# Chainring and crankarm clearance to the chainstay



## bike867 (Feb 2, 2011)

How close can the crank arm and chainring be to the chainstay and not contact it? I realize that any flex in the bb will probably not change the distance between them, but if the crank arms or chainring flex, then it does matter.

I'm just wondering how close is too close.


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## Wookiebiker (Sep 5, 2005)

It they are not rubbing, it's not a big deal...However if there is contact, even now and then...they are too close. 

Sorry for the crass response, but that's about as simple as it gets since there is no standard distance they should be separated by.

I've had some bikes where the clearance was close and others where there was a lot of clearance. I suspect that a bike with larger chainstays such as a Cervelo or other carbon bikes will have less clearance than a steel or Ti frame.


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## bike867 (Feb 2, 2011)

It's a titantium frame and both crankarms and chainring (50) contact the frame...albeit just barely, the chainring being the biggest offender. So, if one was to have the tubes "manipulated", how much manipulation is necessary to ensure there will never be any interference?


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## gamara (May 20, 2002)

If its contacting the chainstays, then its a design problem. Depending on the type of crank system you are using, you might want to consider adding some spacers to the the drive side if its an outboard bearing system. If thats possible with your brand. If not then you might want to consider using either a triple crankset which has a longer spindle length or any of the older isis, square taper or hollowtech systems which allows that particular crank to be used with whatever spindle length that you require. Either one of these scenarios though will require you to purchase a new crank & bb though. BTW with the triple option even if its an outboard system or whatever it doesn't matter because you just want the longer spindle length. You can just remove the granny gear & use that triple as a double without having to go out & change your front shifter as well.


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## Wookiebiker (Sep 5, 2005)

bike867 said:


> It's a titantium frame and both crankarms and chainring (50) contact the frame...albeit just barely, the chainring being the biggest offender. So, if one was to have the tubes "manipulated", how much manipulation is necessary to ensure there will never be any interference?


Did the crank come with the bike? If so, I'd say it's a warranty issue for the shop to look at.

If not, my guess is the Q-Factor of the crank is too narrow for the frame. As mentioned by "gamara" some spacers on the drive side of the crank would likely solve the problem...again, I'd check with a shop to see if they have something that would work, one or two 1mm spacers should cure the problem.

I suppose the other question is what chain rings are you running? If you have a 53x42 that could be part of the issue since a lot of bikes don't really take a 42 tooth chain ring into consideration these days...if it's a 39 tooth, that's a different story.

If the spacer option doesn't work out...there is always the option of a compact crank which with a 34 tooth inner ring should solve half the problem.

I would also contact the maker of the frame and get their suggestions for fixes...if it's a custom frame it sounds like they screwed up the design and may need to make a new frame for you, or at a minimum fix the current one for free.


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## Peter P. (Dec 30, 2006)

If you're running a single 50T chainring and it's a stock road frame, then it's not a design problem. Road bikes can't handle 50T inner chainrings.

If it's a 50T outer ring on a double, then if the 50T is hitting the chainstay, then I would expect the inner ring would be doing so as well. You need to check your CHAINLINE, which means verifying your chainrings are spaced properly relative to the cogs in back. In some cases, the frame is flawed, but I doubt that. In many cases it's a matter of mating the crankset with the correct length bottom bracket to achieve the proper chainline. Simple math and a metric ruler are about all you need. A typical measurement for 9 speed, double chainring bikes puts the space between double chainrings 41.75-42.6mm from the center of the bottom bracket. Taking these measurements is easy or have a local shop do it for you.


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## Kontact (Apr 1, 2011)

Cross frames frequently are built with clearance for no greater than 46 or 48 rings. Since we have no idea what kind of bike this is...

You can't "manipulate" a titanium frame. Fix the problem with components, or with a frame warranty.


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## satanas (Nov 8, 2002)

^ What, hammers don't work on titanium? Since when? 

But yes, agreed there should be clearance assuming a non-weird build. Also, FWIW, I've had ~1mm clearance between the chainwheel and light gauge steel chainstays and zero contact. How much crankarm clearance you might need really depends on your riding style, plus frame and component flex. I'd say "enough that it doesn't contact" - however much that might be. I've found that 5mm is usually plenty and 3mm may be okay, or not, but this will depend on the aforementioned factors.


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## bike867 (Feb 2, 2011)

I sent the frame back and the tubes were manipulated (aka: dented) to allow room for the chain ring and the crank arms. However, the left arm is still quite close (1mm) and I just put some pedals on and I can "tink" the chainstay, even without a chain installed. So, back it goes for more denting. I'm guessing with full torque available with a chain installed, that contact would even be worse.

It's a titanium road frame built with a 68mm bottom bracket and a 135 rear end to handle running disc brakes. The tubes used for the chainstay are quite large and are pretty straight, so there just isn't enough room with Sram Rival 50x34 cranks and GXP bottom bracket. I think Sram has a much lower q-factor and the builder didn't accommodate for the components being used in the build. I'm guessing this is why you see so many mountain bikes with square chainstays: it allows for more crank/heel clearance with a 135mm rear end.


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## satanas (Nov 8, 2002)

The straight chainstays are the problem. I had a short chainstay MTB frame built once where I asked the builder to make sure there was crankarm clearance, suggested bending the stays, and provided the crankset and BB to enable testing. When the frame was finished I asked the builder if he'd had the cranks on and if they cleared okay, the response being "Yes, it all works fine." I duly had the frame painted and went to install the cranks and there was <5mm overlap - aaaargh! My fault for not checking myself but still, sometimes I really hate framebuilders. Fortunately the hammer and indenting tool didn't destroy the paint when the stays were wrapped with a few layers of cloth...


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## gamara (May 20, 2002)

This is a design problem. Whoever this builder is, they are not doing it correctly. They are trying to remedy the problem post welding when this should have been addressed during the design stage. What they should have done was to flare the chainstays to provide proper clearance. Once everything gets welded up, there is not much you can do in terms of manipulation of the tubes without affecting the handling of the bike or compromising the strength of it because the tubes have no where to go so to speak.


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## satanas (Nov 8, 2002)

While there's no doubt curving the stays is the best way to avoid this problem, a little indenting here and there isn't exactly unusual, plus hitting expensive things with hammers can be fun! (If they're not yours.) FWIW, old Cannondales had chainstays that looked like they'd had several unexpected encounters with presses or suchlike and were still plenty stiff.


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## bike867 (Feb 2, 2011)

While I agree that this is a design issue, I would doubt that the small reliefs being added to pretty massive titanium stays would affect the overall strength and rigidity of the rear triangle. The stays probably should have been s-bend type which would have allowed for the low q-factor cranks being used on the frame. The builder should have taken this into account, but has not and now is working around the defect.

The question becomes: how close is ok? 2mm? 3mm? 5mm? Which way does a bb flex under power...does pressing down only INCREASE the clearance such that as long as it clears it will only get MORE clearance as you put the power down, or does that gap decrease at nearly horizontal pedal/crank location?


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## satanas (Nov 8, 2002)

It's very, very hard to put a number to "how much clearance is enough" due to the factors previously mentioned. I've had 3mm clearance on a light-gauge touring frame and no contact ever and 3mm clearance on a MTB frame where the LH crank touched every time I pedalled hard while seated.

BTW, I've indented stays quite a few times (standard frame building practice for chainwheel clearance) and never really noticed excessive flex afterwards. Sometimes it's the only way you can use some cranks on certain frames without destroying the chainline; I had to do this recently on an old Giant alu MTB frame so my Ritchey cranks could turn.


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## 2cflyr (Apr 9, 2002)

we need some obvious questions answered:
what make/model frame is it?
what size is the BB shell?
do you have the right BB for said shell? 
if you have the right bb for the shell, did you put the spacers on correctly for the BB? (assuming it has spacers like on an external BB cup.


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## Kontact (Apr 1, 2011)

I think it is sad that you need to ask a forum how your frame maker should do business. All this stuff is standardized and is part of knowing what you're doing when you hang the "custom framemaker" shingle out. I can't imagine this amateur frame builder's ti wonder was cheap.


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## satanas (Nov 8, 2002)

I'm reluctant to say this but even (some) framebuilders are human and can thus make mistakes - some way more often than others in my experience. What really counts is that any problems that might arise are resolved successfully. All this stuff makes a good case for buying off the peg, provided you can get something suitable, which is what I try to do these days. Personally, I like the idea of seeing a CAD drawing so there's a chance of spotting potential problems before the torch is fired up.


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