you may be unaware of the trim function of a Shimano front mech?

Big push of the brake lever to shift into the big ring, allows use of the smaller end of the cassette, then a small click of the 2nd lever to shift the mech back a bit to eliminate chain rub in the bigger sprockets. Another click of the 2nd lever to drop it onto the inner chainring, no chain rub in the big sprockets, then a little nudge of the brake lever to shift the mech slightly outwards to access the smaller sprockets.

No faffing with cable adjusters.

Have a look on Shimano Tech Docs for the full service instructions for your front mech / shifters

capt_slog

keef66 wrote:

It sounds to me that you may be unaware of the trim function of a Shimano front mech?

Big push of the brake lever to shift into the big ring, allows use of the smaller end of the cassette, then a small click of the 2nd lever to shift the mech back a bit to eliminate chain rub in the bigger sprockets. Another click of the 2nd lever to drop it onto the inner chainring, no chain rub in the big sprockets, then a little nudge of the brake lever to shift the mech slightly outwards to access the smaller sprockets.

No faffing with cable adjusters.

Have a look on Shimano Tech Docs for the full service instructions for your front mech / shifters

What he said, but I have to admit that I found that the 'big ring out a bit further' trim works easier than the one at the other end.

OP doesn't mention the chain being used, just check it's the right one. The tech documents often talk about clearances of zero to tenths of a millimeter in setting up, this doesn't leave much room for error if the chain is too wide to start with.

vermin

keef66 wrote:

It sounds to me that you may be unaware of the trim function of a Shimano front mech?

Big push of the brake lever to shift into the big ring, allows use of the smaller end of the cassette, then a small click of the 2nd lever to shift the mech back a bit to eliminate chain rub in the bigger sprockets. Another click of the 2nd lever to drop it onto the inner chainring, no chain rub in the big sprockets, then a little nudge of the brake lever to shift the mech slightly outwards to access the smaller sprockets.

No faffing with cable adjusters.

Have a look on Shimano Tech Docs for the full service instructions for your front mech / shifters

Just been reading the docs online and, you're right, I didn't know (perhaps embarrasingly). Having said that, I've never encountered this issue on other bikes. I've only been riding modern road bikes for about 3 years. These have used 5700, 6870 (cheating really) and Veloce and I have never had cause to trim them. I'll certainly be giving it a try tonight.

Regarding the chain - I don't know what's on it, but it's just whatever Cannondale (or the shop) fitted.

vermin

@keef66 Hit the nail on the head, thanks for your help. Bikes should come with instruction manuals!

keef66

vermin wrote:

@keef66 Hit the nail on the head, thanks for your help. Bikes should come with instruction manuals!

Indeed they should! I had my bike built up from scratch, but either the OEM kit came with no instructions, or the bike shop just binned them. Either way I was left to find stuff out the same way you have. Just remember there's no such thing as a silly question. Lots of people on here who just love to solve other people's problems. Shimano Tech Docs, Sheldon Brown and the Park Tools web site are permanently bookmarked on all the PCs I use

vermin

And @Ben6899, I had a good look and it seems to have been set up pretty well, but I now have a note of your set-up guide, so thanks for that too.

Ben6899

vermin wrote:

And @Ben6899, I had a good look and it seems to have been set up pretty well, but I now have a note of your set-up guide, so thanks for that too.

You're very welcome, old bean.

Tjgoodhew

I have an identical group and crank on my caad8. I always suffered from chain rub in the extreme gears even with trimming.

Being a noob to road bikes i just thought that was normal until i took it to the LBS for a service. It came back shifting perfectly at the front, no chain rub when trimming and has been ever since.

Once you get it right it works perfectly and just as good as any of the higher spec groups