Chain skipping when pedaling hard in lower gears.Shimano Acera drivetrain

Chris130:
I guess I threw you off a little. I mean the 2 smallest cogs in the rear & the smallest ring in the front. So that would be "small-to-small" right? Is this "cross-chaining"? Tell me more about this cros-chaining thing you speak of. Is this an expected symptom for the way I'm riding? Is it due to the quality of my drivetrain (Shimano Acera)? When the kid in the store (around 130 lbs) tried to repeat the slippage, he couldn't. But when I (around 210 lbs) dig into it, it starts to skip. The guys in the LBS adjusted the derailleur, checked the chain, & a couple of other things to no avail.


Ardent:
Good point. I understand that it's the "el-cheapo" of Shimanos but nevertheless I still want to get to the bottom of the problem or at least better understand what's going on & learn a thing or two in the process. I'm pretty much a novice. I figured I'd work up to better components as I get more into riding.


It sounds like I can just chalk this up to cross-chaining & low quality components. Would a different setup (such as an XTR) correct this problem? Is there anything else I could try?

Thanks
James

Ja, OK, let's say you have a 3x9 drivetrain - 3 chainrings and 9 cogs on the cassette. In general, most of your trail riding is done while in the middle chainring. On most stock setups, your chainline is such that you can use safely & routinely use all 9 gears in the back when in that middle ring, no problems. However, when you are in the Big Ring (literally the biggest chainring) up front, you should ideally try to stay in the 3 smallest cogs in back (smallest in diameter, least amount of teeth). Likewise, if you are in the granny ring up front, you should try to stay within the 3 biggest (closest to your wheel) cogs in the back.

This will ensure that you have a properly straight chainline. Stand behind your bike and look at your chainline from the cassette up to the chainrings. It shouldn't "angle" too much either towards or away from the bike. Standing there, you can picture when you are doing big-big or small-small, the chainline is at a hefty angle - hence the term "cross-chaining". Not good. Your goal when shifting is too keep that chainline as "straight" as possible. Cross-chaining is hell on your drivetrain; it puts a lot of undue stress on the chain, and it wears down the teeth on your cassette and rings alarmingly fast.

On a 3x9 drivetrain, there aren't really 27 completely unique gearings - closer to half that number, in fact. That's one reason why 2x9s and 1x9s (or single-speeds, haha!) are increasingly popular. In theory, you can use all 27 gears, but in practice, you definitely shouldn't. Your drivetrain will explode in short order.

Also, you are pretty big dude, so that adds stress to the drivetrain - that itself isn't a big deal, but it doesn't help either. Here are some tips to consider: When you shift gears, do NOT do it "under load" (in the middle of a heavy pedal mash). Either pause your pedal stroke for the split-second when you shift or just unweight your pedal stroke during the shift. Also, anticipate shifts ahead of time - trying to downshift while struggling to grind up a hill is going to end badly. Remember that your derailleurs are just brute-force mechanisms - they don't operate well under a heavy load (or hard pressure, or however you want to think of it). Higher-end systems (XTR or X.0) are indeed much more forgiving and consistent, but nothing's bomb-proof.

Cheers, Chris

Agree w/Chris130...sounds like you could be crossing up your drivetrain. You should never be in the smallest cog in the front and the smallest one in the rear. As was pointed out, most of your riding will be done in the middle chainring, while your steepest uphills will be in the small (granny gear) chainring, and your fastest downhills will be in the big ring in front.

You're safe to use pretty much all 9 of your rear cassette in the middle chainring, but should dfinatly stay in the lower (bigger ring) half of your cassette if you are in your granny gear, and stay in the upper (smaller rings) half of your cassette if you are in the big ring up front.

If you find yourself in your smallest front and smallest rear, chances are you should have shifted to the middle ring up front long before you get into that combination. Short term use isn't bad (to get you out of a sticky spot where you are NOW) but you should try to shift into a better chainline as soon as you can.

You'll find that whatever gear you are pedalling in now will be the equivalent of the same gear (roughly) if you went up one chainring and down two cogs, or down one chainring and up two cogs...or vice versa.

Tim