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Old 08-31-08, 10:23 PM   #1
AndyG
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Rear Sora compatibility with bigger cassette

Newbie here. I recently bought a Marin Portofino road bike. I discovered it was lacking in hill climbing ability due to the gearing (12-24 8-speed rear). So I bought a larger cassette (11-32), without realizing that the max capacity on the Sora rear derailleur was 27t. This bike uses the (sorry I don't know the correct term) shifting apparatus where you push the brake lever levers inward to shift. I searched Shimano's website to see if I could find a compatible rear derailleur that would accomodate the larger sprocket on the cassette (32t maximum), but the largest I could find for any of their road products was the same as the Sora (27t). I'm sure there's a good reason for that, but I don't know what it is.

In the MTB section of their website I did find derailleurs that would accomodate the larger sprocket sizes (like the Deore LX line), but I don't think they are compatible with the road-style shifters on my bike (I'm not sure about that though). I'm under the impression that the road-style shifters on my bike require a type of derailleur that works with them, and they are different than MTB bikes use.

I'm hoping someone with some expertise on the forum here can give me some advice/guidance as to how I can best accomplish making my bike more hill-worthy. This whole cassette / derailleur issue is confusing (and expensive).

Thanks.
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Old 09-01-08, 01:45 AM   #2
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All indexed Shimano rear derailleurs and rear shifters (except pre-9-speed Dura Ace) use the same cable pull ratio; so they are compatible. (It's the front derailleurs that are not perfectly compatible.)

You're on the right track, get any Shimano MTB rear derailleur rated for at least a 32T max cog size (which is the vast majority).

Another thing: if you want to keep the same orientation for the rear shifter (i.e., brake lever for down-shift, thumb lever for up-shift), then get a Rapidfire (traditional, high-normal) model vs a Rapid Rise (low-normal) rear der.

Lastly, you should have got a new 8-speed chain when you changed out cassettes (or added extra links if the chain was new enough). Size the chain like this: http://sheldonbrown.com/derailer-adjustment.html#chain .
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Old 09-01-08, 06:59 AM   #3
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That bike has a 30T front chainring, even with that on the 24 rear you couldn't do it?

IMO a 32T rear gear is massive overkill on a road bike, I can't imagine the kind of hill you would need to go up on the road. However my Sora equipped bike had a 28T rear gear that worked fine and would probably be enough. That cassette you got is probably even a mountain bike cassette, which you do need gear like that to get up a hill through the dirt.

I have the Shimano 11-28T rear cassette sitting here it has 200 miles on it, if you want to return that one and take mine off my hands for like 10 bucks plus shipping this may solve your problem fine (and be cheap). My bike climbed very well with it, I just didn't need that much gear around here so I went with a 12-25.
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Old 09-01-08, 09:16 AM   #4
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Thanks for that JT. Based on those specs it looks like the Shimano Deore LX RD-M531 SGS should work. They describe it as traditional "top normal" (not high normal, but I assume it's the same thing), and it is the rapid fire. I did neglect to get a new chain with the sprocket, so I'll have to add that to the list.

RL - I agree the cassette I bought is overkill for 98% of my needs, but I have two things working against me. One is, I'm old. Two, I want to use the bike to commute to work and I've got one short but very steep hill. I barely made it with stock gearing on a test ride over the weekend, and ended up very sweaty by the top (and that's in riding shorts and wicking shirt). In street clothes, with our morning humidity, it'll be worse and I don't want to stink the place up since I don't have anywhere to change.

Thanks both for taking time to respond.
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Old 09-01-08, 10:18 AM   #5
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Originally Posted by AndyG View Post
Thanks for that JT. Based on those specs it looks like the Shimano Deore LX RD-M531 SGS should work. They describe it as traditional "top normal" (not high normal, but I assume it's the same thing), and it is the rapid fire. I did neglect to get a new chain with the sprocket, so I'll have to add that to the list.

RL - I agree the cassette I bought is overkill for 98% of my needs, but I have two things working against me. One is, I'm old. Two, I want to use the bike to commute to work and I've got one short but very steep hill. I barely made it with stock gearing on a test ride over the weekend, and ended up very sweaty by the top (and that's in riding shorts and wicking shirt). In street clothes, with our morning humidity, it'll be worse and I don't want to stink the place up since I don't have anywhere to change.

Thanks both for taking time to respond.
I totally hear ya, I'm just thinking give something that will fit a shot before buying a derailer and spending more money, you would be surprised how much difference there is even with only 4 teeth, I know because I have had both. I'm young and there are some hills that kick my ass on my 30t front 25t rear, the 28 was MUCH easier when i had it. I changed to the more aggressive gears since most of the riding I do is flatter and I wanted a beefier midrange. Plus probably in about 2 weeks once you get used to it and built up a little (even though you're old hehe!) that hill will probably be nothing to you. JMO...
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Old 09-01-08, 10:20 AM   #6
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I agree the cassette I bought is overkill for 98% of my needs,
So how good is a levy that's overkill for flood protection over 98% of it's length? I guess you could wait a couple of weeks for the river to go down.
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