HELP! problems with Praxis rings on SISL chainset

[Svetty]

I have recently purchased a set of Praxis rings for my Cannondale Super-six Evo. Previously it was fitted with FSA rings on a compact 110 BCD Hollowgram SiSL chainset, using DA 7900 mechs and cassette etc. and everything worked fine.

With the Praxis rings fitted I have problems: when in the small ring I am unable to use the 4 smallest rear sprockets without the chain rubbing on the inside surface of the outer ring. Looking at the 2 chainrings it appears that they are very close together - there is little spacing between them. As I say, everything worked fine with the FSA rings and the rear wheel is centred in the dropouts with no chain-line problems apparent - the frame is nearly new.

I assume this is something which must have occurred before. Can anyone advise on a remedy? Would shims on the chainring bolts to increase the space between the rings be a feasible fix?

[carlislegeorge]

I have had the same issue with Praxis versus DA rings...although through many adjustments, the rubbing on the large ring has been limited to the lowest two gears on my cassette (12-13). In reality, the same thing happened with the Praxis rings on my DA 7950 cranks. Didn't get appreciably worse switching to the Cannondale crank arms.

I hope someone has a fix...

[thisisatest]

The issue is that Praxis designs their rings "old school" in which the horizontal distance from one ring's teeth is pretty close to the other ring's teeth. With 7900, Shimano moved the outer ring teeth to the right, and to prevent the chain from jamming between the two, took up the extra space with ramping. Not only do you get to cross-chain more with this setup, it also greatly reduces the chance of dropping the chain to the inside when shifting to the smallest ring.
If Praxis made all their rings like that, then it would be difficult for their rings to work on an older Sram group, or Shimano 7800, or Campy. Hopefully they come out with a different option, but these are forgings, not just a different cnc code.

Sent from my EVO using Tapatalk 4 Beta

[Svetty]

Understand that 7900 chainring spacing is sl wider but don't understand how groupset is relevant as chain position is relative to chainrings and sprockets not mechs.

Front chainline is 42mm on my bike. This is less than the industry 43.5 standard as I understand it. Can anyone else with an Evo and Hollowgram confirm this or is my BB/chainset insertion a bit off?

[Svetty]

Ha! Found this:

viewtopic.php?f=3&t=114357

Confirms my thoughts re chainline distance of 42mm being too small eg the chainrings are too close to the midline on this design and the Praxis rings are not the fundamental problem.

Don't suppose Cannondale are likely to be sympathetic to the point of offering to ease my way to a new BB spindle. ...

[thisisatest]

Like you said, with the same cranks and FSA rings it was fine.
Having the rings farther apart means that, when in the small ring, the chain can be at at greater angle back-and-out before the chain rubs the large ring. the FSA rings probably say N-10 or DA79. Those rings are close to Shimano's spec of having the teeth of each ring farther apart .

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[Svetty]

Have ordered 109mm SISL2 bearing and spindle kit. Expensive way to solve the problem but at least it is a 'proper' solution, not a bodge job. Makes the Praxis rings bloody expensive though!

[Svetty]

Update. Installed 109 mm spindle - left original BB in situ. Found needed 4 shims on drive side to get chainline spot on. Consequently had to file several mms off non-drive spacer to get correct preload.
Only problem now is that there is considerable bearing shield friction vs the bearings where they rotate with spindle (despite greasing spindle surface and correct spring washer compression). Maybe this will settle after a few miles. Alternatively I could fit the ceramic bearings that came with the spindle. ....

[carlislegeorge]

Update on my version of the issue as discussed above. I contacted Praxis and they swapped out the large (52 ) chain ring, indicating they have done some recent redesign, but unfortunately the problem of chain rubbing on the inside surface of the outer ring in my lowest 3-4 gears is not improved. I think I'm going to put the Shimano 7950 50/34 crankset back on so I can use all my gears (until I can afford the 9070 upgrade).

My setup is as follows:
Praxis 52/36 rings
Cannondale Hollowgram SL crank arms
SRAM PF30 BB
Dura Ace 12-27 cassette
KMC X10SL DLC chain
Parlee Z5 SLi frame

[Svetty]

Interesting, wasn't aware that Praxis have modified their design...
Might give them a call.

[DamonRinard]

Hi Svetty,

Why not move the two chain rings ~1mm apart? Use 5 tiny spacers, one at each mounting bolt.

Cheers,

[eric]

You can get chainring spacers as thin as .6mm. They're in the QPB catalog so any LBS should be able to get them.
I have used them to solve this same problem on other cranks.

http://www.ebikestop.com/chainring_chai ... %29469.php

[goodboyr]

Based on this thread, I installed the 0.6mm spacers on my compact Quarq Elsa with praxis 52/36 rings. Before I had rubbing on last three cog. Now only happens on last cog. Everything else is fine including slope and zero offset. I used the problem solvers 0.6 spacers.

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[Dammit]

I have Praxis 52/36 rings on an SiSl SRM with 7970 Di2.

Would it be best to space the rings out a little to replicate that distance between teeth that Shimano use?

[goodboyr]

Yes. Based on my experience the 0.6 spacers are a bit too thin. Looks like. 1.0 would do the trick.