A few Bottom Bracket question
 

So long story short, I'm basically over hauling my 2008 Giant Rincon and I'm having a blast doing it. Replacing nearly everything on it.....but I have some bottom bracket question.

The bottom bracket is a Modus sealed bearing cartridge bc-73, 68x113 (also says BC 1.37x24T...not sure what the hell that is..Threading type??). The front derailleur used a bottom bracket mount that was approximately 2.5mm thick.


1. Since I'm not using a front derailleur (switching to a 1x10 drivetrain) will I now need a 2.5mm spacer for the BB?

2. If I replace the BB with another cartridge unit do I need to get a 113mm length or different length.
2a. If I do need a different length how to I calculate it? Just add 2.5mm to it?

3. Could I replace the cartridge BB with an external BB?
3a. If so would it only require to BB shell to get chased and faced to do so?

4. The weld's where the BB meets the chain stays go all the way to the edge of the BB. Will this effect the ability to face the shell?

EDIT: here's a picture of the BB welds
https://farm8.staticflickr.com/7596/...f7cfb9bf_c.jpgBB Weld by gremlin75, on Flickr


Ok I think that's all the BB questions I have for now. Thank you all in advance.

68 is the width of the BB shell the BB is meant for. 113 is the axle length. 1.37 is dia in inch and 24 is 24 threads per inch.
If you go to 1x10 you should use a slightly shorter BB to get the chainline right.
If you'd only removed the fder, then you would have needed to add a spacer.
Now, start by figuring out your chainline, then buy the appropriate BB.

An external bearing BB requires a new crank designed for it

Yeah I know the sizing of the BB (though wasn't positive on the thread info so thank you), that info was just put in there incase it helped with the other question. My assumption was that since I was going from a triple up front and that the chainring will go in the middle spot the chain line should be fine. Is this not the case?

Yes I understand that, thats half the reason I'm looking into possible switching to an external BB. I just want to make sure that switching BB types is as simple as just getting my BB shell faced and then screwing in the new BB. So is that the case? If that is the case question #4 is what I'm most concerned with.

EDIT: added a picture of the BB weld to the first post.

Going from a triple to a single is quite a step. I assumed you were going from a double to a single.
But yeah, with the remaining ring on the middle position, chainline should be good.
I'd be in no hurry going to external BB. They tend not to last as long as square taper, and the supposedly increased rigidity have never impressed me.
But if you insist, do get the shell faced. Bearings not entirely perpendicular to the axle is suspected to be a considerable contributor to short BB life.
Don't worry about the weld, the tool can deal with that.

I'm going from 3x8 to 1x10. Main reason for the change is the fact that I'm in the middle ring 90% of the time. So since I'm over hauling the whole drive train I figured I'd go to a 1x10

Only thing that has me thinking about going to an external BB is that I want smaller crank arms and there are next to none square taper crank arms available. Those that are tend to be 175mm and I've seen no 165mm. So switching to external BB will open up the crank possibilitys. Probably won't end up switching but rather just get a un55 bb and use my current cranks (by far the cheapest good option)

Since I will no longer have the 2.5mm spacer on the drive side should I move to a 115mm length bb to keep the chain line the same?

Glad the weld will not be an issue if I do decide to face the bb shell . Thank you very much!

If this was me I would go new crank shimano with external bb and it would be a double. This gets you a nice crank, cheap but great BB, do not face the shell it probably is fine. Then if you want a single pretend that the front only has one chainring. This to me is almost a no brainer. You can get a shimano crank and bb for less than 100$. Personally I love ext BB easy to service no creaks and solid. The press fit is my beef.

Would be looking at this route, only thing that would be looking at, is how much is a complete re-build costing vs a new bike, the Giant Rincon was an entry level bike back in 2008, and the cost of doing all the replacements the OP indicates they have done will soon be more than a new bike, especially as the OP is doing this all at once.

Do you really want to spend that kind of money on an entry level bike?

Wash off the dirt better, first..

BB spindle length is in relation to the crankarms you will fit on it.



Yea triple to single is easy just take off the outer and the granny chainrings , and leave the Middle one.

yes, yes I do. The frame geometry is perfect for me. The re-build will end up costing around a grand after all the parts are bought (only thing left stock will be frame, headset, Handle bars, stem, and possible cranks)

So yes it will be more expensive then a new bike but the frame is still in great shape, fits me perfectly, and the other hardtails I've tried lately have not felt right.

Yeah the bike was put away dirty after my last ride last year. Too cold right now for me to take it out and wash it. Getting out side and washing the frame is on my list of things to do this week.....after it gets above freezing.

Still unsure in the BB length. I get that the length has to do with the cranks. So if I keep the stock cranks (more then likely what I'm going to do at this point) but have the 2.5mm spacer removed from the drive side a 113mm bb will be moved 2.5mm closer to the frame. That's why I'm wondering if I need to go with a longer bb length so make up for the lack of a 2.5mm spacer.

what is the measured chain line in the back ? parallel line to the CL thats where that 1 cog would be so chain bends no more to the tiny cog than to the big one.

thats why the middle chainring lies where it is..


wonder away .. Or buy several lengths and try each .. No clue what crank you have and I cant help over the web anyway,,

Drop by a Shop ... 1st hand in Person is Better.

So there is no way to say that if the BB moves 2.5mm closer to the frame on the drive side, while using the same cranks as before, then the BB needs to be X amount longer?

Ok well thay sucks as I don't have any local shops that are very helpful with this type of info.

If you remove the outer chainring you may need different (shorter) chainring bolts to affix the old middle (now only) chainring in the middle position due to the missing outer chainring not being there to space the "sandwich" the same width.

The bolts are already taken care of. Using raceface spacers (they go where the old outter chainring sat) to make up the difference

only real question I have left is the BB length.

If my thinking is correct (and it rarely is!) to get the same chain line from from the new 1X10 set up with out the front derailleur (2.5mm spacer) as I had with the old 3x8 with derailleur I will need a 118mm length BB. In theory that will add back the 2.5mm on the drive side that I'm losing without the derailleur (moving the single chan ring back to the same spot the old middle rig was). It will also add 2.5mm to the non-drive side but that won't effect the chain line so I don't really care about that. Does that sound about right or is my theory completely wrong??

Or you can just add in a 2.5mm bottom bracket spacer and use the same bottom bracket you were using with the E-type derailleur mount.

The BB is getting a bit worn so it's about time to replace it anyways. I Could get another 68x113 and throw a spacer on it but would like to now have to use a spacer to get the same chain line.

You would have to do the math. Standard sizes would be 113mm and then 115mm as the next size up. That would not be enough as you would be loosing 2.5mm without the spacer and only gaining back half of 2mm -or 1mm. You could jump to 118mm which should be just right, but your left crank would be out 5mm further than before.

Another idea would be to drop down to 110mm instead, and then flip your single ring to the outer side of the spider rather than leaving it in the middle position. This would leave you pretty close to your original position and look nicer than having the empty slot on the spider where the outside ring used to be, and the ugly spacers you are using to reuse the longer 2-ring bolts. There is no guarantee that that the old granny ring mounting tabs wouldn't rub against the chainstay tubes though. You would have to measure and be sure you have the clearance, but it should be OK as you used to have the granny on there plus the chainring bolt and you are only moving it over 3mm.

I'm using a raceface 30T chainring so the only place it can mount is where the middle chain ring used to be. Mounting it on the outside ring slot will put the chain right on the crank arm. I actually just threw it onto the spider and I'm using the long granny ring bolt without the spacer. The 30T ring has a built in spacer that way the chain doesn't rub on the spider and the holes are threaded. And now that I'm typing that out I realize that the built in spacer is going to push the chain ring another 1-2mm toward the frame (really need to replace the battery in my caliper so I can stop using the analog scale)......

Think I'll probably go with a 68x118. The extra 5mm on the non drive side won't effect the chain line and shouldn't effect my peddle stroke.

If you're using the same crank and go with the spacer and the same BB length if you decide to go back to a triple you're all set