Replacing old RSX chainrings and/or crankset

[esassaman]

My old RSX chainrings are worn out. I'm getting so confused by all the crankset/chainring options out there... help! What I'd like to do is gear down to handle big hills under touring load easier. The biggest cog on my current rear casette is 28T, and 26/36/46 chainrings (110mm BCD/74mm BCD standard Shimano road triple, apparently). I have an RSX 7-speed STI setup. Entire drive train is worn and needs replacing - casette, chain, rings. Big trip coming up.

Ideally I'd like a new SRAM PG-730 7-Speed Cassette (12-32T) and something more like 22/32/44 chainrings.

Problem 1. Rings are hard to find and I don't think I can get the size I want. Also everything I can find says "9 speed chain compatible" but I need something 7 or 8 speed compatible. Or does it matter when it comes to chainrings?

Problem 2. Scrap the entire crankset and get a new one. I have an RSX A413 crankset in a 68mm BB shell. Now from what I understand if I get a replacement I need something with a 45mm chainline, just like my current chainline, is that correct? Shimano LX comes in the exact ring sizes I'd like but they have something like a 50mm chainline. Won't work, right? And once again they are all advertised as "9 spd chain compatible" but that won't work on a 7 speed, I assume? I understand the 7/8 speed chain is wider.

Argh!

[esassaman]

Wait... If I buy, say, a Shimano XT M752 crankset, and then a compatible Shimano ES-51 BB that is 118mm, which is my current BB spindle size... does that mean my chainline will be correct? 45mm? Is this the solution for the chainline problem, just get the right size BB? It's still a 9 spd crank, so I'm screwed on that though, right?

[DaveB]

A 9-speed crank used with a 7/8-speed chain may cause some chain rub in the middle or granny ring and the smaller cogs.  Otherwise it will work.  You could, of course, use a 9-speed chain which will match the crank and is compatible with a 7-speed cassette.

You cannot get a significantly smaller chainring on your RSX crank as the granny rings bolt circle (74 mm) is limited to 24 teeth as the minimum.  You could find a older square taper MTB crank and pair it with a road 68 mm shell width bottom bracket of the correct length.  Most newer MTB cranks use a 73 mm bottom bracket shell width and these won't work on your frame.

[Pat]

Not sure where you are planning on traveling, or how much weight you are planning to carry.  My wife and I just did a test run from San Francisco down to Santa Cruz.  With Surly and weight, I was pushing (literally in one instance) 105 pounds and my wife 90.  We were down in the Granny gear on those climbs.  We have a 24T and 12-34 rear cogs, with nine speed chains.  As I am 61, I do not think I could have made it over Devil's Slide with any other combination.

Good luck in your search,

Pat

[RussSeaton]

Use your existing old 7 speed crankset.  Get new chainrings.  TA and Stronglight sells very nice ones.  Expensive though.  Salsa and a few others sell lower cost ones.  It does not matter what gear number the rings are for, they will fit your crank and shift your 7 speed just fine.  7, 8, 9, 10, they all will work fine.  Have no idea why you say rings are hard to find.  110mm and 74mm bcd 5 bolt chainrings are very common.  Many companies make them.  Every bike shop in the world can get them.  The 110mm bcd allows rings from 33 to the 50s teeth.  The 74mm bcd allows down to 24 teeth.

I have a 1991 Shimano Deore DX 7 speed crankset.  I changed the touring bike from 7 speed cassette to 10 speed cassette.  I replaced the rings with TA Zephyr 44 outer, Stronglight CT2 33 middle, and I have a 20 tooth inner ring.  Uses an Avid adapter thing.  10 speed Shimano chain.  10 speed SRAM 11-32 cassette.  Shimano 5700 STI shift levers.  Shifts perfectly fine.

For you I would suggest a 48 or 46 or 44 outer ring.  A 33 or 34 middle ring.  And a 24 tooth inner ring.  With the 12-32 cassette (12-14-17-20-24-28-32) you will have a low of 24x32.  Low enough.  The high will be plenty high no matter what outer ring you get.  Any chainrings marked for 7-8-9-10 will work fine with your 7 speed chain and cassette and crankarms.  Since your crank is 74mm bcd for the inner ring, 24 tooth is the lowest inner ring you can go.

I like chainrings spaced 10 or 12 teeth different.  14 teeth difference will also work.  But 10 or 12 different makes a good difference between rings.  So 44-34-24 rings is very good.  With a 7 speed 12-14-17-20-24-28-32 cassette, you will use the 44 and 34 rings for 99+% of your riding.  The 44x12 gear is high enough.  If you need to go faster, coast.  Rings of 46-34-24 will give you a little higher high, 46x12.  Some might be happier with a higher high gear.  If so, fine.  The 24x32 low gear is about 20 gear inches.  Low enough.  Not super duper low, but its low enough to get a loaded bike up just about any hill or mountain.  Might have to stand up and work hard for a short distance.  20 years ago I toured Europe with a low gear of 24x32 and made it up the Dolomites and Pyrennes and Alps.

[staehpj1]

Use your existing old 7 speed crankset.  Get new chainrings.  TA and Stronglight sells very nice ones.  Expensive though.  Salsa and a few others sell lower cost ones.  It does not matter what gear number the rings are for, they will fit your crank and shift your 7 speed just fine.  7, 8, 9, 10, they all will work fine.  Have no idea why you say rings are hard to find.  110mm and 74mm bcd 5 bolt chainrings are very common.  Many companies make them.  Every bike shop in the world can get them.  The 110mm bcd allows rings from 33 to the 50s teeth.  The 74mm bcd allows down to 24 teeth.

I have a 1991 Shimano Deore DX 7 speed crankset.  I changed the touring bike from 7 speed cassette to 10 speed cassette.  I replaced the rings with TA Zephyr 44 outer, Stronglight CT2 33 middle, and I have a 20 tooth inner ring.  Uses an Avid adaptor thing.  10 speed Shimano chain.  10 speed SRAM 11-32 cassette.  Shifts perfectly fine.

For you I would suggest a 48 or 46 or 44 outer ring.  A 33 or 34 middle ring.  And a 24 tooth inner ring.  With the 12-32 cassette you will have a low of 24x32.  Plenty low.  The high will be plenty high no matter what outer ring you get.  Any chainrings marked for 7-8-9-10 will work fine with your 7 speed chain and cassette and crankarms.

+1 to all of that.

[esassaman]

Thanks so much for all the advice. I had a hard time tracking down the chainrings I wanted. The TA 46T outers are pretty easy to find, but not so much the TA 44T's! I ended up ordering a 44T outer I found on probike and the 34T inner and 24T granny from wiggle.com with a Shimano HG50 chain (7/8 spd). The SRAM 7 sped 12-32 casette is sold everywhere (like REI) so I'll pick it up there.  So I have a nice 10 tooth spacing between all three smaller rings it's going to make my life easier on the big peak I'll be hitting here in WA state the first week of our 2500m across part of the Northern Tier route. I really liked the fact that the TA's were pinned for STI, some of the other ones were not. So I went with TA's all the way even though they were kinda pricy. What the hell you only live once and if I can't spend my hard earned money on my bike, what's the point of it all, right? 

I kinda got stuck on the issue of replacing the entire crankset because of the chainline issue. I believe the 113mm BB would have set my chainline at 47.5 instead of the required 45, which I believe is not a big deal, but in the end it seems like more of a risk, though replacing the whole shebang was really not much more expensive than the chainrings, but there's really nothing wrong with my cranks or BB. If it ain't broke...

Thanks again for the help.