Switching from 54t to 50t possible on the Rotor 3DF cranks?

[kaotickelly]

Hi,

Does anyone know if it's possible to change the Rotor 54t chain ring from the 3DF cranks to a non-Rotor branded 50t?

Would that (50t) be an issue with the 39t inner ring?

Thanks,
Kelly

 

[c3024446]

It's all about BCD's. If your crank has a 130 BCD you should be able to find a 50T for it. They exist, i just looked

Why do you want 50/39 anyway? the "regular" configuration is 53/39, not sure if changing to a 50 will be worth it. You are sacrificing a little top end and not getting any more in the low end since you still have your 39 little ring.

Riding in the little ring and the 13 or 14 in the back is ok, you don't have to be a hero in the big ring all the time

 

[kaotickelly]

Thanks c3024446

The problem is that the chain rubs when the inner goes past 6/7th (14/15t I think) gear, so I have to shift to the larger ring, but 53 feels too heavy for me on 4th/5th (after up shifting to maintain speed), and any lower it'll rub too. So I wanted to go down on the large chainring so I can have a comfortable cadence in 4th/5th and beyond.

Open to other configuration suggestions...

 

[c3024446]

Is your bike 10spd? I would fiddle around with the front derailleur and make sure that is set up ok. Adjust it so when you are in the little ring and big cog, the fd cage is just missing the chain. If you have a barrel adjuster this should be easy, if not, it's hard to do! That should allow you to use more of the smaller cogs, and hopefully not rub when you are in the big ring as well.

Some road front shifters have trim where it takes a few clicks to move into the big ring (my Campy chorus has this, not sure if Shimano and SRAM do), and having 1-2 clicks will move the FD a bit to allow you in the 6-9th cogs while still in the little ring and the 3rd click shifts into the big ring.

 

[kaotickelly]

Yes, 10 spd, and I have tried to fiddle with it to get the best range out of the two rings.

Unfortunately there's only 1 click on the smaller ring to get to the larger ring, and another click on the larger for it to trim to the 9/10th gear. So I'm out of luck there.
So at the moment the small covers 1-7, and the large 4-10 on a 25-11 cassette, I might think about going to a 28-12. Wonder if that would help.

 

[c3024446]

So you can't make it so the first spot covers the big cogs at the back in the little ring, first click covers the small cogs in the back and still in the little ring, and the second click puts it in the big ring and covers everything in the big ring (except maybe the 2 biggest cogs at the back)?

 

[kaotickelly]

Not the last time I tried. First click shifts it all the way to the big ring. In the small ring the chain rubs the fd after the 7th gear.

 

[Cave Dweller]

Assume you have tried moving in the limit adjustments so that in the big ring/smallest cog the FD is very close to touching the chain, and same for the small ring/largest cog?

 

[c3024446]

^ Yeah, try that and work backways. So:

1. Put chain in big ring and smallest cog. Click front shifter as far as it can go to the right.
2. Adjust FD / loosen cable so the cage just clears the chain (probably can only do this if you have a barrel adjuster since the cable is under tension).
3. Then click the shifter down, and it will hopefully drop into the little ring and have clearance to use the little cogs in the little ring.
4. Then click it down again so you can use the big cogs and the little ring, and adjust lower FD limit if necessary.

Let us know if this would work!

 

[kaotickelly]

Yep, did exactly that couple weeks ago. In fact I had to move the upper limit in to prevent the chain from falling off the big chainring when going from small to big.

 

[c3024446]

Bugger! I think i'm out of ideas, unless the FD isn't 10 speed or something. Maybe the bigger cassette or smaller chainring is your only hope.

 

[Alo661]

Not the last time I tried. First click shifts it all the way to the big ring. In the small ring the chain rubs the fd after the 7th gear.

What groupo and year?

 

[kaotickelly]

2012/3 Ultegra fd and rd, 105 cassette, where ever is stock on the current Ridley Fenix.

 

[Alo661]

Should have trim in both small and big rings.

First click should move the derailleur sightly to allow you to access smaller gears (11t,12t etc..) without the chain rubbing on the FD. If yours doesn't have this and the first click shifts the derailleur up to the big ring, too much tension in the cable.

Only 7900 and 9000 DA have no trim adjust in the big ring. All Ultegra/105 etc should have trim in both small and big rings.

 

[kaotickelly]

Ok, thanks Alo, I'll give it another try this weekend.

 

[The Duckmeister]

Sounds like a fairly major maladjustment on the FD.... If the inner limit is set properly, there should only be chain rub without trimming on the 9th & 10th (outer two!) sprockets, and that trim shift should shut it up.

Shift it to 1st, staying in the small ring; there should only be the slightest gap between the inner plate of the FD & the chain, just enough to not get noise in that gear.

Next, check the cable is fed into the derailleur correctly (heaps of people bugger it up!) - do NOT feed the cable up between the pull tab & the anchor bolt, it'll bugger the leverage ratio. Pull the cable up parallel to the seat tube inboard of the derailleur, rest the cable against the tab and bend the cable over the top of the bolt & tighten it. 5700/6700/7900 shifter/FD combinations do need a fair bit of cable tension, but it can vary due to the routing through the frame. Pull the tension on so the trim shift (first click of the shifter) makes the derailleur move - not enough tension & all the first click will do is take up slack.

Shift up to big ring & out to 10th on the back. set the upper limit so there is about the same gap between the outer plate of the FD & the chain as there was at the other extreme. Any more & it's likely to chuck the chain off when you upshift the front. You should be able to shift to 2nd at the back before needing to trim down the front to eliminate rub. 7900 is the only Shimano shifter that doesn't have downward trim on the front; contrary to the earlier post 9000 does (but being specific to that group, it won't be used in combination with any other components at this stage, so is irrelevant to this discussion).