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Crankset destruction

Old 10-12-22, 12:57 PM
  #1  
louky
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Crankset destruction


I am working on my son's 90's Trek 930. He asked me to adjust the derailleurs but I think there are deeper problems. There are 3 or 4 "ripped" looking places on the middle gear of his crankset as pictured above. His chain is also showing excessive wear. I ordered a chain but then read a decal stating to use "IG chain only".
1) I assume he will need to get a new crankset. It is labeled Shimano MC34 and is 175mm. I am striking out searching by this number. What would be a budget to midline equivalent for this?
2) Do the "IG" chains read any differently on a regular chain stretch gauge?
3) What causes these "ripped" looking places on the gear? It seems like it would take an amount of violence to tear the metal like this.
4) If I replace the crankset and chain, I assume I should go ahead and replace the rear cassette, What would be a good budget to midline choice here?
5) Do I really need the Shimano IG chain, and if I replace the cassette and crankset both, will I end up with a different incompatibly with the chain

As always, Thank!
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Old 10-12-22, 01:09 PM
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1. The 175 is 175mm arm length. 170mm should work just fine and even better if your son is "sub adult" size. (I use 165mm because it's much easier on my old knees)
You won't find that model number new, and a used one may be worse than what you have.
You'll "most likely" have to replace the BB (Bottom Bracket) since newer cranks tend to use shorter spindles.
Just match up the tooth counts to the existing rings.

3. HOWEVER, rings have "shifting aids". These will appear to be broken teeth or cut outs or??
IF they are spaced symmetrically around the ring, expect them to just be that.
Look at a new bike and you'll see some type of them, assuming it's not a 1x.
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Old 10-12-22, 01:17 PM
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Bill: Thanks for the prompt response. I think I will pull the crankset and clean it up to take better pictures and add them to the thread. If I have to replace it anyway, I'll already have this done.

BTW, there is someone on a number of the small engine forums with your same name who I have depended on in the past. Is that you?
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Old 10-12-22, 01:34 PM
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Originally Posted by louky
....BTW, there is someone on a number of the small engine forums with your same name who I have depended on in the past. Is that you?
Yep
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Old 10-12-22, 02:16 PM
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Holy crap! If the rest of the bike looks like that it's time for a new one.
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Old 10-12-22, 02:19 PM
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The crankset appears OK except for dirt and corrosion, There does seem to be a "bite" taken out at the root of one of the teeth, but odds are the chain's roller would bridge that, so it's probably OK. Not OK if the chain settles doo deep into that spot, and doesn't smoothly engage the next tooth.

As for chain "stretch". That's independent of the chain, and about basic geometry. Bike chains and sprockets have 1/2" pitch, and there are working tolerances. Wear causes the chains to appear to stretch, and the pitch is now beyond tolerance, thereby increasing wear.
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Old 10-12-22, 02:42 PM
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Those "ripped" teeth are intentional shift aids, to help lift the chain up and onto that ring from the smaller ring. There seem to be two primary methods for doing this -- the twisted or ripped teeth on your rings...and inserting or riveting very short pins near the outer circumference that end up looking like little buttons on the inside face of the ring. The pins and twisted teeth do the same thing...they're just different ways of getting there. Interestingly, the modern Suntour cranksets (lower grade stuff on hybrids and lower end MTBs) seem to use pinned shift aids vs. Shimano's typical method of twisted shift aids.

louky I'm guessing your crankset is an STX or Alivio crankset. It looks very "Shimano" to my eyes.
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Old 10-12-22, 03:48 PM
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hokiefyd: Thanks for the response and possible good news on the crankset. That might explain why the "rips" seem to be equally spaced around the gear. I am basing the chain wear on one of those gauges that hooks and drops through a tooth, and it falls all the way through so I'm sure it's worn. My main issue after adjusting the FD: It shifts fine going from large to small, when going up from small to middle it is fine, when going from middle to large, there is a clunk, but it does go into the gear. I hope the chain wear is the reason for this, and that a new chain will help.
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Old 10-12-22, 03:53 PM
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FBinNY: Thanks for the response. I am going to pull the crank and clean it up to take more pics. The BB seems to be stiff so this step won't be wasted. It would be good news if I can save the crank and get an improvement with a new chain. The "clunk" I am hearing when shift up to the largest gear may just be the (worn) chain snagging on this point before finishing the shift.
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Old 10-12-22, 07:04 PM
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Originally Posted by louky
hokiefyd: Thanks for the response and possible good news on the crankset. That might explain why the "rips" seem to be equally spaced around the gear. I am basing the chain wear on one of those gauges that hooks and drops through a tooth, and it falls all the way through so I'm sure it's worn. My main issue after adjusting the FD: It shifts fine going from large to small, when going up from small to middle it is fine, when going from middle to large, there is a clunk, but it does go into the gear. I hope the chain wear is the reason for this, and that a new chain will help.
Don't trust your chain checker until you checked it.
My PARK shows .25% on a new chain.
Actually measure the chain with a scale/tape.
I measure a 3' length for better resolution, using the 1 & 37" marks on the tape to avoid any error from the hook/end.
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Old 10-12-22, 08:35 PM
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Originally Posted by hokiefyd
Those "ripped" teeth are intentional shift aids, to help lift the chain up and onto that ring from the smaller ring. There seem to be two primary methods for doing this -- the twisted or ripped teeth on your rings...and inserting or riveting very short pins near the outer circumference that end up looking like little buttons on the inside face of the ring. The pins and twisted teeth do the same thing...they're just different ways of getting there. Interestingly, the modern Suntour cranksets (lower grade stuff on hybrids and lower end MTBs) seem to use pinned shift aids vs. Shimano's typical method of twisted shift aids.

louky I'm guessing your crankset is an STX or Alivio crankset. It looks very "Shimano" to my eyes.
Yes - I believe MC34 identifies the crank as a STX crank (1996 / 1997 )

They can be a challenge to find NOS with 170 mm arms

One possible source :

https://www.ebay.com/itm/23384519321...Bk9SR9Ls__L5YA

.

Last edited by t2p; 10-12-22 at 08:45 PM.
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