March 07, 2005

bicycle problems Any monkeys good with the spanner and know a bit about bicycles. I have a problem that needs urgent fixing to prevent me from going over the handlebars a second time.

I'm a bike novice, I recently dug out my 10yr old mountain bike. Its a reasonable bike Marin Muirwoods. My problem is when I am pedalling hard and the pedal is at the apex / top and I push down hard it just slips. There is no resistance. It is usually accompanied by a metallic crack. I got the rear cannister? and chain replaced but to no avail. Dont think it is slipping into a different gear either. Could it be the crankarm? A couple of days ago I was standing up pedalling hard to get some speed and suddenly there was no resistance and i ended up over the front of the bike. Fortunately a kindly pedestrian dragged me off the road. Any help much appreciated

  • I am far from an expert, but can you tell if it's just the chain slipping? look at the sprockets and see if they are all chewed up. might look to see if the gear shifting mechanism is set up right or if its sort of sitting in between gears. Im sure there are a pile of others here who know a ton more than I do though...
  • replaced the chain and the rear cartridge / sprockets? so shouldnt be that. got to nip out for a couple of hours. Praying there will be an answer waiting.
  • I think the word you're looking for is "cassette," if the thing you replaced looked like this. Technically, any given bike may use either a cassette or a freewheel - they look about the same and I don't understand what the difference is. I had a bike that would toss me on the ground now and again in exactly the manner you describe. I also replaced some parts (well, the chain - the bike was too old and too cheap to spend more than that on) to try and fix it, and it didn't help. If the bike was used for a long time without regular chain replacements, the cogs on both the chainring (the front cogs) and cassette can become cupped as the chain wears and lengthens, causing poor gear shifting. In extreme cases, an inch or two of chain may stick to the cogs when the chain is under a lot of tension, then all come loose at once with a metallic clank and the feeling that the chain has slipped. To fix this, the chain, cassette, and chainring must all be replaced. My problem bike did this sometimes, but this wasn't serious enough to throw me off the bike. The problem with my bike was that pedaling very hard could actually bend the bottom bracket enough to put the chainring out of alignment. Sometimes it would be so far out of alignment that a few inches of chain would jump off the chainring. Once that happened, pedaling resistance dropped suddenly, and I would fall off the bike. This would also relieve the tension on the bottom bracket, swinging the chainring back into alignment. So the chain would roll right back onto the chainring it was supposed to be on. After I got up, cursing, I'd see nothing wrong with the chain or the cogs. I only figured this out because I happened to glance down at the chainring while this was happening once. The solution in my case was to buy a better bike, with a stronger bottom bracket. But I'm not an expert or even a serious bike rider, so take all this with that in mind. I don't know of any myself, but I bet there are bike discussion boards out there that could answer your question more accurately.
  • Um. I know a bit about bikes, but I don't have enough information. Here's some things to do: Flip the bike upside down, and give the pedals a hand crank. This will allow you to mimic peddling without, you know, risking your life and limb. When is there no resistance? Try holding (with gloves) the rear tire while cranking the pedals. What's happening? Is your chain popping loose? Or are the pedals just not applying force? I'm not sure why you got the chain replaced. When the chain is bad, it's pretty obvious that it's the chain. I don't know what you mean by "cannister." Rear gears? I assume that's just some sort of different terminology?
  • Sounds like Lagged has your problem down. I've never had it nor heard of it, really (at least, nothing similar without obvious damage). But my bike's heavy as shit and built to last.
  • If you've only replaced the chain and rear cassette, then the only remaining culprit is the chainrings. (The ones on the crank, look like this. If the slipping occurs more often on some chainrings than others, then it's definitely the problem. Replace the offending chainrings (and the chain too, if it's been on more than a few months) and you should be safe to go. Two other outside possibilities: Maybe you've got the wrong chain type, tho I've never heard it causing symptoms like you describe. A 10 year old bike probably has a 7 or 8 speed rear cassette. Make sure you didn't put on a 9 speed chain. Or, it's very unlikely but possible that your bike originally came with Suntour components. (They went out of business in 1995) If that's the case, then you need to replace just about everything, because Suntour stuff is not compatible with the usual Shimano standard.
  • Thanks for the help. lagged that sounds feasible. I did get the rear cassette (thanks for the correction, i did say I was a novice)and the chain replaced. That leaves the chainring as the only suspect. Can you explain in laymans terms what you mean by the bottom bracket? Is it a support to the chainring? Did your chainring leave the vertical and becoming skewed?
  • Peachy - I have a Marin (a Hawk Hill) which does exactly the same thing to me. In my case, it seems to be related to the rear chain guide not quite lining up with the cog, and the whole thing behaves as though it's in the middle of changing gears, the chain jumping and skipping in the gap between cogs, and not quite engaging with either. I get it fixed by taking it to a bike store and getting them to adjust the alignment, and then wait 6 months or so for it to start drifting again.
  • Peachy - the bottom bracket is the part that connects the two pedal arms together. I.e. it's the axel that the pedals (and your feet) spin around. (Pic)
  • Can you explain in laymans terms what you mean by the bottom bracket? The Bottom bracket is the tube that holds the crank, it's bearing, and the pedals, along with the lower frame tubes that hold it all in place. Any bike will have some flex in this area, but my problem bike was so weakly constructed that the chainring would indeed leave the vertical (very slightly) under heavy pedaling. And, as polychrome points out, if the chain slipping off the chainring is indeed the problem, it may be mitigated by adjusting the front derailleur properly.
  • it may be mitigated by adjusting the front derailleur properly. I should have said: it may be mitigated by adjusting the derailleurs (meaning front and rear) properly.
  • I go with the bottom bracket thing, I had a friend who used to race mountain bikes and he ended up going over his handlebars when his bottom bracket broke. Have it checked out by the bike shop, best advice on this thread.
  • thanks all, Your comments will be invaluable when i take it into the shop and ramble on at length in an attempt to sound knowledgeable.
  • It sounds like your chainrings. If the chain and rear cog-set were worn enough to reqire replacing, chances are that the chain rings were worn as well. I am concerned that the shop that did the original work didn't advise you about the chain rings at the same time. Is this a bicycle shop you are dealing with?
  • I just returned from the bike shop myself. My bottom bracket is making a click-click with each revolution of the pedals. I have to bring it back because I was not prepaired for the five hour wait the told me to expect.
  • Wow all this talk of bikes and bike shops reminds me of a story. Probably won't help any, but I'll tell anyway: A couple of years ago I was mowing lawns and it was really hot out and I got thirsty. Luckily across the street from me there was a store that called itself the "North End Dairy". They had a big "enjoy coke" sign out front. So I assumed I could buy something to drink there. Turns out it's a bike shop. Go figure. True story.
  • It might also be your Knute valve. Or your Johnson Sway Bar. Tell the shop to look at those.
  • Contrarily to our suppositions here, my bike mechanic says it's most likely your free-hub body. Is the groupo 7 speed Acera, or 8 speed Olivio? Also NEVER go into a bike shop and tell the mechanic what to fix, because they will fix that, whther it's the ptoblem or not.