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dannyG

T-mag chain alignment conundrum

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Hi guys,

I bought this bike a while back and I have only just started riding it regularly. Unfortunately the chain is rubbing a groove into the chain stay! I don't know a great deal about bike mechanics but it seems as if the bb axle is too long. Then again the bb states on it that it is for a 68mm shell. Can anybody enlighten me? Any help is greatly appreciated.

Thanks!

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Hi

The 68mm part of your bb measurement refers to the width of the shell

If you remove your bb from the frame and measure the actual axle part, it most likely will be 128mm (could even be labelled on the bb as a graphic or sticker I.e. 68x128mm)

If this is the case it may be worth looking at a narrower axle bb. For example 68mm x 122mm (narrower axle should bring your chain a little closer inbound and stop the rubbing on the stay. May even be able to go down as far as a 68mm x 113mm (or there abouts)

Check the bb is in the correct way too - should be labelled on the bb: L & R (left and right)

Hope this helps and you get the problem sorted anyway :)

Cheers

Barry

Edited by trials26
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The method above will work but it is a lot of trouble for what your problem is!

Get a new sprocket for the rear with centered teeth. Such as an echo sprocket :)

This problem is very common as well and a centered sprocket always sorts it out :)

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The method above will work but it is a lot of trouble for what your problem is!

Get a new sprocket for the rear with centered teeth. Such as an echo sprocket :)

This problem is very common as well and a centered sprocket always sorts it out :)

A centered sprocket will make it worse?

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A centered sprocket will make it worse?

How exactly would it make It worse? I'm on my phone so the pictures aren't very clear. :P

I've seen this as the chain is rubbing a hole into the upper tube of his chain stay? Kinda like the echo frames if you use a sprocket with teeth on the out side? A centered sprocket will pull the chain towards the wheel and away from the tubes?

In could be missing something obvious as the pictures are really small for me :)

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How exactly would it make It worse? I'm on my phone so the pictures aren't very clear. :P

I've seen this as the chain is rubbing a hole into the upper tube of his chain stay? Kinda like the echo frames if you use a sprocket with teeth on the out side? A centered sprocket will pull the chain towards the wheel and away from the tubes?

In could be missing something obvious as the pictures are really small for me :)

It will pull the chain closer to the frame but then his chainline would be terrible

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It will pull the chain closer to the frame but then his chainline would be terrible

Just looked on the laptop and i see what you mean now :P his chain line isn't too great now either.

Looks as if he will need a different bb :/

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Check the bb is in the correct way too - should be labelled on the bb: L & R (left and right)

The cups are threaded differently, you cant put it in the wrong way :P

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The cups are threaded differently, you cant put it in the wrong way :P

you can swap the cups over though

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you can swap the cups over though

unless im being a complete moron (which im sure ill be corrected if i am ;) ) one cup is left hand thread, and the other is normal thread.

So you cant put the left cup in the right side and vice versa?

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unless im being a complete moron (which im sure ill be corrected if i am ;) ) one cup is left hand thread, and the other is normal thread.

So you cant put the left cup in the right side and vice versa?

You can take the cups off the main body off the bb and swap them over. Although I doubt it would make a difference

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Hi

The 68mm part of your bb measurement refers to the width of the shell

If you remove your bb from the frame and measure the actual axle part, it most likely will be 128mm (could even be labelled on the bb as a graphic or sticker I.e. 68x128mm)

If this is the case it may be worth looking at a narrower axle bb. For example 68mm x 122mm (narrower axle should bring your chain a little closer inbound and stop the rubbing on the stay. May even be able to go down as far as a 68mm x 113mm (or there abouts)

Check the bb is in the correct way too - should be labelled on the bb: L & R (left and right)

Hope this helps and you get the problem sorted anyway :)

Cheers

Barry

Thanks Barry. I tried out an old bb with a 113mm axle and it did the trick!

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