Chainline road bike question

[Robbos]

Here's a doozy, at least for my non-math proficient brain. I have a nice classic lugged road bike I'm soon building up (while convalescing from surgery, what better time!). I have a full 7700/6500 9 speed triple groupset to build it with, but I'm tempted to run a first gen XTR M900 crankset in a semi-compact set up (50, 36, 30). I'm trying to figure out chain line, I have a 113 72mm bb but that has no lip on the cups so I have probably +/- 2mm each way. Based on the Sheldon Brown crib sheet, any one have any thoughts on this set up? Any one attempted it perhaps?

[Cicli]

Set the center ring 44mm from the center of the bottom bracket shell.

[oliver1850]

I think a 113 spindle will be a bit longer than you need. Sheldon's info says a 113 will give a 50 mm chainline with that crank. 107 spindle should give 47.5 chainline according to Sheldon.

If you ever want to sell that crank.......

[Kontact]

I wouldn't use the 72mm BB.

1992-95 XTR would have been based on 130mm rear chainline and a 68mm shell, so I would use that kind of crank with an appropriate width spindle - which is the 107mm listed in Sheldon's notes. (I have a '96 Litespeed MTB that is still 68/130 chainline rather than 73/135.)

A UN-55 in 68/107 is $15. I doubt you'll find an XTR 68/107 UN-91 in new condition, but it won't do anything but weigh a few grams less than the UN-55.

[Robbos]

I think a 113 spindle will be a bit longer than you need. Sheldon's info says a 113 will give a 50 mm chainline with that crank. 107 spindle should give 47.5 chainline according to Sheldon.

If you ever want to sell that crank.......

Thanks for the info. Sell the crank? Never! I've wanted one for so many years. Its in my top 10 list of favorite bike bits. Oh and that crank in the photo isn't mine, stole that from the web. Mine is no where as pretty!

[oliver1850]

Thanks for the info. Sell the crank? Never! I've wanted one for so many years. Its in my top 10 list of favorite bike bits. Oh and that crank in the photo isn't mine, stole that from the web. Mine is no where as pretty!

.

[merckx]

That chainring combination will not shift well. The shift from the 50 to the 36 will likely drop to the 30. Try it. Maybe I am wrong. You may have more success if you can locate a 40t middle ring.

[Peter P.]

You need a 107mm BB for that setup, and for proper chainline. However, the chainstays may not clear the 30T granny ring. The frame may not have a chainstay crimp for triple chainring clearance and if it does, it may be for a 24/26T ring. Frames aren't necessarily designed to fit all types of cranksets and chainring combinations. For instance, some cyclocross bikes are not designed for road cranks with 39T small rings because a typical 'cross bike runs a 34T or similar small ring.

I wouldn't mess with chainline to get it to work. But if you want to do so, get the 107 and install it, checking for interference. Work up in length from there if necessary until the granny ring gives you enough clearance so it's not taking off paint.

[oliver1850]

That chainring combination will not shift well. The shift from the 50 to the 36 will likely drop to the 30. Try it. Maybe I am wrong. You may have more success if you can locate a 40t middle ring.

Not sure why that should be so. The FD-7703 installation manual states "minimum difference between top and intermediate" chain rings as 14T, total chain ring difference as 23T.

[oliver1850]

I wouldn't mess with chainline to get it to work. But if you want to do so, get the 107 and install it, checking for interference. Work up in length from there if necessary until the granny ring gives you enough clearance so it's not taking off paint.

Or start with the BB you have and estimate how much shorter spindle your setup will handle. My guess is that most lugged road frames will accommodate a 30T ring with the suggested 107 spindle.

[merckx]

Not sure why that should be so. The FD-7703 installation manual states "minimum difference between top and intermediate" chain rings as 14T, total chain ring difference as 23T.

On a triple, when shifting to the middle ring from the large ring, the chain tends to skip over it if the small and middle rings are close in size. As a result, the chain will drop to the small ring. When this happens it requires another shift up from the small ring to the middle ring. A better ring combination is to create a 50-40-30.

[oldpotatoe]

JIS 107mm with 68mm cups..beautiful cranks. Shift with deliberation from big to middle(low pedal pressure, constant crank spin)..and it'll be fine. Really not that much different than this..53/39/30

[palincss]

1992-95 XTR would have been based on 130mm rear chainline and a 68mm shell, so I would use that kind of crank with an appropriate width spindle - which is the 107mm listed in Sheldon's notes.


I've got XTR M900 triples on four road bikes, one 135 and three 130mm, standard road bike bottom bracket shell. I'm using a Dura Ace 103mm bottom bracket on one, Phil 107 on two, and an SKF 107, I believe, on the last.

[Robbos]

I've got XTR M900 triples on four road bikes, one 135 and three 130mm, standard road bike bottom bracket shell. I'm using a Dura Ace 103mm bottom bracket on one, Phil 107 on two, and an SKF 107, I believe, on the last.

How do you find the shifting? What ring combos do you have?

[Kontact]

I've got XTR M900 triples on four road bikes, one 135 and three 130mm, standard road bike bottom bracket shell. I'm using a Dura Ace 103mm bottom bracket on one, Phil 107 on two, and an SKF 107, I believe, on the last.

So that's what happened to all the DA 103 BBs that are impossible to find for DA 7410 cranks!