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frame flex and BB choice

Old 05-24-13, 03:12 PM
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frame flex and BB choice

Is there a difference in the amount of flex that will be felt in a given bottom bracket area when a square taper BB is installed versus an external bearing bottom bracket? I've read that the external BB is "stiffer" but I'm not sure what that is specifically referring too.
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Old 05-24-13, 05:22 PM
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It's the entire bb/crankset design that collectively reduces flex, but yes, the newer ones with large, hollow spindles and external bearings have the potential to be stiffer than traditional cranksets.
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Old 05-24-13, 05:32 PM
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The large hollow spindle does contribute. Moving the bearings outboard also means there is less unsupported spindle length to flex.
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Old 05-24-13, 05:43 PM
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None of it matters.
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Old 05-24-13, 06:51 PM
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Originally Posted by davidad
None of it matters.
Pretty much correct unless the flex is so bad the chainrings hit the front derailleur cage under load. Bottom bracket flex is only partially, and not even majorly, controlled by the spindle and bearings. A lot of the stiffness, or lack thereof, is inherent in the frame as the bottom bracket shell stiffness is a major part of frame flex. So a soft frame with an absolutely rigid bottom bracket spindle set will still flex under load and a very stiff frame won't flex much even with a relatively flexible bb spindle and bearing set up.
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Old 05-24-13, 06:55 PM
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No one thing matters absolutely, but there are many factors, and each contributes a bit. Some people can feel the difference from one BB to another. I'm not one of those super strong riders, but I can make some chainrings hit the front derailleur. I could probably feel an improvement in one of the new designs. Would it make me faster or less fatigued? Maybe not.
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Old 05-24-13, 07:04 PM
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Originally Posted by davidad
None of it matters.
It does matter when it reduces chain rub on the front derailer cage and the need for trim adjustments, but some square-taper cranksets prevent flexing somewhat better than others.
Stronger/heavier riders will notice.

Although the bb flexing actually allows the left crankarm to move independently of the chainrings, the stored energy is said to be recovered at the bottom of the pedal stroke.
This asymmetry may affect the left leg's ability to produce maximum output though, since the leg effectively feels full resistance later in the stroke, after the brain has attempted maximal force. This timing of the left leg's peak effort/force thus changes with increases in pedaling force, so an asymmetric loss of legpower can be expected under maximum acceleration as when sprinting or maintaining speed up a very steep climb.
A racer will appreciate the more-symmetric pedaling resistance offered by an "oversized" bottom bracket spindle.

My own disfavor with non-square taper bottom brackets (and modern cranks) has to do with the typical unavailability of spindle length options, but I am considering upgrading my race bike with a modern crankset and using a double in place of the current (leftover from my MTB days) square-taper triple on my cyclocross bike, which has 132mm (modern) dropout spacing.

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Old 05-24-13, 07:20 PM
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The win for me with external bearing (besides the potential weight savings and additional stiffness) is that from a mechanical standpoint it's just a better design. It eliminates the press fit of the crank/spindle interface, instead replacing it with two simple bolts on the Shimano units. Once loosened the left arm can be pulled off by hand. Therefore the crank will never "wear out" from being installed and removed too many times and is much less likely to be put on too loose and subsequently ruined when it loosens up.
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Old 05-24-13, 08:14 PM
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The reason I ask is because I just had a new frameset built up with new components and I chose a Deore crankset with external BB. Part of this is because I've read that external BB's are stiffer and I'm a really heavy rider and part is because I wanted the option to switch to indexed shifting. However, in the future I may decide to switch to a more traditional, silver road crank that is square taper and I'm curious if it's possible that I'll experience flex at that point that I'm not experiencing now.
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Old 05-24-13, 08:31 PM
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What's inside or outside the BB shell won't affect frame flex either way. When folks speak of stiffer BB, they're referencing spindle flex, not frame flex.

By moving the bearings closer to the crank arms, external BB systems reduce the length of the spindle subject to flex, and thereby make it stiffer. Of course that assumes that the spindle itself is equally rigid.

IMO the entire issue of spindle flex was exaggerated all along, but marketing requires that every "cure" needs to find a disease.
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Old 05-24-13, 10:29 PM
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i can't help but think that about 8 inches of crank arm plus a couple inches of pedal axle plus pedal cleat and mostly plastic and fabric shoe, not to mention BB/frame flex, would more than out distance any flex afforded by 1/2 inch of steel supported by a typical cartridge spindle's bearing.

try standing beside your bike and lightly bouncing up and down on one pedal. observe what is flexing. i think you'll see the crank arm flex and the BB/frame.
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Old 05-24-13, 11:32 PM
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I've found that the difference in stiffness between internal and external BBs is far outweighed by the frame's flex. That said, my new external BB crankset is indeed noticeably stiffer when pedaling out of saddle.
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Old 05-25-13, 12:07 AM
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frame flexes ?, no crank choice will eliminate the flex..

start with a stiff frame if that is your priority, or sit down , smooth out your pedal style so it does not stress-flex the frame..

Last edited by fietsbob; 05-25-13 at 12:10 AM.
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Old 05-25-13, 12:47 AM
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One of the main reasons for oversized spindles and external bearings is that it allows the designers to preserve rigidity while designing lighter and lighter components.

This extends to the frame itself, which in recent designs positions the widely-spaced bearings more directly in the load path without any cantilevered bearing cup mounting. This shortens the "load path" which is critical to maximizing the strength-to-weight ratio.
Not without problems, some of these "BB30" style bottom brackets have creaking issues.

But the external cups are an interim way of merging enhanced-design cranksets with traditional frames, and do so without many problems.
They particularly address the bearing-durability problems that the previous ISIS bottom brackets suffered.

Square-taper bottom brackets really were being used beyond the limits of design when it comes to off-roading, where a rider's weight may come down hard with the crankarms horizontal, and I've seen many a bent or fractured square-taper spindle.
Not to mention that with a square-taper bb spindle, the left crankarm has that extra flex between the arm and chainrings.
But it all depends on how strong, heavy and hard-riding the rider is. It kind of amazes me that big, strong track riders used square-taper bb's back in the day, or spindly steel frames for that matter. Some of today's chemically enhanced riders might break such equipment in short order I would imagine.

Shimano's Octalink cranksets and BB's were much stiffer than square-tapered bb's, so really solved the main problems of square tapers, but the newer integrated bb spindles have the ability to allow even lighter and stiffer crank mountings while likely being cheaper to produce overall.

Last edited by dddd; 05-25-13 at 12:59 AM.
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Old 05-25-13, 01:30 PM
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Originally Posted by dddd
One of the main reasons for oversized spindles and external bearings is that it allows the designers to preserve rigidity while designing lighter and lighter components.

This extends to the frame itself, which in recent designs positions the widely-spaced bearings more directly in the load path without any cantilevered bearing cup mounting. This shortens the "load path" which is critical to maximizing the strength-to-weight ratio.
Not without problems, some of these "BB30" style bottom brackets have creaking issues.

But the external cups are an interim way of merging enhanced-design cranksets with traditional frames, and do so without many problems.
They particularly address the bearing-durability problems that the previous ISIS bottom brackets suffered.

Square-taper bottom brackets really were being used beyond the limits of design when it comes to off-roading, where a rider's weight may come down hard with the crankarms horizontal, and I've seen many a bent or fractured square-taper spindle.
Not to mention that with a square-taper BB spindle, the left crankarm has that extra flex between the arm and chainrings.
But it all depends on how strong, heavy and hard-riding the rider is. It kind of amazes me that big, strong track riders used square-taper bb's back in the day, or spindly steel frames for that matter. Some of today's chemically enhanced riders might break such equipment in short order I would imagine.

Shimano's Octalink cranksets and BB's were much stiffer than square-tapered bb's, so really solved the main problems of square tapers, but the newer integrated bb spindles have the ability to allow even lighter and stiffer crank mountings while likely being cheaper to produce overall.
The big problem with the splined BBs were that to fit the larger diameter spindle in the still same sized shell the bearings had to be reduced in diameter (as in the balls being smaller). And as we all know, the load capacity of a bearing is more dependent on the diameter of the rolling element (ball) then the number of them. So a bearing that is half the diameter (of the balls) but doubled in ball count has half the load capacity. So the "new and better" splined BBs wore out much more rapidly then the "old and slow" tapered square ones.

The move to outside the shell bearings, or to larger diameter shells, allow larger sized rolling elements and the larger spindle. However some of the newer units suffer from poor sealing. Andy.
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Old 05-25-13, 02:15 PM
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Right on all counts.

I would note that Shimano's 2nd-generation Octalink bb's were pretty durable in all areas. They machined the raceways directly into the spindle and cartridge shell to make room for the biggest possible bearings, and I've had good luck with them, especially as compared to ISIS BB's.

The very first (Dura-Ace and XTR only) Octalink BB's were plagued by sealing problems in part because the bearings were adjustable and depended on a precisely faced bb shell to keep everything lined up and sealing.
This adjustable (and rebuildable) design was taken off the market after just a year or so, and the good, newer design appeared to be used for all models from then on.

Last edited by dddd; 05-25-13 at 02:20 PM.
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Old 05-26-13, 01:17 AM
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Just a big FWIW. I have a good quality, strong, alloy cyclocross frame that I use for commuting (Jamis Nova Pro). For two years I used a 6500 series Shimano Ultegra road triple crank with an Octalink BB. Good solid crank, good solid BB - nothing ever came to my attention in its functioning.

This year, for various reasons, I now have a Shimano square taper BB and an older Suntour XCE triple crank on it. I firmly believe that either or both the spindle and the crank itself are flexier than the other setup. I can hear, not really hear, the thing move when I put force on it. And I'm a 60-something 160 lb weakling.

It doesn't really affect my riding, but it kind of bothers me. If I like this new gearing (48-38-28 instead of 52-42-30), I might invest in a better quality set up.

Another FWIW, and off topic. The suntour chain rings have no ramps and pins but shift crisply and perfectly!
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Old 05-26-13, 05:42 AM
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My 2c is there's a good chance you'll enjoy climbing more on an external BB.

Won't make a huge difference, but it should be noticeable - I'm only 65kg BTW.
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Old 05-26-13, 05:52 AM
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corwin1968, The external bearing/crankset design is stiffer simply because there is less fulcrum length between the bearing and where load is applied. The square taper bottom bracket, while not quite as stiff (also dependent on spindle length), is more than rigid enough for the great majority of riders.

With the advent of over sized tubing (and/or better materials) frame flex at the bottom bracket shell isn't as common as it once was for the stronger and heavier riders. However, if the frame flexes, the bottom bracket design is a moot point.

Brad

PS Unless one can torque the chainring into the FD, I don't think the difference between the two designs will be very noticeable.

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Old 05-26-13, 06:28 AM
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Originally Posted by dddd
I would note that Shimano's 2nd-generation Octalink bb's were pretty durable in all areas. They machined the raceways directly into the spindle and cartridge shell to make room for the biggest possible bearings, and I've had good luck with them, especially as compared to ISIS BB's.
That's been my experience too. I have an Ultegra 6503 triple crank on a BB-6500 Octalink bottom bracket. This bottom bracket has over 35,000 mile on it and is still as smooth and free of play as new. I also have a BB-5503 (105 level Octalink bb) on another bike but it only has about 5000 miles so it's too early to say anything about it's durability except so far so good.

I believe ISIS got a bad reputation since it was an open public access design. Therefore, huge range of bottom bracket qualities were sold and the bad ones were really bad. It was an inherently weaker design but the better quality ones were decent. Shimano had exclusive use of the Octalink design and could control the quality much better.
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