Bent Praxis Chainrings?

Discuss light weight issues concerning road bikes & parts.
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by tdudzik

So I ordered myself a pair of 52/36 110 bcd Praxis Chainrings from Fairwheel Bikes, and after riding them for a little while, I've encountered a problem. When its in the smallest cog in the back, or even second to smallest for that matter, the chain will fall off when I pedal backwards. At first I thought it was just a cross-chaining issue, and rode around a bit for it hoping I would "break in" the rings, as well as the new chain (KMC x10sl). It became a problem after I dropped chain from sprints and bike throws.

I called up my LBS, and had them check it out. They noticed that it always fell off on the same tooth, so they marked it and eventually bent it inwards a bit. The chain drop seems to have stopped, but I can still get it off with a quick back-pedal. It makes the usual grinding noise in the lowest three cogs (it's not touching the FD, I promise). The guys at my shop said that they've had two other customers who had bent Praxis rings. Has anyone else had this issue? I know the guy from Fairwheel has an account on here, it'd be great if I could get some input.

Specs: Quarq Riken, 110 bcd, Sram Red Yaw FD

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by metal

A short chainstay length may be the culprit as well. Do you know the chainstay length of your frame?

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by tdudzik

It's a 2012 Cervelo R5. According to their website geometry measurements, the chainstay length is 405 mm.

EDIT: I should add in that I did not have this problem when I had osymetrics on. I ditched those though.

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by sugarkane

Gm of praxis also has an account here..

Back peddling with the chain crossed is asking for trouble any which way you look at it.
I have praxis semi compacts with SR and it throws the chain if I back pedal with the chain crossed in the last 2 cogs, I reckon it could be the profileing of the main ring to aid in getting it up fast. Maybe the climb profile doesn't play well with a crosses chain on the back peddle..
Contact Adam at praxis. He's super helpful and in a much better position to help you than the one they call madcow

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by thisisatest

If the crank's chainring mounting tabs are not machined perfectly flat, they will force chainrings into a slight bend. I don't think that is the issue here, though.
To me, it seems like your lbs took care of the main issue, the chain coming off when pedalling forward. For the issue when pedalling backward, I would go with the obvious answer, the one you're not gonna want to hear: don't pedal backwards. As mentioned earlier, many drivetrains will drop a chain when being pedaled backwards in a cross chain situation, even in the second cog. Sometimes in the third cog.

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by Juanmoretime

I have to agree with Thisisatest, don't pedal backwards. I only pedal my tt bike backwards and that is by hand to zero out my Quarq. Is there a purpose to pedaling backwards?

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by madcow

I too would add that it could be the spider and also that I wouldn't backpedal. However having said that, if the ring is actually bent Praxis is really great with customer service. If you call and talk to them they'll have you send it back and if it's bent they'll replace it quickly.

A bent ring certainly isn't common, but it can happen. Of several hundred sets we've sold this is the 2nd time we've heard of a bent ring. If you don't want to deal directly with Praxis, drop me an email and I'll take care of it for you.

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by AptosBigRing

Hey tdudzik,
Give us a call direct and we'd be glad to check it out for you. Curious to talk live and hear more for sure.
If something is up we'll for sure take care of it and get you shifting smoothly in no time. No problem at all.
We're here to help for sure!

Praxis Works
Santa Cruz, CA

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by tdudzik

Turns out it was the ring. I called up Praxis, and managed to get a replacement. They were VERY helpful. With the new one I can backpedal without worry, and the grinding noise is not present. I highly recommend Praxis, and their customer service is spot on.

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by sedluk

Adam has done a great job taking care of me in the past as well. I even got a t-shirt.

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by djconnel

I just discovered this exact problem with a Stronglight. Backpedal throws chain, front downshift occasionally tosses it over little ring. The ring is old at this point so can't prove it wasn't ny fault, perhaps from packing bike (it's a Breakaway). I was able to improve things by manually bending ring flatter.
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