(I originally posted this in the hybrid forum, but realized it belongs here... I've also expounded on my original post)

ugh. just swapped my crankset on my Specialized Globe Vienna 02 (think base model Sirrus and you're pretty darn close), from the fugly stock Shimano M191 (Altus, I think?) to an Origin8 Trekking Pro... both square shaft.

The Origin8 is sitting much farther from the frame, such that I had to readjust the front derailleur all the way to its outer limit of travel to get it to shift. Meh. The chainline is all whack, in the middle ring, its about aligned with gear 7 of 8

... yes, I thoroughly cleaned the BB shaft and greased it quite well. I torqued the new crank bolts down about as hard as I could with a long arm allen, but I didn't break out my ratchet hex bits from my motorcycle toolkit....

the teeth on the middle ring are sitting just about 54mm from the centerline of the seat tube, at least by eyeballing a small steel ruler...

does anyone know what the 'standard' chainline offset is for a 3x8 700c hybrid ? ok, Shimano's tech page on the M191 crankset says its 47.5mm, and uses a 122.5mm BB, which is quite a bit wider than Sheldon Brown suggests most of the other mountain triples use (same chainline, but wider axle). hmmmm.

so by my calculations, I want a 108 or 110mm wide BB? does this sound right? there's a ton of room between the chain stays and crank ends, so I'm not worried there.

anyone who wants to beat me over the head with my own idiocy, feel free, I have a thick skull, its been road tested a few times.

You need to separate chainline from spindle length because they don't equate. Chainline is the distance of the chain's central line from the frame. Ideally you want the mid point of the chainrings to line up with the mid point of the cassette, so the chain is running as close to staight as possible in the most common combinations.

You can get chainline guidance from a chart or tech site, or can measure from the inside of the dropout to the center of the cassette and subtracting from the OLD hub width. That's the target to shoot for, but you may want to tweak it slightly. For instance I set my chainline up outboard a bit, since I spend most of the time on the larger ring and outer third of my cassette.

The reason that spindle length and chainline don't compute is that cranks have offset between the chainring plane and the "hub" of the crank arm. Modern spindles tend to be shorter because modern cranks have the hubs sitting more inboard compared to the rings, mainly to improve spindle rigidity. So a modern 102mm spindle and crank may yield the same chainline as an older 117mm spindle and crank.

If you can source a spec guide for your specific crank, it'll usually suggest a spindle length which will yield a textbook chainline (for that crank).

this crank apparently has no published specs, at least I can't find anything from the manufacturer, and there was nothing in or on the package. https://www.origin-8.com/?page_id=91&...+ARMS+%26+SETS

no hub changes, either, same wheel, cassette etc. 135mm nut-to-nut, with a 8 speed cassette.

soooo. I didnt measure the original chainline but Shimano says my old FC-M191 crankset is intended for a 47.5mm chainline using a 122.5mm wide bottom bracket.

new crank installed on old BB measures 54mm wide, so I want to take 54-47.5 == 6.5mm off each side, or 13mm total off my BB axle length, right? so instead of the 122.5, I want a 109.5, basically 110mm should be close enough, right?

Without seeing it, your math seems OK. Be sure to check chainstay clearances for both crank arms, and at the chainrings before buying.

Also check the right land left spindle lengths. Most spindles aren't symmetrical, so you want to confirm that you're taking 7mm or so off the right side. The left has more latitude, so check the crank/chainstay clearance there and shop accordingly

I think the Origin-8 site is recommending a spindle length of 113mm ("MTN 175x48-38-28 113 SQR JIS Blk 110/74x5B 765g 3x9"), but doesn't say what chainline they are shooting for with that -- I'd guess it's the road standard of 45mm.

The ideal chainline depends on how wide your rear hub is and which cogs you typically use. Since your rear hub is 135mm (MTB spacing) then the MTB standard of 47.5 is probably appropriate. If the Origin-8 website is suggesting a 113mm spindle for a 45mm chainline, then a 118mm spindle would give you 47.5. None of this seems to match your measurements and calculations, which is a better place to start than my interpretation of a sketchy internet spec.

Did you measure from the center of the seat tube to the center of the middle ring? If so, had you tightened the crank installation bolts before measuring?

Edit: Given that the line I quoted starts with "MTN" it seems more likely that the 113 is meant to give you a 47.5mm chainline.

I measured the 54mm from the centerline of the seat tube to the tips of the teeth of the middle chainring, but i used a little steel machinist's rule as my dial caliber that i usually use for such shenangins was AWOL.

there's definitely a ton of room between the chainstays and crank arm ends, and I think the little ring too but I paid less attention to that (hey, its the same 28t as my old one, so if the chainline is the same, I should be gold there, too, eh?).

a 113mm wide BB would move it in 4.5mm by my count, but I did just notice Sheldon Brown's webpile suggesting that Shimano measures their chainline about 1mm differently than everyone else (they measure to the inside surface of the middle ring, instead of the tip of the teeth) so that just further confuscates the issues... maybe 112mm is the right compromise

It's also possible that the 113 is meant to give you a 50mm chainline. Who knows? There's not a lot of difference between 113 and 110 and depending on your shifting habits the standard chainline might not be ideal.

Chainline isn't some magic precise number. It's a "roughly here" number, so as long as you have clearance, 1-2mm in either direction doesn't matter. Remember that, unlike SS and IGH chains, derailleur chain spend most of their lives on misaligned sprockets anyway.

yup. my goal is to be able to run the big ring smoothly and quietly in the smaller 6-7 of 8, the middle ring in all of them and the little ring in the top 3-4, if I can do that, I'm a very happy camper.

"my goal is to be able to run the big ring smoothly and quietly in the smaller 6-7 of 8, the middle ring in all of them and the little ring in the top 3-4"

If you move the crankset too far in you might have problems with the chain hitting the inside of the big ring while in the middle ring and smaller cogs.

There are three chainlines so to speak in a bicycle drivetrain so to speak:

1. Front chainline - the distance of the center of the rings from the centerline of the frame.
2. Rear chainline - the distance of the center of the cogset from the centerline of the frame.
3. The drivetrain centerline expected by the lining up of the center of the rings in front and the center of the cogs in back.

As FBinNY clearly pointed out, for #3 - it's an ideal - oftentimes what we get is something close to the middle when viewing the line through the rear cogset.

Avoid playing around with the rear spacings of the hub. OEM put their hubs out using specs that on average aim for a rear centerline that's somewhere in the middle of the cogset.

Instead, try to fix the problem up in front by utilizing the correct crankset - bottom bracket spindle length - and spindle type - symmetrical or asymmetrical.

As FBinNY and others have already stated - if you can get it close enough so that you have smooth operation using the middle in front and a large range of the middle cogs in the rear - you are good to go.

Wouldn't hurt either to ensure your rear wheel is properly dished, and the derailleur hanger is properly aligned against that properly dished rear wheel. That way you eliminate the frame alignment as an issue in rough operation and isolate troubleshooting to the drivetrain parts themselves.

=8-)

the wheels came on the bike, it has about 200 miles on it, I think they are probably dished OK, at least the V-brakes are centered without having to play games. they are cheap wheels, I do plan eventually on getting better ones, probably Handspun Pavement Series 5.1's, with Deore LX hubs and Velocity Dyad rims, 36H, and having my LBS tune them up on arrival for whatever nominal fee they'll charge for that, but in the meantime, these wheels are good enough to get me around, noname specialized hubs, alex rims, your basic cheap hybrid stuff. at least they are stainless spokes and not cheap chrome-plate stuff.

i have no intentions of messing with the rear cluster spacing. the derailleur hanger passes my eyeball test (middle gear, middle chainring, bike on work stand, from 6' behind eyeball the derailleur arm against the chainrings, they sure look parallel to me).

right now, when I eyeball the center rear cog against the chainring, the SMALL chainring is about aligned with the middle cog, and the big chain ring is about aligned with the smallest cog, so getting a BB with a axle thats 10-12mm shorter should put things right.

Most road and "trekking" cranks are designed for a 45-47 mm chainline. Mountain bike cranks are designed for a 50mm chainline. If you put a mtn bike crank on the BB you just had the road crank on, it will not fit. I use Sugino mtn bike cranks when building up touring bikes, I use a 103mm bottom bracket to get the correct chainline, because I like STI shifters.

IMO- I'd work with adjusting my chainline in by using the correct size BB. I think you are on the right track. Measure your chainline with the new crank on the old bottom bracket. subtract 46 mm from that measurement. Double the resulting number, and subtracting it from your old BB size should give you the correct BB length, e.g., 54 mm from center of seat tube to middle chain ring with a 118 mm BB. 54-46= 8, 8 x 2= 16, 118 (or whatever size you have)-16= 102mm. From your measurements it seems like you want a BB 16 mm shorter than the one presently on your bike. This assumes you have a 68 mm BB shell.

In the product description, it seems like they recommend a 110 mm for a 50 mm chainline. If you want to pull it in to 45-47 then you need down to the 103 mm BB which I found to work well with mountain cranksets installed on road bikes.

Greasing a square taper BB spindle is NOT recommended.

thats controversial. I know plenty of mechanics who say to lightly(!) grease them.

For sure, I grease my pedal spindles, and about anything else threaded. I usually use green marine grease (or phil wood grease because I have a 10 year old tube, looks like the same thing to me!).


oh, and, Shimano says my old FC-M191 crankset uses a 122.5mm spindle for a 47.5 chainline

as a wrap to this story..

picked up a UN26 110mm BB at a LBS. slapped it in yesterday, and voila, my chainline is spot on dead center. on the middle front ring, its right in the middle between #4 and #5 (of 8). couldn't ask for better. all gears shift sweet, only a bit of FD/chain noise in the crossover extremes of both bigs, and small front plus smaller 2-3 rear, none of what I'd ever wanna use. chain is much quieter in all 'normal' gears, the machined alloy chainrings are just that much quieter than the stamped steel ones.

and a followup to the followup, I finally got an email back from Origin-8 confirming that 113 did mean the axle length, but they didnt' say what chainline offset that was for. since my measurements and calculations lead me to a 110mm axle, and my chainline is now perfect, I can only assume that 113 would have given me a 50mm chainline.... which, yes, would have worked OK too, after all, 1.5mm is less than 1/10th inch.

oh, and when the rain broke yesterday, I got in a brisk 12 mile ride. mmmm, silence! the stock cheap Tourney grade crankset on this bike was /noisy/, with its spotwelded stamped steel triple chainrings.

Good deal! It's nice to get things lined up for the gears you use most often.

I've read some pretty good arguments from both camps. My compromise has been to clean both surfaces really well, rub a very thin coat of grease onto the spindle using a paper towel, then use a torque wrench to get the cranks on tight enough to stay put without deforming anything. So far so good...