Art Odd

I should have checked the chainline on an old steel frame bike before removing the old crankset, but was impatient to upgrade. Based on the info and photos on Sheldon's page, I installed a new BB (Shimano UN55, 113) to go with the square taper crank (Shimano RX100, A550), but the chainline is now significantly further out. Just eyeballing, the large chainring seems about inline with the smallest cog of the 6-speed freewheel.

I have not adjusted the ancient Shimano OEM front derailleur, so it's obvious that the new crank is further outboard than the old. I measured the distance to the inside of the inner chainring (43mm) and the inside of the outer chainring (50mm). It does look like if I adjust the upper and lower limit screws, the old FD might still work, but that doesn't address the chainline issue. I have not installed the new chain yet.

If the FD can't be adjusted to shift properly, what is my next move? A new FD? A shorter BB spindle? The smallest spindle length for the UN55 is 107; is the 6mm reduction all on the DS, or split evenly? There is plenty of clearance between the crankarm and chainstay, so that's not an issue. Thanks for any help.

hueyhoolihan

i would start by measuring the length of the bottom bracket spindle (i'm assuming from the sheldon page link that you were trying to match up spindle from a loose ball BB) that was removed and comparing it to the one that replaced it. taking note of whether or not it was symmetrical.

if that's not possible, it's plain from your post that the spindle length of the un55 that is in there is too long so, something shorter, like that 107 may well be what you need. i wouldn't start messing around trying the change the FD. if the spindle is too long, it's just too long and needs a shorter one. that is the best solution, IME.

BTW, IME, most cartridge BBs have symmetrical spindles, many older loose ball spindles were asymmetrical. the world has moved on and decided that asymmetrical BB spindles weren't really necessary, i guess.

if you go to the 107mm BB from the 113mm BB you'll get 3mms back on the drive side and 3mms back on the non=drive side, i think.

RoadGuy

In my experience (I'm no expert, as I've changed about a half dozen bottom brackets from conventional ball and cone to cartridge bottom brackets on square taper only), when you get the clearance between the chainstay and the small chainring correct (with the right length spindle), the chainline will be okay.

What would be adequate clearance? Does your frame deflect when you pedal hard? If so, how much? I don't want grind marks on my frame from the chain or the chainring.

I would measure the clearance between the small chainring and the chainstay, and subtract the clearance that you want to keep, then double the remainder. That should tell you the maximum length that you can remove from the spindle length. For example: let's say you measure 5mm clearance between the small chainring and the chainstay. I like seeing at least 3-4mm between the chainring and the chainstay, myself. Subtract 3mm from 5mm, and you get 2mm. So let's say 2mm is the excess clearance. Double 2mm and you get 4mm. Subract 4mm from the current 113mm, results in 109mm. So, in this example, we would be looking for a replacement bottom bracket with a spindle length of at least 109mm.

It's not uncommon to have to do a little trial and error fitting of different length bottom brackets when replacing a crankset and bottom bracket. When I bought my Schwinn 754, it had been partially upgraded to Shimano RX100 double from Suntour GPX double. The crankset had been changed to RX100, but the bottom bracket (was a ball and cone conventional bottom bracket with a non-symmetrical spindle, must have been 109 or 110 that came with the Suntour crankset) was not, resulting inadequate (IMHO) clearance between the small chainring and the chainstay. I tried a 113mm cartridge bottom bracket, but the clearance was inadequate. I ending up going to a 118mm model to get the clearance that I wanted between the chainstay and the small chainring.

Art Odd

Thanks for the replies. I measured the old spindle and it was 140mm, but it was a cottered crank, so it extended all the way through the crank arms. The DS was 4mm longer than the NDS. I measured the distance between the inside of the small chainring and the chainstay, and it was over 10mm. I think the 107mm spindle version of the UN55 it going to be my best bet. Or do they make anything shorter? Maybe I should be considering a Tange BB that comes as short as 103mm?

ThermionicScott 

I use a 107mm BB with my RX100 double crank. The 39T inner ring has enough clearance to the chainstay, but I don't think I would go any shorter or use an inner ring bigger than 42T. Hope this helps.

hueyhoolihan

you should be able to see whether the difference between a 107, which will move the crank in 3mms, and a 103 which will give you two more, is critical. don't forget it's the chainline that's important. if the chainline is right there will be clearance to the chainstay. they may both work...

ThermionicScott 

One other thought, @Art Odd -- did you torque the cranks adequately (25-30 ft-lb)? That can make the difference of a couple mm.

Art Odd

Yeah, the current chainline is right at 50mm, so I need at least 5mm. The 103 may do the trick.

fietsbob

What is the rear chainline distance? center of the bike to the center of the rear gear cluster (cass.F/W)

that is what you match, up front. IE: middle ring of triple (or 1by), or gap between the 2 chainrings on a double.

Chainline is parallel to the center of the bike..

Wilfred Laurier

This is the right answer. The chainline can be measured at the freewheel as well as at the crank.

You need two measurements to do the chainline calculations: The OLD (distance from the inside far of one dropout to the other - likely 120mm on an old bike - and the horizontal distance from the inside of the drive side dropout to the 3rd cog on your 5 speed freewheel (call this value X).
THen the calculation is this:

Chainline = OLD/2-X

Then you can measure the horizontal distance from the centre of the seat tube to the middle of the chainset, as described by Fietsbob, and compare the two numbers, and decide on the correct bottom bracket length.

Art Odd

Maybe I didn't use the correct terminology; the distance from the center of the seat tube to between the two chainrings is 50mm. As best as I could measure, the inside of the large cog on the freewheel is 93mm from the left dropout, and the outside of the smallest cog is at 123mm.

I have been assuming that the distance from the middle of the seat tube to between the two chainrings should be 43.5mm, so I need a shorter BB spindle by approximately 10mm (5mm per side). I have not torqued down the crank arm bolts (but they are snug) in case that might reduce the distance a bit.

Wilfred Laurier

You measure the chainline at the crank after the arms are properly torqued. Measuring before they are torqued is like deciding on the fit of a shirt without buttoning it up. If you remove the driveside crank bolt from the bottom bracket, how deep in the hole is the bb spindle? If it is still like 7mm back, then there is still loads of room to tighten, which will greatly affect chainline. N.B. this is not a good way to judge the installation of a crank, and if the BB spindle is flush with the face then you have gone too far and likely ruined the crank.

Art Odd

After torquing things down a bit, the current crank set "chainline" is now about 47.5mm instead of 50mm.


Using this formula, the chainline should be 45=126/2-18. So, maybe I only need to drop the spindle length about 3mm on the DS, so the 107 might work. Thanks, everyone.

ThermionicScott 

That doesn't sound too far off, then! Mine measures about 43mm with the 107mm BB. It's possible that your crank is a little "fresher" than mine, as with repeated removal and installation they very gradually work their way inward.

I bet that'll do a good job for you, even if it doesn't end up "perfect."

aljohn

Just a thought.... When I fitted all the components from a Decathlon on to my New Ribble frame I automatically used the longer side of the spindle for the chain set, a triple Campag. The chainset was much too far out, so I turned the axle around and the middle ring was bang on line with the middle sprocket. In a hurry to finish building the bike I left it like that (temporarily!) with the left crank 1/4" further out. That was 4 years ago, I forgot to get a symetrical axle. If your axle is asymetrical can you fit it the other way round? It would perhaps give you a better idea what length to go for, before buying one and finding out it's still not quite right. Best of luck.

easyupbug 

I find chainlines so far off in so many bikes (even as sold) I don't even bother to to measure before pulling a crankset, I just measure the rear chain line and work to it as reasonably close as I can.