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Discussion Starter #1
I have gone through several external bottom brackets and now I have road and mountain versions of the Chris King bottom brackets. According to their website, there are specific cups for road use and mountain use, but the only difference is the cup width (see the blue square on the bottom left of the link). The bearings are identical. The plastic center sleeve they come with can be converted to fit 68mm and 73mm bottom bracket shells. They also come with spacers to help align the crank.

Last night, while doing some bike maintenance and gathering parts for a new bike build I compared the two different cups. The road cups are wider than the mountain cups in the section that actually sits outside of the frame, but the overall width of the cup, including the threaded part are the same. I then tried the road BB cups on the mountain bike (73mm BB shell) with XT cranks and they fit perfectly without the need for spacers. When using the mountain BB cups, I have to use the spacers. So, my question is, why is there a need for specific road or mountain external BB cups? Was it just this combination of crank and BB that fit well together?

Sorry if this is difficult to follow.

The spacers I mention are the black ones seen in this picture I found online.

 

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The Punk Hucker
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Theoretically they are the same. Same width, same axle size and should be interchangeable. The problem you could encounter is the center sleeve being shorter for the road version so it won't fit on the MTB version with all spacers to mimic a 73mm shell.

Or something along those lines, I can't think clearly.
 

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I don't know road standards too well, but I'll add my 2 cents

Mountain bbs are designed to work universally with 68mm and 73mm wide shells that are both common, so they need a number of spacers to fit a 73mm compatible bb on a 68mm shell. The picture you linked is such a configuration, with two spacers to make up for the shell width difference and one extra that I'll mention later. Road bikes are almost universally 68mm (King only sells a 68mm BB) so they are not designed around any spacers and wouldn't be universally useful for MTBs

Even on a 73mm wide shell, most mountain external bbs are designed to have a single spacer on the drive side. The spacer is there so you can replace it with an e-type derailleur or chain guide if needed. Road bbs are not designed to use such a spacer and that may be one reason for some extra width in at least one of the cups. That might explain how the road bb fit properly without the drive side spacer

However, this is all assuming the spindle length (or overall width between outside edges of the installed cups) is actually the same between mountain and road cranks, which I'm not 100% sure about.

BTW, you mention using spacers (plural) with your XT on that 73mm bb, but there really should only be one on the drive side. Just checking.
 

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ups and downs
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From the CK website

Road and Mountain cups are almost always specific to discipline-specified crank systems. While the bearings are identical for both versions, the overall width of the cups is what designates use for a certain system. Cups are designated with two identifying marks per cup ( as shown above) along with installation direction and torque values. There are, however, instances of compatibility where a "MTN" cup will be used for a "ROAD" crank. Please contact us or ask your favorite mechanic for advice.
 

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Discussion Starter #5
boomn said:
BTW, you mention using spacers (plural) with your XT on that 73mm bb, but there really should only be one on the drive side. Just checking.
Thanks for the thorough explanation. I can usually find ways to make things work, it's nice to also know why they are made the way the are. I tried one spacer on the drive side, but when putting the non drive side crank on, it wouldn't compress enough against the bottom bracket, leaving some side to side play in the entire crankset. I put another, thinner spacer from my parts bin on the non-drive side and that took care of it. I just felt the road BB, with the wider cups took care of the need for any spacers, making for a cleaner look (in a mostly unnoticeable area, but still). I'll try it out for a while and see how it works.

Thanks again.
 

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Discussion Starter #6
PissedOffCil said:
Theoretically they are the same. Same width, same axle size and should be interchangeable. The problem you could encounter is the center sleeve being shorter for the road version so it won't fit on the MTB version with all spacers to mimic a 73mm shell.

Or something along those lines, I can't think clearly.
That makes sense. The nice thing with the CK center sleeve is that you can move an o-ring over between two slots, making it compatible with 68 and 73mm shells. Which raises another question... how necessary is that center sleeve. I'm assuming it is to keep moister and dirt from coming in on the inside of the bb cups. Is there any other purpose for it? If I have a frame that is sealed (no vent wholes, sealed water bottle bosses), I should be OK to leave the center sleeve out, correct?

Sorry for all the questions, I'm just wondering about all these things. I could care less of any weight savings a sleeve will do, but if I can omit an unnecessary part, I will.
 

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The Punk Hucker
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julioardz said:
That makes sense. The nice thing with the CK center sleeve is that you can move an o-ring over between two slots, making it compatible with 68 and 73mm shells. Which raises another question... how necessary is that center sleeve. I'm assuming it is to keep moister and dirt from coming in on the inside of the bb cups. Is there any other purpose for it? If I have a frame that is sealed (no vent wholes, sealed water bottle bosses), I should be OK to leave the center sleeve out, correct?

Sorry for all the questions, I'm just wondering about all these things. I could care less of any weight savings a sleeve will do, but if I can omit an unnecessary part, I will.
Some people remove teh center sleeve to save weight. It's there to protect bearings from water.
 

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Omitting the tube will save 7g according to another thread. I don't think you can completely seal out moisture it always finds a way in, if not by the seat tube or head tube you still can have condensation accumulate in the BB shell and without a weep hole in the bottom for drainage corrosion can effect the spindle and bearings.
 

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Discussion Starter #9
Thanks for all the info. I like the design, look, and ease of external bottom brackets and hollow crank spindles. I'm just trying to understand them better. Square tapers lasted me much longer, but they do weigh much more compared to the externals and the squares would start to round off with time. I'm hoping these CK external BBs last longer than the ones I've used in the past.
 

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The Punk Hucker
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julioardz said:
Thanks for all the info. I like the design, look, and ease of external bottom brackets and hollow crank spindles. I'm just trying to understand them better. Square tapers lasted me much longer, but they do weigh much more compared to the externals and the squares would start to round off with time. I'm hoping these CK external BBs last longer than the ones I've used in the past.
Same here, but I'll be giving Token a try. Cheaper alternative with what seems like a good rep and reviews.
 
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