Cyclocross - Likely Story?

MY JTS drops the chain onto (but not engaged with) the smaller chainring when shifting at the front. Only in this direction (large to small), and not even with terrible chainline. Since it's still newish, I'm letting the dealer sort it out. The mechanic is convinced it's somehow due to either a mismatch b/w chainrings and 10 speed chain, or (his #1 theory) there's not enough diff b/w large and small chainrings. Chainrings are 36/46, freewheel is 105 12-25, cranks FSA gossamer compact megaexo.
Me, I'm thinking the FD just need to throw more. Simplest solution and all...
So...is the mechanic talking crapola? Is there a chance he's right?
Asking this before he goes swapping chainrings on me, thanks :)

For 10.00 you can get a jump stop.. This will cut it out completely.. They also have smaller ones like the Deda Dog Fang and 3rd Eye Chainwatcher for about 10-15.00.. They are a little less reliable.. Good Luck.. I know its a pain..

http://n-gear.com/whatis.html

http://sheldonbrown.com/harris/derailers-front.html#deflectors

Thanks for the links...much appreciated, and fine looking gizmos to boot! Problem is, the chain isn't going past the smaller chainring and dropping, it's simply not quite making it onto the smaller chainring. Kinda like the sideplate of the chain sitting on the teeth of the chainring but not falling and engaging. Effect is like popping into neutral..a pisser since I'm chopping down for a reason i.e. hill.
Methinks Occam's razor...the FD isn't adjusted properly. All that chainring size and component interface rationale seems horse puckey :dunno:

I would re-post in bike mechanics forum, they seem to have answers to everything..

Good idea, will give it a shot :)

I have had this same exact problem on a '06 Jamis Nova, w/ 50/36 Truvativ compact double w/105 front der. and 12/26 105 rear cluster--pretty much a similar set up to yours. The chain doesn't make it all the way over to the small chainring and skates there--really annoying when you're going uphill. I usually remedy the problem in the short term by hopping my rear wheel. It was *really* bad with Truvativ 9-speed chainrings, a switch to their 10-speed chainrings helped some.

Fiddling with the derailleur barrel adjusters and stops helps some, so I'm inclined to think it's a "throwing" problem, but I still haven't completely solved the issue. Ask in bike mechs, I'd be interested to hear the answer.

My local LBS wrench also thought it was a parts mismatch (shimano/truvativ), but I don't think front derailleurs are really that finicky (as opposed to rear)

Interesting...esp since you already have the larger diff b/w chainrings that my mech is probably looking to as the solution. Empathetic wry grin at the skating description, spot on! :D

I’ve had a similar problem with front derailleurs and small chainring differences. The problem was that there was not enough throw to the derailleur because the derailleur side plate has steps in it which the chain didn’t drop far enough down to engage. There is probably a slight step in your derailleur side plate that the chain doesn’t see with the 10 tooth difference (46/36) you are running. Try raising the derailleur slightly higher up so that the lower chainring works but not so high as to interfere with shifting onto the larger chainring. So the height you would need would probably be the same as if you were running a 48 tooth ring. Some front derailleurs specify a minimum ring difference but like all these specifications you can make slightly out of specification situations work with some tuning of the setup.

Outstanding, thanks :)

I'm a bike mechanic, 6 years as a profession, 25 years as a racer/enthusiast I've been seeing this problem a lot with 10 sp. drivetrains and non-shimano cranks... front derailleur position is finicky, but the usual culprit is 9sp. chainring spacing to slightly narrower 10 sp chain, according to Truvativ tech support. Some inner rings are marked S-9/C-10 or S-10... hope this is usefulif you're still having problems.

yes, this was brought to Truvativ's attention with my Nova, and 10-speed rings were installed (as opposed to the stock rings, which were the 9/10 spd rings).

Still have the skating problem, although not as bad as before.

Combined with the Giga-X pipe bottom bracket problems I've been having, I'm going to try to avoid Truvativ components if I can in the future. Not so impressed with Shimano 10 speed 105 either.

The majority of the first run of the '06 Jamis Quests (Shimano 105 10-spd bike) were incorrectly supplied with 9 speed rings on a Truvativ compact crank. They all exhibited the same problem being discussed here. The switch out to 10-speed rings for the most part solved that problem.

I don't know that it was so "incorrect." the first response my LBS got was that there was no incompatibility using the 9 speed rings with a 10-speed system, and those rings were installed stock on the Nova as well as the Quest. Sounds like crappy testing on Jamis and/or Truvativ's part to me.

Thanks for the input :) Inner chainring says 52/36 and 50/36 along with WB077blahblah, while with out ring is purely FSA 46T. I've been off the bike while on holiday, will see how Jake goes today and get back with the mechanic if there's still skating. Sounds like a bigger chainring is in order, along with a check for compatibility...

I don't know that it was so "incorrect." the first response my LBS got was that there was no incompatibility using the 9 speed rings with a 10-speed system, and those rings were installed stock on the Nova as well as the Quest. Sounds like crappy testing on Jamis and/or Truvativ's part to me.
That must have been early on when the '06's were out and about, because by late spring, the Jamis warranty guy knew the drill, and had those polished 10 speed rings at the ready for replacement. The 9 speed rings worked fine the majority of the time, but periodically the chain was getting caught riding the top of the inner ring when downshifting front der off outer ring.

That must have been early on when the '06's were out and about, because by late spring, the Jamis warranty guy knew the drill, and had those polished 10 speed rings at the ready for replacement. The 9 speed rings worked fine the majority of the time, but periodically the chain was getting caught riding the top of the inner ring when downshifting front der off outer ring.

correctamundo. chain also had/has a lovely habit of falling off the big ring to the outside under load. And it's not a derailleur problem.

I swear, that bike---it's cursed. Great frame and all, but I just keep on discovering new componentry problems.

I took all these excellent suggestions onboard...then developed a headache at the thought of going back to the bike shop (for like the 7th time since I bought the bike) and did the obvious: adjusted the front D. Bear in mind, the mech at the bike shop has had 3 chances to do this, on top of my going there with the skating problem and suggesting maybe it needed adjusting to give more throw at the front.
Long story short: no skating issues, some slight noise at extremes of bad chainline.
I hate the bike shop :(
Thanks again for everyone's input, will be handy down the track if problem resurfaces :)