Drive Chain Efficiency - Big vs Small

The discussion of chainring size has gone on for years and years…

Are larger chainring and sprocket combinations faster than a smaller chainring and sprocket combination - given that each combination is the same (or nearly the same) ratio?

There are a bunch of factors to consider. How does chain-link friction, the added weight of more chain links, added mass, chain stretch, and drive chain wear contribute to the drive chain efficiency?

From my journey as a rider, through to designing new track cycling products, the idea of how we can get the most energy through our drive chain has occurred to me many times. These are a few of the ideas I think are worth some discussion.

I want to begin by comparing two similar gear combinations that make a respectable 95.4 inch gear (+/- 0.1 of an inch). 46x13 is the small chainring with small sprocket. And 60x17 the large chainring with large sprocket. 

Weight

Weight can be crucial for some when it comes to cycling. The heavier the mass of you and the bike, the more energy is required to speed up or slow down. Although weight doesn't affect the constant speed of the rider, if you use all your energy up getting up to speed, you could have less energy to keep powering on.

Chains are the heaviest part of the chain drive system, so adding more links can have a negative impact if weight is important to you. The larger 60x17 gear combination requires 9.95% more chain length than the small 46x13 combination. Although it doesn’t sound like much, this extra weight alone may outweigh the advantages of larger gear combinations. I would love to see the numbers crunched.

To add to the extra weight of the chain, a 60-tooth dinner plate style chainring has 91.9% more mass than a 46-tooth chainring (of the same dinner plate design). The larger chainring has almost double the weight. The extra weight may not be as noticeable with light-weight carbon chainrings, but with aluminium, this could add another 200 grams to the bike.

As for the sprocket, the weight of a few added teeth could be considered negligible, but it all adds up.

Chain stretch

When energy is passed through the drive chain, some of it absorbs into the chain while under tension. The longer the distance between the tangents of the chainring and sprocket, the more links to absorb the energy being put through them.

I had thought that there was a larger distance between the chainring and sprocket on the larger 60x17 gear combination than the 46x13. Upon measuring them, I discovered this was not the case. As illustrated in the diagram above, the distance between the tangents on the larger 60x17 combination is 1% shorter than the 46x13. 

When comparing the two configurations at the same power output, this section is subjected to the same force but spread across a slightly shorter distance. Each link has to take on a bit more energy, but the overall stretch is reduced. 

A great example to visualise this stretch is to compare the chain to a pair of springs. One spring is 100mm long, and the other 200mm long. If both springs are to be expanded out to 400mm, The energy required to extend the shorter spring is double. The slightly shorter length of chain between the sprocket and chainring will stretch less and absorb less energy than the longer chain length of the 46x13 configuration. 

This theory assumes that the rear wheel position in the dropout remains unchanged between the small and large gear configurations. For best performance, move the rear wheel as forward in the dropout as possible to reduce the length of chain under tension. 

On a side note: My track chain of choice for great power delivery is the Izumi Super tough track chain. The chain is designed to be stiffer and stronger than the rest of the Izumi range. The stronger the chain, the less energy lost in chain stretch. Izumi claim that the super tough chain has 3x the durability of a standard track chain - which tells us something.

Chain link bend angles and friction

Chain lubricant helps reduce the friction created between the chain and chainring/sprocket. It also helps reduce friction as the chain links rotate while passing around the chainring and sprocket. The larger diameter the chainring and sprocket - the less each chain link needs to rotate. The tightest the chain links kink is around the rear sprocket, so I will use this to compare the difference between the 46x13 and 60x17 configurations.

The larger 17t sprocket has 4.3% less chain link rotation than the 13t sprocket; reducing the amount of friction generated. 

Drive Chain Wear

In my experience, Track cycling drive chain equipment lasts for a long time. The single speed chain configuration doesn't endure cross chaining and gear changes that chain and gears experience on a road bike. Worn drive chain equipment is not ideal. Slop, chatter, play, it all absorbs a fraction of the power going through the system. 

With the larger combination 60x17 drive chain, the proportion of chain under tension in relation to the limbo is reduced - meaning that each chain link experiences high forces less often. As discussed above, each link is subjected to a marginally greater force, but the frequency of it occurring for each link is reduced. 

Another thing to note, is the more teeth on the chainring and sprocket, the more that the tension forces are distributed over a greater number of teeth. This reduces pinpoint loading on the teeth and chain links.

Summary

As a track cyclist, and into product development, I have a passion for thinking about how we can improve the sport we love. Of course I don't know everything, but I thought I'd share a few points regarding drive chain efficiency that had gone through my mind. I encourage you to discuss the ideas, contribute knowledge and challenge my thoughts on the topic.