Rubbing Chain

[w00hoo_kent]

So, with the Synapse if I'm in the smallest two cogs, on the big ring the chain rubs while riding. But if I put it up on the work stand and run the pedals it sits true, no worries. I can see the chain is rubbing because there are marks on the bottom half of the links. It's only when I'm right up in the hardest gears, only when I'm pressing on and only happens on the right leg downstroke.

Because it only happens in very specific circumstances and I'm a forgetful arse with no mechanical sympathy I've not done anything about it yet, but I know I should. It was noticeable on the Ride London so I stopped at Leatherhead and asked the mechanic, he put it on the stand, showed me nothing was going wrong, made me feel about 2 inches tall and then criticised my gear choice before giving me my bike back (this may be another reason I've been doing sod all about it.).

Thoughts on where to look? I'd checked the crank torque a couple of days before the Ride London as part of my stopping the clicking noises run through. (it's a Synapse, it was a dirty seat post). I presume that'd be the first place to look again although wouldn't have expected it to come loose again.

 

[uphillstruggler]

So, with the Synapse if I'm in the smallest two cogs, on the big ring the chain rubs while riding. But if I put it up on the work stand and run the pedals it sits true, no worries. I can see the chain is rubbing because there are marks on the bottom half of the links. It's only when I'm right up in the hardest gears, only when I'm pressing on and only happens on the right leg downstroke.

Because it only happens in very specific circumstances and I'm a forgetful arse with no mechanical sympathy I've not done anything about it yet, but I know I should. It was noticeable on the Ride London so I stopped at Leatherhead and asked the mechanic, he put it on the stand, showed me nothing was going wrong, made me feel about 2 inches tall and then criticised my gear choice before giving me my bike back (this may be another reason I've been doing sod all about it.).

Thoughts on where to look? I'd checked the crank torque a couple of days before the Ride London as part of my stopping the clicking noises run through. (it's a Synapse, it was a dirty seat post). I presume that'd be the first place to look again although wouldn't have expected it to come loose again.

Can you not let the limit screw off a little to let the front mech travel a little further? it may be when you're putting the power down with your right leg that the frame is flexing.

 

[w00hoo_kent]

I can give it a go, but on the work stand it's sitting pretty comfortably in the middle of the throw for the front mech. I vaguely remember seeing something about it being possible for the mech to be twisted slighty, it's the bottom of the mech that is catching the chain links, rather then hitting them completely flush (from memory the rubbing shows on the bottom third of the link, I'll try and take a picture when I'm at home, I'm on the other bike today.).

 

[raleighnut]

Sounds like frame flex to me as well, but check the BB for movement too.

 

[IanG1]

Sound like possible frame flex to me too? try sitting on the bike holding the brakes on and pushing down hard in the drive side pedal while looking at the BB area. You will be suprised how much stuff flex's under load. Other than that make sure your front mech is aligned properly and set at the correct height on the big ring and maybe just tweak the limit screw 1/4 turn to see if any improvement.

 

[Globalti]

A Synapse is a bike with excellent lateral stiffness so the movement must be very tiny. The advice to check your limit screws is good. Check also that you are pushing the gear shifter right across when going up to the big ring, sometimes they can go a bit further.

 

[w00hoo_kent]

Definitely pushing the shifter all the way, I've got in to the habit of giving it a second tweak when I shift up to the big ring now just to be sure.

I have to admit to a small worry that the pothole I hit that buckled the wheel may have ended up damaging the frame after all, but I presume I'd have seen big evidence of a crack by now if there was one.

 

[jagman.2003]

My opinion would be....there is some frame & crankset flex causing the chain to rub. Right side because you are probably stronger with right leg.
Shimano front mechs now have the trimming option. Presuming you have Shimano setup. The lever can be moved just a tad more & it will trim in for the extra required movement. Larger cassettes need just a little more movement.
Setting up can be fiddly, but there are some videos on the net that explain it well.
Hope that helps.

 

[w00hoo_kent]

It's a 105 groupset with compact crank on a 2014 bike so I guess it'll have this, I'll have a surf around the web over the weekend.

 

[jagman.2003]

It's a 105 groupset with compact crank on a 2014 bike so I guess it'll have this, I'll have a surf around the web over the weekend.

I've done this on Tiagra & Ultegra & both work well now. Good luck.

 

[w00hoo_kent]

Thanks, I'll let you know how it goes after I've had a play.

 

[Nigelnaturist]

BB movement, it happens when you put power down with the right as this is the direction it will move, power on the left moves it away from the cage, might just be a a matter of tightening the cable tension to move the cage out a little more, providing the stop screw is not set to far in.