chain jumping from small ring to large ring all by itself while climbing up hill

[mountainraine]

yesterday and also a couple days ago my Silveo 1.5 did something odd while i was climbing uphill in my small ring it jumped onto the large ring! so I immediately put it back into the small ring and it jumped again pretty much immediately at which point I unclipped and jumped off then walked the bike up the hill ( no fun) this isn't the first time I have had issues with the chain. Recently it also gets stuck on the large ring ( like jammed on the teeth) and I can't pedal so I have to hop off and yank to get the chain off the large ring. It's a little frustrating. I've had the derailer worked on by two bike mechanics and then I've tried adjusting a wee bit. Any one else had the jumping from large ring to small ring experience?

 

[ak-tux]

@mountainraine, it sounds

@mountainraine, it sounds like "chain suck" to me (See here ). Probably Chainring has some wear or a tooth distorted.

In addition, probably the front derailuer is not properly tuned or faulty.

My 2 cents..

 

[Jeremy S]

I used to have

I used to have chainring/front derailleur problems on my Sofrider, the chain would sometimes lock up or fall off. One time I noticed that some of the teeth on the large chainring were actually bent sideways, I hammered them straight with a rubber mallet and that seemed to help. Another time I had the bike shop tune the FD, and they decided to shorten the chain. That also seemed to help. Eventually it worked OK.

You might check how your cable is run from the handlebars to the FD. Check that there are no sharp bends in the housing, and that any exposed cable is straight and tight (like a guitar string).

Basically there are a lot of factors that could mess with your shifting. It seems like a mechanic looking at the bike should have the best chance of fixing this, I'm sorry they couldn't help so far! Assuming you paid them, take the bike back and tell them they did it wrong.

 

[mickjordan]

Chain jumping

Jumping from small to large ring is very weird, as this usually requires considerable increase in cable tension to achieve it during normal shifting. Dropping back can occur quite easily if there isn't enough cable tension to hold the chain on the larger ring.

You can sometimes get the rear derailleur to spontaneously move from a small to a larger ring, as the tension required is much less and only slightly too much tension on the cable can cause it.

So back to the FD which, BTW, is the most complicated bike part to adjust. If the cage is close to the left edge of the chain when in the small ring, and also perhaps the cage isn't quite properly aligned, I can just about imagine that the chain could find a way to engage the cogs of the big ring and start shifting. However, I can't actually imagine how it could actually shift onto the ring without a significant increase in cable tension. So I wonder how that could be happening? Well one way, which I experienced on my Silvio, is that your boom is not tight enough, so when you pull on the bars, which you might well do when climbing, you extend the boom which pulls the cable and effectively shifts up. So check your boom clamp. It's a good idea to put a piece of tape at the join so that you can see if things are changing over time.

 

[BentBierz]

You Are So Right...

So back to the FD which, BTW, is the most complicated bike part to adjust.

Amazing the number of people that don't understand this or, worse, believe the complete opposite. In my experience, the number of improperly adjusted FD's outnumbers those set up properly.

 

[Jeremy S]

I put black electrical tape

I put black electrical tape on my boom when I adjusted it. I can tell if anything moves, otherwise it blends right in.

 

[Doug Burton]

Boom slippage

Mountainraine,

The only mechanism I can think of that would cause the FD to shift UP is slippage of the front slider tube.

In a few cases, we shipped upper slider tubes that were not slotted. These won't tighten properly no matter how hard you pull the slider clamp down. If the slider slips forward, that will effectively shorten the derailleur cable, which is what the shifter does to shift the derailleur up.

To check this, look underneath the upper slider tube near the boom clamp. There should be about a 2" long slot visible, with a round hole at its end. This slot allows the clamp to compress the upper slider tube around the lower slider tube firmly.

Please let me know what you find. This is not terribly dangerous as the cable will stop the sliders from separating, but it is not how we want your bike to work for you.

Best,

Doug

 

[marshall2389]

Another driving force for

Another driving force for upshifts is a crossed chain. If you are in the small chainring and smallest (or small) cog then your chain could be crossed to the point that it almost rubs the large chainring. Maybe hitting a bump or something caused the chain to grab the large ring. When it upshifted did the chain rub on the front derailleur? I ask to try to determine whether the derailleur moved up or not.