Crankset & BB Question

[Justin Kennedy (Brooklyn, NY)]

I bought a mountain bike on Craigslist and have been using it with an Xtracycle Free Radical conversion for a few months as a kid hauler and cargo bike. The crankset and bottom bracket it came with are part of the Shimano Hollowtech group: BB is Shimano SM-BB51 and the crank is a Shimano XT two-piece compression thing like this with a built in spindle. I've been having some issues with the crank creaking and popping when I ride and would like to replace it with something simpler and more classic looking, like a Riv Clipper wide/low. 

Question is, do I have to replace the entire bottom bracket or can I use the SM-BB51 and install a new spindle? The current crankset has a spindle built in/attached.  Would this work? I'm kinda clueless when it comes to this Hollowtech stuff and not sure what's compatible/interchangeable. Thanks in advance. 

[aeroperf]

Justin—

To answer your last question first, no, that spindle won’t fit.
Any way you look at it you would have to replace the entire bottom bracket.
The crank creaking and popping is more than likely due to the bottom bracket itself.

If you want the cheaper solution, just remove the crankset, and replace the bottom bracket with a new BB51 or a Shimano BB-MT800.  These things are only about $25 and can be replaced reasonably easily with the right tools.

Are you dead set on the looks thing?

You need to remove the bottom bracket, and replace the bottom bracket with a square taper sealed unit like a UN-26.
https://www.amazon.com/SHIMANO-BB-UN26-Square-Bracket-68x113-mm/dp/B001EIAZ7A/r

If it is a dedicated mountain bike, it will need a 73mm sealed unit.  You will also need to find the right width spindle to keep the chainline at 50mm.  Somebody on the list with a  Suzie/Wolbis might help you find a spindle, since I’ve heard (but don’t really know for sure) that it has a 73mm bottom bracket shell.

[Justin Kennedy (Brooklyn, NY)]

Okay thanks for the info. I'm not equipped or experienced with bottom bracket replacement which is why I was hoping to salvage what's already there and not have to remove and replace it. But maybe this is an opportunity for me to figure it out. Will need to invest in some tools I assume.

[Benz Ouyang, Sunnyvale, CA]

Justin, I will suggest investigating what's causing the creaking and popping. Quite often, what appears to be from the crank may not actually be from the crank. Especially with an Xtracycle Free Radical conversion, there are more opportunities for creaks and pops. Unfortunately, troubleshooting such a thing is best done with a bit of experience, so you may need to bring it into a shop.

[Justin Kennedy (Brooklyn, NY)]

Thanks Benz. It's tough to tell exactly where the pops and creaks are coming from but it seems to happen only during hard pedaling and kind of feels like it's coming from the left (non-drive side) crank arm which is attached with hex bolts. The whole crankset has always annoyed me, mostly aethetically with this hi-tech look, but I haven't bothered to tinker with it too much.  You're right–the Xtracylce extension could be part of the popping and creaking as well since it's attached pretty close to the crank area. I'll try to do some further investigation especially now that I know a crank replacement will require more than just a swap-out. 

[aeroperf]

What Benz said.  It may be from somewhere else - the pedals, or even the seat.

However, a failure mode for Hollowtech is the bottom bracket creaking, and if you’re going to replace it, it is relatively easy.
If you’re going to take the old one off and replace it with another BB51 (or SRAM equivalent), all you need is something like a Park Tool BBT-9.  “like” because JTDEAL makes one for less than half that price, etc.
Save and reuse any spacers - https://www.youtube.com/watch?v=xZNJM1Hi7jc

They do tend to look like the chainrings were stamped out of old garbage can lids, but Hollowtech cranks do have a nice stiff feel.  I’ve put them on a Homer, and a couple of other bikes, because I like that feel.

[Benz Ouyang, Sunnyvale, CA]

On Tuesday, August 25, 2020 at 9:45:23 AM UTC-7 aeroperf wrote:

However, a failure mode for Hollowtech is the bottom bracket creaking, and if you’re going to replace it, it is relatively easy.
If you’re going to take the old one off and replace it with another BB51 (or SRAM equivalent), all you need is something like a Park Tool BBT-9.  “like” because JTDEAL makes one for less than half that price, etc.
Save and reuse any spacers - https://www.youtube.com/watch?v=xZNJM1Hi7jc

Hate to contradict but don't get SRAM BB cups. They are usually GXP which is 24mm/22mm instead of the straight 24mm that Shimano Hollowtech cranks require. In other words, Shimano Hollowtech cranks won't fit SRAM GXP cups, and vice-versa, even though they are visually similar.

I second aeroperf's recommendation of taking a closer look at the BB. Hollowtech cranks are easy to disassemble, with a simple 5mm hex wrench, a rubber mallet (or a block of wood), and the special tool TL-FC16 for the pre-load cap (usually comes with the crank and shops will have excess that they may give you for free). Once the crank is out, you can manually feel for bearing roughness and determine if the bearings and thus cups actually need to come out; if they do, then you'll need the BBT-9 tool. Do note that there are plastic sleeves separating the cartridge bearings from the crank spindle for Shimano-branded BB cups; there's no metal-to-metal contact. The plastic sleeves deform slightly to help in alignment, and they are typically what creaks, especially if the bearings are reluctant to spin freely.

Good luck!

[Joe Bernard]

You might want to let a bike shop dig into this unless you're committed to buying two types of BB tools and learning all about it. I'm big on the buying and learning, but it's not a small project for a first-timer and you're not even sure where the problem is yet. Could be a headache!

Joe "ok now I broke something else, this is getting expensive" Bernard 

[spencer robinson]

Sage advice Joe Bernard