hey guys, i kind of have a cranking/grinding w/ gears shifting on their own frequently....i've got no clue what causes it, but its annoying. how can i remedy this? it happened today bad when i started on high front cassette, which this is the first time i've tried staying in high front cassette...i've been staying in mid and just shifting up on very steep hills. once i downshifted into the middle front gear it was fine. i'm still a newbie, so go easy on me

By "high front cassette", I think you mean your large chainring, as in the biggest cog that's between your pedals. Is the grinding/shifting coming from the rear cogs or the front? If it is the rear, it could be that your rear derailleur needs to be adjusted. How many miles does your chain have on it?

When i say "high front cassette" i mean its 3 gears. i tried it in 3rd gear, was having issues, but tried it in mid(2nd gear) and remedied that issue....but i'm still having issues w/ my rear derailleur and chain....i've got about 140 miles on it as of today.

you're probably cross chaining- using the big gear in front and the low gears (biggers cogs in back) at the same time.

oh, so if i'm on low rear gears, i need to be in low front gears, with mid rear gears, i need to be mid front, and high rear i use high front?

Um...

Depends.

Since you've got a (presumably fairly new) Specialized Sirrus, it shouldn't be doing what it's doing. If it's a brand new bike, it might be time to take it into the shop for an adjustment. Sometimes things go a bit wonky during the first 100-200 miles of riding, and most shops will offer a free adjustment as part of buying the bike.

If it's not brand new, I would tend to get it looked at anyway. An aluminum framed bike should not have so much flex and slop in the setup that it's shifting by itself. That suggests some of the components are wearing out, or perhaps the frame isn't all that it should be. Given the way Specialized sets up the Sirrus, I think it's not very likely that cross-chaining would cause this all by itself.

You typically dont want to be in the largest chainring(front), and the largest cog(back), or smallest chainring, and smallest cog. With 3 chainrings you have more options if you stay in the middle chainring and just move your rear derailleur.

OK... checked your page and couldn't see any mention of what bike you have so here are a few generalizations that can cause the issues you described.

1) If you have an indexed shift system it needs to be trimmed.
2) Cable housing ends may not be clean and have burrs. This causes cable drag and even worsens the issue described in #3.
3) Cable housing or cable itself is obstructed. I've seen people come into the shop complaining that their bikes shift automatically and the culprit turns out to be a handlebar bag, bottle cage, frame bag or other item attached to the bike that interferes with the cable routing somewhere along the line. Turning the bars slightly, bumping the offending bag or even an oscillation of the bag while pedaling changes the tension of the cable and causes the derailleur to shift automatically.

As others have mentioned, the chainline can be suspect... that is even a modern multi-speed chain has it's flex limits and the further the selected chainring and cog combo are out of alignment with each other, the better the chances are that it'll be noisy and tend to skip between gears... especially if the chain isn't properly lubricated. So choose a chainring/cog combination that gives you the ratio you need and puts the least amount of flex/strain on the chain.

If it's "ghost shifting" like that, you've got a problem with cable tension for that derailleur. About a $10 job for adjustment at the bike shop; or you may even be able to fix it yourself.

Did you recently buy this bike at a local shop? If so, take it back to them. With only 140 miles on it, they will almost certainly readjust it without charging you.