Cyclocross Gearing with two chainrings

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Permalink Reply by Thomas on September 8, 2011 at 6:05am

A small chain ring with 36 T is not long enough for fast courses and you have to change very often between the big and the small chain ring. 38 T can indeed be used in 90 % of the time. I don´t need to have a 46T chain ring but a smaller combinations like 42/38 or 44/38 tend to get clocked on muddy courses. So I returned to 46/38 in combination with 26 to 13 in the rear. This has also the advantage to be sufficient for some training on the road.

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Permalink Reply by stickboybike on September 8, 2011 at 6:35am

Definitely different regions require different combos. good luck this season!

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Permalink Reply by skibikejunkie on September 14, 2011 at 9:40am

I switched from 46/36 to a single ring 42. I haven't looked back.

 

The 46 was too big to be useful, and front shifting was always compromised in the mud. The 42 covers the full range of what I need. YMMV.

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Permalink Reply by Joe B on September 21, 2011 at 10:09am

Chuck,

Just this week I replaced my 46t ring with a 42. I left the 36t ring in place. I rode it in practice last night for the first time. I found that I could mostly leave the front on the 42 for a range of conditions including mud, off camber and small slopes. The shifting on my Force FD was a bit wonky - with the chain moving itself to the 36t kind sometimes when I was riding 23-25-26 cogs, I think I just need to adjust the limit screws (open to ideas from other readers)

Joe 

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Permalink Reply by stickboybike on September 22, 2011 at 9:42am

check you bb and see if there are spacers to be used, if any on that bike. Sounds like your center-line for the front rings is off. Not that you should ride the lowest gear in the rear and the 42t on the front but it shouldn't drop voluntarily, especially when the chain is nearest to the inner frt derailleur plate when riding the Big-Big gear combo.

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Permalink Reply by David whitlock on September 22, 2011 at 11:47am

Hum, that's very odd - I'm assuming the only variable that has changed is the front ring.

 

How's your FD shifting (besides the ghost into the 36) up and down? 

Did you shorten your chain after switching to the smaller ring?

Did you lower your FD after the switch? The cage should be 1-3 mm off of the 42 ring. 

  *If you're running a braze on front derailleur it has an upper installation hole for crank sets with 50 or fewer  teeth

Is there any lateral play in the crank set? 

The limit screws shouldn't be responsible (IMHO) for dropping down from the 42 to the 36 when running the upper cogs on the cassette, unless something is really out of wack, but FD's can be tricky little bastards! 

 

Sorry for all the questions, but it's hard to diagnose problems over the internet :)

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Permalink Reply by Joe B on September 22, 2011 at 11:53am

Thanks gents for your good advice.

@ stickboybike, wouldn't the chain line remain the same regardless of the ring size (it's mounted in the same place)?

@ David W, here are the answers:

1. FD shifting is crappy. Hard to shift up (easy to go down). Feel like I have to push the double-tap lever WAY hard to the right to get the FD to move over. Was that way prior to switching rings. When I first installed the 42t, it would over shift (chain dropped off to the outside). Adjusting the limit screw fixed that.

2. Did not shorten my chain.

3. Did not lower my FD.

4. No lateral play. BB and cranks are nearly new.

 

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Permalink Reply by David whitlock on September 22, 2011 at 1:31pm

Let’s work backwards:

Check for a crimp in the FD cable or a sharp bend somewhere in the housing.

Make sure nothing is clamped over the FD housing on the handlebars such as a lighting mount.

Have you been washing the bike with a pressure hose (gumming up the cables)?

Check the FD cable tension when in the 36, it should be tight, but not strung like a banjo.

When you place the chain on the largest cog (26?) and in the 36 ring there should be around 1mm of space between the chain and the inside FD cage.  The same is true with the smallest sprocket and the 42, but with 1mm from the outside cage. (I use a penny to measure - 1.4mm)

Make sure your FD cage is parallel with the 42 ring. * Very important *

Lower the outside FD cage to 1-3mm from the 42 ring.

 

Check the 42 for bent teeth

Check your chain's length (in the small sprocket and the 36 check for the chain rubbing together after the bottom pulley on the RD).  If so, remove a link or two and recheck; hopefully you're running a KMC chain since the missing link is super easy to remove.  If you have a Shimano, the chain is probably directional and the logo should face the outside.

Once things are set up properly you can check for misalignment issues with the chain line, RD hanger, so on and so forth.   

 

Whew, my fingers hurt from typing :)

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Permalink Reply by stickboybike on September 22, 2011 at 2:31pm

Good points David. You up on your tech stuff.

 

Chainline...I wasn't sure if you swapped out the crank and bb when putting on new rings. not assuming anything but sounds like you retained the same bb and crank.

 

SRAM does require a very forceful front shifter movement. Many ppl tighten the 5mm anchor bolt at the ergo/hood, thus crushing the plastic body down onto the front derailleur cable. IMHO, for SRAM derailleur/shifter, run either Campagnolo shifter housing/cables or an XTR cable/housing kit.

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Permalink Reply by andrew on September 23, 2011 at 12:50pm

definitely lower the fd...that should help - although with the rings so close in size I'm not 100% sure if you want to move it super close to the big ring as you would with a 39/53. you might run into issues with the small ring. let us know how it ends up.

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