Chucking the chain, me or the bike?

Paul 8v

Not sure where this went, I think it is a mechanical issue so I thought I'd put it in here.

This has happened to me a couple of times now and thank god I've kept it rubber side up but last night it happened and I nearly took out some team mates where I lurched over the road where there was suddenly no resistance!

When I've gone for a really big effort for a sprint the bike has thrown the chain off the big ring on to the pedal side, I can't work out why, it's already been in the gear so it's not from overshifting, is it possibly flex in the frame, chainring or something? I'm a bit puzzled.

The bike is a Trek Domane 4.3 and the chainset is a Shimano non-series one which is essentially a 105 but with silver bits on it (RS500?)

Many thanks

Paul

JayKosta

Sounds like maybe a frame flex issue to me - especially if you are heavy and strong.
Other concern is the position of the inside edge of the front derailleur cage - ideally the cage should be centered over the chain. If your shifter has 'trim' capability then it can be used to center the cage after a shift is complete.

Jay Kosta
Endwell NY USA

drlodge

My guess is I think same as Jay just said. Something is flexing (frame and/or chainset) and when you put your left foot down, the chainring moves to the inside of the frame and the inside edge of the front derailleur then pushes the chain to the right (outwards).

Which rear gear are you in when sprinting? I would think its more likely to happen when in a large sprocket, but sprinting at a fast speed would imply a higher gear and hence smaller sprocket...have a look to see how much clearance there is between the chain and inside edge of the FD cage when in your sprinting gear, I bet its not much and tweaking the trim may help.

Paul 8v

I think you might be right guys, I'm reasonably heavy for a cyclist at about 82kgs and I can put out a lot of power in a Sprint (albeit not for very long!) I'm going to check the front mech cage when I get home as that sounds the most logical thing. Not sure exactly what gear I was in but we were doing between 40 and 45 kph, I was at a lowish cadence and I had to really crank it so it must have been one of the smaller sprockets like an 11 or 12

Ben6899

drlodge wrote:

My guess is I think same as Jay just said. Something is flexing (frame and/or chainset) and when you put your left foot down, the chainring moves to the inside of the frame and the inside edge of the front derailleur then pushes the chain to the right (outwards).

Which rear gear are you in when sprinting? I would think its more likely to happen when in a large sprocket, but sprinting at a fast speed would imply a higher gear and hence smaller sprocket...have a look to see how much clearance there is between the chain and inside edge of the FD cage when in your sprinting gear, I bet its not much and tweaking the trim may help.

I agree with Jay and Dr L, but I also think it must be a setup issue as well. The derailleur not only derails the chain, but it also acts to keep the chain on the ring. My guess is there's too much daylight between the large chainring and the outer derailleur cage plate.

I hope you get it sorted - sounds scary as censored , what happened last night!

Paul 8v

Thanks Ben, It was pretty scary, thankfully the guy behind me was a good mate of mine and had the reactions of a ninja cat! I'll definitely sort this front mech tonight :-)

Ben6899

Paul 8v wrote:

Thanks Ben, It was pretty scary, thankfully the guy behind me was a good mate of mine and had the reactions of a ninja cat! I'll definitely sort this front mech tonight :-)

Ninja cat. We have one of those... spends most of the time not being very ninja - on the end of the bed, asleep.

Let us know how you get on.

Paul 8v

Haha, so does ours :-) I'll let you know my findings

desweller

I get this symptom when my front derailleur is not running parallel to the chainrings.

meanredspider

I kinda doubt frame flex adding more than a small contribution that kicks an already existing issue over the edge. It's not as if the FD is mounted very far from the BB. As above, I'd be 99% sure that adjustment of the FD in one or several of its axes is the cause.

Paul 8v

My mate who is a mechanic said he would have a look and we're going to check front mech, chainring bolts, BB, everything basically and make sure it's all tight and has the correct alignment. Hopefully that will sort it

arthur_scrimshaw

Did you ever resolve this?

I'm getting a similar probem, on the big ring and shifting to smallest rear cog (top gear) on a downhill section at a fair lick. It is a rough road surface, but I've been riding this section for years and only just started happening. Only thing that's changed recently is new wheels, much harder ride. I'm wondering if I'm backing off the load when changing up at the same time as hitting a bump is throwing the chain? Chain is the right length.

Paul 8v

I've adjusted the front mech and it hasn't done it again yet but I've not gone for a big sprint yet. I really think front mech's are the most agricultural bit of a bike to be fair, awful things! I hope you get yours sorted, as above I would check the alignment, if everyting is tight etc.

arthur_scrimshaw

Thanks for getting back to me, will do. Thinking about it I had a bit of chain suck a week ago and it may have move the front mech or bent the cage a bit?

Paul 8v

The cages are pretty tough but it's definitely possible you've moved it though. Get a token or k edge chain catcher if you haven't already got one by the way. My token was the best £8 I've spent!