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50/34 172.5mm -> 52/36 170mm - any reason not to make the change?
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I'm in the process of upgrading my road bike from 8 to 11 speed, and while searching for parts, I came across a good deal (ETA: not half price, but at 2/3 of list price) for a new FC-5800 52/36 170mm crankset

I would be hesitant to use it for the road bike (currently 170mm 50/34 front, 11-28 in back), as when not in peak shape, I need easy gears for climbing (and I want to keep the cassette ranges the same across bikes for easy wheel swaps).

However, it could be great for the TT bike (currently 172.5mm long 50/34 front, 11-28 back), and I could swap the current TT crankset to the road bike (assuming no issues with cornering etc.)

TT bike with 170mm 52/36 pros:
  • on the dumb trainer with virtual power, I'm happy for the larger small ring
  • when racing, I'm usually in rather good shape, so not worried about climbs (had done my favorite climb with 42x28 after my A race last year, and set a PR)
  • a more open hip angle can only be good as I understood it
  • currently I'm averaging ~95 cadence in races, so going a bit lower shouldn't be a problem

cons:
  • even in races, I'm riding using the middle of the rear cassette rather than the small cogs, so it could be an overkill

Thus I think it buying the 52/36 is a good choice.

Is the above reasoning sound? Or I'm overlooking something? Thanks!
Last edited by: bableves: Jan 17, 20 7:49
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Re: 50/34 172.5mm -> 52/36 170mm - any reason not to make the change? [bableves] [ In reply to ]
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if you're not using the smaller cogs, I see no reason for buying bigger chainrings
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Re: 50/34 172.5mm -> 52/36 170mm - any reason not to make the change? [bableves] [ In reply to ]
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Just so you know your gearing on 170mm cranks is 1.01 percent higher because of the shorter crank, and then 2 teeth on 50 is 4 percent higher gearing so overal in the big ring you are going to 5% higher gearing in the same cog in the rear. On the 34 to 36 it is going to by 7% higher gearing. So your lowest gear got 7% bigger and your highest gear got 5% bigger.

Other than that, its a fit issue.
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Re: 50/34 172.5mm -> 52/36 170mm - any reason not to make the change? [bableves] [ In reply to ]
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How much is the new crankset?
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Re: 50/34 172.5mm -> 52/36 170mm - any reason not to make the change? [jimatbeyond] [ In reply to ]
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jimatbeyond wrote:
How much is the new crankset?

75 EUR for the discounted vs 115 EUR list price. Need to update the original post that it's not actually half price, I was comparing to another model/site probably or the discount sticker influenced the math in my brain.
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Re: 50/34 172.5mm -> 52/36 170mm - any reason not to make the change? [jollyroger88] [ In reply to ]
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thanks - yes it is not a case of actually needing bigger changes, but coming across a sale where the chain ring happens to be bigger :)
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Re: 50/34 172.5mm -> 52/36 170mm - any reason not to make the change? [devashish_paul] [ In reply to ]
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thanks! I only considered gear ratios, didn't expect the difference in crank length to reach 1% (as have been using 170, 172.5, 175 on various bikes without really noticing)
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Re: 50/34 172.5mm -> 52/36 170mm - any reason not to make the change? [bableves] [ In reply to ]
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I would get FC-R7000 with 160mm arms and 50x34 chainrings for the TT bike.
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Re: 50/34 172.5mm -> 52/36 170mm - any reason not to make the change? [jollyroger88] [ In reply to ]
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Bigger chainrings are supposedly more efficient, even if it results in cross chaining.

You’ll need to adjust your saddle height slightly when you switch to shorter cranks.
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Re: 50/34 172.5mm -> 52/36 170mm - any reason not to make the change? [bableves] [ In reply to ]
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Go with 50/36. The perfect compromise. It's what I have on all my bikes.
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Re: 50/34 172.5mm -> 52/36 170mm - any reason not to make the change? [bableves] [ In reply to ]
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Change your gearing and your crank set up because you think it really makes sense, not because you can bag a bargain and save yourself a measly €40!
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