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I've got a bike that I have had for about 8 weeks and it has already been back to be serviced twice. It's just returned from the second service and I've noticed that it is not shifting from 14th to 7th gear - 2nd at the front and low gear at the back to low at the front and low at the back.

If I change down to 12th or 10th it will comfortably change down to the low gear at the front. Is this a common problem or should I get it looked at again?

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Note that one generally doesn't describe bike gears as "14th" or "7th", but rather like "large ring on front and 2nd smallest cog on back" or whatever. From your description it's hard to tell whether by "low" you mean smallest or the lowest/easiest gear. Please clarify which cogs you're switching from/to. –  Daniel R Hicks Jul 5 '12 at 11:46
    
I was a bit unsure how to describe it, when I'm speaking on low I'm talking about smallest cog on the front, smallest on the back. –  Tom Jul 5 '12 at 13:06
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You shouldn't be doing that anyway. Your chain simply isn't designed to stretch so far laterally. With a seven-speed cassette in the back, you should at most only use the largest 5 cogs with the small chainring, and the smallest 5 cogs with the large chainring. When switching chainrings, you should ideally be somewhere toward the middle of the cassette. –  Stephen Touset Jul 5 '12 at 14:56
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Adding onto @DanielRHicks's comment, part of the reason we don't typically describe gears in a 1st through whatever is that bicycle gears aren't linear the way that cars' gears are. There's often overlap in the gear ratios. E.g., The biggest ring (highest gear) on the front and a middle-ish cog on the back will be very similar or even identical to the middle ring on the front and the smallest cog (highest gear) on the back. More info about how that works can be found here: sheldonbrown.com/gain.html Warning: If you're not mathematically inclined, this will make your head spin. :) –  jimirings Jul 5 '12 at 15:05

2 Answers 2

If you can, with the chain in one of the middle cogs in the back, shift to each of the front chain rings then your front derailleur is likely in proper alignment and adjustment. This question and answer cover how to adjust the derailleur if you want to learn how to do it yourself.

What you are describing, shifting to small ring in front and small cog in back, or big ring in front-big cog is called "cross chaining" and should be avoided. If you ride cross-chained it causes:

  • Significant stress on the chain, and may cause failure.
  • Added friction and wear on the drive train.
  • Possible derailleur failure if you cross chain "big to big"
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Tom- A good way to think about your chain line and how to avoid cross chaining is to keep your chain parallel to your bicycle (an imaginary line from wheel to wheel). –  WTHarper Jul 5 '12 at 15:03
    
Note that many bike shops will not regard this as a fault (but they should give this explanation as to why). With cheaper/sloppier derailleurs it can be impossible to get them to do this, or do it reliably. If you must, the solution is more expensive parts. –  Kohi Jul 6 '12 at 0:23

If you mean that you're having problems changing from the large front chainring to the small front chainring when the smallest rear sprocket is selected, then you should be able to adjust the travel on the front derallieur. If you're not confident tweaking the appropiate screw on the front changer then ask your shop.

Do you have this problem only when riding, or is it the same if you change gears with the back wheel off the ground? You might need a friend to turn pedals while you try this.

There is sometimes a compromise between tricky changing and throwing the chain off at the front and then you have to decide which is most inconvenient - I'd have tricky changing.

As an aside, it is generally recommended that you avoid running large front chainring to large rear sprocket or small front chainring to small rear sprocket.

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