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Crank/bottom bracket issues

by Cycle_Jim
Wed, Sep 04, 2013 17:01
6

Few weeks ago I bought a fixed gear bike, its an old frame with newer parts. Frame is a falcon, looks pretty nice - but not too nice (the idea is that it can be locked up and not potentially stolen) To save money I used the chainset that was on the bike originally but had the bottom bracket replaced. (as well as headset etc)

After a couple days riding there was a ticking/clicking noise. At first I thought it was the rear wheels bearings had gone, but it was a new wheel so seemed odd. I was (and still am) convinced that something to do with the bottom bracket or chainset isn't right. I took it into the shop and they said they'd take a look, serviced the bottom bracket and told me nothing was wrong. So I took it for a ride and the problem is still there. I'll describe it and hopefully some suggestions will come to light...

If I pull on the cranks, wobbling them back and forth (counter-clockwise) they move, about 1mm. There was noticeable side to side movement, though this has gone since the BB was serviced. The cranks felt like they were grinding. When moving the cranks clockwise, looking at the chain there is a noticeable movement in the chain-line, perhaps nearly 2mm, on every rotation.

So what do I do? Is it just me?

The shop said if I'm still not happy they will replace the BB with a better one. Could the cranks be an issue due to the age?

I don't exactly want to start throwing money at a bike I've just bought.

Should I ask for my money back? Or ask for new cranks to be installed? The bike is going to be used to commute, so its going to save me money. But I'm moving away from home in 3 weeks, so ideally this needs to be sorted out soon. I don't want to go uni have a bike on my hands that is broken and need to pay for repairs and/or be getting the bus and spending money on that!

Thanks in advance for any suggestions!

If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.

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6 comments

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Cycle_Jim | 10 years ago
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Thanks for all the suggestions!

Will try it all after the weekend and then get back to you!

I'm hoping to get another shop to look at it next week but I know he's pretty busy.

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velotech_cycling | 10 years ago
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I'd drop the cranks off, undo the non-gearside cup a good few turns, ensure that the gearside cup is screwed hard in against the frame, then tighten down the NGS cup - if, as Yorkshire Whippet says, a 73 mm BB was fitted into a 68 shell (assuming you are BSC threaded which AFAIK all Falcons are / were), that should eliminate any problem in that area, though I would question why the shop didn't fit a 68 mm shell BB in the 1st place.

Don't forget the driveside cup tightens anti-clockwise if it is a BSC threaded frame.

Next port of call is the chainring / chainring bolts - are the bolts that retain the ring on the spider all fully tight?

If so, put a suitable indicator against the downtube (a pencil, say) with the tip against the inside of the chainring and turn the cranks - see if the side-to-side play at the ring is in one small area (maybe a bent chainset spider arm or section of chainring) or if half the ring swings to a greater or lesser extent away from the indicator, the other towards - that would indicate that either the chainset has not pulled onto the square taper straight (possible but unusual) or that the BB axle itself isn't machined square (also possible, but unusual).

The noise may of course have nothing to do with the crank / BB as such - do check the pedal threads are greased and the pedals properly tight (LH pedal tightens anti-clock, remember) and that there is no movement of the rear wheel in the dropouts. Both of these things can & frequently do present as BB-related problems.

Good luck!
Graeme - Velotech Cycling Ltd

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Yorkshie Whippet | 10 years ago
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Remove the cranks and give the bb a quick nip, starting with the drive side then the non-drive side. If the non-drive side cup is buried in the frame it is possible the bb shell is too short ie 68mm rather than a 73mm. Then refit the cranks and tighten them.

I'd also go to another shop, you can not service a sealed unit.

Worse case is that the bb shell on the frame has been damaged, ie threads stripped.

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Cycle_Jim | 10 years ago
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Sprocket brand new along with a new chain. I'll check chainline but that as far as I know is set up right as is tension, I'll check if it is/isn't right in the morning!

It's a sealed square taper 7 But there is definitely sideways movement, so perhaps bb or cranks are bent

My thoughts were the combination of new sprocket/chain and old chainset causing issues?

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cat1commuter | 10 years ago
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Is it a new chain on an old sprocket? That will produce a ticking / clicking noise. You can tell by looking at the teeth of the sprocket if they are worn. (Same is true for chainring, but they don't wear as fast.)

Is it a sealed bottom bracket? I imagine it is if it is new. Square taper sealed bottom brackets are usually reliable.

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cat1commuter | 10 years ago
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Is the chainline of your cranks correct? If it is too wide or too narrow, the chain might be rubbing on the cog teeth. This is more of a problem with 1/8" chains, not 3/32" 8-speed chains which are designed to work with non-straight chainlines across derailleur blocks.

Is your chain too tight? Only 1 mm back and forth sounds like high chain tension to me.

There shouldn't be side-to-side movement (viewed from above) in the chainline. That indicates either a bent bottom bracket or bent cranks.

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