Installing BB - Spacers and alignment

[Robrider]

I changed my BB yesterday and am having some problem with the alignment of the chainrings relative to the casstte after i put everything back together....

 

My frame BB part is 72mm wide, and the instructions say that if its 72mm, put one spacer (2.5mm) on the right side cup only. For other options (68mm wide frame), the instructions say put 2 spacers on the right and one on the left. (So in the instructions there is no option where the L and R use the same amount of spacers).

 

Ok, so thats pretty simple and obvious, and I put everything back together. Now whats bugging me is when the bike is in the middle chain ring and biggest gear in the back, the chain is really skew. as you pedal slowly the chain  even shakes from side to side as each link comes off the cassette, because of the angle. So I'me really worried about how much wear this will cause. When the cassette is in its smallest, the chain is pretty much straight.

 

So my question... Are the instructions more of a guideline, and should i take the one spacer off the right cup, which will bring the chainring closer to the frame and more inline with the cassette. will this have any other implications (like L and R pedals at different distances from the frame, or with the seals in the central sleeve (because the cups will now be closer together than recomended)).

 

My logical side says just take out the spacer. But then why would the instructions so clearly be wrong?

 

Other information, if relevant:

XT drive train, no baby chainring

Aerozine BB

Merida HFS hardtail frame

 

[Johan Bornman]

The MTB cranks are designed for a 75.5mm space between them. In the old days we had different length BBs to accommodate different frames but nowadays one size crank fits all. But not all BB shells are the same width and 68mm and 73mm ones are available. The spacers make up for the difference.  Two 2.5mm spacers make 5mm - the difference between 73 and 68.

 

The spacer set-up works, as far as I know, with Shimano, FSA, Race Face and perhaps Aerozine...I don't know about the latter. If your instructions say so, then they're also part of the spacer brigade.

 

With a 73mm BB shell, you still need one spacer on the right hand side, as you've done. Now you're worried about your chainline.

 

The biggest gear at the back is a poor judge of chainline. Put it in the middle gear at the back i.e. with four empty sprockets below the chain and 4 above it. Shift the front setup to the middle chainring as well. Now look to see if your chain is in a straight line.

 

If you only have two chainrings, create an imaginary chainline in the centre of the two. It should be a straight line.

 

Do that first and let us know what you find.

 

 

 

[koukie]

I have the exact same problem with my new Morewood Zula frame, Also just one spacer on the right side and still even in the fourth gear at the back the chainline is skew. If I go to the third gear the chain makes a weird sound that does not sound good. This is now when I'm in the middle blade in front.

 

I did try to take the only spacer that their now is out but then the chain touches the frame when I'm in my granny  gear.

 

What I do now is to just use my granny gear much more, because I cant really go to the second and third gear at the back when i'm in my middle blade.

 

Will a 10 speed setup work better, because then I only have the two bldes in front so then I can take the spacer out that is there now so that the crank can come closer the frame. I dont really want to go this way because 1st to much money and I like my gripshifts.

 

Or can you buy a smaller middle blade for the XT crank so that I can take the granny gear out to remove the spacer and in that way bring the crank closer the frame. This would be ideal, if its possible.

 

Feedback would be really appreciated, if their are other Zula riders I would like to know if they have the same problem.

[Johan Bornman]

I spoke to Robrider this morning and I couln't get to the bottom of the problem. Having said that, I didn't have the opportunity to examine the bike. He did indicate that the chainline was nowhere at the desirable position.

My advice to Robrider was to somehow, find out what the preferred chainline is for his bike - a Merida IIRC.

Koukie, you have to do the same. From the frame manufacturer, find out what the preferred/intended chainline is.

Chainline is the distance from the centre of the seat tube to the centre of the middle chainring. In a roadbike's case it would be to halfway between the big and small blade.

 

Once you have what it is supposed to be, you can start fiddling with the crank spacing to get it right.

 

Lets start there. The solution is not to fit other cranks or grind off the granny gear mounts on your existing cranks. This has to work.

[koukie]

Thanks, but with my XT crank there is no way that I can get the crank closer to the frame, because if I take the spacer out that is in there now the chain touches the frame(swingarm) when in the granny gear.

 

Maybe this is just how it should be? i'm used to my hardtail frame that I could easilly uses up to my 2nd gear when in the middle front. Maybe i'm just parranoid because the angle of the chain is not the same as my hardtail anymore, but still its making a noise that does not sound good as the angle of the chain gets bigger as I shift up.