Hi all. Here's a conundrum/question regarding the above details for anyone who is intheknow about these things:
I had re-built my '89 Brodie Romax a while back, replacing the old (not original, as I had a shop re-build the bike back in 1999, which shop installed a 112mm spindle length UN-71 XT one-piece unit) one that came out of the frame with a like (112 mm/XT) part, thinking that that would be the correct arrangement.
>
> Alas, I have found that with the set-up as it is, any time I try to shift the chain to the smallest front sprocket (granny ring) and then shift the rear down from the biggest cog to progressively smaller ones, as soon as the chain goes from the fourth cog to the fifth, sixth, and seventh ... the chain invariably ends up interfering (rubbing) against the left side of the middle chain-ring ... causing me to now understand that more than likely, the bottom bracket spindle length is a tad too short.
>
> And, now that I think about it, it does seem to make sense; because after I got the bike back from the shop that re-built it in 1999, I did notice that the bottom bracket they had installed wasn't quite perfectly centered in the frame's shell, but was sitting over a bit (three or four threads) toward the drive-side of the bike. I had wondered about it back then, but didn't pay too much attention to it at the time, as all the shifting/gears and drive-train functioning seemed to be working just fine.
>
> I guess what had happened was, that shop installed a 112 mm spindle-length bracket into the frame, but found that because the spindle was a tad short, they ended up 'cheating' the thing over a few threads toward the right, and then lock-tighted the whole thing into place (to get the chain-line aligned).
>
> That said, I now can understand that the 112 mm spindle length is most likely a bit short for the frame, yet I wanted to check with you first - before I go installing something else into it's place (I do have a nos UN-72 unit with a 115 mm long spindle on hand, and am hoping that switching over to it would correct the chain-line problem). Can you please help me with this conundrum and give me some advice on what the best b/b axle length for my '89 Romax frame should be (I am installing some very low mileage Shimano M-730 crank arms with a low mileage set of Shimano M-900 SG-X chain-rings this time round, for the final drive-train).
Your insight and expertise will be greatly appreciated!!
PS: the current chain-line measurement, as the bike sits, is about 48 mm spot-on from frame-center to the middle chain-ring
PPS: To further complicate matters, I am going from having a newer 135mm Deore rear hub squeezed into the frame (only thing available to me at the time my rear wheel failed in 2012), to a once again period correct 130mm Shimano XT rear hub fitted - which, as I understand, will have a slight effect on the chain-line spacing
:roll:
I had re-built my '89 Brodie Romax a while back, replacing the old (not original, as I had a shop re-build the bike back in 1999, which shop installed a 112mm spindle length UN-71 XT one-piece unit) one that came out of the frame with a like (112 mm/XT) part, thinking that that would be the correct arrangement.
>
> Alas, I have found that with the set-up as it is, any time I try to shift the chain to the smallest front sprocket (granny ring) and then shift the rear down from the biggest cog to progressively smaller ones, as soon as the chain goes from the fourth cog to the fifth, sixth, and seventh ... the chain invariably ends up interfering (rubbing) against the left side of the middle chain-ring ... causing me to now understand that more than likely, the bottom bracket spindle length is a tad too short.
>
> And, now that I think about it, it does seem to make sense; because after I got the bike back from the shop that re-built it in 1999, I did notice that the bottom bracket they had installed wasn't quite perfectly centered in the frame's shell, but was sitting over a bit (three or four threads) toward the drive-side of the bike. I had wondered about it back then, but didn't pay too much attention to it at the time, as all the shifting/gears and drive-train functioning seemed to be working just fine.
>
> I guess what had happened was, that shop installed a 112 mm spindle-length bracket into the frame, but found that because the spindle was a tad short, they ended up 'cheating' the thing over a few threads toward the right, and then lock-tighted the whole thing into place (to get the chain-line aligned).
>
> That said, I now can understand that the 112 mm spindle length is most likely a bit short for the frame, yet I wanted to check with you first - before I go installing something else into it's place (I do have a nos UN-72 unit with a 115 mm long spindle on hand, and am hoping that switching over to it would correct the chain-line problem). Can you please help me with this conundrum and give me some advice on what the best b/b axle length for my '89 Romax frame should be (I am installing some very low mileage Shimano M-730 crank arms with a low mileage set of Shimano M-900 SG-X chain-rings this time round, for the final drive-train).
Your insight and expertise will be greatly appreciated!!
PS: the current chain-line measurement, as the bike sits, is about 48 mm spot-on from frame-center to the middle chain-ring
PPS: To further complicate matters, I am going from having a newer 135mm Deore rear hub squeezed into the frame (only thing available to me at the time my rear wheel failed in 2012), to a once again period correct 130mm Shimano XT rear hub fitted - which, as I understand, will have a slight effect on the chain-line spacing
:roll:
