auxym

I recently got a new triple crank for my tourer -- Shimano FC-M371. Unfortunately, after installing it, I was unable to adjust the front derailer to shift to the granny ring reliably. With the Low limit screw completely out, and no tension, it will *just barely* move to the granny, with the ring gear combinations and some luck. OK, a small heel kick to the chain also helps it

So I have bad chainline. My current BB spindle is 116 mm. Shimano's spec for the M371 is 123 mm. I went ahead and bought a 123 mm unit from my LBS.

I'm just about to install it (pulled out the old unit), and I'm having second thoughts. Shimano's spec probably assumes a front derailer of the same groupset (Altus). I have a Sora 9sp on, from a few years ago at that. The spindle is 7 mm longer in total, that means the crankset will move 3.5 mm outboard, correct? That doesn't seem like a lot. By you guys experience, will this be enough to make granny shifts good? Considering it *barely* shifts right now, sometimes.

Thanks a whole lot!

Kontact 

Cranks and spindles are built to produce a chainline, not match a derailleur. The derailleurs are also built to go to a chainline, not for a particular crank.

If anything, your bike is a road bike and the crank is an MTB crank, so it may be further out rather than further in. Did you get a BB for a 68mm shell or a 73mm?

And 3.5mm is a fair amount for crank chainline. But there could always be a problem with your derailleur, too.

dedhed

Sora FD says 45mm chainline. https://bike.shimano.com/en-EU/produ...D-R3030-F.html
Altus crank says 50mm chainline. https://bike.shimano.com/en-EU/produ...0/FC-M371.html

hermanchauw

cny-bikeman

You might as well go ahead and try a dry fit, but you need to understand that spindle lengths assume a spindle and crank that are built for each other. Changing a spindle without changing the crank may not produce the chainline one is expecting, though of course it will change it by the difference in spindle lengths.

auxym

I did get a 68 mm BB, matching my frame. Oddly, I can assure you that the crank is definitely farther inboard than the stock FSA Vero triple was.

Most resources I found asserted that 9 sp groups were pretty much all inter-compatible, ever between mountain and road.

Anyways, I know I'm frankensteining stuff a bit around here, but that's what you have to do these days to get a granny ring with brifters. I guess I'll just go ahead and try it out, based Kontact's a comment that 3.5 mm is "a fair amount", and my own intuition.

nfmisso

There are many levers to play with, in addition to BB axle length, you can offset the BB is the frame by using a spacer between the frame and the BB. Conveniently for BSA threaded BB, freewheel spacers, cassette spacers and BB spacers are the same. A bag of these comes in very handy: https://www.amazon.com/Wheels-Manufa.../dp/B0025UINFC

draganm

I'm tackling this issue as well on 68mm BB shell. I have an older Exage M500 Shimano crankset , it's way too far in-boasrd,, and the spindle I have for it is 2mm longer than the one it's supposed to use. I have decided to go Sram , Their crank spindle boss is practically flush with the small cog, where the Shimano is recessed by quite a lot. I know this isn't a scientific approach, but I can always get a shorter BB, which is easier than starting with your small ring slammed against the frame and messing with spacers, which i don't like the idea of using in an area that is prone to creaking anyway.

Shimano

SRAM

Kontact 

That isn't the way it works. What makes you say the crank is too far inboard? On a road bike, the center chainring should be 45mm out from centerline. Is it a road bike or something else?

2_i 

A drive-side spacer with your current 116mm spindle, that was already mentioned, can help. Otherwise, I ride a quad so I had to look for all kinds of tweaks to make standard components to work with it. In the derailleur I remember doing some filing to increase the range by which it swung. It sounds like do not need much to make it work.

Kontact 

I'd recommend finding the correct pairing of frame, crank, spindle and derailleur before anyone breaks out the files.

draganm

like the OP, with lower stop-screw backed out completely the FD only goes in far enough to engage center chain-ring.

Mt. bike, Access CL hard-tail frame (Performance house brand )

draganm

files are too slow, angle Grinder baby!

Kontact 

That explains that the crank is too far in for that derailleur, but it doesn't mean that the derailleur isn't at fault.

auxym

Well here's my final update.

123 mm works. The ideal length is probably a tad shorter (around 120 ?), since the big-ring shift is definitely at the limit (H-limit is fully backed out), but it works, so it stays for now.

No file (or angle grinder) was used

Again, thanks for all you guys input, it's invaluable stuff!

draganm

yeah it was pretty far in , some grind marks on the frame from when the crank would flex under heavy load. Not something you would expect with a 123mm spindle. For goodness sakes, the longest one I saw available to buy is 127mm

We'll see, I've ordered the SRAM S600, so even if it's off at least it will match the bike . Like the OP, I'm hoping to get lucky and everything falls into place, worth a try for 48 bucks. I have read that BB fitting is more of a " try it and see" than an exact science, and I've always had plenty of luck on my road bikes, but the Mt. triples are a different story.

Kontact 

No, it is an exact science, and I have tried to explain several times in this thread how you go about getting it right. Did you try measuring your chainline? It's a lot easier than returning stuff that doesn't work.

draganm

well i'll just have to politely disagree on that , and exact science doesn't require spacers or angle grinders : )

if I was keeping the old crank I probly would have spent more time on it, but they were old crap from 1990, so I'l start fresh with the new SRAM cranks and go from there. Regardless, the most I could have moved the old cranks out was another 2mm, not nearly enough.
I am slowly transitioning all our bikes to SRAM, I might not be getting the very best but I'm more willing to accept little problems when the parts cost 30% less. I'll report back on how it fits , you will probably get a chance to say " TOLD YA SO!"

Kontact 

I was a shop service manager for years. It is an exact science, and I know that because of decades of experience that I'm betting you don't have.

I'll never understand why amateur mechanics post their problems on forums and then ignore any and all advice offered.

nfmisso

++++ total agree, bicycle components are Engineered.

Take apart an automotive engine, say in your Mercedes Benz, and you will find things that are shimmed to fit. Parts and assemblies can economically be made to limited tolerances and often there needs to be an adjustment during assembly. These adjustments are based on MEASUREMENT, which is a SCIENCE, not a random guess.

draganm

well I don't own a shop, but i have built about a dozen bikes in the last 30 years, so not exactly a total newb either. It's fine to have an an opinion that it's an exact science, but just in this thread we have 2 different people who used the recommended BB spindle length, or longer, and wound up with a crank that was too far in or too far out. Just saying results don't lie,

well your analogy would be great if all bicycle parts were made by Mercedes, but they're not. We have Japanese, Various other Asian, American, and quite a few European companies all trying to maintain some kind of standard , both between older and newer generations of drive-trains, all the while constantly cooking up new "standards". The bottom bracket crank interface and number chain-rings being one area where some of the greatest changes have been made on bikes. I can remember in the last few decades on more than one occasion where there were changes made to what was even considered to be the "ideal chain-line".
It just seems like calling this an exact science is a bit of stretch.

Kontact 

In this thread we had two different people assemble mystery bikes out of unknown combinations of parts that resulted in functional problems that may or may not have been the result of chainline issues.

So it is science, if you pay attention to everything involved in building a bike rather than assuming that problems are the result of uncontrollable random discontinuities.

For you or anyone having the problems described in this thread:
1. What make and model frame is it, and what is the correct chainline for that frame and the speed crank used?
2. What make and model crank is it, is that crank designed to work with that sort of frame, and is the BB spindle the recommended one for that use?
3. What make and model front derailleur is it, and it is designed for used with that kind of frame, crank and seat tube thickness.


You can grope around in the dark, or you can learn about bicycle mechanics and stop leaving things to random chance. I've never worked in a bike shop where it was necessary to just "try stuff" to see if it would work. Everything has a manual or a spec, regardless of whether it is Asian, US or European. Use them.


Your problem is that you have gotten away with doing things without understanding them for long enough to think that is normal. It isn't normal to not know how the parts of your bike are speccd.

draganm

LOL, we're many, many posts into this thread where plenty of information has been supplied about frames, cranks, FD's , etc., yet for all your scientific specificiationism i didn't see you recommend a single fact in terms of BB spindle length or parts to match up. Others did , and didn't even need to be condescending about it.
It's not even worth debating, I will continue to depend in my intuition and apparently amazing luck in assembling bikes since 99% of the time I get the results I want. I'll be sure to report back after I've tried some random parts i picked by throwing darts at a catalog.

Kontact 

Neither of you provided the frame, crank, BB spindle length, chainline number and front derailleur model.

As for your bike, I couldn't say based on the little you provided, but I would point out that you tried to put an Exage crank built for 45mm chainline on a bike built for 47.5mm chainline without adding the full 5mm spindle length necessary - you added 2mm. No wonder it didn't work. But I can't tell if that's the only reason.

But since you already moved on to your next guess, there wasn't much point in fixing your old crank problem.

draganm

thanks for the info, appreciated. I put the old Exage crank back on and measured a chain-line of 44.5mm to center chaining, so it's presumably too far in . I will do more measuring once the SRAM cranks arrive.
Shifter are SRAM X3 and FD is a high mount X7 clamp-on.
thanks,