Some Inherent Weaknesses of External Bottom Brackets

j e SS e · Jun 28, 2009

ShadowsCast said:
Terrible connection. Terrible logic.

Not really, the point is what doesn't bend will break. Bridges and bicycle drive trains are very similar in that they have forces exerted on them which have to be transferred. If the force is greater than what can be transferred and absorbed you have increased material fatigue and failure.

It's pretty basic, really.

How long to you think a rigid bike would hold up in a DH race?

Same principles apply, the energy has to go somewhere....in this case it's absorbed rather than sent through the bike and rider where it would inflict some damage.

Another oxymoron is that everyone wants stiffer frames and parts, but a smoother ride.

Problem is, all that impact energy has to go somewhere...and I see an awful lot of premature failures on these boards, especially frames.

I've explained my reasoning, can you explain yours, re: "terrible logic" ?

scooter2468 · Jun 28, 2009

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

Let's not also forget that just because keirin racers do something, it doesn't necessarily mean it's better. They are as subject to myth and tradition as anyone. Besides, even if replacing the grease with oil for one event does reduce drag, by how much? I bet not enough to be significant.

j e SS e · Jun 28, 2009

aka brad said:
I thought about that, but there are 140mm suspension forks on 26" wheel bikes and I haven't heard that they were causing the same level of headset problems as 29ers. Although 1.5 headsets are available, 1 1/8 is still remains standard.

That 140mm is absorbing most of the energy before it gets to the HS.....

Most 29ers are run either rigid or with relatively little travel, so more energy goes straight to the HS with less buffer, this is compounded by the fact that it is a longer lever.

gibbed · Jun 28, 2009

One thing you may not have thought of. Because an external BB spindle is larger in diameter and stiffer, it will flex less under load. Because it flexes less, the bearing's inner races do not have to tolerate as much angular misalignment under load. This will lead to lower friction under load and better power transfer.

Even though the external BB has higher friction losses from the larger seals on the larger bearings, I feel the increase in stiffness is worth it for grinding out slow starts on a SS or taking big drops to flat.

bad mechanic · Jun 28, 2009

j e SS e said:
Especially with increasingly "laterally stiff, vertically compliant" frame designs.

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

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

You can go narrower and stiffer and still last as long provided it's properly engineered. The one thing narrower, stiffer, and long lasting might not be is lighter.

Square taper bottom brackets may have lasted a long time, but a lot more crank arms died with square taper than with external bottom brackets. I prefer replacing cheap bearings compared to expensive crank arms.

Roasted · Jun 28, 2009

I have yet to understand how external bottom brackets are getting the rave they have. I've been on square tapers since the beginning of time. A while ago I acquired my first Octalink bike, which so far the stock Octalink cartridge that came with it already outlasted the square tapered replacement I recently put in my other bike. I definitely think I'll be sticking with Octalink for quite some time.

As it was said before, I too believe that BB30 will replace the external bottom brackets. However, I don't see BB30 taking off for quite some time. It's had some decent exposure yet not as many people have picked up on it as I thought they would by now.

I've ridden several external bikes and, yeah, they're nice... but them being nice doesn't seem to fill in the big shoe of why some people swear by them. Maybe I'm just naive, I just don't see the rush in it.

I didn't even have a reason to go from squared taper to octalink in the first place. It just so happened the bike I liked had one, however, I'm happy with my purchase. In the future, as I build onto this bike, if there ever comes a time where I'm in need of a new frame and BB30 is taking off with a solid reputation, I may jump on that bandwagon. Till then, I'll pass on the externals.

ISuckAtRiding · Jun 28, 2009

bad mechanic said:
You can go narrower and stiffer and still last as long provided it's properly engineered. The one thing narrower, stiffer, and long lasting might not be is lighter.

Square taper bottom brackets may have lasted a long time, but a lot more crank arms died with square taper than with external bottom brackets. I prefer replacing cheap bearings compared to expensive crank arms.

amen to that. i've replaced my own square taper crank arms well into the double digits (and yes, they were torqued properly).
Couldnt get them to not round out.

KeylessChuck · Jun 28, 2009

gibbed said:
One thing you may not have thought of. Because an external BB spindle is larger in diameter and stiffer, it will flex less under load. Because it flexes less, the bearing's inner races do not have to tolerate as much angular misalignment under load. This will lead to lower friction under load and better power transfer.

Even though the external BB has higher friction losses from the larger seals on the larger bearings, I feel the increase in stiffness is worth it for grinding out slow starts on a SS or taking big drops to flat.

See some of the original discussion above. It's not really the bearings that are in question, but the flimsy cups they ride in.

ShadowsCast · Jun 28, 2009

j e SS e said:
Not really, the point is what doesn't bend will break. Bridges and bicycle drive trains are very similar in that they have forces exerted on them which have to be transferred. If the force is greater than what can be transferred and absorbed you have increased material fatigue and failure.

It's pretty basic, really.

How long to you think a rigid bike would hold up in a DH race?

Same principles apply, the energy has to go somewhere....in this case it's absorbed rather than sent through the bike and rider where it would inflict some damage.

Another oxymoron is that everyone wants stiffer frames and parts, but a smoother ride.

Problem is, all that impact energy has to go somewhere...and I see an awful lot of premature failures on these boards, especially frames.

I've explained my reasoning, can you explain yours, re: "terrible logic" ?

Crank arms have a fatigue limit; crank arms that are flexing/bending/absorbing or whatever else you'd seem to think they do end up breaking. By your logic cranks would always be breaking at their least flexible, or thickest point. Time has shown that to be false. If you ever looked through pardo.net you would have seen nearly every time the crank broke at a thinner more flexible part, usually at the "vanity groove," a spot they had machined material away so the logos would not be rubbed off by use.

The only time something needs to bend to absorb the energy is when it isn't strong enough (or can't be made strong enough) to absorb it without bending. Given the relatively low forces cranks need to handle, in comparison with skyscrapers and bridges, they don't need to flex, and aren't built to flex; they are designed to transfer all the energy into the drive train and the frame without bending to dissipate it.

ShadowsCast · Jun 28, 2009

Roasted said:
I've ridden several external bikes and, yeah, they're nice... but them being nice doesn't seem to fill in the big shoe of why some people swear by them. Maybe I'm just naive, I just don't see the rush in it.

Some people swear by them because of the situations most everyone has agreed they excel. Downhill, large jumps, drops to flat... On square taper the spindle and interface become the weak link, not bearings, seals or longevity. With a spindle that large it is a lot stronger, and able to handle those kind of forces.

I agree with you that I can't believe BB30 hasn't at least spread a little bit more than it has... It seems like it's still just Cannondale's few high end models and that's it.

aka brad · Jun 28, 2009

bikeny said:
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.

Mark

aka Brad said:
1)I errored when I said fewer contact points. What I meant was the contact points were father apart.

Let me clarify my clarification. Most external BB have fewer (larger) bearing held in a cage, therefore, there are less bearings and less contact points. I don't know if this is true of Chris King, but it is true of Shimano and Race Face, with the Race Face DH BB having the full compliment of ball bearings; twice as many as their XC BB.

gibbed said:
One thing you may not have thought of. Because an external BB spindle is larger in diameter and stiffer, it will flex less under load. Because it flexes less, the bearing's inner races do not have to tolerate as much angular misalignment under load. This will lead to lower friction under load and better power transfer

.

The problem is the external BB cups are much less support then the cartridge housed inside a BB shell. Therefore, the exterior BB will flex much more than the bearing being housed inside the BB shell. This causes more angular misalignment, which is why external BB bearings have such a short life; Chris King will only warranty their BB bearings for 5 years normal wear and tear not covered, and you have to grease them every 3 months or so; this compared to their headsets, which are warrantied for 10 years.

ISuckAtRiding · Jun 28, 2009

aka brad said:
Let me clarify my clarification. Most external BB have fewer (larger) bearing held in a cage, therefore, there are less bearings and less contact points. I don't know if this is true of Chris King, but it is true of Shimano and Race Face, with the Race Face DH BB having the full compliment of ball bearings; twice as many as their XC BB.

.

The problem is the external BB cups are much less support then the cartridge housed inside a BB shell. Therefore, the exterior BB will flex much more than the bearing being housed inside the BB shell. This causes more angular misalignment, which is why external BB bearings have such a short life; Chris King will only warranty their BB bearings for 5 years normal wear and tear not covered, and you have to grease them every 3 months or so; this compared to their headsets, which are warrantied for 10 years.

every external bearing bottom bracket i've worked on did not have a cage. They have sealed cartdrige bearings. The balls are nut to butt to eachother, meaning there are far more balls needed to fill the circumfrence of the bearing race as compared to any other type of bottom bracket. They probably have smaller balls, if anything, because the wall thickness of the bearing needs to be a bit more compact.

Miker J · Jun 29, 2009

Octalink worked very well for me. Still does. Only problem I've had is the BB drive side loosening. Not many threads on that side. I must have a dozen of the BBs and 5 cranksets l still use. Got'm cheap when friends move onto "other" designs.

Still like square tapers for xc riding. Burned through my PW BB bearings in less than a year - go figure. My Shimano square tape is still going strong after years of abuse.

aka brad · Jun 29, 2009

ISuckAtRiding said:
every external bearing bottom bracket i've worked on did not have a cage. They have sealed cartdrige bearings. The balls are nut to butt to eachother, meaning there are far more balls needed to fill the circumfrence of the bearing race as compared to any other type of bottom bracket. They probably have smaller balls, if anything, because the wall thickness of the bearing needs to be a bit more compact.

"The balls are nut to butt to eachother" ? I don't know what this means. Also the ball bearings are 35% larger than an ISIS internal BB. What make exterior BB did you work on. Most the sealed cartridge bearings I have seen are caged on the inside. Kinda looks like this.

Brad

ShadowsCast · Jun 29, 2009

That means the balls are touching, with nothing between them.

bad mechanic · Jun 29, 2009

aka brad said:
Also the ball bearings are 35% larger than an ISIS internal BB.

From what I understand, this was the big problem with ISIS, that the bearings in it were too small, and therefore prone to failure. I saw this with my own eyes as my friends' ISIS bottom brackets kept failing. Generally, bigger bearings are a good thing as they handle large loads betters.

I have to wonder if part of the problem people are having with external bottom brackets isn't the fact they're not facing their bottom bracket shell before installation.

I also have a hard time believing there's any appreciable flex in an external BB's cups since they're of such a large diameter.

aka brad · Jun 29, 2009

bad mechanic said:
From what I understand, this was the big problem with ISIS, that the bearings in it were too small, and therefore prone to failure. I saw this with my own eyes as my friends' ISIS bottom brackets kept failing. Generally, bigger bearings are a good thing as they handle large loads betters.

I have to wonder if part of the problem people are having with external bottom brackets isn't the fact they're not facing their bottom bracket shell before installation.

I also have a hard time believing there's any appreciable flex in an external BB's cups since they're of such a large diameter.

Again my take is that the bearings are hanging outside the BB shell. When pressures are exerted against the bearings, rather than being surrounded by a structural housing, they must rely on the rigidity of cups that are simply screwed into the shell. Therefore, rather than the fulcrum of the axle exerting its weight through the bearings and races that are contained by shell, the fulcrum becomes the interface between the cup and shell; specifically the interface between the bearing cup and shell. Even if the BB shell has been properly surfaced, it only means the bearings start out aligned. With an internal BB, the part most likely to flex is the crank arm, with an external BB the weakest point is the BB bearing cup.

bikeny · Jun 29, 2009

forwardcomponents said:
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

Where did you find any reference that the Phil cups are made of steel? I have been searching for info on this, but have not found any. The Phil Wood site does not have any info at all.

Mark

Edit: Nevermind, I found a bunch of store listings that say steel. Interesting that the Phil site has no info, though.

EBasil · Jun 29, 2009

aka brad said:
Again my take is that the bearings are hanging outside the BB shell. When pressures are exerted against the bearings, rather than being surrounded by a structural housing, they must rely on the rigidity of cups that are simply screwed into the shell. Therefore, rather than the fulcrum of the axle exerting its weight through the bearings and races that are contained by shell, the fulcrum becomes the interface between the cup and shell; specifically the interface between the bearing cup and shell. Even if the BB shell has been properly surfaced, it only means the bearings start out aligned. With an internal BB, the part most likely to flex is the crank arm, with an external BB the weakest point is the BB bearong cup.

With so much esoteric analysis, it seems that we Luddites have forgotten that external BB cranksets perform as apparently less flexy than internal sets, be they square taper or not. While it's true that many have ground through the cheapass bearings used in many ExBB cranksets, there's not an apparent issue of FLEX at the BB.

Either way, Shimano announced today that they're returning to BioPace, cantilever brakes and the UN-72, based on the illuminating exposure of their craven marketing, in this very thread. We've done our job fellas! :thumbsup:

boomn · Jun 29, 2009

bikeny said:
Where did you find any reference that the Phil cups are made of steel? I have been searching for info on this, but have not found any. The Phil Wood site does not have any info at all.

Mark

Edit: Nevermind, I found a bunch of store listings that say steel. Interesting that the Phil site has no info, though.

link 1
link 2

Some Inherent Weaknesses of External Bottom Brackets

j e SS e

scooter2468

j e SS e

gibbed

bad mechanic

Roasted

ISuckAtRiding

KeylessChuck

ShadowsCast

ShadowsCast

aka brad

ISuckAtRiding

Miker J

aka brad

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