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PostPosted: Mon Feb 03, 2014 10:23 pm 
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Posts: 217
The teeth are totally asymmetrical, severe case of shark fin you got there. Notice how the left flange is a lower angle than the right?

You need a new ring, unless you can run it backwards so the fresh flanges bear the load.

You should check your chain's wear with a caliper, if it's already elongated too much it will kill the new ring quickly, and your cassette too!

Hth,
Marin

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PostPosted: Tue Feb 04, 2014 2:42 am 
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Joined: Mon May 05, 2008 5:15 pm
Posts: 480
here's a few pictures

small ring to the biggest cog
Image

Image

space between the cage and ring is about the thickness of a penny.
Image

top pic of the FD at zero .
Image

chain length from small ring to largest cog
Image

chain in small ring and middle cog
Image


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Posted: Tue Feb 04, 2014 2:42 am 


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PostPosted: Tue Feb 04, 2014 3:01 am 
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Posts: 372
Do the problems exist with a different wheelset? I had a friend that had a jumping chain on mtb (similar to your problem), and the freehub was culprit. I don't remember what the exact problem with the freehub was. Odd problem you have. Everything looks fine from the pictures. Nice bike too!

Since you have extra bike/parts. I think one the easiest but tedious ways to solve this is swapping parts out one at a time. Good luck!


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PostPosted: Tue Feb 04, 2014 4:57 am 
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Location: Irvine, CA
Are you certain your sprocket is not on backwards? This same thing happened to me.


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PostPosted: Tue Feb 04, 2014 7:18 am 
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Marin wrote:
The teeth are totally asymmetrical, severe case of shark fin you got there. Notice how the left flange is a lower angle than the right?

You need a new ring, unless you can run it backwards so the fresh flanges bear the load.

You should check your chain's wear with a caliper, if it's already elongated too much it will kill the new ring quickly, and your cassette too!

Hth,
Marin


I don't think so. The steep side of each tooth is actually on the load-bearing side. This is how Praxis shapes their teeth. I have a brand new one and it's almost indistinguishable from this. Praxis rings shift very aggressively, so you can sometimes get over shifts if you don't adjust them just right, but this doesn't even require a shift to pop the chain off.


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PostPosted: Tue Feb 04, 2014 7:31 am 
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Photos show a chain line that's quite reasonable. That's not usually a problem on Cervelos anyway, so I'd have been surprised. The problem isn't at the rear end. I'd still suggest you shift up and down a bit and see if you get the chain hanging on the top of the teeth of the small chainring (actually with the inner side plate on the small chainring and the outside plate leaning against the big chainring. That's the easiest explanation. The shifting ramps on Praxis large rings work really well, but they can cause the chain to fall off the big ring and in between rather than right onto the small ring. Again, Praxis rings offer very aggressive shifting -- for Di2 they are really fast and Di2 automatically adjusts for them. For manual shifting, you have to get it just right and then they're superb but they are a bit less tolerant than your basic Dura Ace 9000 rings.

If not that, then next would be to swap out a different chain. I've found that Praxis chainrings are slightly thicker in the teeth than other rings -- it may only be in the heavy coating they get, but some chains occasionally stick on them. You get a slight bit of catching as the chain comes off the bottom or get the chain riding a little high as the chain engages when spinning without any pressure on the chain. The KMC is on the narrow end of 11-speed chains, or at least a little more prone to sticking on rings like alternating-thickness single speed cross rings. Try an Ultegra 9000 chain (same as the Dura Ace but half the price and without the extra cutouts). I'm guessing the problem might go away with that. If the Ultegra works (and it's about the most tolerant of all the 11 speed chains out there today), it was just something peculiar about the chain. You might find that with more wear on the chainring, the problem will disappear for the KMC chain as well.


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PostPosted: Tue Feb 04, 2014 8:54 am 
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To the OP, are you running the ring with the side that is facing up in the pictures towards the frame or towards the crank arm?

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PostPosted: Tue Feb 04, 2014 2:38 pm 
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mnmasotto wrote:
Are you certain your sprocket is not on backwards? This same thing happened to me.


I will double check it again ..


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PostPosted: Tue Feb 04, 2014 2:43 pm 
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Marin wrote:
To the OP, are you running the ring with the side that is facing up in the pictures towards the frame or towards the crank arm?


from the first picture on pg1 with the praxis marking towards the crank.


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PostPosted: Tue Feb 04, 2014 2:50 pm 
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Location: Geneva, Switzerland
The small chainring is backward.
Look at the picture that shows both rings. On the large ring, the steep side of the tooth carries the chain, right side of the tooth (left side of the notch). On the small ring it's backward. Flip it.


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PostPosted: Tue Feb 04, 2014 2:53 pm 
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My problem is not on shifting and chain dropping off .. the problem is when I'm spinning on the small ring and try to hammer or spin it fast or try to stand up and do a short sprint. what I'm afraid of is trying to tackle a hill and you start to put power on the crank and next thing you know that chain dropping off. perhaps a chain catcher will help.


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PostPosted: Tue Feb 04, 2014 2:56 pm 
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basilic wrote:
The small chainring is backward.
Look at the picture that shows both rings. On the large ring, the steep side of the tooth carries the chain, right side of the tooth (left side of the notch). On the small ring it's backward. Flip it.


ok I will try that ..


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PostPosted: Tue Feb 04, 2014 3:01 pm 
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Location: Geneva, Switzerland
yes, understood.
Look at your 3rd pic above where you see the teeth profiles of both rings clearly: they're inverted. When you press on the pedal in the small ring, the shallow ramp of the tooth will tend to slide under the chain roller. in any case it'll be easy to verify if I'm wrong.
ps: there is a delay on the internet, we talk past ach other...


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PostPosted: Tue Feb 04, 2014 4:15 pm 
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Location: Toronto
basilic wrote:
The small chainring is backward.
Look at the picture that shows both rings. On the large ring, the steep side of the tooth carries the chain, right side of the tooth (left side of the notch). On the small ring it's backward. Flip it.


^^ this.

I went back to OP's first picture.

If that face that we see is facing the crank then it's obviously backwards. Both from the assymetric tooth profile and the fact that the chainring bolt holes are countersunk.

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PostPosted: Tue Feb 04, 2014 5:04 pm 
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bikerjulio wrote:
basilic wrote:
The small chainring is backward.
Look at the picture that shows both rings. On the large ring, the steep side of the tooth carries the chain, right side of the tooth (left side of the notch). On the small ring it's backward. Flip it.


^^ this.

I went back to OP's first picture.

If that face that we see is facing the crank then it's obviously backwards. Both from the assymetric tooth profile and the fact that the chainring bolt holes are countersunk.


Thanks I may have to check the M3/praxis crank I have on my other bike If I have it backwards too.. although that one never gave me a problem. Thanks I will reverse that tonight and let you guys know .. thank you !


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Posted: Tue Feb 04, 2014 5:04 pm 


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