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PostPosted: Tue Aug 18, 2015 10:51 am 
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I have a Dolan Scala time-trial bike with Ultegra derailleurs and a power2max power meter on a Rotor 3D crankset. Crankset BCD is 130mm and current chainrings are 53/39 with an 11-25 10-speed cassette on the rear. On flat and slightly downhill sections of TTs, I find myself wanting a bigger gear. It’s not that I’m spinning out, it’s more that I find my natural cadence in those situations is about 85-95rpm and once I hit that in the 53x11 gear, I find it hard to increase the cadence to generate more power. I feel as though I’d be more able to increase power by maintaining that 85-95rpm but in a bigger gear.

I have a flat/downhill 6kmm TT in a stage race coming up in 2 weeks and I’d like to explore changing chainrings for that but I’m not sure what my options are. Would something like a 55T outer chainring make sense and work with my setup? What inner chainring would go best with this, a 42T? Any links to places with them in stock (in black preferably) with relatively quick delivery to Ireland are much appreciated.

In terms of swapping them out, is it just a case of undoing the chainring bolts and replacing the chainrings (i.e. don’t need to unscrew anything on the crankset itself) and possibly swapping for a longer chain, and then readjusting front derailleur? Thanks


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PostPosted: Tue Aug 18, 2015 11:17 am 
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I'm running 54-42 for a similar reason, you just have to raise the mech, and I think your chain should be ok as its not a big jump.


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PostPosted: Tue Aug 18, 2015 11:49 am 
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safcstuie wrote:
I'm running 54-42 for a similar reason, you just have to raise the mech, and I think your chain should be ok as its not a big jump.

Thanks. Do you notice much difference in 54x11 vs 53x11 or would you prefer to have a 55T chainring? Not sure I set it up correctly, but using Sheldon Brown's gear calculator with it set to 'KPH @90 RPM' seems to suggest 53x11 gives 54.8Km/h, 54x11 gives 55.8Km/h and 55x11 gives 56.8Km/h.

For a 6km flat TT, I'd aim to be generating somewhere in the region of 370-410watts (I weigh 91Kg) so I doubt I'd be troubling those top speeds on the flats, but would look to hit them on any slight downhill slopes, so am wondering whether 55T is a bridge too far or if 54T will give me plenty of extra power.


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PostPosted: Tue Aug 18, 2015 12:30 pm 
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Which P2m are you running?
If its the classic, you can run into trouble with some of the aero rings not clearing the back of the unit.

Have to remove the powermeter to change rings :?

You could try an osymetric- i'm on a 54, but with a 39 round inner ring. Shifting ain't great between the 2, but other than getting to the start, and warming up i never use the little un.

You then can have problems with mech and tyre clearance issues. I'm on a slice, and once the spacer block is fitted to the mech, the gear cable only just clears the tyre (22mm ultremo TT). Any more run out in my disc and i could have problems...


Not sure whether in the real world if you would gain anything by going to a slightly bigger round ring.Better to learn to spin a bit more :wink:

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PostPosted: Tue Aug 18, 2015 1:42 pm 
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It's a Classic Rotor 3D (24mm axle) version alright. By removing the power meter to change the chainrings, do you mean using the Rotor lockring tool to remove the axle/crank from the power meter, changing the chainrings and then re-assembling? If so, that's not too much hassle so I would do it.

However, it's more concerning if there would still be issues with the aero rings not fitting - is this because the derailleur has to move up the seat-tube and so runs closer to the tyre? Or is it that the aero chainring goes closer to the chainstay due to the increased diameter? Haven't tried osymmetric chainrings so would plan to stick to normal rings. Will take a look at my setup and take a photo and see if there looks like there'll be space for the larger chainrings. I'd settle for a 54T and do feel it's worth the hassle as in the short-term while I'm working to increase my cadence, I do feel I could more comfortably pound a bigger gear at 85-95rpm than I could spin a 53x11 at over 100rpm, so I'm leaving some power on the table at the moment.


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PostPosted: Tue Aug 18, 2015 3:20 pm 
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yes, for aero chainrings, you have to remove the chainset from the bike, then remove the powermeter (and chainrings) complete from the crank arm using the tool. Then swap the rings over normally before reassembling. You cant physically get the filled in chainring around the arm and the battery compartment on the pm.
Once its on you might still have clearance problems depending on how thick your chainring is- it could bend the back of the p2m.

As for the other problem, it depends where the braze on mech hanger is mounted.
Could you get the front mech high enough to clear the larger chainring?
The osymetrics come with a spacer block that raises the mech up and back from the original braze on, so pushes the cable at a more extreme angle. I had a 52t which gave no problems, but the 54 (equivalent to a 56 at its highest) is pushing it. I changed to a shimano mech just so i could hopefully get away with a different pull angle.

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PostPosted: Wed Aug 19, 2015 10:34 am 
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Thanks jekyll man. I have taken some photos and uploaded them here: http://1drv.ms/1Pz0iWZ - looks like there might be space for an increased chainring without hitting the chainstay and the braze-on for the derailleur has about 15-20mm it can rise by. Cable angle doesn't look like it should be an issue?

In terms of options for the chainrings, only aero ones I've come across are Rotor NoQ and Praxis Works. Praxis one is a set of 54T and 42T rings: http://www.praxiscycles.com/product/5442-tt-ring/ and the Rotor ones seem to come as separate inner and outer chainrings. If I went for a 55T outer, would it work ok with 42T inner?

Part of me is thinking that if it's worth making the change, it's best to go 55T, part of me is thinking just go with the Praxis 54/42 combo as you know those rings work together and it's less likely that a one tooth increase will cause space problems.


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PostPosted: Wed Aug 19, 2015 12:02 pm 
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I run either 55-42 or 54-39 on my TT bike. I prefer lower cadence myself.

There is an added benefit of a 55 ring - in that it will slightly improve your chainline in the middle of your block. Less sideways pulling and resistance on the chain. Plus a 55T looks badass.

If going to 55-42, just make sure you won;y need your granny gears (39-25, etc). Probably not a real issue.

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PostPosted: Wed Aug 19, 2015 4:31 pm 
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Right, I've decided I'm going to go with a 55tooth outer chainring. For hillier TTs, I can always change the rings back to my 53/39 if needed, but reckon I'll get value from the 55T in downhill sections where I feel underpowered today. Will continue to work on increasing cadence in the interim.

So 55-42 seems like the combo to go for. Before I purchase, be good to understand if these are the right products (I'll be in the US next week so will get them delivered to the hotel):

1. 55T Rotor NoQ aero outer chainring in 130mm BCD: https://www.racycles.com/equipment/comp ... rings-6196
2. 42T Rotor NoQ inner chainring in 130mm BCD: https://www.racycles.com/equipment/comp ... rings-6197

Note the inner chainring is normal, not aero, which makes sense, just want to confirm those two chainrings will play nice together?

While not good practice, would using an 11-28 cassette work with this setup? I don't find there to be much gappiness at the bottom end of an 11-28 (doesn't affect my rhythm anyway), and I can see that having the 28T there might be less hassle than changing down to the inner chainring on more shallow inclines. Yep, I know chain crossing is not good but I don't spend hundreds of hours on my TT bike so if I can get away with it, it'd be some useful flexibility.


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PostPosted: Wed Aug 19, 2015 6:28 pm 
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Those are the correct chainrings. And running a 11-28 cassette won't be an issue. I train on 55-42 with 11-32 on my bike (REALLY hilly area).

P.S. They don't make aero inner rings. No point.

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PostPosted: Mon Aug 31, 2015 9:04 pm 
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So I got the Rotor 55-42 chainrings. As mentioned, had to remove the power meter from the crankset, but that was fairly straightforward (I never used sealant glue so it came apart easily when I put it in a vice). Got the new rings on easy enough but problem with the chainring bolts brushing off the chainstay - or maybe it was actually the brake calliper (it's an integrated brake behind the bottom bracket). So I took it off and turned the chainring bolts around so that the side with the less protruding ends is facing inwards and this is just about clearing the chainstay/brake calliper.

If it does prove problematic and starts brushing off the chainstay/calliper under strain, should a 55-39 combination work (I know the 39 has no issues with brushing off other parts), or is that gap between the 55 and 39 chainrings too much of a gap when shifting and/or for chain length? I put on a new chain and sized it by running the chain directly over big sprocket (11-28 cassette) to big chainring (bypassing the rear derailleur) and then adding 2 links, and it seems fine.

Changed the aerobar TT shifters at the same time as I had damaged the inside of one of the shifters after reassembling it incorrectly after changing gear cables (the little black bit that falls out and seems to control indexing got blunted) and I'm having a problem with rear shifting - when in the small sprocket at the rear, when I shift up, the first click (and sometimes second click) does nothing. After that, it shifts reasonably ok, but the initial shift is a problem. It's these Dura Ace 7900 bar end shifters: https://www.chainreactioncycles.com/acc ... 1921867879 and it's Ultegra front and rear derailleurs (couldn't find 10-speed Ultegra bar end shifters online).

To double-check I hadn't set the derailleur up wrong, I went back and redid the adjustments, following this video https://www.youtube.com/watch?v=wQncKmddahk . All seemed ok, though I did notice the b-tension screw doesn't seem to have the usual effect as I don't notice the jockey wheels moving up or down as usual. Wonder if there could be an indexing compatibility problem or perhaps something I did incorrectly in the derailleur setup (which was basically to set High limit screw, undo and redo cable to reset tension, adjust tension with barrel adjuster, then do Lower limit screw)?


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PostPosted: Mon Aug 31, 2015 9:41 pm 
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You'll probably get away with the 39 as its doubtful you'll ever be wanting it in a hurry from the big ring while tt'ing.
As I said before, i'm running a dafter combo and "get away with it" most of the time.. cue tempting fate here...

The only reason I can think of for you to be having clearance issues now is the thickness of the inner ring- its pushing the nut heads further in. It's not like the bolts are in a different place.
(or your spider isn't sat on the crankarm correctly)....

Don't know how the shimano shifters function so dunno what could be wrong, other than maybe outer cable is too short or not sitting on its stop properly at the shifter.
For setting the mechs up, you should really set both limit screws with the cable disconnected. That way the cable tension isn't holding anything where it shouldn't be.
Indexing should then be a piece of pee. shift 2 or 3 gears up and then adjust tension until theres no chain rattle and it goes up and down the cassette properly. Limit screws shouldn't be touched...

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PostPosted: Mon Aug 31, 2015 10:16 pm 
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54-40 , 11-28 here. (10 speed). 54 is fast. It's not the 54x11 is 'needed', but having a bigger big allows for chain to stay more in the middle of the casette for me. the small ring is just for aesthetics.... only used to go to and from start line ;)

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PostPosted: Mon Aug 31, 2015 10:37 pm 
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Thanks, good to know 39T should work in the unlikely event I need to use it. So jekyll man, is the following what you recommend for mech setup?

1. Shift to smallest sprocket and big chainring and then disconnect cable from rear derailleur and ensure it's fully pulled through (i.e. no slack at shifter - something I didn't specifically check for).
2. Set Higher limit screw to position chain directly under small sprocket (by hand).
3. Push chain onto largest sprocket by hand and adjust the Lower limit screw to position chain directly under largest sprocket.
4. Screw barrel adjuster all the way in, pull cable tight by hand, and redo cable bolt on rear derailleur.
5. Shift to 3rd sprocket up and then turn barrel adjuster counter-clockwise if it's not shifting up smoothly, clockwise if it's shifting up too quickly.

Have a short 6km TT in a stage race this Saturday so am hoping to have the 55T ready and working smoothly to pound out the wattage....though I've just travelled from US West coast to Dublin and the jet lag is a b1tch so hopefully get into decent enough form by Saturday to make this all wortwhile!


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PostPosted: Tue Sep 01, 2015 8:10 am 
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You should be able to set the limit screws with the chain off completely. it shouldnt really matter.

You dont want the barrel adjuster screwed all the way in- how can you slacken the cable if its found to be too tight?


And good luck with the race!

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