Steve B.

I’m looking at a new Emonda, has an Ultegra Di2 system, also a 52/36 crank. I would prefer a 50/34 crank, not sure that’s a swap the shop wants to do for me, so I might just buy an 50/34 Ultegra crank. Is this as simple as removing old crank, installing new, sliding down and positioning the F derailer, checking and adjusting the shifting ?. Can the 50/34 use the same bottom bracket as the 52/36 ?. Trek specs says it’s a Praxis T47 threaded if it matters,

roadcrankr

Sounds like a straight swap to me. And moving the FD down a few millimeters.
You can recover much of the cost by eBaying the 52/36 as a new take-off, as long as you avoid putting any miles on it.
Another option worth considering involves obtaining a cassette with an added tooth or two.
I recently went from 11-28 to 11-30 and it gives me a huge leap in the low gear. Love it.

Kontact 

You can't just slide the derailleur down because there is a tension screw behind it. Follow the installation directions and you'll be fine.

Steve B.

I’ve 2 Di2 systems so know the drill. As well and to roadcrankr, it’s a 12 spd with the 11-30 cassette, so only option is an 11-34 cassette. Shimano would do well to make a 12-25 I. 12 spd.

choddo

You can just replace the chainrings surely, not the whole crank?

veganbikes

That is odd the shop wouldn't do it for you? If you are talking they won't give you a free crank then sure, it would be an oddity for someone to do that However they would do the labor and might do some swapping where you pay the difference type of thing. In the end though it would be a pretty simple swap and some front derailleur adjustment should you wish to do it yourself.

You shouldn't need to swap the B.B. unless you want a different B.B. but honestly Praxis makes fine quality stuff so I wouldn't worry about it.

SoSmellyAir

An 11-speed Shimano 12-25 cassette has the following cogs: 12, 13, 14, 15, 16, 17, 18, 19, 21, 23, 25. The gaps are already very close; what additional cog would you add for a 12-speed version?

Yes, but buying both chain rings by themselves is often barely less than buying the entire crank with chain rings installed.

Steve B.

[QUOTE=SoSmellyAir;23180608]An 11-speed Shimano 12-25 cassette has the following cogs: 12, 13, 14, 15, 16, 17, 18, 19, 21, 23, 25. The gaps are already very close; what additional cog would you add for a 12-speed version?

20. I use an 11 spd. 14-28 currently, also know as a Junior cassette. I think Shimano may have discontinued this in the 12 spd. systems. It goes, 14-15-16-17-18-19-20-21-23-25-28. It’s a really nice cassette for somebody not racing, especially on the standard 50/34 crank as you can sit on the big ring all day.

Steve B.

Had not gotten to a point of asking the shop, will probably end up going that route. Depends on the cost. I was really just questioning would I need a different b-bracket and I didn’t think so.

veganbikes

No you shouldn't need a different bottom bracket for any reason unless this is some really odd OEM Shimano crank specifically for Trek that nobody has mentioned but that would be a strange thing and highly doubtful so you should be alright. The crank will cost what it does but they may if it is still a new bike comp labor or like I said they may give you the difference to get a new crank.

choddo

99.9% sure it's the same T47 IB Praxis BB - 24mm standard Shimano spindle that my Trek Checkpoint has.

My recommendation if you're going to do it yourself - get the Praxis T47 tool, not the Park Tool one. It has a special chamfering on the teeth which narrows towards the diameter of the bearings whereas the park tool one has a big flat face which nicely scratches your paint around the bottom bracket unless you put a rag over the BB. And with such a thin area to grip, a rag makes it harder to engage the tool.

It also has a nice 24mm (or 22 if you flip it) protrusion which keys it in place so it won't easily move and I expect makes it easier to tighten it up to 40Nm without it slipping off (which is what the Park Tool one also did for me - I've now bought the Praxis tool but not tried it yet)

Kontact 

Weird answers. Of course he doesn't need a new BB to swap one current Ultegra crank for another current Ultegra crank. It doesn't matter what kind of bike it is - it is already set up for the Ultegra crank.

choddo

Haha. I know. I don’t know why it entered my head that he would need to even touch the BB, sorry for mentioning/confusing. (But for future reference, OP, use the Praxis tool if you do)

SoSmellyAir

I was riding an 11-speed 12-25 cassette and went to an 11-speed 12-28 cassette instead. The 18T was nice to bridge the gap but not as useful as having a 28T as a bailout cog. Given that I could manage without the 18T, I am not sure that a 20T would be that useful between the 19T and 21T cogs.

Russ Roth

Not sure which shop you're buying the bike and crank from but I can't imagine any of the ones I know would charge you for installing and adjusting the front derailleur, unless you're buying the crank online. But check with the shop, ones I've worked at have done swaps for a small up charge in exchange for the old crank. Might not benefit you as much financially, might do better selling the "old" crankset, but for ease and convenience it can be the better way if the shop offers it.

Steve B.

The near straight block gearing is very useful on mostly flat terrain like Florida, Delaware and Long Island where I live. The 28 cog gets me up about any of the local hills. If I am riding upstate, I take my grave, bike which has a 46/30 and 11-34. I would be riding this bike mostly local.

SoSmellyAir

I agree that straight block gearing (i.e., a single tooth gap) is nice for maintaining cadence but at a certain point (i.e., past the 17T cog for me personally) there is diminishing returns. For example, having a 20T cog between the 19T and 21T cogs gives two consecutive shifts having only a ~5% difference in gear ratios:

Bicycle Gear Calculator (gear-calculator.com)