3

On my bike, I currently have a 50 x 34 crankset:

enter image description here

I would like to mount bigger chainrings: I was thinking about a 53 (or a 52 at least) as the bigger one.

I see that my BB has "36 x 24" written on it. Is it possibile to just replace the chainrings without having to replace the BB? Or in my situation I also have to replace it?

Thank you, and sorry if it is a trivial question, I'm really a newbie in these mechanical aspects.

enter image description here

Improve this question
3
  • 2
    Just a comment, but do you really need the bigger chainrings? If you spin up to 110rpm cadence the 50t chainring should get you all the way up to 65km/h. I only spend a few seconds at those speeds on my rides (and even more rarely do I have the desire to push down on the pedals to go faster). Much more often would I like an easier gear, especially for easy rides or bikepacking. Gear calculator: ritzelrechner.de/…
    – Michael
    Commented Aug 19 at 10:25
  • 1
    What speed are you travelling in 50:11 that you feel the need for another gear?
    – Criggie ♦
    Commented Aug 19 at 10:33
  • Thank you also for these considerations: in fact, I'm also wondering if I really need a bigger chainring. The main reason for which I'm planning the upgrade is that I do some races on flat circuits, and I'm feeling a bit "stuck" when approaching minimal descents (example: -1%). In these occasions I start "milling" the 50x11, but having the feeling of not reaching the maximum, possible speed. Since pro racers also use 54+ gears for sprints, I wonder if I could gain extra speed with this upgrade.
    – Andrea
    Commented Aug 19 at 11:58

1 Answer 1

Reset to default
4

The bottom bracket has no effect on the size of the chainrings you can install. The 36x24 (commonly known as BSA) merely indicates the thread size.

Generally the size of the chainrings you can install only depends on the pitch (or bolt) circle diameter (PCD/BCD) and number of bolts (or arms).

For good shifting performance they also come in matching combinations. 53 teeth big ring with 39t small ring, 52t with 36t, 50 with 34

You can install 53/39t Ultegra R8000 chainrings without issue.

Improve this answer
5
  • Just a heads up, 4-bolt chainrings won't install on that crank because it's a 5 bolt version.
    – Weiwen Ng
    Commented Aug 19 at 12:08
  • Thank you, Michael. Sorry, I just recognize an error in my question (I just edited the title). Despite the BB, I was also curious about the crankset. In fact, I heard that - for some chainrings' upgrade - you need to replace the whole crankset, while in other situation you can just replace the rings. Sorry for the confusion, in Italy we have totally different names for these components :)
    – Andrea
    Commented Aug 19 at 12:09
  • The PCD only affects how small ring you can install, since we're going big, it puts no physical limitation. The small/big combination is not just about tooth count. Shimano has a marking system to identigy compatible small/big rings. But if you buy a pre-packaged set, then it's all good. Note that if your current drive train is old, you may have to change the chain and cassette too to ensure smooth operation. At which point you can also consider getting same sized chainrings and a smaller cassette instead.
    – Torben
    Commented Aug 20 at 12:07
  • @Torben: True, there are also a few other details, for example asymmetrical chainrings (i.e. bolts not evenly spaced). Or special shapes like what Shimano does for their big rings which can introduce incompatibility even if PCD and bolt number matches. I didn’t want to make the answer overly complicated. For OP’s Ultegra FC-R8000 crankset all that matters is that they can upgrade to Ultegra 53/39t chainrings.
    – Michael
    Commented Aug 20 at 12:16
  • 1
    Here's something to consider: Cost of R8000, Ultegra chainrings $140 (52/36) to $160 (53/39). Cost of R7000, the 105, crankset with 52/36 or 53/39 chainrings can be had for as low $120, but typically sell for between $140-160, depending on the length of crankarms you desire. The weight penalty between Ultegra and 105 cranksets is 60-75 Grams depending on chainring configuration (larger difference btw models with 53/39 rings). Availability online is still relatively decent. It's much less fiddlely to swap an entire Hollowtech2 crankset than chainrings on one, tho neither task is difficult.
    – Jeff
    Commented Aug 30 at 21:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.