Some Inherent Weaknesses of External Bottom Brackets

aka brad said:

..... The first is the fact that external bb are not supported by the inside of the bb shell. External bb bearing are in cups outside the bb shell. This causes the spindle to wobble, reducing smooth pedaling and bearing life. Further, the whole point of external bb are larger bearings for larger dia spindles. Unfortunately larger bearing also mean there are less points supporting the spindle; this also increases the tendency of the spindle to wobble. .....

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How do you mean "less points supporting the spindle? Lager bearings have a larger internal circumference which, in theory, should mean more contact points. I may be inclined to think that square taper bearings are wider and therefore less prone to wobble. But they are closer to eachother so wobble is a bit more exaggerated. Tits for tats. But I would love some more info.

My main gripes about external bearings are the seals and the tiny bearings compared to square taper. The spindle is bigger and therefore less flexier. But hey I think that those differences and neglible compared to possible frame flex and lower body flex. Also modern square taper BB are pretty bomber and need zero maintenance (except for those 10' drops). Compare that to the multitude of extra parts/setup for a External BB.

BB30/90 should solve a lot of the above mentioned problems. But I anxiously await the first damaged frames from not perpendicular axles/bearings. HS cups are also pressed in but the bearings are supported bij the cups not the frame. Which is also my main gripe about some of those internal HS.

aka brad said:

Recently there has been some discussion about bottom brackets and specifically the superiority of external bottom brackets (bb). While researching why Shimano and Campagnolo did not have external bb in their Track/Pista gruppo, some interesting facts came up regarding the weaknesses of external bb. The first is the fact that external bb are not supported by the inside of the bb shell. External bb bearing are in cups outside the bb shell. This causes the spindle to wobble, reducing smooth pedaling and bearing life. Further, the whole point of external bb are larger bearings for larger dia spindles. Unfortunately larger bearing also mean there are less points supporting the spindle; this also increases the tendency of the spindle to wobble. My take on this is there is a point of diminishing returns with bearing size and the old square taper seemed to have gotten right. Another issue with external bb is the tightness of the seals, which causes more drag on the spindle of internal bb. Keirin racers are known to still use loose ball bb, even though Octalink is NJS certified. They do this because the loose ball are not sealed and they can use oil instead of grease when they race, both reducing bb drag. I have said this before, that I believe that the BB30 will probably replace external bb, as it still supports the bb bearings inside the shell and still allows for a larger dia spindle. If there is a weakness with square taper, it might be its inability to withstand some 10'+ drops. Yes, the larger dia spindle will withstand the drop, but it's only a matter of time before the poorly supported bearings fail. So you might want to think twice before you upgrade to to external bb. The only reason I can think of is to use the uber cool Forward Components EBB converter.

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I am glad that somebody initiated this discussion. I have been thinking about this issue as well.

While looking at the Phil Wood site, I noticed something very interesting. As everyone knows, Phil Wood is noted for two things: very high quality bearings and seals, and square taper bottom brackets. What many people do not know is that Phil Wood also makes outboard bearing bottom brackets, as well as the bearings and seals for them.

https://www.aspirevelotech.com/images/Phil_Wood/OutboardBearingCupsWW.jpg

It is interesting to note that the Phil Wood outboard threaded cups are made from stainless steel, not aluminum. No doubt these are heavier, but one can only speculate that they are much less flexible, and provide a more stable housing for the bearings. Phil Wood obviously feels that it is possible to make an outboard bearing bottom bracket that is up to his high standards for bearing durability. The industry people I have spoken with have all told me that his outboard bottom bracket replacement bearings outlast all the others. Obviously it is possible to make a bearing for this system with exceptionally good life expectancy. The difference is in the details, as usual.

As for bearing size, both of Phil Wood's systems use balls that must be almost exactly the same size. The dimensions of their square taper cartridge bearings are 17mm x 30mm x 7mm. The dimensions for their outboard system cartridge bearings are 25mm x 37mm x 7mm. Do the math, and you will see that the difference in ball diameter is probably 0.5mm, assuming that the races are equally thick. It is possible that both systems use identiacl ball sizes. Tha fact that both systems use cartridges that are the same 7mm width suggests that this is probably true.

Phil Wood is obviously confident that outboard bearing systems can have the durabilty that his customers expect, and experience seems to be proving him right. I think that the issues associated with currently available outboard bearings have more to do with failures in bearing cup, bearing, and seal design rather than any inherent flaws in the system itself.

Are external seals absolutely necessary on outboard bearings? Since the introduction of cartridge bearings and cartridge bottom brackets, one lip seal has been the design standard. All of these designs, including good examples such as Phil Wood, or the venerable Shimano UN-72 square taper cartridge bottom bracket, use one lip seal on the bearing, with no external seal. The bearing industry has always claimed that external seals are preferable, be they lip seals, dust seals, or labyrinth seals. They are better than one seal if designed properly. Despite this, excellent products have been made without them, as the two examples show.

Phil Wood
https://www.cambriabike.com/Images/product/phil_wood_steel_bottombracket.jpg

UN72
https://farm1.static.flickr.com/4/3937021_609d2ab488.jpg?v=0

Both of those bottom brackets are fine examples of square-taper specimens, but this discussion is comical in the manner of scientific "proof" that the world is flat. I bet we can find another thread (or nine) about how rigid bikes are far more efficient, faster, smarter, better, whatever, than suspension bikes. C'mon, who was around for all that?

I've been to the edge, from there I stood and looked down. I've ridden square tapers, built bikes with square tapers and broken square tapers. I've ridden external, since 1995 on my main ride, built with external and worn out bearings in external. The earth may be flat where you are, and that's GREAT -- I am stoked for you. But despite all your Creation Science and various apocrypha, things are not as you think. Cute, charming and even quaint, but for those of you getting all worked into a froth trying to convince us, also silly.

If you like square taper, if it makes you feel core or fast or happy, by all means: use it in good health! Get yourself a qualtiy product with a short spindle and some strong cranks. The only thing you'll be flexing is your bottom bracket shell, and it will last a good long time. There's no need to leave the valley, all you need is here.

aka brad said:

Recently there has been some discussion about bottom brackets and specifically the superiority of external bottom brackets (bb). While researching why Shimano and Campagnolo did not have external bb in their Track/Pista gruppo, some interesting facts came up regarding the weaknesses of external bb. The first is the fact that external bb are not supported by the inside of the bb shell. External bb bearing are in cups outside the bb shell. This causes the spindle to wobble, reducing smooth pedaling and bearing life. Further, the whole point of external bb are larger bearings for larger dia spindles. Unfortunately larger bearing also mean there are less points supporting the spindle; this also increases the tendency of the spindle to wobble. My take on this is there is a point of diminishing returns with bearing size and the old square taper seemed to have gotten right. Another issue with external bb is the tightness of the seals, which causes more drag on the spindle of internal bb. Keirin racers are known to still use loose ball bb, even though Octalink is NJS certified. They do this because the loose ball are not sealed and they can use oil instead of grease when they race, both reducing bb drag. I have said this before, that I believe that the BB30 will probably replace external bb, as it still supports the bb bearings inside the shell and still allows for a larger dia spindle. If there is a weakness with square taper, it might be its inability to withstand some 10'+ drops. Yes, the larger dia spindle will withstand the drop, but it's only a matter of time before the poorly supported bearings fail. So you might want to think twice before you upgrade to to external bb. The only reason I can think of is to use the uber cool Forward Components EBB converter.

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I think you have some good points and some that are not so good. First, on a mountainbike, you cannot use a square taper BB with loose bearings, no seals, and oil. It just won't last long. Sure, it will feel great for the first ride, but after that it will get gunked up. But it does bring up one of the main complaints about external BBs, namely the drag associated with the bearing and seals. This is definately noticable when spinning the cranks by hand, but I am not sure how much of a difference it actually makes will riding. I am not sure what you mean by less points of contact, I assume you mean less balls in the bearing? I don't think that is correct. I would think the external BB, because it uses larger I.D. and O.D. would have more balls, or at least larger balls which can support more load. As far as spindle wobble, I would think you would get less with the external BB system. Basically because the spindle is supported further out and because the spindle itself is much stiffer. Now I agree that if the BB shell was wider to the point that the bearings ended up in the same place as an external BB system, that would be better than everything else. I am not familiar with the BB30 system, maybe that is what it does.

I think the needs of Keirin racers are drastically different than that of MTB riders. A little extra drag is perfectly acceptable if it results in longer lasting bearings and a stiffer system.

Mark

bikeny said:

I think you have some good points and some that are not so good. First, on a mountainbike, you cannot use a square taper BB with loose bearings, no seals, and oil. It just won't last long. Sure, it will feel great for the first ride, but after that it will get gunked up. But it does bring up one of the main complaints about external BBs, namely the drag associated with the bearing and seals. This is definately noticable when spinning the cranks by hand, but I am not sure how much of a difference it actually makes will riding. I am not sure what you mean by less points of contact, I assume you mean less balls in the bearing? I don't think that is correct. I would think the external BB, because it uses larger I.D. and O.D. would have more balls, or at least larger balls which can support more load. As far as spindle wobble, I would think you would get less with the external BB system. Basically because the spindle is supported further out and because the spindle itself is much stiffer. Now I agree that if the BB shell was wider to the point that the bearings ended up in the same place as an external BB system, that would be better than everything else. I am not familiar with the BB30 system, maybe that is what it does.

I think the needs of Keirin racers are drastically different than that of MTB riders. A little extra drag is perfectly acceptable if it results in longer lasting bearings and a stiffer system.

Mark

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Enough with all the "balls" and "loads", we're talking bike parts here.....

One problem is everyone's obsession with q-factor. It seems like everyone wants their feet tied together rather than spread apart a bit for better balance and power.

The other problem is everyone's obsession with stiffness. Whatever doesn't bend will break. There needs to be some flex to help bear the load. Bridges flex. Skyscrapers flex. If they didn't they would fail prematurely. Crank/BB/Spindles need to have some flex in there somewhere, too. Especially with increasingly "laterally stiff, vertically compliant" frame designs.

Narrower and stiffer but lasts as long or longer ain't gonna happen.

Internals are much better supported and the bearings, typically, last much longer.

The industry is loving it though, think how many more BB they sell now than when everything was square tapers that last forever.

When things work too well, in any industry, the tendancy is to "improve" the design, which results in something that requires more maintenance and/or replacement. It's how they make more money. Manufacturers don't make as much money with product designs that don't need maintenance or replacement.

bikeny said:

I think you have some good points and some that are not so good. First, on a mountainbike, you cannot use a square taper BB with loose bearings, no seals, and oil. It just won't last long. Sure, it will feel great for the first ride, but after that it will get gunked up. But it does bring up one of the main complaints about external BBs, namely the drag associated with the bearing and seals. This is definitely noticeable when spinning the cranks by hand, but I am not sure how much of a difference it actually makes will riding. I am not sure what you mean by less points of contact, I assume you mean less balls in the bearing? I don't think that is correct. I would think the external BB, because it uses larger I.D. and O.D. would have more balls, or at least larger balls which can support more load. As far as spindle wobble, I would think you would get less with the external BB system. Basically because the spindle is supported further out and because the spindle itself is much stiffer. Now I agree that if the BB shell was wider to the point that the bearings ended up in the same place as an external BB system, that would be better than everything else. I am not familiar with the BB30 system, maybe that is what it does.

I think the needs of Keirin racers are drastically different than that of MTB riders. A little extra drag is perfectly acceptable if it results in longer lasting bearings and a stiffer system.

Mark

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A few clarifications. 1)I errored when I said fewer contact points. What I meant was the contact points were father apart. 2)The problem with the idea of the spindle being supported by points father apart is the poor support of the bearings. In this case it is an aluminum cup outside the shell threaded into the shell. With an internal bb, the side loading forces on the bearings are mitigate by the beairngs being supported inside the shell. With an external bb, there is no outside rigid shell holding the bearings in line. The interior bottom bracket bearings are held perpendicular to the spindle to prevent side loading and premature wear or failure of the bearings. The external bb cups are screwed into the ends of the shell and therefore not are held into place by a surrounding rigid steel housing. Therefore the bearings simply cannot be held as secure, creating more side loading and wobble.3) Finally, I did not bring up the Keirin racers to suggest that we should be using unsealed bearings. I simply wanted to demonstrate that seal drag does count for something. The nature of an external bb means both bearings need to be sealed on both sides. And while you may not feel it when you are riding, it does slow you down; so why have more of it is not necessary?

aka brad said:

A few clarifications. 1)I errored when I said fewer contact points. What I meant was the contact points were father apart. 2)The problem with the idea of the spindle being supported by points father apart is the poor support of the bearings. In this case it is an aluminum cup outside the shell threaded into the shell. With an internal bb, the side loading forces on the bearings are mitigate by the beairngs being supported inside the shell. With an external bb, there is no outside rigid shell holding the bearings in line. The interior bottom bracket bearings are held perpendicular to the spindle to prevent side loading and premature wear or failure of the bearings. The external bb cups are screwed into the ends of the shell and therefore not are held into place by a surrounding rigid steel housing. Therefore the bearings simply cannot be held as secure, creating more side loading and wobble.3) Finally, I did not bring up the Keirin racers to suggest that we should be using unsealed bearings. I simply wanted to demonstrate that seal drag does count for something. The nature of an external bb means both bearings need to be sealed on both sides. And while you may not feel it when you are riding, it does slow you down; so why have more of it is not necessary?

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OK, you make some good points again. You may be correct that contact points are further apart, I have not looked at the actual bearings being used, but there are definately more of them in and external BB, so that should increase the load carrying capacity. Although, if the same size ball is being used, the distance would be the same. I have very limited experience riding an outboard bearing system, but here is my take: The external BB system is stronger than a sqaure taper system. I believe it is also a stiffer system overall. I also think there is more drag than on a square taper system. This can certainly be felt when turning the cranks by hand. BUT, I don't think this is noticeable at all while riding, and probably has a negligeable effect. If you have tests that show otherwise I would love to see them. And the bearings also seem to not last nearly as long. I'm not sure if this has to do with the quality of the bearings, the sealing being used, or the added movement you are referring to. Maybe a combination of them? And again, I think using the Keirin racers as an example just has no relevance. Of course they will use the system with the least friction, because the other factors don't matter to them. They are also not allowed to use any other system!

So let's get theoretical here for a minute. If you were to design the ultimate BB system, what would it be? Here is what I think:

Wider BB shell, maybe 90mm.
Slightly larger diameter BB shell.
Still use threaded BB shell, as pressed in bearings is just asking for trouble.
Use bearing similar to those used on a square taper BB, just bigger I.D. and O.D, using the same size balls and seals.
Use a spindle size similar to that of the external BBs.
I also like the idea of using a splined interface that is preloaded and then clamped to the spindle.

I think that is it!

Mark