# Help! Crank arm rubs front derailleur



## Simeonbl (Mar 23, 2011)

Hi first post here!

So I bought an 80's apollo hybrid because bicycle theft round here is pretty bad and my last bike got stolen from the garage in the flats Iived at the time. So I bought this cheap old thing.

Anyway not being very knowledgeable on things at the time so correct if my jargon's wrong. I ended up having to replace the rear and front derailleur's and the bottom bracket of which the cup is epoxied to the frame:crazy: oh and a new chain. Any parts have been cheap so far and I haven't spent much. Oh I've used all shimano tourney stuff because it's cheap and that's what the frame is supposed to have on it according to it's decals:lol: 

The problem I have now is when on my largest chain ring the crank arm rubs hard against the FB. It's adjusted properly and is square with the crank set. So I'm assuming the crank set isn't clearing the frame enough what should I do?

The chain ring is really worn but actually runs quit smooth! DO i need a new crank set?


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## FBinNY (Jan 24, 2009)

If it's a multi-speed crank (double or triple) there's plenty of clearance for a correctly adjusted FD, but it's not unlimited.

If as you say the FD cage is parallel to the chainring, then I suspect that the outer limit is set to far out. Back off the lever and tighten it a bit then shift back and forth. Keep doing it trial and error until the FD moves out only enough to shift smoothly and reliably and it should clear the crank arm.

BTW- in some cases it may be necessary to have the FD not quite parallel, but toed out very slightly for best shifting.


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## durielk (Jan 8, 2011)

Go to parktools, and look at the "wrenching" help to figure out how to adj you FD. It is probably too far out as FB states.


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## Simeonbl (Mar 23, 2011)

It's not the outer limit, it's as close to the chain as physically possible without actually touching the chain. The crank arm pushes the fd back into the inner chain ring it needs about 4mm-5mm further in to clear but obviously it won't go into gear then. Do you think the crank arms bent?


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## FBinNY (Jan 24, 2009)

Simeonbl said:


> It's not the outer limit, it's as close to the chain as physically possible without actually touching the chain. The crank arm pushes the fd back into the inner chain ring it needs about 4mm-5mm further in to clear but obviously it won't go into gear then. Do you think the crank arms bent?


Sounds suspiciously like it is. 

Crank arm to chainring clearance is typically 7-12mm. to allow for the chain and the FD cage. Road and older cranks tend toward the low end of the range, and mtb toward the higher, to allow more angle off a chain coming off the smaller sprockets.

Measure the gap. It's also possible that you somehow mounted the outer ring outboard of where it should be. It's hard to do, and requires some creativity but I thought I'd mention it.


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## vladvm (May 4, 2010)

1. you should have 3mm space between crank teeth and bottom of fd cage (put a penny in between)
2. should have outside fd cage parallel to crank ring
3. adjust outer limit


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## Simeonbl (Mar 23, 2011)

theres 6-7 mm between the chain ring and the crank arm. If the outer limit is set to touching the chain 0-0.5mm there's 8-9mm's between the chain ring and the fd's outer plate.


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## FBinNY (Jan 24, 2009)

OK, 6-7mm is fairly narrow, and the crank might be bent. OTOH I checked my bike and I only need about 6mm for the FD to clear the chain. 

FD cages vary in the amount of clearance needed based on the shape of the cage's outer plate, Older FD's had flat plates and needed very little clearance. Modern ones, especially triples and mtb cages need more clearance because they're ribbed and have steps increasing the effective thickness.

If your cage is stepped or otherwise thicker, you have two choices. Either replace the crank with one having more clearance, or find an older thinner caged FD. 

You can measure and agonize all you want, but you can't use your existing hardware any more any more than you can pour 2 quarts of milk into a 1 quart bottle.

BTW- what size chainrings do you have? I have a bunch of older FDs of various models, and might have one that'll work within your clearance. Or I also have a bunch of square taper crank arms, one of which might be OK.


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## Simeonbl (Mar 23, 2011)

Just went over the limits and re did everything as everyone has suggested still get the same problem. The crank set only takes 2 chain rings one on the inside one on the outside. 

When I first got the bike the bottom braket had threaded one end and the pedal fell also the cup had threaded on the frame and fell out hence the epoxy. I don't whether that would have caused any damage?


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## Amfoto1 (Feb 16, 2010)

I'm a bit suspicious of the bottom bracket replacement. Are you sure it's the right type/size for the bike? I can't help but wonder because you had to epoxy one of the cups into the frame.

Maybe the new BB has shifted the entire crankset toward the LH side of the bike, either due to being a different design or because a spacer of some sort is missing? Different BBs have longer and shorter spindles and are designed for different dimensions on the frame. Some use a spacer, too. 

Measure the distance from the inside face of the crankarm, right at the pedal, to the bike's down tube. It should be the same on both sides. Or you might be able to spot misalignment looking straight downward from the top or by looking along the chainline from behind.


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## Simeonbl (Mar 23, 2011)

36t and 48t. I actually have an old FD but it to has the same problems but less so, still knocks it out of gear. The crank arms got the spider incorporated if you know what I mean. 
Now if I end up getting a new crankset then theres another problem I have in that case I can't adjust the bottom bracket axel because it's epoxied and the the end bolt on the axel bolt if thats the correct only goes on half so a thinker spider could cause problems.


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## Simeonbl (Mar 23, 2011)

bb is 127mm long I had to replace it with a like for like one side is longer for the crank set. But who knows whether thats the right one maybe a 130mm will do it? The cup with the spacer/adjuster (if thats what its called) is as far in as possible to get maximum length on the crank end.


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## Simeonbl (Mar 23, 2011)

Actually scrap that it's 55mm between the crank arm and down tube both sides.

By the way guys thankyou so much for your help this bikes been the bain of my existence since i got it.


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## FBinNY (Jan 24, 2009)

Keep focused on the problem, which is localized within the crank arm with the 6mm chainring/arm clearance, and the required clearance for the FD, which is greater than that.

No changes you make elsewhere can change either of those, one of which will have to go.


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## Simeonbl (Mar 23, 2011)

True that didn't accur to me the bottom bracket will make no difference. Right so how else can I get enough clearance maybe a different crank arm? Do they put the dimension for things like the distance between the chain ring and the crank arm?


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## FBinNY (Jan 24, 2009)

Simeonbl said:


> Right so how else can I get enough clearance maybe a different crank arm? Do they put the dimension for things like the distance between the chain ring and the crank arm?


They don't usually publish that spec. so you have to ask for it. As i said earlier, I have a bunch of new crank arms, and can probably help you out. Let me know the crank length, chainring bolt circle, and whether it's for a square taper BB or not. 

If I have something, I'll check and confirm the arm to chainring diatance.


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## Simeonbl (Mar 23, 2011)

I'm in the uk though! 170mm crank length, square taper chain, bcd is 86 also noticed the stronglight logo on the chainset so it must be a stronglight 99 set.


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## Simeonbl (Mar 23, 2011)

Here's a pic of the chain set. I also now know that the left crank arm doesn't match.


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## tober1 (Feb 6, 2009)

this happened to me and my FD was slightly bent. It still shifted fine etc, it was just slightly bent. I literally took a wrench and bent it in slightly....fixed!


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## Simeonbl (Mar 23, 2011)

the FD is brand new the old wasn'tand had the same problem


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## Simeonbl (Mar 23, 2011)

Think I found my answer

"Mech rubs crank – 1997.07
My front changer either rubs the chain or knocks on the crank when I am in top gear. There does not seem to be a position when there is no noise, which is irritating.
The set up is a Campagnolo changer and Stronglight 80 chainset. Is it just a bad combination — this didn’t use to happen with my old Suntour changer — or is there anything I can do?
Simon Rose – Hampton, Worcs
I’ll assume you’ve adjusted the front mech cage height (to clear outer chainring by 1 or 2mm) and angle (parallel with chainrings) and declare that this problem commonly occurs when new designs of front mech are used with old designs of chainset. Front mechs now have wider cages, so they don’t need to be moved a bit to stop the chain rubbing when you shift at the rear — hence front shifts can be indexed too. And whereas chainsets used to be as narrow as possible — to minimise pedal track (Q-factor) which improves the bio-mechanics of pedalling and reduces frame flex — modern designs have been widened to suit the wider cage. Such is progress.
So your solution is either a modern, wide-spaced chainset or an old (second-hand nowadays) front mech, or to take a hammer and pliers etc. to the new one — so as to make its cage a bit slimmer!
Chris Juden" from http://www.ctc.org.uk


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## Simeonbl (Mar 23, 2011)

Any Ideas on a replacement chainset? I was thinking a shimano one seeing as the rest of my transmission is shimano.


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## Simeonbl (Mar 23, 2011)

Anyone know of a compatible sis index chainset for road bikes around the same price as the tourney stuff. So Far this is all I can find my FD goes to 48t http://cgi.ebay.co.uk/SHIMANO-FCM15...keparts_SR&hash=item1e61f5f13b#ht_1880wt_1139


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