FastJake

I'm about to tear down and rebuild a near and dear MTB soon. It's setup single speed with a 36T Biopace chainring. I would like to upgrade to a Shimano external bearing crank (Hollowtech II.) But I can't seem to find one that'll work.

I need:
- works with a 73mm BB shell
- 110 BCD, 5 bolt

There doesn't seem to be anything that fits that description. The compact road cranks look to be for 68mm shells only, while the MTB cranks are only using 4 bolt super compact rings these days. Am I missing something? Will a road triple work? (Assuming I can find one in 110/74 rather than the apparently more common 130/74...)

tomacropod

Road triples are still road cranks - still 68.

Octalink/square taper are your best bet.

- joel

JiveTurkey

I see no options for a 73 mm HT2 bottom bracket working with a 110 or 130 BCD chainring.

FastJake

Darn. Looks like I'll stay with the old square taper crank that's on there now. I'd rather give up ext BB (and not buy any new parts) than give up my Biopace ring...

Thanks for the info.

tomacropod

Actually, you could run a Middleburn x type with 110bcd atb ring.

zukahn1

I have never seen or heard of a modern BB with 73 shell so I think your out of luck on this one.

FastJake

Have you heard of mountain bikes?

HillRider

It's a lot of metal to remove but I think it's possible to have a 73 mm shell faced down to 68 mm.

Dan Burkhart 

It's a lot of material to remove with a hand powered facer, but a machine shop could do it pretty quick.

Garthr

Yes ... who says an external bearing BB crankset is an "upgrade" ? Just becasue others claim it's so superior does not make it so. I have never utterted or even thought once .... "oh, that darn crank and BB flex .... it ruined my ride !". The only thing that ruins a ride is a chronic case of upgrade-itis ! Keeping up with the Jonses .... etc.

I ride Sugino XD cranks with Tange BB's. Awesome combo !

Dan Burkhart 

Yup. Nothing but square taper and Ashtabula in the house here, and they work just fine.

joejack951

External bearings cranks, aside from being stiffer, are also an upgrade by virtue of being lighter weight, offering better shifting and compatibility with the latest groups (especially 10 and 11 speeds), and to many people they look a lot nicer too.

Surely, your Sugino XD is an upgrade over something like this: https://www.niagaracycle.com/categori...70-black-4-arm

How are the Jones' doing, Garth? Nice crankset you got there

Andrew R Stewart 

So could a guy with experience using a grinder, a chasing tool and a hand facer. Mark off the width edge needed with masking tape. grind away bits of the edge with a grinder getting close to but not on the tape edge. Clean out the shell of grinder crap and chase the threads. There might be some fresh cutting done deep inside the shell depending. Then a simple facing up to the tape edge. I have done this a few times back when i had a shop that sold Specialized. So if you get a Specialized MtB that is speced with a 73 shell but has a 68 shell blame me. Andy.

Andrew R Stewart 

++1. I never heard of a rider who didn't get up the hill or past the finish line because of flexibility. But i have known many who didn't finish because of being too worn out or beat up by the ride. Andy.

HillRider

Would you remove all 5 mm from the nds, all 5 mm from the ds or 2.5 mm from each side?

Andrew R Stewart 

Well that depends... If i was doing my own bike I would do whatever made the chain line and clearances work. But a customer's bike would only be a centered situation. Andy.

FastJake

I thought about trimming the shell, but I don't feel like going through all that.

Yes, the square taper crank that's on there works just fine. Would an external bearing crank change the way the bike rides? Probably not. But I love the bike and I love the awesome design of Shimano's 2 piece cranks. They are elegantly simple compared to square taper. And I like how they look.

Kimmo 

/shudders

I know that stuff works, but... yuck! 1-piece cranks in particular (with the notable exception of LOOK's wild proprietary number) are just so agricultural.

A SRAM external BB crank may eventually fail at the spline, and the bearings likely won't last as long, and that little sleeve is a PITA when it doesn't stay in the bearing when you try to remove the crank, and you'd struggle to name a less performance-critical major component of a bike anyway, but I don't care - the stiffness to weight and the aesthetics are worth the price, for mine. Each of my bikes is rocking Rival cranks, one new, one s/h... I'll see how they go.

I've often found I get better life out of my stuff than average due to my low weight and careful wrenching (for instance I haven't had a single issue so far running a couple of 16/20h wheelsets); my commuter looks like a 10yo race bike with a light on it.

I'm pretty sure the difference I've felt swapping out square-tapered cranks for ext-BB ones wasn't all placebo, and I'm only 65kg.

I think you feel the pedals are more rigidly part of the bike, and the stiffness makes more difference through the pedals as attachment points rather than to the very slightly improved power transfer, if you see what I'm getting at.

We're talking infinitesimal movement here of course... you feel slightly less flex than you'd expect landing a jump or whatever, and it just contributes to an almost subliminal sensation of quality and performance, which colours your perception of the bike in the opposite way a creak does. IMO this placebo factor is not to be sneezed at.

As for power transfer, maybe you could say that the movement may be small, but it's repeated 90 times a minute, and the more power transferred, the more is wasted, in a non-linear fashion... it's obviously a tougher argument to make that anyone under 80kg or so will really notice better transfer or just assume they have.

But anyway, there's simply no question a 24mm pipe > a solid rod of much smaller diameter.

dsaul

Trimming the BB shell would get a 68mm bottom bracket to work, but you should check the crankarm clearance if you're going to use a road crank. They are narrower than MTB cranks and may hit the chainstays. In any event, it's no loss if you trim the BB shell because most external bottom brackets come with spacers to make them 73mm for use on mtb cranks with a 68mm BB.

Garthr

To me the issue with external bearings is it's an afterthought . Why put the bearings on the outside when they can be inside ? Hence, carbon frames are now designed just that way, designed arounf the bearings being inside. But external bearings to me are like a retrofit, not OEM. Original-Equiptment-in-Mind

Of course, now there are all these differing standards for BB's ! It's rather crazy really. Every Manufacturer seems to want to make their own standard. Some of these standards will fall away, leaving the frame owner SOL or limited in BB selection. Square Tapers will be around forever.

joejack951

There are good reasons for putting the bearings outside, whether there are in threaded cups on a standard frame or pressed into a BB30 frame. While external bearings may not have been intended for a frame made in 1990, I'm fairly certain any carbon frame made today with a threaded bottom bracket shell is intended to use external bearings.

You do realize there are two common and a third less common square taper "standard" out there?

https://sheldonbrown.com/bbtaper.html

So many standards! And that doesn't even begin to address the various issues with spindle lengths (symmetric and asymmetric) and bottom bracket shell threading options (British, Italian, French, Swiss, all of which affect square taper BBs.

BB30 uses standard industrial bearings. Definitely zero chance of those going out of existence.

HillRider

For one thing, internal bearings limited by the 1-3/8" (34.9 mm) internal diameter of English bottom bracket shells so the bearing can't be larger than will fit in them. One of the downsides of using a larger diameter spindle like Octalink or ISIS was that the bearing had to be smaller to keep the same overall diameter. External bottom brackets eliminated that restriction.