Crankset destruction
#1
Junior Member
Thread Starter
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!
#2
Really Old Senior Member
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.
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.
#3
Junior Member
Thread Starter
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?
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?
#4
Really Old Senior Member
#5
Senior Member
Join Date: Jul 2013
Location: Nor-Cal
Posts: 3,767
Bikes: lots
Mentioned: 7 Post(s)
Tagged: 0 Thread(s)
Quoted: 1958 Post(s)
Liked 2,932 Times
in
1,489 Posts
Holy crap! If the rest of the bike looks like that it's time for a new one.
#6
Senior Member
Join Date: Apr 2009
Location: New Rochelle, NY
Posts: 38,544
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 139 Post(s)
Tagged: 1 Thread(s)
Quoted: 5703 Post(s)
Liked 2,430 Times
in
1,343 Posts
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.
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.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
#7
Senior Member
Join Date: Feb 2017
Location: Northern Shenandoah Valley
Posts: 4,124
Bikes: More bikes than riders
Mentioned: 36 Post(s)
Tagged: 0 Thread(s)
Quoted: 1438 Post(s)
Liked 749 Times
in
559 Posts
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.
louky I'm guessing your crankset is an STX or Alivio crankset. It looks very "Shimano" to my eyes.
#8
Junior Member
Thread Starter
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.
#9
Junior Member
Thread Starter
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.
#10
Really Old Senior Member
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.
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.
#11
Senior Member
Join Date: May 2022
Location: USA - Southwest PA
Posts: 2,980
Bikes: Cannondale - Gary Fisher - Giant - Litespeed - Schwinn Paramount - Schwinn (lugged steel) - Trek OCLV
Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 1359 Post(s)
Liked 1,803 Times
in
1,038 Posts
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.
louky I'm guessing your crankset is an STX or Alivio crankset. It looks very "Shimano" to my eyes.
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.