helixbikes

Forgive my n00b'ishness:

I'm trying to build a clone of https://yeticycles.com/bikes/arc - but am confused about one aspect: The ARC uses a SRAM DUB BB92 to fit the SRAM X1/XX1 crankset - which has a 91.5mm shell width, with boost spacing. For reference, I build with bamboo, and have built all my frames so far as BSA68's, but had issues with fitting larger tires.

I went shopping for where I could buy a BB92 shell, and it seems like no one sells pressfit shells. (I assume this is for a good reason, but can't figure it out): Nova, Paragon, Frame Builder Supply, eBay, TiCycles, BikeFabSupply don't appear to stock them. However, It seems like T47 shells are pretty easy to come by in this width (92.5mm, to allow for facing). My concern is that SRAM doesn't list T47 DUB as supported for X1/XX1, but I can't see why not: https://www.sram.com/en/sram/models/fc-xx-1-c2 mentions only:

* DUB:BSA73
* PF89.5
* PF92
* PF30 MTB73
* BB30 MTB73
* BSA MTB68SP

I assume based on this that I should probably just buy a 73mm shell to get the most space for the chain stays (because I also have to watch out for pedal strike), but I'm just genuinely curious - why are press-fit shells unavailable for purchase?

I have 3 ideas, but I'm not convinced of any of them: alignment is difficult, no low-volume builders use PressFit, or PressFit shells are so easy for folks to create out of their DIY frames that selling shells isn't economically viable.

Darth Lefty 

I was thinking about this. My Calfee 29er kit came with a 68 bb shell. I wanted to make it 73mm and QR 141. The bent stays help. To get tire clearance modern MTB frames have a yoke, but I don’t know how to do that with bamboo.

You could add to your list that they’re going out of fashion.

Salsa Timberjack 29er / 27.5+ titanium shown to illustrate the drive side yoke, and the extreme bend in the NDS

unterhausen

It's a real pain to get them right and very few builders want to take the risk. Big companies don't care if they have to eat a few frames because the press fit bb is bad. And they have the market power to just ignore customers that have problems with them, if the internet is to be believed. No small builder has that luxury. Seems like there was a time when someone offered a reamer/facer for a press fit bb, but a number of people got burned in that deal.

I have been doing a lot of frame prep recently and i was thinking about how you could do it with machine tools. I think I have heard of people using boring bars on a mill for press fit bb. Fixturing would be an issue, but it is doable

helixbikes

Thanks for bringing up yokes - I hadn't considered that before. I'm building a new Calfee kit, and plan to make it 73mm/BOOST. This time I have bent stays, but I guess I was just a bit nervous given how bad the fit issue was on my first build.

Paragon sells Yoke kits, but at an exorbitant price: https://www.paragonmachineworks.com/...stay-yoke.html

I did neglect to mention that they do sell PressFit shells, but not for any size compatible with an XX1 crankset. I guess I'll just end up going BB73, but I'm still surprised that there is no love for T47 for their MTB components.

unterhausen

lots of companies use a flat plate on the drive side instead of a yoke. I was always put off by the price of a yoke. I don't think the price is exorbitant though, it's a complex piece of machining.

duanedr 

I have done a Paragon 44mm HT's this way. It wasn't as hard/time consuming to indicate it in as I was expecting. I would clamp the DT with blocks for more rigidity and take light cuts. A bigger machine would make this much easier.

unterhausen

Did you flip the frame to do the other side, or come in from one side. I would like to be able to do it on my mill, but I think that will be tough because it has a fixed head and I don't think it has enough room to get the bike in there.

wsteve464

Not sure if this is what you are looking for but wheels manuf makes various BBs for the T47 flush and out board bearings.

https://wheelsmfg.com/bottom-brackets/t47.html

From looking at the Paragon site it seems that T47 has taken the place of the press fit BBs and they no longer make any PF bbs except PF30.

If you are looking for a wide BB Paragon has T47 up to 108mm wide, this can't be used with a Sram crank but some of the smaller vendors such as race face make cranks with a wider spindle which can be adjusted for chainline with shims between the crank and BB. Also the Q factor will be wider but it will give you more crank clearance.

Canaboo

I'm pretty sure Rock West Composites sells carbon PF BB shells in whatever size you need.

This one? https://www.rockwestcomposites.com/s...hell/45295-c92

Canaboo

There's a few pretty easy ways of getting around that by using high density foam as a core for a carbon yoke joined to the bamboo or you can aggressively bend bamboo with heat and retain most of it as the core before adding carbon etc over it.

duanedr 

I have one of the small Grizzly mills similar to what Andrew has. The HT was 90mm Paragon so that helped. I used a tube block on the TT near the HT and put it in the vice with the BB up in the air. Then I went at it from the bottom of the HT. I had to stop after doing the bottom seat to extend the boring bar further out of the boring head to get the top seat. It really wasn't that bad to set up with my smallish tools.

Serenity5

nova supply now has press fit bb's 69 and 73mm widths

Canaboo

Those are not finished to final dimensions.