Broken teeth --> broken chain?

Hey folks, 
I've broken the same chain several times now.  It's going in the trash but... Wondering if the two broken teeth on my middle chain ring could be a contributing factor?  
I've searched this forum and the 'net, but haven't found a solid answer.  
Any thought? 
Cheers,
Flydyl

If you have two broken teeth, and you broke your chain twice, its more likely you are just hard on your drivetrain.  

Uh.... yes. 

You don't really expect it to shift right or not break a chain with 2 broken gears right. Newbie here to gravel riding but on my 3 race I hit something and kind of tacoed(bent at 2 places) the outside ring and it wouldn't even shift to the top gear ring it just keep falling all. I think you have found your problem.

Zman

Put your bike up in a stand and watch what the chain does as you shift front chainrings. When the chain first begins to leave the ring it’s currently on to engage the chainring you are shifting to, it initially will be only engaged to one tooth (and crooked at that). If you pedal like Sasquatch at this point during the shift you may get broke/bent teeth and or bent/taco chainrings. Technique involves planning your shift sufficiently ahead of time to allow chain to transfer(one revolution) and at the same time not putting load/tension on the chain. The front derailleur shifts better/quicker when you ease-up a little so you don’t lose any speed and wear/tear is much reduced.

I think Skldmark has the theory I like: without those teeth--and under hard-pedaling and shifting--the chain gets torqued sideways, causing the sideplates (?) of the links to bend.  That's what seemed to happen on the last one (it wasn't a broken pin, it was a bent sideplate or whateveryoucallit). 

Dang.  Replacing the chain is easy.  But I doubt I've got another 9spd middle ring to use, and I've seen those sell for a pretty penny.  

Flydyl wrote:
Dang.  Replacing the chain is easy.  But I doubt I've got another 9spd middle ring to use, and I've seen those sell for a pretty penny.  

Sorry, I do not subscribe to the missing teeth causing the problem. Chainrings are aluminum and forces that could bend a chain's plate would be bending the soft chainring teeth. 

I would be more inclined to believe the problem is shifting the RD where it is possible to really torque on the chain while forcing it to move between steel cogs.

The basic physics of the drivetrain consists of two lever arms:
1. Pedal axle -> BB -> chain on ring
2. Chain on cassette cog -> axle -> tire

If the small chainring is half the size of the large, the tension on the chain will be twice as much when in the small chainring than the large.

The longer the crankarm the greater the leverage on the chainring and thus the greater the force (tension) on the chain. 

If one is in a small chainring it is possible to tear up the chain - especially if the rider is big and powerful.  The smaller the chainring the greater the forces.  Since this is a triple crankset, it likely has a very small chainring.

I would definitely replace the chain, but I would not replace the chainring. Instead I suggest being more mindful of when and how you're shifting the RD.  If a chain is under heavy load (lots of tension), most FD's will not shift as they cannot deflect the chain sufficiently to make it move.  The RD OTOH will shift under very heavy load and the chain will be undergoing very high twisting/tearing forces.

BTW - using the larger chainring can reduce wear on one's chain..

48/28
 - is the same gear ratio as
24/14
- But the tension of the chain in the 24/14 is twice as much
- Higher tension means the grit in the chain is wearing it faster
- Additionally fewer teeth share the load so the chainring and cog will wear faster

Disclaimer: no I'm not advocating riding in higher gears (lower cadence), I'm just saying that for a given cadence the drivetrain will wear slower with a larger chainring combination.


Hope this helps,
Greg

[QUOTE=NoCoGreg]

Sorry, I do not subscribe to the missing teeth causing the problem. Chainrings are aluminum and forces that could bend a chain's plate would be bending the soft chainring teeth.—�\n\n\n\n\n I also don’t think missing chainring teeth are the cause of the problem(maybe the result). But I do think chainring teeth can be damaged (as well as the chain) during a torquey front shift. Usually it’s the teeth right next to the pickup pins. That is also the place where a chainring is most likely to be folded in(board) if shifted under enough load and while the chain is only engaged with the transfer tooth. The chain link on this tooth is being twisted on the tooth like a can-opener as well.

[QUOTE=Skldmark]

NoCoGreg wrote:

Sorry, I do not subscribe to the missing teeth causing the problem. Chainrings are aluminum and forces that could bend a chain's plate would be bending the soft chainring teeth.—�

I believe that if teeth actually broke off, there would be signs of bending in the remaining teeth and more likely bent teeth would have caused a mis-shift prior to breaking off.   

Aluminum is very soft compared to the heat treated steel used in chains so it is unlikely an aluminum chainring would cause damage to a chain.  Cassette's are made of hardened steel and have long been known to be able to tear apart chains. 

Bent and/or broken chainring teeth are most common with mountain bikes where the the chainrings have hit something such as a log or rock.  

Most chainrings and cassettes for indexed drivetrains have short or missing teeth to improve shifting.  These are easily mistaken for missing or broken.  Here's a link with some additional info on how this works:  
https://www.purecycles.com/blogs/bicycle-news/shift-ramps-and-short-teeth-how-geared-chainrings-work

Disclaimer: Not having seen the chain nor chainring I'm speculating based on prior experience and info I've read.

Careful with that ax Eugene.