I'm contemplating increasing my road bikes gear range. I have a race coming up that has some significant climbing. My question is if I want to increase the teeth on my rear cassette will that be limited by the type of gears I have at the front? My set up currently is a Shimano Claris FD 2403 front mech, Shimano HG50-8 11-30t cassette. Would I be able to increase the cassette to 32t or more without it being incompatible with the front mech?
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Stay with an 8 speed cassette - anything else will require a new brifter/shifter.– Criggie ♦Commented Mar 8, 2019 at 21:22
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If you can't get a bigger big cog on the back, the other option is to fit a smaller inner gear on your front chainring. Again, this is limited by various things, including cost!– Criggie ♦Commented Mar 8, 2019 at 21:23
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What model rear derailleur does your bicycle have? With that information, it's possible to calculate whether or not you can safely use a 32t or more cassette.– Andrew HenleCommented Mar 8, 2019 at 22:01
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I run a bontrager triple crank, the large chainwheel having 53 teeth. My rear drive is Shimano Claris, SGS cage. Cassette is a SRAAM 8 speed 11-32. This set up works great. My B-adjust bolt is one or two threads shy of fully in/tight. I don't ever run the large-large combo, but the derailleur makes the shift smoothly– JeffCommented Mar 10, 2019 at 6:34
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2 Answers
Front derailleurs don't care what size cassette you have. However, the front rings do affect the size of cassette you can run, because rear derailleurs have a total capacity specification. Total capacity is a measure of how much chain slack the derailleur can take up. The capacity required is: (difference in cassette large/small sprocket tooth counts) + (difference in chainring large/small sprocket tooth counts). Note the difference is what counts, not the actual sizes.
If you have a FD-2403 front derailleur you must have a triple setup, so presumably you have the RD-2400-GS (long cage) rear derailleur and 50/39/30 crank. The derailleur has a max sprocket size of 32 and total capacity of 41 so a 11-32 cassette will work fine ((32-11)+(50-30)=41).
People have expressed opinions on this site that running a cassette with a large sprocket 2 teeth larger than what Shimano states works OK, but you can't violate total capacity. Doing so risks shifting into large chainring-large sprocket and snapping your chain and/or ripping your derailleur off.
If you do fit an 11-32 cassette you may need a longer chain length. You will want to run a new chain with the new cassette anyway.
Update: If you really want to run a bigger cassette you could put an 8 or 9 speed MTB derailleur on. Shimano used the same derailleur actuation ratio for road and MTB 7, 8 an 9 groupsets (and road 10). Just as an example, an Acera M360 8 speed derailleur (from 2008, but still available) has a total cap of 43 and max sprocket of 34. (Specs can be found in this).
answered Mar 8, 2019 at 21:55
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ripping your derailleur off Which then finds its way into the spokes of your rear wheel, destroying that too. Commented Mar 8, 2019 at 22:03
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Oh man great answer thank you so much! If I want to go up to a 34t would just changing the derailer fix that issue?– AJ S.Commented Mar 9, 2019 at 1:10
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Your cassette won't affect your front mech. You may need a longer chain, and your rear derailleur might not be designed to handle the larger cassette, but that's another question.
Also, the difference between a 30-tooth and 32-tooth gear is barely noticeable. Go for a 34-tooth low gear.
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Is the asker likely to have derailleur compatibility issues with a 34-tooth cassette? Commented Mar 8, 2019 at 19:08
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According to Shimano a 34 tooth cassette requires a medium-cage RD, a short-cage RD handles a 32 cassette.– CarelCommented Mar 9, 2019 at 8:44