Chainline question

bmadau said:

I've got a Blur and I suspect that my chainline isn't right. How do I check this? Could some of you with Blurs measure the distance from the seat tube to the big ring?

If I try to shift to big/big combonation (not that I'd want to) the chain instantly falls off of the big chainring, and sometimes when I'm down one in the back. I think it should be able to stay in these gears... My deraileur stops seem to be adjusted an aweful lot over to the side too...

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The best way to measure chainline is tho use the tool specifically designed for it. I believe that Park makes one.

The cheap and quick way to check is to have the chain on the middle ring up front and the middle cog out back. There should be no sideways bend in the chain as you sight it form behind the rear derailler..

Thanks.. I wasn't aware that there was a tool.. I'll look into that and maybe check with my lbs.

bmadau said:

Thanks.. I wasn't aware that there was a tool.. I'll look into that and maybe check with my lbs.

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The tool may be very expensive.....I have no idea. You really can get a good idea if your chainline is right just by doing what I said earlier. If the chain looks straight in the middle cog and middle ring...sighted from behind the derailler, then your chainline is most likely close enough.

If your chainline is correct, but you're still having those issues, you may have a frame alignment issue.

Good Luck!

p.s. If you're going to the LBS.....Have them check the chainline and frame alignment. Hopefully, they have the tools to do both.

bmadau said:

I've got a Blur and I suspect that my chainline isn't right. How do I check this? Could some of you with Blurs measure the distance from the seat tube to the big ring?

If I try to shift to big/big combonation (not that I'd want to) the chain instantly falls off of the big chainring, and sometimes when I'm down one in the back. I think it should be able to stay in these gears... My deraileur stops seem to be adjusted an aweful lot over to the side too...

Click to expand...

Just curious, what size of bottom bracket or crankset are you using? knowing this will make it easier for us us to find out where the problem is...

I did like you guys said, and in the middle ring and #5 in back, the chain looks visibly straight as far as I can tell. I'm about 90% (there's a story behind it) certain its a 73 x 118mm BB. FSA platinum pro ti isis, and an FSA carbon pro team issue crankset.

bmadau said:

I did like you guys said, and in the middle ring and #5 in back, the chain looks visibly straight as far as I can tell. I'm about 90% (there's a story behind it) certain its a 73 x 118mm BB. FSA platinum pro ti isis, and an FSA carbon pro team issue crankset.

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Besides checking your front derraileur alignment and installed "angle" (whe viewd from top) and stop-screws, it sounds like a source of the problem could be the BB. The Blur is a 68mm BB shell.... so maybe your drive side arm is too far outboard and the frt. der. adjusted to reach out that far isn't working correctly, throwing the chain off in the process.... just a thought.

I know that with 3 different sets of cranks, the small ring is always very close to the lower pivot bearing/cap assembly. Maybe 2-3mm?

Honestly though, it may be as simple as getting the frt. der. adjusted properly! And you should never be in the big/big combo, very bad! It puts too much strain on all the drive line parts.... Big ring for smaller cogs, middle for middle, granny for the big ones, generally. Keith Bontrager has written lots of stuff about gearing & chainline... check it out if you can find it!

Blue Shorts said:

The tool may be very expensive.....I have no idea. You really can get a good idea if your chainline is right just by doing what I said earlier. If the chain looks straight in the middle cog and middle ring...sighted from behind the derailler, then your chainline is most likely close enough.

If your chainline is correct, but you're still having those issues, you may have a frame alignment issue.

Good Luck!

p.s. If you're going to the LBS.....Have them check the chainline and frame alignment. Hopefully, they have the tools to do both.

Click to expand...

The chainline alignment guage is about $25 online (plus freight, so same as $35 from LBS, eh?). But if that's not ie, you need to check the entire bike, so you also need a frame alignment guage & straightening tool, as well as a derailleur alignment guage.

....Probably just as easy (and cheaper!) to take the frame to the shop where you bought it and have them give it a thorough going-over. (I'm going to have them do this to my new frame before I build it up.)

Thanks, I've considered that. I will in a little bit when I have a few bucks to spend. for the most part, it shifts fine (actually great, I love my sram drivetrain).