Changing cranks from Praxis to Ultegra (BB advice)

Question

I purchased a Specialized Roubaix Comp 2018 last year, I was not intending to purchase this bike but Evans had a 40% off sale and the deal was too outrageous to pass. I was originally looking at bikes with Shimano 105 groupset because I was given a friend's old stages 105 left hand side power meter. This power meter has been sitting around for months and months. I recently got an indoor trainer and have started 'properly' training. Reading power has been a big step up in my understanding of pacing and I'm very keen to get a power meter on the bike for when I'm out and about (once the world lets us out of lockdown...).

My problem is that I have no idea if my bike will take an Ultegra crankset. The currently fitted crank is a Praxis Zayante. The bottom bracket is not explicitly stated, most sites say "Included w/ crankset" (source: specialized).

Looking on the bike itself, the BB has external cups and say M30, so it looks like the BB is the M30 THRU BSA Threaded.

Now I understand very little about bottom bracket standards and honestly reading about them has confused the situation for me more...

Do I simply just unscrew the Praxis and fit a Shimano Ultegra BB such as the 6800?

Will my frame fit a different bottom bracket? I could not find any numbers for my frames BB sizing. Do these bikes adopt a standard or are they all slightly (stupidly) different?

The Shimano BB looks very cheap, this is a luxury/performance bike and I'd expect such a crucial part to cost a little more, are there better (less creaky) options than the vanilla BB for not much more?

Authors note: I recognise an Ultegra crank with 105 LHS is scuffed but cheaper than getting a 4iii for my Praxis (which is seemingly unavailable)

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Edit: Here is a picture of the external cup on the LHS

Answer 1

If it is an M30 Thru BSA, then the cups will say 'M30 THRU'. As they say M30 and only M30 then they are the M30 BSA version. The latter suits crank spindles which are 30mm drive side, 28mm non-drive side, as the Zayante crank is.

If they say anything else (like press fit) then (future readers) can match that to other models on the Praxis website before proceeding. For example, it's possible to get thread-together units for pressfit frames which don't look that different but have different wording on and are not BSA.

If BSA is confirmed, then the frame is threaded to the BSA standard. As the Shimano spindle is a smaller diameter (24mm) then yes you would need to change the bottom bracket, but this is trivial with a BSA threaded frame. That is to say, yes you unscrew the Praxis and screw in a Shimano.

It does look like the Praxis BB may require a different tool to the Hollowtech II items, but the shop can check this and take care of it, maybe make them aware beforehand to avoid delay if they need to get a tool in. I think having the shop do the whole job is a fair idea, it represents minimal risk to your wallet, even if parts are more expensive at retail.

Happily you can mix and match the Shimano BB, new chainset and 105 LH crank.

I know of no reason to distrust Shimano BBs; the more expensive ones are lighter and smaller items which might make them look less 'rugged' perhaps, but they are tailored to perform a certain way, i.e. light and racy.

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If the bottom bracket cups say anything additional to "M30" then it is a different model. As Weiwen Ng has sleuthed in a comment above, it seems 2018 Roubaixses may have either BSA threaded, or pressfit bottom brackets, so measure twice, order once if reading this in the future.

Answer 2

From the picture of the left hand side BB cup comparing it to pictures on the Praxis bottom bracket web page here I think it's certain you actually have a ISO (aka BSA or English) threaded bottom bracket shell. This can be confirmed by looking at the drive side cup which should have an indicator for the direction to tighten it. Some Praxis BBs fit in press-fit shells but still thread together, the right hand side of these have no tightening direction indicator as they do not rotate in the frame.

If you you want to switch to a Shimano crank this makes it very easy for you. All you need to do is remove the Praxis BB, install a Shimano Hollowtech II Road external bearing BB and any Shimano road crank will drop right in.

You mentioned in comments that you were thinking about a Hambini BB. I don't believe he makes BBs for threaded shells (although I seem to recall he made a custom one for someone and it was featured in a YouTube video). Also, having a threaded shell means you don't need a Hambini BB - the point of having one seems to be to overcome issues with press-fit shell designs.