How To Avoid Dropping Your Chain When Changing Down On The Front.

[Lovacott]

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.

 

[Blue Hills]

well you certainly shouldn't be standing on the pedals when changing down anyway. So that's an advance.
I wouldn't stand up at all when pedalling, uphill or not, unless I was in the Tour and champagne, mass adulation and willing women were waiting over the hill.

 

[battered]

You shouldn't change gear under load. You definitely should NEVER EVER drop onto the bottom ring under load. Full stop. I have broken enough MTB chains to learn this.
Also, if you do shift under load and you drop a chain, this is probably because the BB flexes under load. If you stand on your right pedal and shift, the chainring will be slightly further to the R than normal, making a chain dump more likely.

 

[Lovacott]

Also, if you do shift under load and you drop a chain, this is probably because the BB flexes under load. If you stand on your right pedal and shift, the chainring will be slightly further to the R than normal, making a chain dump more likely.

I haven't dropped a chain for quite while but a year ago, I was dropping my chain almost daily.

On the back end, I can change up and down with impunity, but on the front, it works differently.

The front mech is simply a "tugger" which pulls the chain left or right and if you counter that force by loading the pedals, you create sideways torque and that is what creates the snap across to the left.

I know it's basic stuff, but it's taken me the best part of a year to get to grips with it.

I now consider myself to be a bit of an expert on gear changing

 

[matticus]

I create sideways torque

?? Whatever next!

 

[battered]

I haven't dropped a chain for quite while but a year ago, I was dropping my chain almost daily.

On the back end, I can change up and down with impunity, but on the front, it works differently.

The front mech is simply a "tugger" which pulls the chain left or right and if you counter that force by loading the pedals, you create sideways torque and that is what creates the snap across to the left.

I know it's basic stuff, but it's taken me the best part of a year to get to grips with it.

I now consider myself to be a bit of an expert on gear changing

Bear in mind also that the rear cassette is designed to shift under moderate load, that's why the sprockets have climb ramps to make this easier. It's common to have the chain across 2 gears. The front isn't like this, it's really not designed to be loaded when it's bent across 2 gears and if you do, you'll break it.

 

[Drago]

Never unshipped a chain.

 

[Lovacott]

Bear in mind also that the rear cassette is designed to shift under moderate load, that's why the sprockets have climb ramps to make this easier. It's common to have the chain across 2 gears. The front isn't like this, it's really not designed to be loaded when it's bent across 2 gears and if you do, you'll break it.

The rear mech does whatever I want it to do under any load without issue. But it's a different bit of engineering to the front mech.

The front mech simply tries to shove the chain left or right and if there is something trying to stop it (a load), it won't do as it's told.

 

[Lovacott]

I can't say why it doesn't happen to me now but I guess it's a case of adjusting things correctly.

It's probably because you know how to ride a bike.

 

[cougie uk]

A chainkeeper would be worth fitting if you do drop your chain.

 

[Lovacott]

A chainkeeper would be worth fitting if you do drop your chain.

I've fitted one on the CF road bike just in case.

Maybe they should be fitted as standard?

 

[Poacher]

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.

Exactly this. The front mech has to deal with whatever tension the rider is applying to the chain via the pedals, whereas the rear mech only has to deal with the tension the spring(s) in the rear mech apply to the chain (which looks a bit slack, by the way. )

 

[rrarider]

Changing down whilst standing on the pedals is most ill-advised, as it is often very painful, particularly for men.

 

[Lovacott]

Changing down whilst standing on the pedals is most ill-advised, as it is often very painful, particularly for men.

I've found that out on more than a couple of occasions.

 

[sleuthey]

That sideways torque is like an elastic band being bent in the middle.

I think that’s one aspect of it. I think the other is that when the chain is under tension it’s like the elastic band under tension and requires a larger force from the shifter to move it a certain distance. When it finally leaves the cog, due to the force applied, it picks up such momentum thatit overshoots the next cog and derails.