Can anyone in here educate me on bottom brackets and cranks? I just picked up a "kit" bike off CL and am trying to put it together. The guy sold me a mountain bike frame with a road bike crank. The problem is the bearings are too wide for the axle on the cranks. The axle is 108.2mm long and the 100.8mm, this only leaves 7.6ish mm of crank engagement on the splines and there is no stop, causing the cranks to squeeze the bearings. Long story short I have decided that I will just buy the right BB and cranks and stop jacking with the wrong ones.

Here is the problem...I have no idea how to buy the right size! I know that the bottom bracket on the bike is 73mm wide, and that I have to put 5mm of spacers under the non-drive side and 2.5mm under the drive side bearings to keep the cranks form hitting the chain stays. The cranks are FSA SL something or other.

Can anyone give me tips on getting the right parts?

Thanks in advance.

The FSA crank set is made for a road frame and only comes in 68mm English or 70mm Italian BB shells. Mountain bike cranks and BB would fit best or go back to the older road bike 106 or 600 road square taper cranks and then use the square taper BB (same as mountain) probably 1 to 2 sized longer to clear the inner ring to the frame. The BB comes in both 68 and 73 but because of the chain line with the bigger rings the BB axle will need to be a little longer to clear the frame.

I'm sorry, but.
Okay, just had to get that out.



Not uncommon for a MTB frame.

OK this is where you lost me.
I understand that you need to
have a certain distance between
the crank arms and the frame,
but why spacers? Is this some
kind of external bearing crank?

Usually, you accomplish this with
your choice of spindle length and
tapers.

BB units (sealed bearing units) come
in a wide variety, including some that
ought to fit your BB shell. My advice:

Take it to a LBS or local bike coop and
show the whole "kit bike"
(sorry, thought I was done) to someone
that can look at it with an educated eye
for chainline. There's a chance you can
get by just with a sealed BB cartridge
for around 30 bucks (for the part), assuming
it will work with your cranks.

Good luck, amigo. I wish I could say I've
never made a similar move, but I'd be lying.

Respectfully,
Mike Larmer

Figuring this stuff out is part of the fun, and the only reason that I bought the bike, er, box of parts. I have been to one bike store with less than stellar results, the guy I talked to had less of a clue about bikes than I do.

If I find a BB that is listed as "Bottom Bracket, 68 73 x 113mm", I think that I understand that it will fit in a 68 or 73mm bottom bracket, but what does the 113mm indicate? Is it the space between the crank arms? Or is it the length of the shaft?

OK, but in passing, it is often the case
at any bike store that there are only one or
two individuals who actually know their ass from
first base, bicyclewise. Your job in the future is
to figure out who that reliable source is.


What this designation indicates, as you have
correctly surmised, is that the unit in question
will work on either a 68 or 73 width BB shell
(courtesy of spacers or different width threaded
installation rings that thread into the same
threads as the cups in a traditional BB spindle
and cup unit.

The 113 is, indeed the length of the shaft overall
including the tapers. So if you have 73mm lost
inside the shell, you need to figure out how much
additional length you require for your particular
use. In addition, you need to figure out if you
want a longer length (say for a double or triple
chainring) on the drive side. You also need to
be aware of the chain line (which you can eyeball
usually from the back and front) so that the chain
runs roughly straight between the front cogs
(chainwheels) and the middle cog on your rear
cluster.

Also, if we're talking square taper cranks, you
need to be aware that either ISO or JIS taper
usually works with most cranks, but just to make
the whole thing more amusing, not always.

Look here:https://www.sheldonbrown.com/bbsize.html

and here:https://www.sheldonbrown.com/bbtaper.html

and finally here:https://www.sheldonbrown.com/cribshee...mbrackets.html
but if you're dealing with standard BSA
threading and a sealed replacement unit,
this last is mostly extraneous information.

What you're doing is not particularly simple
to calculate, but will prove quite educational
if that is your purpose.

Lastly, look here at the Phil Wood site:

https://www.philwood.com/products/bbhome.php

https://philwood.com/store/page30.html

Although you don't want to buy one of
these for a CL basket bike, the last reference
is a great educational resource in the variety
of Square taper BB axle lengths, tapers, and
configurations that exist in the various bikes
that have been produced over the years.
Phil Wood makes something to fit almost
everything, except your pocketbook.

That's a bottom bracket for a english threaded 68 or 73mm wide shell with a length of 113mm.

What you need to do is stop worrying about bottom bracket. You need to buy a crank - the crank itself will dictate what bottom bracket you need to buy. It will spec the length, and you know your shell width is 73mm. That is all you need to know.

Truly spoken like a guy who sells bike stuff
for a living.

I mean, why try to use the crank he already
has, eh?