CX: 1x front chainring offset

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jever98
Posts: 914
Joined: Wed Feb 18, 2009 12:02 pm
Location: Seattle

by jever98

Hi,

I am running a 1x system with a power2max pm on my Cannondale SuperX. For the gravel season, I had a 44T FSA Megatooth at the front (came with the power meter), which gave a nice chain line. For CX, I changed to 40T, using a RaceFace chain ring, only to realize that the chain line isn't the same:
- The Race Face is completely flat
- The FSA offsets the chainring slightly inboard

The chainline of the RaceFace isn't great: on the outside, it's too far out, on the inside, too far in (9-11 have too much chain angle).

I'd like to source another chain ring with the same offset as the FSA 44T, but am unsure if the FSA 40T will have the same offset. And I can't find offset information on the web.

Does anybody have a FSA 40T chain ring and know its offset?

Cheers
----
No longer in the industry

jfranci3
Posts: 706
Joined: Tue Jul 26, 2016 5:21 pm

by jfranci3

wouldn't it be easier to shim the cassette out a mm and/or the crank spider in a mm? maybe put a thinner seal on your BB?

The FSA looks like 3mm inboard. The Raceface is probably flat.

by Weenie


jever98
Posts: 914
Joined: Wed Feb 18, 2009 12:02 pm
Location: Seattle

by jever98

Shimming in the spider isn't an option, because there is not enough space between the frame and the power meter.

I might just try another FSA chain ring.
----
No longer in the industry

erik$
Posts: 121
Joined: Sat Nov 15, 2008 5:26 pm
Location: Trondheim, Norway

by erik$

Not FSA, but my Praxxis 40t has an offset which seems to play well with the 1X setup. Came stock on the Specialized Crux so should be well tested. I have not found that many alternatives myself and not many brands list their offset numbers. Maybe most of them have no offset?

simoncx
Posts: 161
Joined: Sun Nov 15, 2009 12:27 am

by simoncx

I'm pretty sure the sram cx1 chainring I had was offset to the inside but I dont remember how much, wolftooth and raceface was flat.

jfranci3
Posts: 706
Joined: Tue Jul 26, 2016 5:21 pm

by jfranci3

jever98 wrote:
Mon Oct 07, 2019 2:31 am
Shimming in the spider isn't an option, because there is not enough space between the frame and the power meter.

I might just try another FSA chain ring.
That's what the shim does.

So youve got a flat ring and you need to go -3mm backwards. You've got outter position (OP), 3 (?) mm spider (SP), inner position (IP). At the spindle, you've got the Spider (SP), Lock Ring (LR), Shim (S+), bearing seal (BS), and bearing (B).

You're trying to move the ring on the OP -3MM, so you've either got to buy a new ring or put move things around.
Plan A) If you have a super thick (BS), you can get a thinner one - you're worried about collisions though.
Plan B) Put a 1.8mm spacer (10sp/11sp spacer) behind the cassette if you've got the space and the cassette lockring gets a good bite on the freehub. You'd still be 1.2mm off, but thats better. https://amzn.to/2AZx7Ng or https://amzn.to/33lrzcj
Plan C) OK, so move the 1x ring from the OP to the IP. You now have the ring where it needs to be +3 instead of -3mm. You need to move the whole drive side OUT 3mm with the ring on the IP. This will give you MORE space between the PM and frame. Either put a shim or wavespring in between the LR and BS. . https://amzn.to/2IDMPSv or https://amzn.to/33ilNrE or https://amzn.to/33fuTFI

Marin
Posts: 3524
Joined: Wed Jan 22, 2014 11:48 am
Location: Vienna Austria

by Marin

jever98 wrote:
Mon Oct 07, 2019 1:56 am
Does anybody have a FSA 40T chain ring and know its offset?

Cheers
I have FSA Megatooth 38 and 40t rings and they are both offset, probably by the same amount.

jever98
Posts: 914
Joined: Wed Feb 18, 2009 12:02 pm
Location: Seattle

by jever98

Thanks Marin, that should work.
----
No longer in the industry

joejack951
Posts: 630
Joined: Fri Feb 18, 2005 6:50 pm
Location: Wilmington, DE

by joejack951

jfranci3 wrote:
Wed Oct 09, 2019 11:10 pm
jever98 wrote:
Mon Oct 07, 2019 2:31 am
Shimming in the spider isn't an option, because there is not enough space between the frame and the power meter.

I might just try another FSA chain ring.
Plan B) Put a 1.8mm spacer (10sp/11sp spacer) behind the cassette if you've got the space and the cassette lockring gets a good bite on the freehub.
I've never seen a lockring/cassette pairing that had enough engagement that removing 1.8mm worth of it wouldn't be cause for concern. While in theory this could work to help the offset problem, I see this as merely a quick way to producing an unusable (read: stripped threads) freehub body.

by Weenie


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