Niloc

Ok, so here's the set-up: I've built up a new cyclocross bike by moving over a bunch of components from my old one that got wrecked. I run a 2x8 drivetrain. I run 8-speed because I have a flat bar set up with Suntour XCpro thumbies that I love. These are great thumbies that index well with Shimano 8-speed cassettes. You don't need a lot of gears for cyclocross racing, and with a double up front, eight in the back is plenty. I figure the wider spacing between cogs will resist mud clogging better and the system will be tough and durable for difficult cyclocross conditions. There's really no need to go to 10 speed for CX, with skinnier lighter materials that wear out faster. Of course the bike companies are not about to go back to 9 speed or lower for any discipline, even if it would make sense.

But I digress, here's the problem: This set up worked well on my old bike with a FSA compact front derailleur friction shifted by the XCpro thumbie. My old bike had the cables running underneath the BB for a bottom pull FD. My new frame is set up better for cyclocross with all the cables running on the top tube for a top pull FD. So I needed to get a new FD. I could have put a pulley on and used a bottom pull FD, but that kind of defeats the purpose of having the cables running along the top tube out of the muck. A pulley right between the chainstays is going to get mucked up fast. Besides my old FD clamp was too small for the new seat tube. I went the local used shop and tried out a few different used top-pull mountain bike FDs I found, but none of them would work. So I went ahead and ordered a new Shimano CX70 top pull FD. It is designed exactly for this application - top pull on a CX bike with a compact chainset. It mounted up nicely and a shifts between the chain rings fine. The problem is it is that the clearance at the front end of the cage is very tight around my 8-speed chain, which causes lots of rub when the chain is on the big ring. No problems on the small ring, but on the big ring pretty much have to fine trim the FD for every cog to avoid contact. This is not going to work, and any bit of muck or misalignment on the race course is just going to make it worse.

So obviously this FD is designed for 10 speed chains and my 8 speed chain is too wide for it. So here are the solutions I can think of:

1) Try a 9 speed chain. The interwebs have told me it is in-between 8 and 10 speed in width, so the fit would still be tight, but not as tight. I think you can run 9 speed chains on 8 speed cassettes fine. The difference in width is all in the outer plates and pins. Between the plates is the same, which makes sense since 8 and 9 speed cogs are essentially the same thickness. My RD is 9 speed and I suspect my chain rings are too. Of course a 10 speed chain would work best with the FD, but I'm pretty sure it would not be good with the rest of the drivetrain. Besides the whole point is to avoid skinny 10 speed stuff.

2) Try to bend the FD cage open at the front. I think it's made of steel so maybe if I clamped a crescent wrench on to the outer cage I could pry it open a bit.

Does anybody have any experience with either of these approaches?

gsa103

The lower grade Shimano stuff still runs 8-speed, so you can get a cheap FD that should work better than a hacked solution.

8-speed Acera FD $15
Shimano Acera M360 Top Swing Front Derailleur | Chain Reaction Cycles

SRAM 3.0 FD $12
SRAM 3.0 7-8sp Front Mech | Chain Reaction Cycles

AnkleWork

Maybe mount the FD a little higher?

Niloc

gsa103, those look like the derailleurs I found in the used parts bin that didn't work. I was bummed b/c they were only $5! The problem, I think, is that they're designed for triples. I couldn't even get close to getting them to work.

AnkleWork - smart idea. Maybe I'll try it a little higher than recommended, see what happens.

dsbrantjr

How did they not work? What did/didn't they do? Move too far or not enough? Cage didn't clear something/rubs? What size rings are you shifting? There may be a simple workaround, but "don't work" is a little vague to be able to suggest a solution. Help us help you by giving some details.

FBinNY 

My eyes glazed over trying to find your post to the actual problem, so I'll simply focus on the limits of trying to straighten the FD cage.

It's often (not always) possible to spread a cage slightly, to improve clearance for chains coming from various angles, but you can only get so much.

The problem is that the cage is bridged at the top, so if the chain is passing fairly high in the cage, you can't spread that critical area. If all you want is increased trim latitude for chain angle, note where the chain passes at the back of the cage, and if that's away from the bridge, you can insert some sort of a cam, like a wrench handle through the cage and rotate it to spring the cage open there.

Note that some cages are made of steel tempered to softer than spring temper, and others much harder. The very hard cages have limited ability to bend, and may crack instead when you try to spread them. If you;re unsure you can do a file test on some hidden portion. If you can't file it, you can't bend it either.

krobinson103

I've run a 9 speed chain through an 8 speed derailler and it works fine. It also got rid of the rubbing. On my beater I have a 9 speed chain for the Deore FD and a 7 speed claw derailler at the back. That messes with the indexing just a tad. Not enough to be annoying for its purpose (ride and forget) but if I was riding for fun or racing it would be annoying. I'd say you have a 1 speed latitude in chains for any given setup.

himespau 

I'd bet the problem lies with cable pull. The mtb derailleurs you tried from the bin have mtb cable pull, but the XC derailleur you tried has road bike cable pull. The trick is figuring out which your shifter has (thumbie, so I'm guessing mtb?). If that's the case, it could be the curvature of the inner plate of the mtb FDs you tried was wrong. What size rings do you use? Maybe something like an 8 speed touring FD (I think I've got a top pull LX FD which was meant to handle a 48T big ring) would work for you better. If your levers have road bike cable pull, you'll probably need problem solvers if you want to use top pull FDs.

Niloc

So just to provide closure here, I raised the FD slightly and that helped some because the cage tapers narrower towards the top. I also put a 9 speed chain on and that solved the issue entirely. I can shift fine in the rear and front, no problem. I do have to trim the FD a little at the extremes, but I think that's just normal anyways. I don't have to trim it every couple cogs like I did when I was running an 8-speed chain (and the derailleur was a little lower). Thanks for the advice everyone. And, FBinNY, you're correct my original post was waaayyy too long

Bezalel

Go with a 9-speed chain FDs are designed to be compatible with chains 1 generation back. and RDs simply hold the chain by the inside.

trailangel

Yes 9 speed chain.
I had the same problem running 8 in back with 10 speed Front DR
Plus KMC 9 speed in not as wide as a Sram 9 speed..... it got down to that!
Plus I spread the front DR slightly.. not much. Works great.