Why the big ring is smoother

The big ring. Big and smooth. Uhm... yeah.

One of the guys I sometimes ride with mentioned the other day that he preferred riding in the big chainring as often as he could. Yes, part of it was vanity. But part of it was that he felt that the drivetrain felt smoother on the big ring than the small, even when the gearing was equivalent. He couldn't figure out why.

I'm not an engineer or a scientist, nor am I an artist (as you're about to see in the following diagrams), but I do have an explanation about why you feel smoother on the big ring. And no, it's not because you think you have impressive quads.

As the chain moves through the drivetrain, there are two mechanisms by which additional friction is generated. One way is by running the chain off its centerline so that the plates start to rub laterally against each other. This happens most often when you cross-chain (e.g. big ring/largest cog). This is something that many of us have noticed since the resistance in the chain is usually quite noticeable in that position.

The second way to increase friction in the chain is by decreasing the diameter of the chainring or cog that the chain runs through. Any easy way to start thinking about this is by understanding when the least friction is generated in a chain as it's being pulled along.

Straight chain - least friction

From a conceptual standpoint, a straight chain generates the least friction/resistance as it's being pulled length-wise, probably because there is the least amount of interaction between the links on the chain. On the other hand, when a chain is doubled back on itself in a tight loop and moved, there is a lot of resistance because the links and plates are rubbing against each other quite a bit. In particular, there is a lot of interaction at the point where the chain "turns".

Tight turn - high friction

Now obviously, these are extremes and the reality is that your chainrings and cogs fall somewhere in between. But assuming that the big chainring is more like a "straight chain" and a small chainring is more like a "doubled over chain", it is reasonable to think there is less friction and resistance on the big ring.

Big ring (left) has less friction that small ring (right)

Chainrings and cogs aren't the only place where the diameter of the chain movement makes a difference. In the tour last year, several riders (including Andy Schleck) used a modified Berner rear derailleur that had significantly larger pulleys. The idea was similar... increase the diameter to decrease friction and make the drivetrain smoother.

Example of a Berner pulley

So there you have it. Not only do you look like a stud in the big ring... you also look smooth. Your momma would be proud.