Hi,

This is my first time building (putting together) a bike and I decided to go fixed gear. I need help figuring out what bottom bracket to get.

the frame is a Soma Rush 53cm:
https://www.somafab.com/rush.html

and I was wondering will the Alien crank set + bottom bracket fit with the frame?
https://alienbikes.com/gear.html

yes

I know that the Alien BB is 107 but I can't find that anywhere on the Soma site.

Lets say I'm going to buy a different crank set and BB, what should I keep in mind?

check out chainline. The Alien site mentions a 42mm chainline which is where most hubs are designed to have the cog.

additionally, keep in mind threading

for the rush and alien crankset you will need a 68x107 bb to get a 42mm chainline. bb spindle length is crank specific, but for the soma you will english threading no matter what crank/bb combo you choose. incidentally both of these components are part of my new build as well

The length of bb spindle you need depends on what cranks you get. Two cranks on identical length spindles may put the chainring in different places.

However long a spindle you wind up getting, make sure your bb is english threaded with a 68mm body. That's what your frame's bb shell is set up for.

Also, if you are getting square taper cranks, it's good to make sure that the taper on your spindle is correct*. There are two standards, ISO and JIS. Which one you want will depend on what cranks you get. Most cheaper cranks are JIS but check and make sure.

*You can mix tapers but it affects chainline and you have to compensate for that by changing bb spindle length---it's slightly easier to just get the correct taper.

I'm really confused right now.

So the if I get the Alien crank set I'm good to go? are they english threaded?

How do I know if a crank set is english threaded?

On the Soma site it says that the Bottom Bracket Drop is 58mm, what does that mean?

Thanks!

Crank sets aren't english threaded. BB shells are.



This is a bb shell. The bb screws into those threads there, and of course the threads on the bb have to match the threads on the shell or it won't work. English threading is one standard for bb shell threads and bb threads.

Think of a line from your front hub to your back hub. Bottom bracket drop is how far below that line the centre of your bb shell is. On a lot of track frames, bb drop is somewhat less than on a normal road frame, (i.e. the bb is higher). This is to keep the cranks from striking the banking of the track when you're going slow.



Sheldon Brown's bike glossary is an excellent place to get answers to these kind of questions: it's quicker than this forum and generally a lot better (no offense to us).

https://www.sheldonbrown.com/glossary.html

rush+alien crankset+68x107 bb=good to go

so this BB should also work, right?

https://store.somafab.com/irdqubobr.html

(just checking, 107x68mm, #28844)

anyone?

yes

nice post mander

stupid question-when doing a conversion, is it safe to assume that the original bottom bracket size for the double chain wheel/cassette will be the appropriate size for the fixed final product?

just to let you know, since this happens pretty often at the shop i work at, if you plan on taking the bike to professionals after you're done building it up and asking them to check your work - they will likely charge you for the time it would've taken for them to do it.

i don't know what your mechanical aptitude is so i can't say whether or not you ought to have someone else do it for you but there are bike coops in many cities where you can get wrench time and free advice from people who know how to do the work. otherwise, consider just taking the parts to your LBS and have them put it together for you, maybe they'll even let you sit in on the process.

ive completely dissasembled and rebuilt my bikes, wheels and all, but im assembling from just the frame and have to buy a bottom bracket for it, so don't want to screw it up and im having trouble finding a clear article on this minutiae for a fixed gear bike. this hasn't been an issue on the road's i play with

also volunteered @ said coops and have the tools, just want to be sure like i said. i do appreciate any help you can give

gotcha. yeah, there needs to be standards on what information should be displayed on things like BB which are really detail oriented.

personally i just hit up the qbp book, the numbers are consistent and clear.

gunna plead shamefull ignorance on this one-qbp book?

Get this crankset and bottom bracket combo, it's inexpensive and better than the cranks mentioned in this thread.

https://www.bikemannetwork.com/biking/p/CC-CRTRACK

the quality bike parts catalog. most lbs have one.

well specifically, what am i looking for with the qbp? is there a conversion that tells me if i have x-chain line, y drop spacing, z chain ring/cranks, i will need q size bb for a fixed conversion?

i do appreciate your patience

well in the qbp catalog, i'd say more than in any one spot, you'll find a lot of stuff you can use, at decent rates, whose complete technical information is readily available. that way you can easily match up x number with x number with x number without having to research every part to make sure it fits. as for chain line thats really give and take. some people have tons of problems with it, others don't. my chainline was perfect the first time without any adjustments and i doubt worrying about it before hand would've made much a difference.

Sheldon knows

Often the inside chain ring on a road double will give you a good chain line with a 42mm hub. Check out Sheldon's BB database linked above. Also, the Bob's Garage videos on FGG give a pretty good tutorial with pictures on how to measure and adjust chain line.

That depends on where you want to run the chainring. On a standard shimano double crankset on its standard/ recommended size bb, the inner ring is at 41, and the outer is at 46 mm. This means that with a standard 42mm-ish track hub, the inner ring position will be just about perfect, since 1 mm either way is inconsequential.

On the other hand, not everyone wants to run a single ring on the inner position. Among other advantages, the outer position looks a bit cleaner. So, if you get a bb spindle that's 3 mm shorter on the drive side, the outer ring position will now be around 43 mm---again, within 1 mm of 42 mm is close enough to perfect. As a bonus, moving the crank in means your q will be a few mm narrower, very slightly reducing your chance of a pedal strike.

The one issue that can arise from moving the outer ring position in to 42 mm is, the bike's designers may not have intended for the crank to come in that close and so parts of the crank may rub on the bike frame. You have to figure out for yourself whether this will be a problem with your own frame and crank.