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PostPosted: Fri Sep 12, 2014 9:17 pm 
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Joined: Mon Sep 07, 2009 3:10 am
Posts: 372
So this is not necessarily a weight weenie issue. I take care in building my bikes and have several weight weenie bikes and I think a lot of forum members are interested in the finer points in bike building.

One of the things that I have noticed is that, even with a chain catcher. Every so often when shifting or going over rough roads, a chain will slip off the smaller chainring. It will not slip all the way off because the chain catcher does its job and keeps the top of the chain on the smaller ring. The lower part of the chain will hang and as you pedal the chain will rotate and find itself back on the smaller chainring.

What should not be a problem often turns into a problem because of the damm chainring bolts. The bolts have an edge on them that will grab the chain and suck it into the bike. This is a bad problem and could damage the frame if you pedaled hard enough. I have never damaged a frame this way but I have scratched the paint and have seen the potential for a much bigger problem that I do not want to deal with.

So I understand that this is a very rare condition, the chain comes off the smaller ring very infrequently. Some may argue that the chain should never come off the small ring on the inside if the bike is properly adjusted.

What I have noticed is that all the chainring bolts that I have seen have an edge that can grab a chain. I am not sure if all bolts are like this. Maybe someone sells a bolt that has a smooth rounded edge, if not there may be an opportunity to make a better chainring bolt. What I have done is round off the edge on my chainring bolts and it seems to fix the problem.

I use a black marker and color the bolt to match the black anodized. Here is what I have done and thought that I would share with my fellow weight weenies and see if anyone else has had this issue.

Image


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Posted: Fri Sep 12, 2014 9:17 pm 


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PostPosted: Fri Sep 12, 2014 9:46 pm 
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Joined: Fri Sep 05, 2014 8:29 pm
Posts: 13
Nice tweak, and just think of the 0.01's of grams saved :D


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PostPosted: Sat Sep 13, 2014 3:52 pm 
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Joined: Tue Dec 23, 2008 2:57 pm
Posts: 671
Location: NYC
Are you sure you are using chainring bolts that are matched with the chainrings? Also, make sure that your chainrings are not worn, where the chain may be sitting lower on the chaining and snagging on worn bolts, as well as snagging on worn teeth. What type of chainrings are you using?
EM3

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My 2013 Cervelo R5


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PostPosted: Sun Sep 14, 2014 12:15 am 
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Joined: Mon Sep 07, 2009 3:10 am
Posts: 372
I am using Praxis rings. The Praxis rings, like most rings have a recess for the chainring bolts. The recess is about half of the edge of the chainring bolts. So what you are left with is about half of the edge of the chainring bolts sticking out on the backside of the chainrings.

What can happen is that this flat surface, lets say 2mm, is just enough to catch a chain and suck it into the frame. When the bolt is rounded, the chain never grabs on the edge of the chainring bolt, it just glides over the bolt and as you pedal the chain will find it's way back on the chainring.

I looked around at numerous chainring bolts. I was surprised that every chainring bolt that I saw for sale has this edge.


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PostPosted: Sun Sep 14, 2014 7:05 am 
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Joined: Tue Dec 23, 2008 2:57 pm
Posts: 671
Location: NYC
sedluk wrote:

I looked around at numerous chainring bolts. I was surprised that every chainring bolt that I saw for sale has this edge.


...interesting. This leads me to believe that this is a Praxis design flaw, where the distance from the bottom of teeth to bolt holes is too short. Ur workaround appears to fix the issue but seems like if ur paying so much for Praxis u shouldn't have to grind down ur bolts. I still don't understand the advantage or allure of Praxis rings compared to high end stock rings from Shim/Sram/Campag?em3

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PostPosted: Sun Sep 14, 2014 10:35 am 
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Joined: Fri May 25, 2007 6:43 pm
Posts: 2020
sedluk wrote:
I looked around at numerous chainring bolts. I was surprised that every chainring bolt that I saw for sale has this edge.
of course, it's a fairly standard part.
You'd be better served by looking at numerous chainrings and seeing how deep the recess is on them. The 5 or 6 different brands I have down stairs all have little to no top hat sticking out.

Also, removing metal from there could (?) give you problems down the line with keeping the bolts tight. (If you've removed too much material.)


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PostPosted: Sun Sep 14, 2014 2:58 pm 
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Shop Owner

Joined: Sat Jun 13, 2009 4:02 am
Posts: 1974
Location: NoVA/DC
The thickness of ring between the chainring bolt/nut and the spider should be relatively standard. What is not is the thickness of the chainring overall. If the chainring starts off with a thicker blank, then the recess will need to be deeper to have the appropriate amount of chainring left over. In essence, there will be more chainring to hide the bolt head.
Most importantly, the chain should not end up there in the first place.


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PostPosted: Sun Sep 14, 2014 8:38 pm 
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Joined: Fri Oct 28, 2011 3:25 am
Posts: 886
Location: United Kingdom of Great Britain and Northern Ireland
A nice example of backwards thinking.

It's not the chainring bolts. It's the chainrings. The recesses have not been cut deep enough. Sounds like Praxis don't know what they are doing.


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PostPosted: Mon Sep 15, 2014 1:05 am 
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Joined: Sat Jun 13, 2009 4:02 am
Posts: 1974
Location: NoVA/DC
It has nothing to do with Praxis. Listen to what you're saying. The chain FALLS OFF during a shift. The chainring bolt protrusion sometimes interferes with pedaling the chain back on without stopping.
The chain should not fall off. I would certainly not be concerned with a chainring's chain re-installation abilities when designing one. There are so many variables that can make that aspect better or worse.


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PostPosted: Mon Sep 15, 2014 1:31 am 
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Joined: Fri Oct 28, 2011 3:25 am
Posts: 886
Location: United Kingdom of Great Britain and Northern Ireland
The chain falls off the small chainring. The falling off of the chain has nothing to do with the chainrings, and is neither here nor there. However, as I read this issue, rather than just 'falling off', instead the chain catches on the heads of the chainring 'nuts' that protrude from the inside surfaces of the small chainring because they protrude further than normal. The reason for their excessive protrusion is that the recesses for the heads of the 'nuts' are not cut as deep as with other chainrings. No?


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PostPosted: Mon Sep 15, 2014 2:07 am 
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Joined: Mon Sep 07, 2009 3:10 am
Posts: 372
Valbrona, you understand the problem correctly. This is not a problem with worn chainrings or something. The chain very rarely ever goes off the smaller chainring when shifting. But when it does, the chain catcher does it's job and keeps the top of the chain on the ring. The tension from the RD pulls the chain back and that is still not a problem. The problem is when you pedal the chain can get caught on the chainring bolts and sucked into the frame.

You can look at google images of "backside of chainring" and see page after page of chainring bolts that stick up just a little. With the chain under tension, you don't need much of a edge and the chain will get lifted, or sucked into the frame.

It is a very subtle problem, maybe that is why there seems to be some confusion. I can understand if someone does not think it is a big problem if every 8,000 miles of riding, someone drops a chain, it gets caught by the chain catcher and some of the time they get chain suck.


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PostPosted: Mon Sep 15, 2014 1:00 pm 
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Joined: Fri Oct 28, 2011 3:25 am
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Location: United Kingdom of Great Britain and Northern Ireland
Suggest you carry on collecting chainring bolts/nuts until you find some that have very shallow heads. Sometimes the traditional ones with a slot but made from stainless steel can have very shallow heads. And rather than sharp edges, the edges can be bevelled, thus reducing the chance of them catching the chain if it unships.


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PostPosted: Mon Sep 15, 2014 2:50 pm 
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Resident Pro

Joined: Tue Mar 18, 2003 9:42 pm
Posts: 2009
perhaps try different chainring bolts. parts of passion and fsa torx are my favorites. very low profile


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PostPosted: Mon Sep 15, 2014 6:19 pm 
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Joined: Mon Dec 26, 2011 9:04 am
Posts: 167
Shouldn't the tension from the RD keep you from getting chainsuck from the chain riding on the inside chainring nut? I have to admit that I've never had nor heard of this problem. I am sitting by one of my bikes at work and it feels like the inside chainring nut doesn't have an edge but is tapered like your homebrew fix.


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Posted: Mon Sep 15, 2014 6:19 pm 


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PostPosted: Tue Sep 16, 2014 2:01 am 
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Joined: Sat Jun 13, 2009 4:02 am
Posts: 1974
Location: NoVA/DC
Check out store.loadedusa.com. if run with the bolt on the inside and nut on the outside, the heavy taper should do what you want.


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