Keep It Big, Keep It Straight

I came across an article on Bike Radar recently that was addressing the uptick in use of big chain rings by Pro roadies. The article is fascinating from the standpoint of how Pro cycling has changed in regard to equipment choices, but what I found most interesting was the technical aspects regarding drive train efficiency. 

What I found there in that article hinges on two basic principles of drive train efficiency and informs all of us how we can make the best use of any chain driven drive train. Both of these ideas were expounded upon years ago and that is where I learned these ideas and have implemented them into my thinking about bicycle components and which has informed my choices about drive train components over the years. 

It's been so long ago that I read about these ideas that I forget where it was I learned these ideas specifically. I know that before the Internet's widespread usage, there was an article published in "Dirt Rag" that was written by Keith Bontrager. In it he attempted to explain how to use a typical for those days triple crank set drive train. 

You can read in the "Bike Radar" piece I linked to about these same ideas, so this is knowledge that has been around for decades, but it is criminally either missing in most rider's knowledge base or completely misunderstood. At any rate, you can break it down into two basic things.

Keep It Big: Chain articulation around cogs is a source of loss of power as the smaller the arc of the cog is the more the chain has to articulate to conform to the cog shape and this causes a loss of energy. Efficient use of a derailleur drive train would be to use the largest cog/chain ring combination that is reasonable and that provides a ratio that allows the rider to do whatever it is that rider is attempting to do, be that climbing, descending, etc. 

There is another reason a rider would consider doing this and that is for the longevity of the drive train parts. Smaller cogs on a cassette contact chain links more often as they spin than larger cogs will. Also, that chain articulation under tension is supported by fewer teeth on a smaller cog, so the combination of frequency of load bearing and the focused wear on fewer teeth means that smaller cogs, when used often, will wear out more quickly than would the same/similar ratio using bigger cogs and chain rings. 

Both reasons are why I try to use bigger chain rings and cogs on my single speed bikes. As an example, I could run a typical 32T x 16T on a 29"er for a gear inch of 58, or I could get the same 58 gear inch by running a more efficient, longer wearing 40T X 20T set up. Of course, the weight conscious amongst us would cry foul, and yeah, some big chain rings will not fit on some bicycles. But the point remains valid. 

Image courtesy of Campagnolo

Keep It Straight: The other thing Keith Bontrager said in that old "Dirt Rag" article was that the straighter you kept the chain the more efficient the drive train would be. This principle of derailleur drive train usage is also expounded upon in the "Bike Radar" piece. 

This idea helps riders understand when to shift, and why. It helps explain why certain combinations, while usable, should be avoided if possible. The commonly heard term, "cross-chain" came out of discussions about how to use triple and double crank set drive trains. The more misunderstood "duplicate gearing" concept is also part of this discussion. Similarly to how one would want to always defer to a larger combination of cog and chain ring, one also would (or should) want to choose  to use a straighter chain line whenever possible over a cross-chained set up. 

Efficiency increases with a straighter chain over that of a cross-chained choice of gear, but again, wear is accelerated with cross-chained choices. And where do we see a lot of severe chain angles now? That's right. In 1X systems. 

Front derailleurs were eliminated in MTB for several design choice reasons not based on drive train efficiency. Gravel bikes ended up adopting 1X to a degree due to the MTB influences, and rarely will you see 1X in Pro level road racing, because efficiency is king there. I would also put forth the argument that consumers see 1X as being "easier" since they don't have to think about how to use a front derailleur. Compounding this is that shifting a front chain ring is harder to do than shifting at the rear and you can see why 1X is so popular now. 

It may be popular, but 1X has shown that it is not as efficient as a multiple front ring crankset by wearing out chains laterally as well as in the traditional way. In fact, many mechanics have posted online that they change 1X chains now based more on lateral wear than based on the traditional way that chains are measured for wear. That's an indication of less efficient operation.

Conclusions: Of course, you don't have to consider any of this and you can have fun riding a bicycle without any knowledge of the above. That said, if you care about efficiency and wear issues, these ideas might help you out.