My bike has a mysterious, intermittent ailment. Itís a 2003 61cm 27-speed Lemond Tourmalet with nearly 12,000 road miles, originally equipped with Shimano Tiagra and 105 components. It has developed some kind of drive train problem that causes the crank arms and pedals to sometimes skip or slip repeatedly, not just when standing on the pedals, but even when subjected to only moderate pedaling pressure. Sometimes I can ride for weeks and hundreds of miles without a problem, and then suddenly it starts slipping so badly that I have to walk it up a hill. The problem goes away before I can get to a bike shop and demonstrate the problem to a mechanic. It happened again yesterday during my commute home from work. After taking the rear wheel off and carefully checking unsuccessfully for the source of the problem, I thought to myself that I would baby the bike the rest of the way home so as to be certain that the problem was still in effect when I reached home and could then take the bike to the bike shop in the back of my car and demo the problem at the bike shop, once and for all. By the time I mostly coasted down the other side of the hill, the problem had vanished again!
When the problem occurs, changing to a different chain ring in the front (I have a triple) or different cluster gear in the back, has no effect. When the skipping or slipping occurs, the only thing I can see as I look down at the drive train is that the lower wheel of the rear derailleur flicks slightly forward each time the pedals skip or jump, but not to one side or the other as if the chain were trying to switch to a lower or higher gear on the cluster. I do not notice any behavior changes with the chain or the gears. There is also no shifting noise associated with this slippage and no visible sign of the chain attempting to climb up to the next larger gear as there is when the chain is on the verge of switch gears. The only noise is the sound of the chain seemingly slipping or jumping teeth and then catching; however, I am unable to visually verify that the chain is actually slipping.
The chain on my bike seemed to begin slipping teeth 6 months ago or so when I would put all my weight on the pedals when standing to climb or accelerate. Then the problem gradually began to sometimes happen even when just pedaling while seated. Obviously, this was both scary and dangerous, nearly causing me to crash several times. Iíve taken the bike into the bike shop several times for quick, non-appointment diagnoses. (the waiting list for an official service appointment is too long) Originally, the diagnosis was that the drive train components on my bike were old and worn out and needed replacing. This is where the story gets really weird. So, I saved my money and between Christmas and New Years, I upgraded and inve$ted in Shimano Ultegra replacement parts for the large and middle chain ring (they were out of stock on the small chain ring) and replacement 9-speed gear cluster. At the recommendation of the bike shop sales person, I bought a 9-speed non-Ultegra replacement master link chain that he used and was touted as being better than Ultegra. I installed all these myself, being very careful to follow the installation instructions. At first, I thought the replacement parts had solved the problemÖ.until the skipping started reoccurring!
It appears to me that the teeth on the chain rings and cluster are straight and Iíve checked for stiff links in the new chain or a problem with the master link. The bottom bracket was replaced by the bike shop sometime last summer. Iíve replaced the shifting cables in the past year due to breakages.The rear derailleur appears to be working properly.
Iím scheduled for a double century in a week and a half. Anybody got any ideas on what kind of gremlin is in my bike and how I exorcise it?