1

I currently have a Shimano LG50 12-25t 8 speed cassette and am finding it a real struggle on steep hills - I am around 13 stone (82kg). This is the first time I have looked at changing the original gear ratios on any bike I have owned.

I want to change to the 11-32t or another cassette but I am wondering if I need a new chain i.e. to lengthen it and also if the rear mech is OK. I ask this as stuff on the interweb says both can possibly be an issue. I believe there may be normal and long leg versions.

The rear derailleur is the original Shimano Sora 8 Speed Rear Derailleur (GS Long Cage).

Current chain is the Shimano Altus HG40 6-7-8 Speed Chain.

Crankset is Truvativ Touro 52-42-30t.

Using the link below it says the rear derailleur will take 37T to 39T or less. So the current cassette is (25-12) + (52-30) =35T 11-28 is (28-11) + (52-30) = 39T 11-30 is (30-11) + (52-30) = 41T 11-32 is (32-11) + (52-30) = 43T

From this it looks like my only option is the 11-28, which would only give a minimal improvement. Am i wasting my time trying to fix this particular bike as it stands?????

Thanks in anticipation. Kev

Many thanks to you all for your responses. I hope I have learnt a lot around this area and it sounds like I was hoping to achieve too much. I have ordered a 11-28t cassette and new chain as this stays within the guidelines from Shimano for the Sora rear mech. I will be v interested to see the difference it makes.

As suggested by someone, I did a lot of shopping around for a slightly smaller 5 bolt chain ring than the 30T that's on there, but hit a massive blank in finding a Truvativ ring that was smaller. Thanks again to all......UR*s

  • The Shimano limits for range are very conservative, meaning that the 11-30 combination could work if the indexing set-up is properly done. Although you certainly need a longer chain. – Carel Oct 5 '18 at 18:51
  • You need a new chain because you have a new cassette as well as needing a longer chain to go around the extra teeth. – Criggie Oct 6 '18 at 3:22
  • 25 to 28 might not sound like a lot, but its still ~10.8% lower gearing, which is roughly the gap between your lower gears. Imagine being in second-biggest-rear-cog which is probably a 22, and having one more click to 25. Then one more click again to 28. Might be all you need. – Criggie Oct 6 '18 at 3:23
1

I think a disadvantage of the 11-32 cassette are the large gear steps. Also consider that switching from 25 to 28 already makes it 12% easier.

You could (additionally) install smaller chainrings, 52 is quite large and instead of 30 you could go down to 28 or 24.

You will need a new (longer) chain if you increase cog size, unless you decrease chainring size at the same time or the chain is currently a bit longer than necessary. A worn chain will work badly on new cogs or chainrings though.

1
  1. Yes, you would need a longer chain.
  2. Depending on how confident you are in your ability to avoid using the "wrong" gears, you can take derailleur chain-wrap capacity guidelines with a grain of salt. You never need to be in the 30/11 or 52/30 combinations, as those ratios are reproduced with other gear combinations, and it's generally preferable to avoid cross-chaining anyhow (going big-big or little-little).
  3. You could try getting a new chain and cassette and seeing how it works; if it doesn't work, you'd need to get a new rear derailleur. I'm a little unclear on the compatibility, but the Sora group has been upgraded to 9-speed, so I think you'd need to get a Claris derailleur instead (I'm happy to be corrected on this point). The current Claris long-cage does have 43 teeth of wrap.
  • Long cage will take 32 easily, even medium cage does. – Carel Oct 5 '18 at 18:54
  • 1
    Cross-chaining is nowhere near as catastrophic as putting a chain on that's too short to go big-big, snapping your derailleur off, and getting it sucked into your rear wheel, breaking about 10 spokes and crashing you out in the process. – Andrew Henle Oct 5 '18 at 19:46
1

The things you need to take into account are the rear derailleur's

  1. Max sprocket size
  2. Min sprocket size
  3. Total capacity (difference in sprocket tooth counts plus difference in chainring tooth counts).

For a 9 speed Sora RD-3300 GS (Source: [Shimano 2004-5 Line Up Chart][1]):

  1. Max sprocket size = 27
  2. Min sprocket size = 11
  3. Total capacity = 37

So technically you are exceeding the max sprocket size. You need a total capacity of (52-30) + (32-11) = 43. That's well in excess of the spec.

If you has a narrower range of chainrings I'd say you could get away with the larger cassette sprocket, but the required total capacity really rules it out. Yes, you could try to avoid the big ring - big sprocket combination but sooner or later you will try to change to that combination and possibly rip the derailleur off.

Basically, older, less expensive bikes with 7 and 8 speed systems used a derailleur designed to work with a narrower cassette range, and got wider spread of ratios with a triple chainring crank, so they are not up-gradeable to bigger cassettes

If you want lower ratios, look at replacing the chainrings for a smaller set., perhaps 48-36-26 instead of your big 52-42-30. I think you might need a new crank to do this as I believe the Truvativ Touro is 130mm BCD. Triple cranks are not expensive though.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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