Wednesday, May 2, 2012

Tech Post - Osymetric and Q-ring set up

Setting up DuraAce 7900 for Osymetric and Rotor Q-ring chainwheels.

This week I watched Bradley Wiggins incredible win of the Tour de Romandie in Switzerland. The final time trial was going to be the tour decider and shockingly Wiggins had to dismount his bike when his chain came off. The team mechanic was in the car following and ran out to rapidly fix it and off he went again to still win the stage by 0.7 seconds and the tour by 9 seconds - almost collapsing at the end. Wiggins uses Osymetric chainrings and I was actually relieved when I saw this happen because until then I thought that it was just my own lack of bike mechanics skill that was giving me similar problems. Bottom line is that those chainrings are very tricky to set up.

First of all I had decided to make things even more complicated than usual by combining a 50T Osymetric outer ring with a 34T Rotor Q-ring inner ring. The Osymetric ring is a special dual camber design and the Q-ring is oval. Osymetric have a 38T inner ring available but I was sure that was too big for the mountians here. The effective range this combination gives is about 54T to 31T (due to the ring geometries) That's a bigger range than a standard triple chainring set up! To begin with it seemed like getting this to work with a standard DuraAce 7900 front derailleur was next to impossible! The set up here is with a 28T to 11T ten speed cassette - but I think that any other ten speed cassette would set up the same.

The derailleur has three positions when moving from inner to in-between and outer - but steps back to the inner awlays in one movement - from either of the other positions. With the inner ring the inner position should allow the first five rear sprockets to work with no rubbing on the derailleur cage. The in-between position then lets you access from sprocket 3 up to sprocket 8 with no rubbing. For most climbing I'd keep it in this position. The limiter bolt is set to allow maximum movement to the inside. A chain catcher is also used on the frame to stop any risk of the chain dropping between the carbon frame and the inner ring. Changing up to the 50T outer ring uses the outer position only. This lets you access all the useable sprockets from 3 to 10 on the rear. The first two big sprockets don't work anyway with the Osymetric ring - they clunk all over the place. The outer limiter on derailleur needs to be set to stop the chain being pushed right off. The Osymetric supplied shims shouldn't be used to widen the space between the two chainrings because if you do then the narrow 10 speed chain falls in between the two rings and just slithers around when you pedal (I think this is what happened to Wiggins). This is also why the outer ring needs the full outer derailleur position - because then you get a clear powerful pull back right across to the small ring when changing down. When changing up to the big outer ring you need to remove all pressure from the pedals for a moment or it will not shift up. When there is no pressure it moves up easily every time. Tension adjustment is done using the trim control on the cable once the cable is secured very slightly lose in the inner position. 

It was tricky getting to this point because there are no instruction about how best to do it. I can now  say though that the combination of 34T Q-ring and 50T Osymetric (110 BCD Compact) works very well. The Osymetric is much more pronounced than the Q-ring and I prefer the feel of it, but they both seriously aid getting the feet over the "dead" spot and keeping up momentum with bigger gears or higher speed. The feeling is much more natural than for round rings.


  1. Thanks for the write-up. Looks like as good a spot as any for mine. I'm running the 54/42 OSymetric with 2012 SRAM Red on my ~2009 Cervelo R3 with a 11-28 cassette. I think I have it pretty well dialed in, but I can drop the chain if shifting big->little while in a lower cassette cog (28t or the 2 next to it), or shifting little->big while in a higher cassette cog (11t or the 2 next to it). I have a chain catcher as well - in my case the one that comes with the new Red deraileur. I chuckled when I read " stop any risk of the chain dropping between..." and thought I should edit that to "to remove risk 80% of the time but exacerbate the issue the other 20%". My problem is that on an inner chain drop the BOTTOM of the chain gets sucked up between the ring and the BB/frame, and the top will get PAST the chain catcher, at which point the catcher is actually an impediment to getting it back on. It's not a dealbreaker, I just have to get filthy and yank the chain out of the right place. I really need to just not go big->little while in one of the larger cassette cogs. I did it yesterday and today and it's not too cool. In the rare event I have a little->big chain drop on the outside of the crank, I can just shift back to the little crank position and pedal the chain right back on easily.
    As for the benefits... I've been riding it about 2 months. I've ridden faster and slower with it. My fitness has a greater margin of error than the chain difference, so it isn't night and day. I do notice the 54t - that's a big ring in general and I get some top end no doubt. I've been using it for a lot of climbing, which isn't really the intended use. I'm thinking of going back to the stock 53/39 on that bike now and swapping the Osym to my TT bike. I run dura-ace there, so your review is encouraging. I also don't have to use the little ring on that bike (triathlons) so that should take that issue off the table. Once I go back to 53/39 I should have a better feel for how the Osym helped or hindered my climbing/road ride.

  2. Hi Perullo, It seems that everyone is having some sort of problem - including Bradley Wiggins who almost lost the Tour de Romandy in Switzerland with a dropped chain in the time trial. It looks kind of like Osymetric needs to design a special front derailleur. I don't get any chain drop with my 34T Q-ring but I do get it on the 50T Oysemetric - falling out to the crank. The problem is that it will not shift up at all with the Dura-ace derailleur if there is any force at all on the pedals - so if you panic and push the lever harder then it jumps right across. I noticed on the work stand that the shifting was always perfect when turning the pedals by hand and sure enough if you spin with zero pressure it changes no problem at all and with no apparent risk of dropping the chain. I tried to set the limiter but it needs the full range of movement to make a clean shift - just without sustained brute force on the shifter lever.

    I'm really enjoying the sensation of both brands and find that it helps my mechanics - it feels much more natural. I don't want to go back to round - so it's worth the hassle.

  3. I should note.. the 2012 SRAM Red Yaw derailleur I'm using here works well. I did NOT have to use the cage spacers with it, and I was able to dial it in well. It has a wider range of motion that works very well with the Osym setup. With my setup, I only throw the chain crankside if I'm in a very tall cog on the cassette (11t, 12t, 13t etc) while going small to big ring. I've done that very rarely and know better. I can also recover it without slowing down, so overall it is a non-issue. I also know the right spot to pause while shifting to the big ring (it's with my right pedal at the bottom/6 o'clock) to get reliable shifts every time. This is a lot more relevant with the Osyms than a round crank since the chain/crank attitude is very different at different crank positions. I found this by studying it while shifting by hand on a bike stand.

  4. Very interesting! I will try the pause at 6 o'clock and see if it helps. It would be good to be able to only "pause" instead of spin with no pressure. Normally I change gear somewhere around the middle of the cassette and so far I've not noticed any connection between chain dropping and cassette with Shimano. I don't get the chain dropping on the work stand no matter what sprocket is being used - but when riding I'd never be in 11 or 12 when on the small chainring - partly because those gears can't be reached with the Dura-ace derailleur not giving enough clearance to prevent rubbing. This may also be because it's 34T to 50T - which is probably a much bigger effective difference in diameter than 54T to 42T

  5. I've been experimenting a fair bit now and seem to have found the best way to set up the Osymetric rings. I can change up to the 50T ring from every gear between 4th and 10th(smallest cog) without the chain being dropped and the key seems to be specifically with the outside limiter screw in the front derailleur. Within the space of 1/4 turn it goes from one extreme to the other - blocking the derailleur too much completely stops the change between chainrings. 1/4 of a turn looser and the chain will move over when on any sprocket all the way though from 4th to 10th, but even on 4th it risks being dropped to the outside (managed that on a workstand!). Somewhere within this 1/4 turn there is a sweet spot where it still changes even from the 4th largest sprocket and will not get dropped even on the 10th (smallest) sprocket. It's a really fine touch. What helps to find it is pausing the pedal stroke at 6 o'clock to make the gear change, then one turn of the cranks with ZERO pressure to take the chain over onto the big chainwheel. When this is used it really then makes it easy to find the correct limiter screw position. Start by having the limiter too tight (more limitation) and being unable to select the big chainwheel and then let it out a tiny bit at a time until you can guarantee a gear change from the 5th or 6th sprocket. Then the chain won't drop.

  6. Interesting. I've found Shimano asymmetric chains drops the chain on the I side less than the SRAM chains. Also, using tt-shifters makes front shifting lots better, since the slow chain movement does not kick the chain in position. Do you find the 2012 Red front works better than the old with osymetric?