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Discussion Starter #1
I have a 2006 5 spot and the bottom bracket shell is offset 2mm to the driveside when measuring from the center of the downtube. Now when I put in a shimano non adjustable 73x113 BB (UN73) my chainline is 50mm instead of 47.5mm. this is old school square taper BB. This same crank and BB gave me a 47.5mm chainline on my other bike which the BB shell is centered with the downtube.

I measured another 5 spot in the store and the BB is offeset to the drive side by 2mm just like mine. The problem I'm having since the chainline is 50mm, the chain falls off when in the center chainring and 1st gear. It falls off just spining the cranks a few turns in the bike stand, without any load. The cranks are raceface next lp with raceface chainrings, cassette is XTR 8spd, chain is scram PC68 8 spd. Chain, chainrings and cassette are all brand new. Front deraileur in not touching the chain. I installed a 110mm BB and the chainline is now 47mm and the chain doesn't fall off anymore, but the chain is 2mm away from the bottom chainstay pivot torx bolt when in the small chainring. The drive side crank is a little closer to the chain stay than the left side crank using this 110mm BB.

What is the reason to have the BB offset to the driveside by 2mm? Tuner must have a reason to do this. The LBS didn't know why and just said to use the 110mm BB.
 

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Team Blindspot
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glader60 said:
I have a 2006 5 spot and the bottom bracket shell is offset 2mm to the driveside when measuring from the center of the downtube. Now when I put in a shimano non adjustable 73x113 BB (UN73) my chainline is 50mm instead of 47.5mm. this is old school square taper BB. This same crank and BB gave me a 47.5mm chainline on my other bike which the BB shell is centered with the downtube.

I measured another 5 spot in the store and the BB is offeset to the drive side by 2mm just like mine. The problem I'm having since the chainline is 50mm, the chain falls off when in the center chainring and 1st gear. It falls off just spining the cranks a few turns in the bike stand, without any load. The cranks are raceface next lp with raceface chainrings, cassette is XTR 8spd, chain is scram PC68 8 spd. Chain, chainrings and cassette are all brand new. Front deraileur in not touching the chain. I installed a 110mm BB and the chainline is now 47mm and the chain doesn't fall off anymore, but the chain is 2mm away from the bottom chainstay pivot torx bolt when in the small chainring. The drive side crank is a little closer to the chain stay than the left side crank using this 110mm BB.

What is the reason to have the BB offset to the driveside by 2mm? Tuner must have a reason to do this. The LBS didn't know why and just said to use the 110mm BB.
Why don't you call Turner and ask them directly.
 

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No, that's not phonetic
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glader60 said:
I have a 2006 5 spot and the bottom bracket shell is offset 2mm to the driveside when measuring from the center of the downtube. Now when I put in a shimano non adjustable 73x113 BB (UN73) my chainline is 50mm instead of 47.5mm.
"Chainline" tells you how far from the central plane of the bike the middle ring sits. One way to measure this is from a point on the frame which you assume to be at the central plane of the bike, such as at the middle of a tube. If the tube is not centered at the frame's central plane, it does NOT change the chainline, only the point from which you are measuring the chainline. The chainline is not dependent on the front triangle, but rather the rear triangle, and how the rear triangle aligns the cassette and chainrings.

Think about this: the reason you measure chainline is to get a feel for where the chainrings sit in relation to the cassette. A wide chainline (e.g., 50mm when paired with a 135mm spaced wheel) tells you that the cassette sits somewhat inboard relative to the chainrings (or conversely that the chainrings sit somewhat outboard of the cassette), and the chain will see a less direct path to the lower-gear cassette cogs. So it is the relation between the chainrings and the cassette which is important. It is a function of the rear triangle and has nothing to do with the front triangle.

You are simply measuring chainline from a reference point on the front triangle as a sort of indirect indicator of the true chainline. (Typically you measure the chainline from the seat tube anyway, but again if this happens to be offset from the bike's true centerline too, it will not necessarily tell you what is really happening relative to the cassette.)

If you are interested in frame alignment, you may want to play with the string method. This may tell you if the bb is centered between the rear dropouts. If it is, then the offset downtube will have no bearing on chainline.

Let us know how you make out.
 

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Natl. Champ DH Poser/Hack
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Dusty Bottoms said:
Bikes in the 21st century are designed to operate with 9 cogs on the cassette. You must ASSimilate.
true. but 8 and 9spd have the same cassette widths and offset therefore not makin a damn diff in this case. maybe ya knew that and were just givin him crap. cheese makes excellent points. do as he suggests. learn from him in the process.

fwiw: my old 8spd burner uses very similar parts as yer spot with a 113 x 73 b/bkt on a 68mm shell with a proper spacer and doesnt have the issues you describe. to me thats strange and makes me think you need to call turner and also check frame allighnment. good luck and report back.
 

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"El Whatever"
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Weird... acccording to Turner, you'd be using a 73X113... and according to Shimano, a chainline between 47.5 and 50mm is OK.

TS makes a very good point that the BB shell does not necessarily needs to be centered on the downtube.

However, the non-drive side of the shell should not be offset. Otherwise, you'd get your non-drive side crank all over the place (not that this matter much, though; while the crank does not hit the stays and your "Q factor" feels comfortable). This should not affect your chainline, though and any offset of the BB shell's drive side face relative to the downtube is irrelevant, as long as the chainline is OK.

Call Turner... they should have the whole scoop on the matter.
 

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Daniel the Dog
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6,761 Posts
I have not had a lick of shifting change

I had an Octalink crank and now have a External BB XT crank. Both work(ed) perfect. NO chainsuck or shifting problems in so long that I have forgot what it is like to drop a chain or miss a shift.

Jaybo
 

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Discussion Starter #9
Response from Turner

I emailed Turner and they say, a 110mm bottom bracket is probably the right one for my application, they said a 113mm BB is just a recommendation. They also told me to use a shimano chain instead of the sram.

I'm using a 110mm bb and have put a couple of easy offroad miles on it on the trails in my back yard and it's working well. I kept the sram chain on it.

I haven't tried the string method to check frame alignment, though I plan on checking it.

Dusty Bottoms said:
Bikes in the 21st century are designed to operate with 9 cogs on the cassette. You must ASSimilate.
:nono: I will not ASSimulate (at least not until my supply of 8 spd stuff is gone) , I like the 8 spd. better than the 9 spd, just call me old school. At least it's not 7 spd :eek: I bought a bunch of 8 psd stuff for dirt cheap after the 9 spd stuff came out. Only thing I wish I could get in 8 spd is a 12/34 cassette.

Plus, the 9 spd would have the same chainline problem as the 8 speed.
 

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Natl. Champ DH Poser/Hack
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sounds like it worked out. still sounds odd but without seein it theres no way for me to make anything like a real judgment. keep ridin pal. and good call on the sram chain.
 
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