Lovacott
Über Member
I've done it loads of times (dropped my chain onto the bottom bracket when changing down at the front).
Not such a big deal with a steel or Alloy frame, but quite damaging for a Carbon Fibre frame.
Early on, when it happened to me, I assumed I needed to adjust the L screw on the front.
I farted around by adjusting in by an eigth of a turn but to no avail.
So I started to think about when it was that I was dropping the chain.
Always uphill, always under load.
So I thought about it a bit more and wondered why?
Here's my conclusion (shout at me if you disagree).
The front mech gives the chain a gentle shove when you change gears.
When there is little load on the chain, it goes where the front mech tells it to go.
But if there is a load on the chain (maybe you are standing up on the pedals pumping up a hill), the chain is firmly gripped to the chain ring and all the front mech is doing, is applying sideways torque to the chain.
That sideways torque is like an elastic band being bent in the middle. At some point, the front mech will win and the chain will snap left.
The force of the chain snapping back will cause the chain to overshoot the gear change and end up on the bottom bracket.
To avoid this, when changing down at the front, you simply have to pedal "behind the load" (keep moving your legs but take off the pressure for a second or so whilst you change down).
It works for me anyway.
Not such a big deal with a steel or Alloy frame, but quite damaging for a Carbon Fibre frame.
Early on, when it happened to me, I assumed I needed to adjust the L screw on the front.
I farted around by adjusting in by an eigth of a turn but to no avail.
So I started to think about when it was that I was dropping the chain.
Always uphill, always under load.
So I thought about it a bit more and wondered why?
Here's my conclusion (shout at me if you disagree).
The front mech gives the chain a gentle shove when you change gears.
When there is little load on the chain, it goes where the front mech tells it to go.
But if there is a load on the chain (maybe you are standing up on the pedals pumping up a hill), the chain is firmly gripped to the chain ring and all the front mech is doing, is applying sideways torque to the chain.
That sideways torque is like an elastic band being bent in the middle. At some point, the front mech will win and the chain will snap left.
The force of the chain snapping back will cause the chain to overshoot the gear change and end up on the bottom bracket.
To avoid this, when changing down at the front, you simply have to pedal "behind the load" (keep moving your legs but take off the pressure for a second or so whilst you change down).
It works for me anyway.
Last edited: