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Bike guy tells me this problem is unavoidable,is he right?

Badass01Badass01 Posts: 13
edited August 2010 in The workshop
Hi guys,

Just bought a brand new Giant Rapid 2 on Saturday...

Following the test ride, there was a noise coming from the chain/crankset area when i was shifting through certain gears.

So, when the guy was prepping my bike before I bought it, he told me that on a double crankset, the chain is going to scrape on the front derailleur (FD) on certain gears. He said this was true for any bicycle, and only a single speed bike or twin crankset bikes would be exempt from this problem.

Is he right? Will me chain keep hitting the FD when I have it on certain gears or was he wrong?

Many thanks... :)


  • nicklousenicklouse Posts: 50,675 Lives Here
    have a read of this

    and even on 2 ring set ups you can get the chain rubbing.
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  • MrChuckMrChuck Posts: 1,663
    It's not really a problem though. No doubt someone will be along to say they have theirs set up with no rub, but my compact double rubs if I'm in the 2 smallest cogs and the smallest ring. I wouldn't worry about it, as you ride more you'll find yourself using the right gears more and the 'problem' will go away.

    Just as a reminder, you should never be in the smallest ring and smallest cog on a triple (if this is what you've got- it's not clear from your post)
  • Badass01Badass01 Posts: 13
    yes i have a triple.

    thanks very much for the links, and info, very useful, i'll ride again and see which gears specifically have the chain rubbing.

    seems a bit silly that the chain rubs on certain gears, almost like there's no point in having them!
  • cougiecougie Posts: 22,512
    No - what it means is that those ratios arent to be used. The gears arent sequential - so you would find that you dont go all the way down from :

    Big ring small sprocket to
    Small ring Big Sprocket like that

    there comes a cut off when the ratios are reproduced - so you'd get to within a gear or two of the bottom of the block, then change to the small ring, and you'd probably need to change to a smaller sprocket at the back.
  • Badass01Badass01 Posts: 13
    i'm confused now, but i think what you mean is that on all bikes some gears will cause the chain to rub against the FD, and this is different from one bike to another, and I will just have to figure out which ones are best for mine...
  • biondinobiondino Posts: 5,990
    It's simple mechanics - you can't have a wide cassette of 10+ sprockets without rubbing. The parts have to be extremely rigid to work properly, so you can't have any kind of give that would straighten out the chainline. If you spaced the chainrings further apart, the trouble this would cause when shifting between them would be insurmountable.

    So, just avoid big ring/bi sprocket and small ring/small sprocket. There's no need to be in them because other combinations will give you almost exactly the same gearing without pulling the chain to its extremes and causing rubbing.
  • wgwarburtonwgwarburton Posts: 1,863
    Derailleur gears arn't sequential like car gearboxes or hub gears, there's an overlap between the ranges available on each front sprocket.

    The highest gears in the small front ring will be higher than the lowest on the middle ring, similarly the highest on the middle ring will be higher then the lowest on the big ring.

    It's a little complicated, and you learn to use a pattern of gearshifts front & rear to suit the terrain (and your riding style, energy levels etc).

    A side effect of the way the system works is that certain combinations force the chain to flex a lot from side-to-side.. this is less efficient and increases the wear-rate... however, this happens most where the overlap is biggest (big front/big rear, small front/small rear), so with a bit of practice you learn to avoid those combinations- you don't really need them and they don't work as well as the alternatives.

    On some bikes these extreme chain angles cause the chain to rub the front derailleur- whether it happens depends on the geometry of the chainrings, derailleur, rear sprockets and chainstay length (that's the frame tube between the rear axle and the cranks, BTW)... It's best to take the rubbing as a warning that you arn't using the gears optimally and maybe choose a different combination to get the same ratio next time.

    Hope this helps.

  • I use this as a rule of thumb on a triple

      Smallest front ring lowest biggest 3 gears on the rear ie 123 Next ring up at the front best with gears 3,4,5 at the rear Largest ring on the front - gear 5 6,7,8, at the rear
    The general rule of thumb is the gear that makes the chain parallel to the frame is the most efficient.

    You can interchange a bit..

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  • dennisndennisn Posts: 10,584
    Here's my take. The chain should never rub the front "D" if you are in the smallest front
    ring and the largest rear cog. If it does then either you haven't shifted completely down
    with the front shift lever or the front "D" is not adjusted properly. If it rubs when in the small front ring and say the third cog in on the rear then most likely you need to slightly "feather", for lack of a better word, the front "D" toward the big ring, just a little, by pushing on the shift lever(this works on Shimano but I am not familiar with the other systems). Also don't use the small chainring with the outer two(and possibly even three)
    (smallest) rear cogs. It causes a bad chainline(chain angle) and can cause the chain to actually hit the teeth on the large ring, causing what's known as chain dink(a slight clicking noise when the chain hits the large ring).
  • dondaredondare Posts: 2,113
    I have a triple front and either 8 or 9 cogs at the back depending on which wheel I'm using. I get away with this because I never switched to indexed gears and therefore can adjust both the front and rear derailleurs to run along any line. If the front one rubs then I simply move it a fraction. So I do not have to put up with rubbing in any gear.
    Is it not the case that at least some systems have indexing for the rear only so that the front changer can be moved to prevent rubbing?
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  • SLX01SLX01 Posts: 338
    I have a compact chainset and mine rubs and states in the manual I got with it that its meant to, so I guess the bike shop guy is right.
  • snailracersnailracer Posts: 968
    The front trigger shifter on most (Shimano-equipped) flat-bar bikes are trimmable to eliminate rub. Of course you still have to trim it every time you shift to the problem gears, and it's not optimal being cross-chained in the first place.
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