Stumped by front derailleur adjustment

dstev55

Hi all,

I've just bought a new road bike, a Btwin Alur 700 from Decathlon. The chain is rubbing on the derailleur in certain gears and I am struggling to adjust it adequately. It was rubbing on the front derailleur when on the small cog on the front and the largest four cogs on the rear, but also rubbing on the other side of the derailleur when on the smallest cog on the rear. I initially took it back to Decathlon but the guy said this is how road bikes are and his is like it and you'll never eliminate chain rub. I know you may still get a little in the most extreme gear, but 5 out of 10 gears, really???

I have tried following this video (http://roadcyclinguk.com/how-to/mainten ... 4ZP529L.97) however if I change limits it just shifts the front derailleur it would catch on more of the smaller cogs as I eliminated the rubbing on the larger cogs. It's as though the gap in the derailleur is not big enough to accommodate the chain moving across 10 gears.

Can anyone give me any pointers on what might be going wrong? The bike has Shimano 105 10 speed however has Btwin's own 50/34 crankset and a Tiagra cassette.

dj58

Are you aware that there is a trim function for the LH STI? In that article you linked to Jon refers to it as micro adjust. From this link http://si.shimano.com/#seriesList/18/57 scroll down to the heading ST-5700 and download a copy of the PDF file SI-6TH0A-002-ENG.PDF It explains Trimming (noise prevention).

dstev55

Thanks, I obviously heard him talk about this but wasn't sure how you did it exactly so that document helps. I don't fully understand it though, does trimming permanently alter the position of the front derailleur or will you have to trim all the time whilst on the bike?

dj58

As you shift across the cassette and you start to get chain rub you use the trim function to move the FD over a fraction so that the chain no longer makes contact with the FD guide plate. Once you figure it out you will do it with out thinking about it.

On my 10sp bike when in the 50T front ring I trim when I am in the 24T, and 27T rear sprockets, I don't use the 50T - 30T combination. The trim works the other direction also 34T front ring 14T, 13T and 12T rear sprockets or whatever combination you are using. I have usually shifted back up to the 50T front ring before then so don't trim those combinations, though you could if needed.

If you have a turbo trainer or a bike work stand, just have a practice with it or next time you go for a ride, find somewhere quiet to try it out.

dstev55

I've just spent a couple of hours working on the bike trying to get it right and I can't figure it out and if anything the bike is probably worse. As the video describes, I put it in to one of the middle gears to try the front derailleur shifting between the small and large ring which is OK, but then when I go through the rear gears, the chain rubs on both the lowest and the highest gears. I don't understand how this can be. How is any amount of adjusting ever going to stop the chain rubbing on both sides of the derailleur when in the same front ring? I looked at the trim adjustment but I don't see how any moving the gear levers changes anything on a permanent basis.

I'm sorry if I sound completely stupid, I am wanting to do as much as my own stuff as possible with the bike but I feel like I've made my brand new bike worse (I've even made a right mess of the end of the cable when reattaching it to the derailleur!) and at the moment I don't think it is even state where it can be cycled.

plodder73

The issue is the rubbing on the largest cogs at the rear whilst in the small chainring. The rubbing when on the smallest cog is to be expected and can be helped by using the trim. I would start by unscrewing the low limit screw on the front deraileur fully out, ie turn it anticlockwise so you know that it's not the limit screw stopping it. What will happen is the chain will probably drop off towards the seat tube, screw it back in just so it doesn't fall off and then try the gears.

If that eliminates it fine tune the low limit screw so it's 1mm when in the smallest chainring, biggest cog. Let me know how you get on.

plodder73

Just having read your last post, it must be your low limit screw that's stopping the derailleur releasing enough to stop it rubbing on the inside plate of the derailleur.

Don't panic, I know it's easier said than done, if all else fails take it back to the shop because the bloke in the shop is talking crap, and it's his fault you have resorted to doing it yourself.

plodder73

Just having read your last post, it must be your low limit screw that's stopping the derailleur releasing enough to stop it rubbing on the inside plate of the derailleur. The low limit screw on 105 is the one nearest the seat tube.

Don't panic, I know it's easier said than done, if all else fails take it back to the shop because the bloke in the shop is talking crap, and it's his fault you have resorted to doing it yourself.

dstev55

OK thanks, think I'll take it back tomorrow and leave it with them until it is sorted!

dj58

looks to me as though it has not been set up correctly from the off. Make sure they explain and show you how to use the trim function before you take the bike away.

dstev55

OK so I took it back to Decathlon yesterday evening and a different person had a look it. He was really helpful and has managed to sort everything out. There is only a tiny bit of chain rub in the most extreme gear on either side of the rear cassette but he said to get some miles on the bike and see how it goes. He also explained how to use the trim function on the front derailleur which has now made it very clear, although it would have been helpful if someone had showed me this before as it is an essential piece of information! Top marks to this guy though and I now have a bike that I am much happier with.

ai_1

Bear in mind that it's generally best to keep the chain line as straight as possible rather than having it running at a steep angle as you would get if the chain is running from the large front chainring to a large rear sprocket or from the small chainring to a small sprocket. This is often referred to as "cross chaining". Cross chaining will produce more noise and more wear and can be helped using the trim function but is generally unnessecary as you can find a similar gear by using a different combination. If you have 2 chainrings and 10 sprockets that's 20 possible combinations but in reality there are 4 of those I would probably never use and a couple more I'd only use rarely. There is overlap between the gear ratios available. So if you're on the big chainring (50) and a big sprocket (say 28) then you have a ratio of 1.786 but the chain is at a steep angle and will be unavoidably noisy. However if you switch to the small front chainring (34) and a smaller sprocket (19) you'll get an almost identical ratio of 1.789 but a nice straight and quite chain. I'm not suggesting you do the maths on the bike. Just be aware that if you're towards the extremes of the cassette while cross chaining you can change front chainring and move towards the centre of the cassette to find a similar ratio with a better chain line. It won't always be as close as the example above but then it doesn't need to be!