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Discussion Starter · #1 ·
Hi
What are the symptoms of wrong BB length?

My front derailer cage rubs on the chain. I am doing up my old bike. I have recently changed my old worn square taper BB for a new ISIS BB and crank. I also changed to a new 9 sp chain. I have an 8 speed set up.

I set the chain on the middle ring and set the front derailer so the chain didnt rub when shifting across the 8 sp cassette on the back. When i shift to the small front chain ring i can only select the inner (largest) 2 cogs on the rear before the chain rubs on the front derailer. If i adjust the front derailer so it doesnt rub when on the small front cog then the chain rubs when on the middle cog. I also cannot reliably change back from the middle ring to the small once i have made that last adjustment.

Any thoughts?
 

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It sounds to me like either you didn't tighten down the cranks sufficiently or the front derailleur needs to be adjusted. The chain line on the chainrings should be between 47.5 to 50.0 mm from the middle of the downtube/bottom bracket to the middle of the middle chainring.
 

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Old man on a bike
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Like PCC says, check to make sure the arms are fully seated on the bb, sounds like you've been adjusting the fd already. Otherwise maybe you did get the wrong bb spindle length, but just what crank and spindle length did you get? What chainline do you have now?
 

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Discussion Starter · #4 ·
Thanks guys,
I just measured the chain line as best i can with ruler and spirit level!. 46mm.

I wondered if the chain line was too long then the problems would be with the small ring? and if chain line was too short then the rubbing on the derailer might be with the large chain ring? perhaps its not that simple.

Like i said, when front derailer is set in middle ring with no chain rubbing, when on the small ring i can only get the 2 largest cogs on the rear before it starts to rub on the front derailer. I did have a spacer ring on the BB drive side to push the crank arms out a couple of mm but the rubbing was worse. I removed the spacer bringing the rings closer to the BB and it is better, but still not rite.

I put in a Middleburn crank, 175mm, using raceface 8 speed rings, 8 sp cassette and a new 9 sp chain.

kev
 

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Old man on a bike
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Just for kicks, what spindle length did you get? You got the bb width that matches your shell dimensions, too, right? Your assessment of too long or too short a spindle is good, but maybe you should describe your fd adjustments? Why did you add spacers? Were your crank arms contacting your frame?
 

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meh....
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Kevan said:
I set the chain on the middle ring and set the front derailer so the chain didnt rub when shifting across the 8 sp cassette on the back. When i shift to the small front chain ring i can only select the inner (largest) 2 cogs on the rear before the chain rubs on the front derailer. If i adjust the front derailer so it doesnt rub when on the small front cog then the chain rubs when on the middle cog. I also cannot reliably change back from the middle ring to the small once i have made that last adjustment.

Any thoughts?
How are you adjusting the FD? I don't know how you are doing it, but I'll say use the cable barrel adjuster to adjust for the middle and big rings, use the low limit screw for the small ring. You will have some slack cable when on the small ring, that's okay.

What you'll do is use the low limit screw and adjust it so that the FD will just barely toss the chain from the middle to the small, that gives you the most cogs you can use when on the small ring. For the middle, use the barrel adjuster to make it shift well and let you use the cogs you want in the rear without rubbing. You might lose 1-2 cogs on either end of the cassette, depending on how you adjust it. If you can't use them all, it's probably best to adjust towards the small cog end of things for the middle ring if you don't use a big ring.
 

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Discussion Starter · #7 ·
Ok, I did the adjustment as you suggested. I set the FD to just select the small ring by using the set screw on the FD. I adjusted the barrel on the shifter to get the middle and large rings. The middle and large rings do not rub at all but on the small ring i can only get 4 of the largest cogs on the rear before it rubs. If i adjust the screws on the FD so it doesn't rub whilst in the small ring then i cannot select the small ring. Do you think i need to move the chain line out a bit.? The ISIS BB i have came with a spacer ring to be put on the drive side if needed. This would move the chain rings further out. I did take that spacer out thinking it was causing the trouble, perhaps i need to put it in again and try the adjustment you described?

cheers
kev
 

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Discussion Starter · #8 ·
The ISIS BB i have came with a spacer ring to be put on the drive side if needed. This would move the chain rings further out. I did take that spacer out thinking it was causing the trouble, perhaps i need to put it in again and try again.

I set the FD to just select the small ring by using the set screw on the FD. I adjusted the barrel on the shifter to get the middle and large rings. The middle and large rings do not rub at all but on the small ring i can only get 4 of the largest cogs on the rear before it rubs. If i adjust the screws on the FD so it doesn't rub whilst in the small ring then i cannot select the small ring. Do you think i need to move the chain line out a bit.?
cheers
kev
 

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Does it rub when using the small chainring on the four or five smallest cogs on the side or the bottom of the derailleur cage?
 

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What is the width of the bottom bracket? Is it 68mm or 73mm? If the ISIS bottom bracket is designed to be used with 73mm bottom bracket shells then you will need spacers if you are installing it onto a bike with a 68mm shell.

With regards to the front derailleur rubbing, check that the front derailleur is straight relative to the chainrings. I usually line up the outer cage with the largest chainring then angle it outwards a degree or two. The cable tension is adjusted so that the front derailleur works fine in both the middle and the large chainrings. When dropping down to the smallest cog it's the lower limit screw that stops it so that you end up with a slightly slack cable when in the smallest chainring. The limit screw is set up so that the largest five or so cogs don't rub but you shouldn't be cross-chaining anyway so you shouldn't shift into the two or three smallest cogs on the back while in the small chainring.
 

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Discussion Starter · #11 ·
I did the adjustments you described and its the best results ive had so far. All rings and cog combos are good except the small ring and the smallest 4 cogs which rub on the outside of the derailer cage. In the past i have had no rubbing on this bike with my old BB and crankset, Perhaps this is the best i can get.? I have tried rotating the FD a litttle but it causes rubbing on other combos.

Its now only rubingon the largest cogs and small chainring. Perhaps i should try putting that spaqcer back into the BB it would push the chain line out another 2mm or so. If i then tried your set up routine i might then get better results.

cheers
kev
 

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Discussion Starter · #12 ·
To Answer a previous question.

It rubs on the outer plate of the FD when in the small ring and smallest 4 cogs.

I do not know what length BB i purchased and i do not have the packaging,

The BB shell size is 68mm i think based on my measurement of its width. I have the spacer removed.

cheers kev
 

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If you measure the bottom bracket shell it'll be a little bit more than 2 5/8" or a little bit more than 2 7/8". Obviously, the smaller dimension is 68mm and the larger is 73. If your bottom bracket came with the spacers and your shell is 68 then you need the spacers, or at least the one on the drive-side. Try putting that spacer in and measure the drive line again.
 

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Discussion Starter · #14 ·
Thanks
I will give the spacer a try tonight and post later. Should i be worried about the BB and cranks after several removals? this will be the 4th removal and install?

cheers
kev
 

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Discussion Starter · #16 ·
OK
I tried adding the spacer and the rubbing on the FD is worse, there are fewer gear combinations that dont rub. I cannot get it to shift to the small ring and the large ring, it will only do one or the other reliably. If i set the FD by adjusting the barrel on the shifter so that the middle ring doesn't rub at all then i cannot select the large chain ring and and when in the small ring, only the 3 largest cogs do not rub.

I think i will have to remove the spacer again and live with the rubing on the small ring and 4 smaller cogs.

thanks for your help. i have learnt a lot about FD set up.
 

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Kevan said:
OK
I tried adding the spacer and the rubbing on the FD is worse, there are fewer gear combinations that dont rub. I cannot get it to shift to the small ring and the large ring, it will only do one or the other reliably. If i set the FD by adjusting the barrel on the shifter so that the middle ring doesn't rub at all then i cannot select the large chain ring and and when in the small ring, only the 3 largest cogs do not rub.

I think i will have to remove the spacer again and live with the rubing on the small ring and 4 smaller cogs.

thanks for your help. i have learnt a lot about FD set up.
You might check the bb while it's out to make sure you have the correct size...
 

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Discussion Starter · #18 ·
Hi
I removed the spacer and the BB lock ring is starting to round off. I guess im just not much a mechanic. I noticed a new symptom that might shed some light. When in the small ring and small cog the chain rubs on the bottom of the FD as well as the side. I lowered the FD and it doesn’t rub on the bottom any more and the shifting the the Large ring is much better.
Presently with the BB spacer removed, I have no rubbing on the middle and big rings, I do have rubbing on the smallest of the 4 cogs when in the small ring.

Now I have lowered the FD height by 10mm or so I would like to try the spacer again but ive rounded of the BB lockring. I think it will stand another removal or 2 but who knows, i may just make it worse. I shall visit my LBS at lunch to see if that have another lock ring. If not i will leave it as is because its the best ive had it so far and perhaps put it into the LBS at the end of the season for a new ISIS BB and let them set it up. At least i will get a season out of the BB i have.
Thanks for your help.
kev
 

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Kevan said:
Hi
I removed the spacer and the BB lock ring is starting to round off. I guess im just not much a mechanic. I noticed a new symptom that might shed some light. When in the small ring and small cog the chain rubs on the bottom of the FD as well as the side. I lowered the FD and it doesn't rub on the bottom any more and the shifting the the Large ring is much better.
Presently with the BB spacer removed, I have no rubbing on the middle and big rings, I do have rubbing on the smallest of the 4 cogs when in the small ring.

Now I have lowered the FD height by 10mm or so I would like to try the spacer again but ive rounded of the BB lockring. I think it will stand another removal or 2 but who knows, i may just make it worse. I shall visit my LBS at lunch to see if that have another lock ring. If not i will leave it as is because its the best ive had it so far and perhaps put it into the LBS at the end of the season for a new ISIS BB and let them set it up. At least i will get a season out of the BB i have.
Thanks for your help.
kev
I didn't know that any of the ISIS bb's used lockrings. Well, if you could lower your fd by 10mm it was way too high, would have helped to know that earlier. It should clear the big ring by about 1-2 mm. You might want to review your adjustment procedure again.
 

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Whoah there, Nellie! I highly recommend that you read this which shows you how to properly adjust your front derailleur.

If the bottom bracket is designed for a 73mm BB shell then I highly recommend that you use the spacers with your 68mm BB. Without the spacer the cranks are 2.5mm out of alignment towards the centerline of the bike. This will cause it to rub one cog earlier on the back of the bike (or you should not experience rubbing on one more cog on the back if you put them in now). Of course, now that you've moved the cranks out by 2.5mm you need to adjust your limit screws to compensate for this. That's why you could not shift into the larger chainring.

When cross-chaining there should be some rubbing at the extremes (small-small or large-large) but not a lot. Different bikes have different amounts of rubbing here. Bikes with shorter chainstays have more of a problem than bikes with longer chainstays. Is the bike a hard tail? You shouldn't be cross-chaining anyway as this increases wear on the chainrings, cogs, derailleur cage, and the chain. While on the large chainring avoid the two largest rear cogs and while on granny gear (the smallest one) avoid the two smallest cogs.
 
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