q. on bike mechanics / placement of front derailleur

On my last mtb ride, which was fairly mellow, I noticed a lot of noise in my little ring. I brought it home and had DH look at it -- the problem was actually pretty obvious once the bike was hanging in the stand.

The front derailleur is mounted such that when I go past about the fourth rear cog while in the smallest front ring, the chain is too low and rubs against the bottom of the derailleur. But the front d. only has about 3mm clearance for the big ring up front, so mounting it lower really isn't an option.

This is a high end bike, XS frame size.

So -- I guess a few questions --

If you had a bike where you effectively couldn't use anything past the fourth cog in the front ring, would you be upset? I know that you're supposed to avoid crossing the chain, but to me it seems there's a big difference between "avoid doing this" and this, which effectively prevents me from using half the cogs. (Okay, I guess I could do it and just suck up the noise, but I suspect having the chain rub against the derailleur like that isn't a recipe for long-lasting bike parts.)

Shouldn't the bike shop have noticed this when building the bike and at least mentioned it to me?

Is there any possibility of a different mix of components working better? (I can get an exact list, but I don't remember it all off the top of my head.) Aside from getting a smaller large ring or a bigger small ring, neither of which sound like wonderful ideas from here.

Is the fact that it took me something like 7 or 8 rides to even notice this an indicator that it's not a big deal?

I do tend to hang out in the little ring -- is this just the universe's way of telling me to learn to shift up front more often?

Take it back to the bike shop! This is not something you should "just get used to". If you've only ridden it 7-8 times, you didn't buy the bike that long ago, right? It could be that it's just a matter of getting the bike tuned because of the initial stretch in the wire etc. At my LBS, that's a free service...

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Originally Posted by SuperRunt

Take it back to the bike shop! This is not something you should "just get used to". If you've only ridden it 7-8 times, you didn't buy the bike that long ago, right? It could be that it's just a matter of getting the bike tuned because of the initial stretch in the wire etc. At my LBS, that's a free service...

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I will definitely take it back to the shop to have them take a look -- I just want to have a good idea of what I want to say and how to approach it.

I actually bought it last fall; I got into a car accident right around the time I bought it, and was too beat up physically and emotionally to really put it through its paces. (I did try riding right after the accident; it was a mistake for me.)

I have had them tune the bike several times since I got it, and they'll certainly keep looking at it as long as I have questions or issues. I've looked closely and I do *not* believe this is an issue with stretch. This is an issue of physics -- when the front ring and the rear cog are both lower than the derailleur, the chain rubs against the derailleur. If the derailleur were any lower, it would hit the big front ring. It looks to me like whoever put it all together put the components in the only places they possibly could go.

I built up the components list myself, so it's certainly possible I chose some stuff that doesn't work well together, or just doesn't work well together in combination with the specifics of my bike geometry. I do have a mixture of SRAM and Shimano, so maybe that has an impact.

I don't know anything about MTB shifting -- but usually with road systems, you can trim the front derailleur a bit by giving the shifter a little "half-shift" tap (seems like the newer Shimano shifters are specifically designed to allow this -- you have to shift "two clicks" for a full shift -- but it's possible on the older indexed systems as well). So if your gear combination is causing some chain rub, you can shift the derailleur just a little to the left or right to alleviate the rubbing.

Any possibility your shifters might allow this?

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Originally Posted by liza

I don't know anything about MTB shifting -- but usually with road systems, you can trim the front derailleur a bit by giving the shifter a little "half-shift" tap (seems like the newer Shimano shifters are specifically designed to allow this -- you have to shift "two clicks" for a full shift -- but it's possible on the older indexed systems as well). So if your gear combination is causing some chain rub, you can shift the derailleur just a little to the left or right to alleviate the rubbing.

Any possibility your shifters might allow this?

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There are some mtb shifting systems that allow this, like grip shifts; I don't think mine is one of them. But I think trimming applies to lateral rub (rubbing against the sides of the chain holder), not vertical (on my bike it's rubbing against the *bottom* of the chain holder).

What I think would need to happen is to somehow get the derailleur to go lower when in the small ring but stay where it currently is in the big ring. I suspect that's not possible.

Aaaahhhhh. I didn't read carefully enough. Could you get a longer-cage front derailleur? (Again, I don't know anything about mountain components, but often road bikes with triples have longer FD's than bikes with doubles...

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I built up the components list myself, so it's certainly possible I chose some stuff that doesn't work well together, or just doesn't work well together in combination with the specifics of my bike geometry. I do have a mixture of SRAM and Shimano, so maybe that has an impact.

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Hope this isn't useless info, as I can't quite remember... :o But here it goes:
I put together my old bike with a mix of SRAM and Shimano. I remember there was an issue with the front derailleur. I had to get one that mounted higher on the frame and had a bigger "range of motion" (can't remember why...Sorry). I also, had to use a specific set of SRAM grip shifts for the shifting to work.

You are absolutely right that your chain should NOT be rubbing on the bottom of your derailleur. You didn't mention whether you have a hardtail or a full suspension bike... if you have a full suspension bike then you have to take into account that the rear of the bike will have some sag which does have some affect on the derailleur. With that in mind, derrailleurs are set up different for the two different types of bikes. However, regardless of which type of bike you have... it should NOT be rubbing and you should have your LBS take a look at it. If you don't, you will eventually rub a hole in the bottom of the derailleur and then it won't be able to pick the chain up to shift. (I've had that happen in my shop)

Good luck!::)

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Originally Posted by SuperRunt

Hope this isn't useless info, as I can't quite remember... :o But here it goes:
I put together my old bike with a mix of SRAM and Shimano. I remember there was an issue with the front derailleur. I had to get one that mounted higher on the frame and had a bigger "range of motion" (can't remember why...Sorry). I also, had to use a specific set of SRAM grip shifts for the shifting to work.

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Thanks, Super -- that is definitely not useless. It gives me a direction for the bike shop -- I could ask about a different front derailleur. I didn't realize they came with different "ranges of motion." I don't want grip shifts, though, if I can avoid it ... but still, this is definitely the kind of info I was looking for. Thank you.

Is your front derailleur Shimano or SRAM? Shimano has different mtb front derailleurs that might work. Questions to ask if Shimano - is your current ft derailleur top pull, traditional or bottom pull, topswing. If it's SRAM, you have a high clamp or low clamp option.

So I'm home now and checked --

The front derailleur is Shimano XT (rear is SRAM X-0; that's what had me confused). The chain rings are the ones that came with XT Hollowtech II cranks.

Earlier today, I called the shop and spoke to a mechanic there. He said that while of course I should bring it in and they would look at it, rubbing issues are common for extreme sizing (XS, XL, etc) on full suspension bikes -- the frames can't be sized that way and meet spec perfectly. He also said that rubbing the chain against the bottom of the derailleur cage, while annoying, is not damaging to the components, just noisy.

All of which would be fine and dandy *if* it had been disclosed to me prior to buying the bike, not left for me to discover. Prior to this I was riding a medium frame Sugar. It was way too big for me, yes, but it's not like I had experience with XS frames. At the time I did express that having a quiet bike was important to me (the Sugar was so noisy I felt like it would come apart at any moment), and I did express concern about getting something as small as the XS, but was assured it would be the best fit. And specs indicated that the standover height on the S would be an issue. Unfortunately, this was a case where they only had the M frame at the shop, and I had to choose sight unseen. So I really don't know if the S would have been better for this issue or not ... and it is true that I have had some gnarly wrist issues, so my biggest concern was getting a bike that would not be too big and put pressure on my wrists from being stretched out. Argh. I just wish I could remember exactly how all those conversations half a year ago played out.

DH postulates that, buying that high-end of a bike, the shop guys assumed I would know this. I don't know that that's a good enough excuse. If I already knew everything, I wouldn't need the shop and would have saved money buying the frame myself and having a friend build it up for me.

My discontent is compounded by the fact that the new bike feels so darn short. I am fairly sure that is just because my old bike had such a ridiculously long wheel base for me. I was able to get away with really not having to move my body much at all. But I'm having to learn how to descend all over again, and feel like I'm way extended over the handlebars all the time. Intellectually I know that this just means I need to learn to get further back behind the seat while descending, but there is still this niggling "what if this bike is wroooong for me?" feeling. And the shifting issue isn't helping, even though, yes, I can always use the middle ring to accomplish the same gear ratios.

Sorry for venting. *Deep breath* Just gotta get out tomorrow and ride the darn bike and remind myself, it's fun, it's fun! I just need to ride it more.

I just asked my DH who is our bike mechanic. He said take it back. I ride a high end xs frame also and my bike certainly does not do that.

Take her back.

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Originally Posted by bounceswoosh

He also said that rubbing the chain against the bottom of the derailleur cage, while annoying, is not damaging to the components, just noisy.

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This is unacceptable.....especially if you are in a reasonable gear (not cross chained)

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Originally Posted by bounceswoosh

My discontent is compounded by the fact that the new bike feels so darn short. I am fairly sure that is just because my old bike had such a ridiculously long wheel base for me.

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Did you get a bike fit?

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Originally Posted by bounceswoosh

Intellectually I know that this just means I need to learn to get further back behind the seat while descending, but there is still this niggling "what if this bike is wroooong for me?" feeling.

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If it's wrong, you'll be buying a new frame and swapping the components. Take this bike back to the shop and if you don't get satisfaction, take it somewhere else. You'll have to pay, but another shop's opinion is worth it.

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Originally Posted by bounceswoosh

I built up the components list myself, so it's certainly possible I chose some stuff that doesn't work well together, or just doesn't work well together in combination with the specifics of my bike geometry. I do have a mixture of SRAM and Shimano, so maybe that has an impact.

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Also - a true "high end" shop would tell you if the components weren't compatible. Having a mixture of shimano and SRAM is very common. I currently have XT up front and SRAM XO in the back.

I agree with all of the others,,,take it to the shop! I also work at a mtn bike park, and our xs bikes do not have that problem. I really think it's simply not set up quite right. Regardless of what size it is, rubbing on the BOTTOM ( not the side plate) of the derailleur is a height/compression issue and is something that is quickly fixed. Whether it's Shimano or Sram is also not the issue.

It sounds like you have a great bike, and although there is an adjustment phase in getting used to the size... in the long run, it truly sounds like it's a better size for you. Mountain bikes do not have as many options for sizing as road bikes, but hopefully one day they will... it's certainly getting better all the time.

Have fun!