Why is the front derailleur on my Shimano gears rubbing against the chain?

Question

I have a 21 speed shimano indexed gear system.

7 cogs at the back and 3 at the front.

I recently adjusted the gears, as the back derailleur had worked itself out of alignment. This went well, and I managed the job with no problems.

But since doing that, I seem to have discovered another problem that I can't figure out how to fix.

When I put the rear derailleur onto either the largest or smallest cog, the chain rubs noisily on the front derailleur.

It's okay on cogs 2 to 6, but on 1 or 7, it rubs against the front derailleur.

I'm certain it didn't do this when it was new, only 8 months ago, and I can't believe this is how the designers would have intended it to operate. After all, friction isn't usually desirable in gearing systems.

So how can I prevent the chain from rubbing on the front derailleur.

Please don't suggest taking it to a bike shop.

The nearest one is over 20 miles away and I can't fit the bike in the boot of my car. Besides, I can't afford to pay a repair man.

Answer 1

This is a surprisingly common problem and there is not an easy answer.

The phenomenon is known as chain rub and on some set ups it is impossible to remove totally.

I have a similar problem on a similar set up on my long distance tourer. All the adjustments are correct, the gears index perfectly but there is occasional chain rub where the side of the derailleur rubs against the chain.

My set up allows me to nudge the front changer slightly with the handlebar changers to get rid of this problem.

I have never come across this problem with a double chain wheel set up.

On virtually all of my bikes this problem does not arise as I usually ride fixed gear or take advantage of hub gears.

Answer 2

It could be something as simple as tightening your front derailleur cable, as they can stretch over time. Any bicycle with derailleurs is going to have gear combinations that make noise, or are altogether unusable (ie-1-7, 3-1). Bicycle chains like to run as close to parallel with the chainrings and cassette cogs as possible. They get pissed off and let you know about it when you try to use gear combinations that force them to run at extreme (relatively speaking) angles. Also, shifting to the innermost or outermost cassette cogs can cause your chain to rub on the front derailleur. This problem is exacerbated by triple chainrings and indexed front derailleurs. Grip shifters have a few extra index points between gears that allow you to "trim" the front derailleur to prevent chain rubbing in the extreme outer or inner cassette cogs. Say I'm riding along in 2-4 and I decide I want to shift to 2-7. I shift to 2-7 and I notice that my chain is rubbing on the front derailleur. I would then shift past the #2 index point to the next index point between #2 and #3. At least this is how I remember using grip shift. My 3x9 CX bike has downtube shifters, which means no indexing on the front derailleur, thus I never have to deal with that annoying clickclickclick rubbing sound. My old Raleigh three speed doesn't have derailleurs, and my single speed mountain bike is pretty much maintenance free. You need to really learn about how a bicycle drivetrain works and how to use it effectively. Then you can decide what will work best for you.

It's always a compromise.

Answer 3

http://www.sheldonbrown.com/derailer-adj...

Answer 4

You didn't indicate what front chainring you were in when you had the rubbing problem, but you should be aware that cross chainline gearing is a bad idea and will typically result in a chain rubbing the derailleur (among other issues that will lead to premature drivetrain wear).  So NEVER use small-small or large-large gear combinations.

You should be able to hit all 7 rear cogs with your chain on the middle ring without interference.

Since it's unlikely that your high and low limit screws have somehow come out of adjustment, I would say that it's likely a inner cable stretch issue.  You can tighten the cable by turning the barrel adjuster counter clockwise.

If you want to make sure it's done right, follow the first answer to the Sheldon Brown website.  By far and away the best mechanical resource online for DIY bike repairs.

Answer 5

This happened to my bike once. I took it into Halfords and they quickly changed the height of the dérailleur and adjusted the gears for no charge. It isn't difficult to adjust and only took a few minutes. 20 miles isn't really that far to cycle to, it would take about 2.5 hours maximum.. and the same on the way back.

Anyway to try adjusting yourself:

I got this from a website: "Before checking limit s***w adjustments, check the [[[derailleur positioning]]]. If the derailleur cage is too far above the large chainring, it is more likely to shift poorly. If the derailleur is too low, it may [[[scrape against the chainrings or jam the chain when shifting]]]."

There is some useful advice here: http://www.parktool.com/repair/readhowto...

Answer 6

There is a small s***w on the derailleur that may me loose. try to tighten it to pull it back into place