Cyclist0103

Hello,

i am trying to find out the size of BB that could work on my mtb because my bike is a specialized mtb. i think i may be dealing with one of a kind bike. allow me to make it short and quick.

i am looking for a square taper BB so i can put my new crankset on my mtb and do some work out or training on my roller. but i measured my current BB on my bike using the caliper and it show me the result following: 73mm

but i am looking for a BB that will move the crankset away from the chain-stay so the teeth won't touch the chain-stay. I am sure you know why. i know that there is no spacer on the sqaure taper that can move the crankset away from chain-stay. if i am wrong, then correct me. so the width from the arm of crankset to the other arm is about 113mm. I dont have the tools to remove the arms to measure the BB from spindle to spindle along with 73mm. so if i want to move my new crankset away from the chain-stay, then the 113mm would be added with 8mm (4mm on both side) for my new crankset to work with my bike so that i would looking for the BB with 73mm x 121mm..

so I would like to hear your opinion about the BB with that size? if you have any question, then ask and i will answer. and by the way, i will appreciate that you would teach me a lot more about the BB and the bike along with the crankset.


addition information on my bike:

the current BB is octalink (i think it is a octalink from what i am looking at).

jimc101

Specialized are a mass market producer of bike, the chances of you having a one off are virtually non-existant.

Not much info to work with, only that you have a 73mm shell (which is the standard MTB shell size for non-press fit BB's).

You haven't ID'd the cranks your going to fit, these will determine the spindle length & the axle type (square taper, ISIS or Octalink)

More info needed with the other parts you are working with, the model of the bike would be useful to.

LesterOfPuppets

what is the make/model/year of the new crankset?

SkyDog75

...which is a standard size for mountain bikes. So far, so good.

Not to be argumentative, but no, I don't know why. If you get a bottom bracket with the proper spindle length for the crankset you choose, and you're not using chainrings that are bigger than the bike was designed for, the chainrings shouldn't touch the chain stay. And just as importantly, your chainline (the distance of the chainrings from the centerline of the bike) should be proper. You can refer to this Park Tool article for a little more info:

Chainline Concepts - Park Tool

Actually, you can use spacers on the drive side of a cartridge-style square taper bottom bracket, but it's typically not needed unless you're using a new bottom bracket -- most of which are symmetric -- with an old crankset that was designed for an asymmetric spindle (typically longer on the drive side). Most newer square taper cranksets are designed for symmetric spindles, though, so just buy a bottom bracket whose spindle is the right length for your crankset. They come in a bunch of different lengths for this reason.

Measurements taken with your current crankset aren't going to be helpful. Cranksets can be designed for use with all sorts of different spindle lengths, so just because your current crankset sits X distance from centerline on your current spindle doesn't mean your new one will, too.

Bottom line: What's your new crankset? Start with the manufacturer's recommended spindle length for that crankset.

TiBikeGuy

While moving the bottom bracket further out may prevent the chainrings from touching the chain stays, it will also produce another problem. The front derailleur may not be able to reach the biggest chainring. Also chainline may not be straight resulting in noise from chain rubbing the front derailleur.

Cyclist0103

I did mentioned what my current crankset is.. and my new crankset is.. so i will say it again. My current crankset is for octalink.. but my new crankset is a square taper. thats why i said "I am looking for a square taper BB".

Let me see: my crankset is FC-M361 with 104/64 BCD and 175mm spindle length for arms. and i also see that its recommended for spindle length on BB which is "LL113" and the model recommended for BB is "BB-UN26 (-E)"

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yes i did think of the problems.. with FD (front derailleur) not able to reach the largest chainring. but like i said in my first post.. if i used the LL113 BB for that crankset.. then the teeth of the large chainring can touch the chainstay but worse.. it can damage the teeth on the chainring.. so i only need to move it away from the chainstay up to 4mm if possible. 2mm should be fine or a 3mm is fine as well. but i think that 3mm is a minimum requirement for the chainring to stay away from the chainstay.. and if its 3mm then the teeth would be about 1 to 1.5mm close to the chainstay.. so please keep teaching me. i appreciate it. i learned something new.. i didn't think of. the chainline has to be straight with the FD. i will keep that in mind.

LesterOfPuppets

I think the -E version is made for use with an E-type front derailleur which has a bracket that fits between the BB shell and the drive side cup. These plates are about 2mm thick, IIRC.

2.5 spacers are readily available.

Wheels Manufacturing BB Spacer > Components > Drivetrain > Conversion Kits & Parts | Jenson USA

SkyDog75

You mentioned what general type of bottom bracket interface each crankset mates to (Octalink versus square taper), but you didn't identify your specific cranksets. It's absolutely important to know the exact model of crankset we're talking about because different model cranksets can require different spindle lengths. Shimano currently sells square taper bottom brackets in at least 7 different lengths. Which one does your square taper crankset require?

In the case of Octalink, there was also a v1 and v2, which you'd need to know in addition to spindle length. Again, someone would need to know the model to give you informed advice.

175 is your crank arm length in millimeters, not the bottom bracket's spindle length. The "LL113" tells you that the recommended spindle length for that crankset (to achieve typical chainline) is 113 mm. The UN26 bottom bracket is a suitable model, but you'll need a UN26 with the appropriate spindle length (113) and the appropriate bottom bracket shell width (73).

Is it possible your new crankset has larger chainrings than your old one? For example, if you replace a crankset that has 22-32-42 tooth rings with a crankset whose rings are 28-38-48 teeth while leaving chainline the same, those larger chainrings could rub the frame's chain stay. Your options would be to use a bottom bracket with a longer spindle, which could screw up your chainline to some extent and/or exceed your front derailleur's reach; or you could use a crankset with smaller chainrings that better fits the contour of your frame's chain stay.

Cyclist0103

ah. yes i knew about these spacer. i even looked at the link you provided.. just before i created this thread. and about the "E" version.. yes thats correct. it mean "e-type" for front derailleur to be mount to the frame with 2 screws but it also will have a plastic that will mount along with the BB. but there is one problem. there is no e-type FD that can handle 48T chainring. which is why i will have to get a clamp for my bike.

yes i know about the octalink v1 and v2. but just so you know we are not dealing with a crankset that require octalink.. but it require square taper. but what you teach me was the spindle length that is required for the crankset to fit on.. thats something i learned. so i understand everything. so thank you. I know what to do now.


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also, LesterOfPuppets and everyone. thank you for teaching me about the BB and the crankset. i know what to do now and i am going to order the BB and the tools to remove my old BB. so thank you. this case is officially closed. so i enjoy this conversation and i learned something else new. have a great cycling!

Cyclist0103

Thank you for your reply.


CLOSED CASE

Nucks

This is old I know but I think in this case the Shimano documentation that recommends the LL113 bottom bracket spindle length is wrong. I have the same FC-M361 crankset that I just installed on my wife's bike and I tried both the LL113 and a 123mm spindle length BB. The LL113 produced an extreme chainline from the middle chainring even to the cog one size smaller than the middle cog of the 7-speed cassette. With the 113mm spindle, the middle chainring was directly in line with the 2nd largest cog in the back. Switching to the 123mm spindle length bottom bracket lined up the middle chainring with the middle cog. There is nothing but confusion regarding this crankset and BB compatibiility. Websites that sell the crankset have conflicting information on the correct spindle length, probably as a result of an error in Shimano's documentation. The documentation is here:

https://cycle.shimano.co.kr/media/tec...9830667768.PDF

BTW, the old BB I removed from my wife's bike was 123mm spindle length although it was a different (Sugino) crankset.

SkyDog75

It's never too late to set the record straight since more people may find this thread when looking for info about their crankset.

Anyway, upon further review... It appears the documentation is correct, but laid out in such a way that it's easy to misinterpret. If you find the FC-M361 crankset in the "Front Chainwheel" table at the top of the PDF and follow the column down, it appears to line up with the BB-UN26 LL-113(E) in the "Bottom Bracket" table. The catch is that those two tables are completely separate; the columns in the top table don't correlate to the columns in the lower table.

If you look just at the "Bottom Bracket" table, the left column states the "applicable chainwheels" for the BB-UN26 LL-123(K) are the "FC-M411 / M361 / M311".

Thanks for chiming in to help clear things up!

Nucks

Ha, ha! I can't believe I got confused over that. I'm not the only one I guess. I see people quoting the documentation and recommending the 113mm spindle length all over the place.