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  1. #1
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    Replace Chainring or Entire Crankset

    I have an older 2006 Specialized Tricross sport that came with RPM forged alloy compact crankset, 48x34T teeth. The bottom bracket is a Shimano UN-25, 113mm spindle. I bent the big chainring so I have been thinking about either replacing it orupgrading to a new(er) crankset that would be lighter/better.

    I know nothing about bottom brackets and was wondering if a replacement crankset is even able to be purchased. Or, is it just easier to replace the chainring. I am currently running a set of 9 speed 105's shifters.

    Thanks for the input.

    Sunny

  2. #2
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    Doesn't seem high end, I would say look on eBay for an crank with a newer interface, But its all up to you though.

  3. #3
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    If you have the money, upgrade (you can replace the BB with a different type if necessary). If you're wanting simple and less expensive, replace the chainring. Neither is a bad choice, probably.

  4. #4
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    Chainring replacement?

    Hi,
    I tried to start a new thread but apparently don't have enough posts yet. Can I hijack this thread? It is similar to my question.

    Have used the MTB forum a few of times - thought it time to join up. Actually I have a question I'm hoping one of you can help me with. Bit of a novice with bike repair so bear with me.

    Have a Fuji Nevada 2001 8-speed

    Seems to be a bit difficult to find parts for, in fact impossible for the bottom bracket. It is still working for the time being.

    The rear Derailleur, cassette & chain were all pretty worn so I replaced them. However as soon as began to pedal experienced what I believe you call Chain Suck. Put the old chain back on - No Problems. I'm assuming that means the front cog has mated to the old chain & needs to be replaced?

    I really only use this bike to toodle around @ the dog park & rarely get above 5'th gear. found a thread on your forum suggesting that instead of replacing the entire crankset perhaps replace only the chainring - which in this case would the smallest with 22 teeth.

    I found a bunch of chainrings with 22T however the bottom bracket width has me a bit stumped. Do I really have to worry about the BB size 68mm x 124.5mm / square vs. tapered if just replacing the chainring?

    The bike came with a SR Suntour aluminum, 22/32/42 Crankset but I'm sure finding something that specific is unrealistic. If I can find a replacement chainring for something like a Shimano Front Chainwheel FC-M340 would that be sufficient?

    I'm hoping by replacing only the chainring I can avoid the bottom bracket business.
    Can you help me choose the appropriate chainring?

    thanks
    I'm not going to buy my kids an encyclopedia. Let them walk to school like I did.
    Yogi Berra

  5. #5
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    If the BB is still in good shape (not loose or grinding), then no need to mess with it. May need to pull the DS crank off to replace the chain ring, if so get a puller if you don't have one. Just match the bolt circle diameter (BCD) of the chain ring you want to replace and have at it. Probably also need a tool for the chain ring bolts.

  6. #6
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    Guess I need to pull it off to understand what I'm really doing here. Purchased a Super B 37pc Home Mechanic Bike Bicycle Tool Kit Set - got tired of paying someone else to work on the bikes. OK with autos, figured bikes can't be that much harder. EDIT: It does look like the Super B has a crank puller. But no chainring bolt removal tool. So I'll need to pick one of those up.

    Thought I did hear some grinding couple months ago from the BB but not there anymore. That's when I discovered that BB has been discontinued. Will eventually have to go with something like a Shimano UN-54 Sealed Square Taper Bottom Bracket 68x127 & adjust for the extra width of 2.5 mm. Or smaller to 123mm but that doesn't seem doable - right? Isn't larger the better way to go?

    thanks again.
    I'm not going to buy my kids an encyclopedia. Let them walk to school like I did.
    Yogi Berra

  7. #7
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    I have a 68x123 and a 68x127 UN-54 BB that have both been used just a few months if you want to save a little money. I used them on 2 of my bikes before switching them to a different BB. As far as width, check your inner/small chain ring and crank clearance to the chain stays. The 123mm BB will move those a little closer to the chain stays and the 127 will move them a little outward. Since it is only 1/2 the width difference, you'll only see a 0.75mm change, so probably won't even notice. You would also need the BB cup install tool as well.:

    Park Tool Co. CWP-7 : Universal Crank Puller for Square Taper and Splined Cranks : Crank Tools
    Park Tool Co. CNW-2 : Chainring Nut Wrench : Crank Tools
    Park Tool Co. BBT-32 : Bottom Bracket Tool : Bottom Bracket

  8. #8
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    Six or so years ago the BB went out on this bike. LBS (the largest bike store in Colorado) told me that BB had been discontinued & I had to buy a new bike. Huh! The very next LBS I contacted said No Problem. $150 later they had me back up & running. Decided it was time to work on these bikes myself. Nothing to troublesome until now.

    There's plenty of clearance on the chain stay however the chain while on the small cog is so close to chaffing on what I would call the inner plate - (part of the front derailleur) .75mm might make a dif. I'm wondering if they didn't replace it with the 68x123 six years ago. Would there be any way to tell except by removing it?

    What is the purpose of that plate? Just something for front derailleur to bolt to? Can I remove the plate, use a dif derailleur?

    I am obviously going to have to use one of these BB's, in fact have had both in my watch list on eBay for few months now. If you're going to give me a deal perhaps I should ask for both. Here are some stats on my bike just in case:
    Bottom Bracket VP sealed, 124.5mm spindle
    Bottom Bracket Shell Width 68mm English
    Bottom Bracket Spindle Length 124.5mm


    Have the crank puller & BB tool, just purchased a Chainring Nut Wrench on eBay.

    thanks again,
    rich
    I'm not going to buy my kids an encyclopedia. Let them walk to school like I did.
    Yogi Berra

  9. #9
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    You could pull the cranks off to measure the spindle length to know for sure. Could sell both BBs for $25.00 plus postage if interested. I used them a month or two last last spring, maybe 100-200 miles on each when I was replacing the old ball bearing BBs on two bikes. I ultimately found I could get a narrower BB in there so no longer needed the wider ones.

    Not sure on the FD plate, I know on my two bikes, I just had to readjust the FD with the narrower BB. On one, I also needed to do a little clearancing of the FD mechanism to let it shift closer to the seat tube.

  10. #10
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    Haven't completely given up on on finding a 124.5mm BB in length yet. I will pull the cranks & measure sometime in the next couple weeks. Right now just glad to have the mountain bike back. Waited couple weeks for all the parts to arrive from eBay & rode the hybrid in mean time. Nice bike but really miss those fat tires in the dirt & sand.

    Thanks for all the help Roger.

    daemoncycler out.
    I'm not going to buy my kids an encyclopedia. Let them walk to school like I did.
    Yogi Berra

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