Bicycle Mechanics - Skipping Under Pressure

Hi everyone!

Got a problem with my rear derailliur. When I put too much pressure on it, like standing up and taking off from a start, many times the gear will "let go". I mean it feels like I broke a tooth off a gear or something, and I nearly go crotch first into the top tube. (Something I don't want!) This usually happens when I'm in 2nd or 3rd gear while on the middle chainring. What's causing this?!

Here's the info:

-2002 Fuji Supreme Hybrid used for commuting to work
-Shimano C-101 Mega Range R/D
-Shimano C-201 7-speed "Revo-Shifters"
-Never seriously taken off-road, (back roads, etc)
-About 100 miles on it
-Ridden in So.Cal weather, (Warm, usually dry)

I've tried adjusting the shifting, to limited success. I've also taken it to the LBS, but I seriously doubt they did anything to it. I'm starting to think it may have something to do with the spring tension involved with the "Mega Range" jump from 24t to 34t, but that's just a newbie guess. Oh, and I have a sneaking suspision that the R/D just plain sucks. :rolleyes:

Any help would be appreciated!

First guess, cable tension is off, to tight or to loose and the chain is trying to hop one way or the other.
However, if it has been adjusted with no improvement and this skipping usually happens when starting off, your freehub or freewheel(Your bike probably has a freewheel) has a damaged ratchet mechanism.
If you bought it from a shop, take it back and bug them(nicely) until they fix it. With only a hundred miles nothing could be worn out and it should be covered by warranty if damaged or a store plan if adjustment is needed.

The thing is, deurailler doesn't carry any load. For deurailler, there is no difference wether you are going up the hill or down. The stress is carried between rear cassette gear and front chainring, the part of the chain that is below the rear cassette gear going through the deurailler, is not under stress and just being pulled up.
Sounds like you have a gear chain issue going on, OR deurailler is not keeping the chain paraller enough to let it roll smooth...

What kind of shifter do you have? My guess is that your shifter is letting out cable - essentially shifting gears when you don't want to.

Also, keep in mind that if you have Revo shifters, and you stand up to put on the power, and your hand is on the shifter when you do... yeah. You may inadvertently actuate it a little.

As a general practice, I'd encourage accelerating from a stop in a seated position anyway :) You pretty much fingered the reason: graceful landings are difficult if your chain breaks, derails, or shifts unexpectedly while standing up! :p

Thanks everyone! These are all really good diagnoses of the problem. I'm tending to lean toward a problem with the racheting mechanism in the freehub, although I hope this isn't the case. I say this because the derailliurs(sp) are pretty much calibrated correctly. I don't have problems with "ghost shifts" anymore, now that the bike is broken in. (Cables stretched out) I don't think it's the shifters because I haven't had to adjust the bike that much in the few months I've had it. But in any event, I need to take it back to the shop and have them look at it. I got the extended warrantee from Performance Bicycle, so hopefully I won't get have any problems.

Just out of curiosity: is this a common occurance with freehubs, or lower-end Fuji bikes? I've never heard anyone say anything bad about Fuji's, and I definately don't abuse the bike. I use it mainly for a real easy commute to work, (like a mile or two), and to go get groceries from the supermarket that's down the street. I also do a few miles around a lake nearby. All are on nearly flat, paved terrain.

Thanks again!

mechBgon,

That's what I thought I did the first few times, but then I got butterfly bars. So the last few times my hands were nowhere near the shifters and it happened!

And also, I'm a easy-going kind of guy. I'm not really into zipping about quickly anywhere on my bike. But sometimes you need to stand up in the saddle for that extra bit of speed. Like when the lefthand turn traffic light goes from green to yellow in about 3 seconds, and you're in the middle of a busy intersection!

So yeah, I really need to have this problem fixed. I'll head down to the LBS armed with all your suggestions as references. Thanks again!

If you are applying pressure to the pedals and it "lets go" without changing sprockets, then it probably is your freehub.

It could happen, but is not normal for a hub with less than, say 7,000 to 10,000 miles on it.

If the bike is mysteriously shifting gears, then it is likely the shifter or cables.

Well, found the problem!

Appears the FRONT deurailler was causing the problem! Their's alot of space between the second and third chainring, and when in second or third gear the chain is too much at an angle, or something. So if you really honk down on it, the chain tries to "straighten" itself out. And the weak link is gap in the front deurailler. I guess this happens because of the play in the 7-speed setup, or something. I'm not sure. The guys at the LBS told me I'm just too powerful for the drivetrain on my bike. But from looking at these wiry road bikers and comparing them to my 210lbs of girth, I have a sneaking suspicion that they were just politely saying I'm too heavy to be mashing the pedals in that gear. Fair enough.

In any event, the bike is more or less adjusted and fixed. Thanks for everyone's advice and diagnostics. I'm very glad that it wasn't a problem with the freehub! And now I have even more reason to ride my bike: to lose more weight! :D

Congra... for identifying the problem and your journey to lesser weight.

Bicycle has been around for a long long time and has very few parts. Till now cyclists still find peculiar problems like this and others. This really made bike interesting and not a boring machine. As I follow some of the threads it is really a blessing to have a forum like this where braining storming could take place and solutions found.

Happy riding!