Bicycle Mechanics - fixed drivetrain issue

about a month ago i bought a fuji track from my LBS and everything was going great until one day my cog slipped (or something like that, my friend said my lock ring needed to be tightened), so i took it in and the mechanic said it's normal for new bikes to have that problem. he fixed it up and it rode great, up until a few days ago when i felt like sprinting part of my commute. i started my sprint and i heard a creaking sound followed by a jerking of the drivetrain (it sounded catastrophic) but nothing more than the normal chain slack was to be felt when i slowed down (thankfully the fixed stayed fixed). i forgot about it and today my friend and i were on our way down the same open road, so i called out a flying sprint to the next light and we took off. as i powered away the same thing happened, but with more strange slipping or something. i'm going to take it in to my LBS tomorrow, but i figured in the meantime i'll post an inquiry here since i don't know how to diagnose the problem myself.

it's a stock 2009 fuji track, and i remember when i rode on a few different rental fuji track bikes at the encino velodrome when i was taking beginner classes over the summer, i could sprint without any strange problems like this, although i'm not sure if those bikes were stock or not...

any ideas?

about a month ago i bought a fuji track from my LBS and everything was going great until one day my cog slipped (or something like that, my friend said my lock ring needed to be tightened), so i took it in and the mechanic said it's normal for new bikes to have that problem. he fixed it up and it rode great, up until a few days ago when i felt like sprinting part of my commute. i started my sprint and i heard a creaking sound followed by a jerking of the drivetrain (it sounded catastrophic) but nothing more than the normal chain slack was to be felt when i slowed down (thankfully the fixed stayed fixed). i forgot about it and today my friend and i were on our way down the same open road, so i called out a flying sprint to the next light and we took off. as i powered away the same thing happened, but with more strange slipping or something. i'm going to take it in to my LBS tomorrow, but i figured in the meantime i'll post an inquiry here since i don't know how to diagnose the problem myself.

it's a stock 2009 fuji track, and i remember when i rode on a few different rental fuji track bikes at the encino velodrome when i was taking beginner classes over the summer, i could sprint without any strange problems like this, although i'm not sure if those bikes were stock or not...

any ideas?

I would think that by now your rear sprocket/cog is tight, but check the lockring for tightness.
Is your chain loose? 1/2" play at the center is about right.

Are your crank arms on tightly?

Check your frame. Especially around the chainstays.

so I took it in to my LBS today and they said my lockring was loose. the mechanic tightened it up and I sprinted a bit and the catastrophic-sounding problem is gone, but if this happened twice within a month, shouldn't I be a little worried about my lockring/cog situation? it was suggested that I come back to the shop in a week to see if it happens again, which then they could try and fix it for good with new parts under warranty, but is there anything to do on my part as a preventative measure, or am I totally at the mercy of my LBS?

Learn how to rotafix and buy your own lockring wrench. Also You may want to opt to replace the ****ty quality cog/lockrings that come stock on that bike with minimum dura ace. And check the hub threads while you're at it.

Keep running your bike like that and you will come back with a stripped hub.

I've had the same kind of pb with the fomula lockring that came with my Fuji track 2009. Now i changed for a better steel lockring and everything's fine.

If this continues, the theads on your hub will be toast.
I also take it that you're running brakeless.

yeah brakeless. I'll look into some new components for the back, but what should I be looking for? I'm assuming steel. any brands other than shimano for comparison's sake? thanks.

yeah brakeless. I'll look into some new components for the back, but what should I be looking for? I'm assuming steel. any brands other than shimano for comparison's sake? thanks.

No.

Minimum good quality cog = dura ace, lockring = dura ace. And it's also the cheapest. The next step up would be EAI/Phil cogs. Which are about double the price.

I would also recommend a surly cog. I paid $35 for mine and I love it. Also greatly reduced drivetrain noise. Make sure you pay attention to your chain size though! 1/8" cog will not fit a 3/32" chain.