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I have an old road bike with 53-39 rings and 13-23 cassette. Although it's generally flat in my area, I like to get to the top of these hills where they put the radio antennas (without dying). Max slope on these is 15%, average 8% and I suffer a lot.

I have spare cogs, rear mech is Shimano RX100 and thought I can fit a 26T in the cassette Which cog should I remove from my current setup (13-14-15-16-17-19-21-23)? I guess either the 16, the 21 or the 23?

I'm already assuming body fitness can be improved ;) Thanks!

Edit: here are the gear ratios http://gears.mtbcrosscountry.com/#700c/23I61I3909X700c/23I61I193X700c/23I61I15271X700c/23I61I15275


Report back (edit 2): I set 13-14-15-16-18-21-23-28 today. I just realized I have a 24 I could swap for the 23 to make a fairer gap, and I still have the 26 available. There is room for the 28T and didn't touch the B-screw. For now, it shifts fine standing, I have still to ride test it. I'll get back.


Conclusion - edit 3 :) Went for a small ride with climb included. Setup is still very close to what it was, introducing the 18 as Grigory said and leaving room for a life-saving 28 (thanks Criggie for the tip).

Shifting is good, I didn't notice much change, but as ChrisH and Criggie pointed out, it probably is a bit irregular on the new cogs. So I will eventually look for a complete cassette. Anyway, many thanks to all. I'll accept Grigory's answer as it addresses the original question with a better alternative to do what I intended.

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    15% is very steep for 39/26, watch you knees. – mattnz Jan 12 '18 at 19:33
  • @mattnz Thanks, I never thought it may not be healthy. Which gear ratio is safe to use? Mind the whole climbs are not very long, less than 1km at 8% average. – GiantChicken Jan 12 '18 at 20:06
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    I personally have RX100 and it takes a 28 tooth on my road bike. YMMV but 26 tooth should be fine. A test fit will confirm given you already have it. – Criggie Jan 12 '18 at 20:57
  • Once you're finished testing by cobbling together what's on hand, then I'd just replace the entire cassette with a 12-28, and a new chain. Maybe even new jockey wheels and a recable too. – Criggie Jan 12 '18 at 21:00
  • @Criggie Glad to know I can go up to 28! That is the way to go if I am to face some climbing. Thanks! – GiantChicken Jan 12 '18 at 21:53
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You can remove two cogs 17, 19 and replace them with one new 18, and then add 26 to the end of the cassette. That would allow you to keep more evenly distributed gear steps in the middle. That is how certain modern 11-speed MTB cassette extenders are sold: along with the biggest 46 teeth cog they also pack something around 18 teeth. So this way two cogs go out and two go in.

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  • Thanks! This sounds like the perfect option to go. I'll look for 18T cogs. In the meantime, should you change only one, which one would it be? – GiantChicken Jan 12 '18 at 17:41
  • I'd change the front rings to 52-36 though. Let it run through your gear ratio calcuator! And you won't mess up the shift-ramp system. – Carel Jan 12 '18 at 18:49
  • @Carel Thanks for the tip! For now, cassettes are cheaper to mess with (would have to change whole crankset). – GiantChicken Jan 12 '18 at 19:51
  • Cranks from the time RX100 existed have 38 tooth minimum (Shimano, Campagnolo had either 39 or 42). – ojs Jan 12 '18 at 20:15
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    @Carel It's 130, so this crank can only go to 38 (don't think there is much difference from current 39 to 38).I've seen double-to-triple 130BCD adaptors that provide a 74BCD extra place. It would be interesting to see if this could be used on the first ring to provide a lower BCD for the second ring. I know it'd probably be easier to just buy a compact, but I'll have it in mind as a future experiment. – GiantChicken Jan 14 '18 at 11:59
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The answer in terms of ratios might not be the same as the answer in terms of being able to shift. Taking out the 16 would be reasonable but then the shift ramps and pickup teeth wouldn't align. This issue will occur going from the 23 to the 26 whatever you remove.

You've got 8 gears in the back so I assume you've got indexed shifting. If you're using friction shifters it's not an issue as you can overshift then back it off.

Further reading from Sheldon Brown

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    Ahm! Didn't think of that. Yes, it's indexed brifters. So unless the ramps in the 26 matches the place after the 21 (swapping the 23), it's not going to shift? New cassette then? – GiantChicken Jan 12 '18 at 17:56
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    I like to experiment, so I'd probably try it, at least if it wasn't a matter of drilling out rivets. The shifting will be somewhere between almost impossible and as good as if you'd bought it with that range. We just don't know where on the spectrum it will fall. You know the effort and budget you're prepared to put in better than I do. With the problem shift being into the biggest cog your options are slightly different as you've got the limit screw to play with but can't take the strain on the shifter in the same way as at the middle, to encourage it to shift. – Chris H Jan 12 '18 at 18:52
  • If riveted, is it bad to file the rivets, provided the pin will stay in between the cogs afterwards? I did it on an MTB cassette and it is working ok. Anyway, I'll try and see what happens! – GiantChicken Jan 12 '18 at 19:47
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    The "rivets" were actually hex screws on all cassettes that I have seen. They are there only to help with installation, older Dura-Ace and some Campagnolo and Sram cassettes don't have them at all. – ojs Jan 12 '18 at 20:17
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    @GiantChicken it will still shift, but may take up to half a wheel revolution longer to get up to the next biggest cog. In reality it just feels a bit slower to shift, like a lethargic teenager. – Criggie Jan 13 '18 at 1:36

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